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| LNK520 LinkSwitch Family (R) Energy Efficient, CV or CV/CC Switcher for Very Low Cost Adapters and Chargers Product Highlights Cost Effective Linear/RCC Replacement * Lowest cost and component count, constant voltage (CV) or constant voltage/constant current (CV/CC) solutions * Optimized for bias winding feedback * Up to 75% lighter power supply reduces shipping cost * Primary based CV/CC solution eliminates 10 to 20 secondary components for low system cost * Fully integrated auto-restart for short circuit and open loop fault protection - saves external component costs * 42 kHz operation with optimized switching characteristics for significantly reduced EMI Much Higher Performance Over Linear/RCC * Universal input range allows worldwide operation * Up to 70% reduction in power dissipation - reduces enclosure size significantly * CV/CC output characteristic without secondary feedback * System level thermal and current limit protection * Meets all single point failure requirements with only one additional bias capacitor * Controlled current in CC region provides inherent soft-start * Optional opto feedback improves output voltage accuracy EcoSmart - Extremely Energy Efficient * Consumes <300 mW at 265 VAC input with no load * Meets California Energy Commission (CEC), Energy Star, and EU requirements * No current sense resistors - maximizes efficiency Applications * Linear transformer replacement in all 3 W applications * Chargers for cell phones, cordless phones, PDAs, digital cameras, MP3/portable audio devices, shavers, etc. * Home appliances, white goods and consumer electronics * Constant output current LED lighting applications * TV standby and other auxiliary supplies (R) + DC Output (VO) Wide Range HV DC Input LinkSwitch D C S (a) VO Example Characteristic Min Typ (CV only) (CV/CC) 10% VO 5% 24%* IO For Circuit Shown Above (b) 24%* IO With Optional Secondary Feedback** *Estimated tolerance achievable in high volume production (external components with 7.5% transformer inductance tolerance included). **See Optional Secondary Feedback section. PI-3853-030404 PI-3577-080603 Figure 1. (a) Typical Application - not a Simplified Circuit and (b) Output Characteristic Tolerance Envelopes. OUTPUT POWER TABLE1 PRODUCT4 LNK520 P or G 230 VAC 15% 85-265 VAC No-Load Input Min2 Typ2 Min2 Typ2 Power 3.3 W 4 W 2.4 W 3 W <300 mW 4.2 W 5.5 W 2.9 W 3.5 W <500 mW3 Description LinkSwitch is specifically designed to replace low power linear transformer/RCC chargers and adapters at equal or lower system cost with much higher performance and energy efficiency. LNK520 is equivalent to LNK500 but optimized for use with bias winding feedback and has improved switching characteristics for significantly reduced EMI. In addition, if bias and output windings are magnetically well coupled, output voltage load Table 1. Notes: 1. Output power for designs in an enclosed adapter measured at 50 C ambient. 2. See Figure 1 (b) for Min (CV only designs) and Typ (CV/CC charger designs) power points identified on output characteristic. 3. Uses higher reflected voltage transformer designs for increased power capability - see Key Application Considerations section. 4. For lead-free package options, see Part Ordering Information. regulation can be improved. With efficiency of up to 75% and <300 mW no-load consumption, a LinkSwitch solution can save the end user enough energy over a linear design to completely pay for the full power supply cost in less than one year. LinkSwitch integrates a 700 V power MOSFET, PWM control, high voltage start-up, current limit, and thermal shutdown circuitry, onto a monolithic IC. February 2005 LNK520 CONTROL VC ZC 0 DRAIN INTERNAL SUPPLY 1 SHUNT REGULATOR/ ERROR AMPLIFIER + 5.6 V 4.7 V SHUTDOWN/ AUTO-RESTART + - /8 CURRENT LIMIT ADJUST 5.6 V CURRENT LIMIT COMPARATOR + I FB HYSTERETIC THERMAL SHUTDOWN OSCILLATOR D MAX CLOCK SAW S + PWM COMPARATOR Q R IDCS LOW FREQUENCY OPERATION RE LEADING EDGE EDGE BLANKING SOURCE PI-2777-032503 Figure 2. Block Diagram. Pin Functional Description DRAIN (D) Pin: Power MOSFET drain connection. Provides internal operating current for start-up. Internal current limit sense point for drain current. CONTROL (C) Pin: Error amplifier and feedback current input pin for duty cycle and current limit control. Internal shunt regulator connection to provide internal bias current during normal operation. It is also used as the connection point for the supply bypass and auto-restart/compensation capacitor. SOURCE (S) Pin: Output MOSFET source connection for high voltage power return. Primary side control circuit common and reference point. S S S S 1 2 3 4 LNK520 8 7 C S 5 D P Package (DIP-8B) G Package (SMD-8B) Figure 3. Pin Configuration. PI-3790-121503 2 E 2/05 LNK520 LinkSwitch Functional Description The duty cycle, current limit and operating frequency relationships with CONTROL pin current are shown in Figure 4. Figure 5 shows a typical power supply schematic outline which is used below to describe the LinkSwitch operation. Power Up During power up, as VIN is first applied (Figure 5), the CONTROL pin capacitor C1 is charged through a switched high voltage current source connected internally between the DRAIN and CONTROL pins (see Figure 2). When the CONTROL pin voltage reaches approximately 5.6 V relative to the SOURCE pin, the high voltage current source is turned off, the internal control circuitry is activated and the high voltage internal MOSFET starts to switch. At this point, the charge stored on C1 is used to supply the internal consumption of the chip. Constant Current (CC) Operation As the output voltage, and therefore the reflected voltage across the transformer bias winding ramp up, the feedback CONTROL current IC flowing through R1 increases. As shown in Figure 4, the internal current limit increases with IC and reaches ILIM when IC is equal to IDCT. The internal current limit vs. IC characteristic is designed to provide an approximately constant power supply output current as the power supply output voltage rises. Constant Voltage (CV) Operation When IC exceeds IDCS, typically 2 mA (Figure 4), the maximum duty cycle is reduced. At a value of IC that depends on power supply input voltage, the duty cycle control limits LinkSwitch peak current below the internal current limit value. At this point the power supply transitions from CC to CV operation. With minimum input voltage in a typical universal input design, this transition occurs at approximately 30% duty cycle. Resistor R1 (Figure 5) is therefore initially selected to conduct a value of IC approximately equal to IDCT when VOUT is at the desired value at the minimum power supply input voltage. The final choice of R1 is made when the rest of the circuit design is complete. When the duty cycle drops below approximately 4%, the frequency is reduced, which reduces energy consumption under light load conditions. Auto-Restart Operation When a fault condition, such as an output short circuit or open loop, prevents flow of an external current into the CONTROL pin, the capacitor C1 discharges towards 4.7 V. At 4.7 V, autorestart is activated, which turns the MOSFET off and puts the control circuitry in a low current fault protection mode. In auto-restart, LinkSwitch periodically restarts the power supply so that normal power supply operation can be restored when the fault is removed. Internal Current Limit Auto-restart ILIM CONTROL Current IC Duty Cycle IDCT Auto-restart 77% 30% 3.8% ICD1 CONTROL Current IC IDCS Frequency Auto-restart fOSC fOSC(low) CONTROL Current IC PI-3579-031004 Figure 4. CONTROL Characteristics. D1 R3 C3 C4 VOUT D3 VIN D2 LinkSwitch D IC C S R1 C1 R2 C2 PI-3578-021405 Figure 5. Power Supply Schematic outline. E 2/05 3 LNK520 D1 T1 VOUT C4 VR1 RTN R3 C3 85-265 VAC D3 D2 R2 C2 R5 R4 LinkSwitch LNK520 D C S R1 U1 C1 PI-3703-030404 Figure 6. Power Supply Schematic Outline with Optocoupler Feedback, Providing Tight CV Regulation. The characteristics described above provide an approximate CV/CC power supply output without the need for secondary side voltage or current feedback. The output voltage regulation is influenced by how well the voltage across C2 tracks the reflected output voltage. This tracking is influenced by the coupling between transformer output and bias windings. Tight coupling improves CV regulation and requires only a low value for resistor R2. Poor coupling degrades CV regulation and requires a higher value for R2 to filter leakage inductance spikes on the bias winding voltage waveform. This circuitry, used with standard transformer construction techniques, provides much better output load regulation than a linear transformer, making this an ideal power supply solution in many low power applications. If even tighter load regulation is required, an optocoupler configuration can be used while still employing the constant output current characteristics provided by LinkSwitch. Optional Secondary Feedback Figure 6 shows a typical power supply schematic outline using LinkSwitch with optocoupler feedback to improve output voltage regulation. On the primary side, the schematic only differs from Figure 5 by the addition of optocoupler U1 transistor in parallel to R1. On the secondary side, the addition of voltage sense circuit components R4, VR1 and U1 LED provide the voltage feedback signal. In the example shown, a simple Zener (VR1) reference is used though more accurate references may be employed for improved output voltage tolerancing and to provide cable drop compensation, if required. Resistor R4 provides biasing for VR1. The regulated output voltage is equal to the sum of the VR1 Zener voltage plus the forward voltage drop of the U1 LED. Resistor R5 is an optional low value resistor to limit U1 LED peak current due to output ripple. Manufacturers specifications for U1 current and VR1 slope resistance should be consulted to determine whether R5 is required. When the power supply operates in the constant current (CC) region, for example at start up and when charging a battery, the output voltage is below the voltage feedback threshold defined by U1 and VR1 and the optocoupler is fully off. In this region, the circuit behaves exactly as previously described with reference to Figure 5 where the voltage across C2 and therefore the current flowing through R1 increases with increasing output voltage and the LinkSwitch internal current limit is adjusted to provide an approximate CC output characteristic. When the output reaches the voltage feedback threshold set by U1 and VR1, the optocoupler turns on. Any further increase in the power supply output voltage results in the U1 transistor current increasing. The resulting increase in the LinkSwitch CONTROL current reduces the duty cycle according to Figure 4 and therefore, maintains the output voltage regulation. Figure 7 shows the influence of optocoupler feedback on the output characteristic. The envelope defined by the dashed lines represent the worst-case power supply DC output voltage and current tolerances (unit-to-unit and over the input voltage range) if an optocoupler is not used. A typical example of an inherent (without optocoupler) output characteristic is shown dotted. This is the characteristic that would result if U1, R4, R5 and VR1 were removed. The optocoupler feedback results in the characteristic shown by the solid line. The load variation arrow in Figure 7 represents the locus of the output characteristic normally seen during a battery charging cycle. The two characteristics are identical as the output voltage rises but then separate as shown when the voltage feedback threshold is reached. This 4 E 2/05 LNK520 Output Voltage Voltage feedback threshold Inherent CC to CV transition point Tolerance envelope without optocoupler Typical inherent characteristic without optocoupler Characteristic with optocoupler Load variation during battery charging Output Current Figure 7. Influence of the Optocoupler on the Power Supply Output Characteristic. PI-2788-092101 Output Voltage Inherent CC to CV transition point Tolerance envelope without optocoupler Typical inherent characteristic without optocoupler Voltage feedback threshold VO(MAX) Characteristic with optocoupler Power supply peak output power curve Load variation during battery charging Characteristic observed with load variation often applied during laboratory bench testing Output Current Figure 8. Output Characteristic with Optocoupler Regulation (Reduced Voltage Feedback Threshold). PI-2790-112102 is the characteristic seen if the voltage feedback threshold is above the output voltage at the inherent CC to CV transition point also indicated in Figure 7. Figure 8 shows a case where the voltage feedback threshold is set below the voltage at the inherent CC to CV transition point. In this case, as the output voltage rises, the secondary feedback circuit takes control before the inherent CC to CV transition occurs. In an actual battery charging application, this simply limits the output voltage to a lower value. However, in laboratory bench tests, it is often more convenient to test the power supply output characteristic starting from a low output current and gradually increasing the load. In this case, the optocoupler feedback regulates the output voltage until the peak output power curve is reached as shown in Figure 8. Under these conditions, the output current will continue to rise until the peak power point is reached and the optocoupler turns off. Once the optocoupler is off, the CONTROL pin feedback current is determined only by R1 and the output current therefore folds back to the inherent CC characteristic as shown. Since this type of load transition does not normally occur in a battery charger, the output current never overshoots the inherent constant current value in the actual application. In some applications it may be necessary to avoid any output current overshoot, independent of the direction of load variation. E 2/05 5 LNK520 LinkSwitch LNK520 D C S PI-3854-031804 VOUT C1 R1 C2 C4 VOUT Typical Characteristic VIN D1 R2 (a) D2 (b) IOUT Figure 9. High-side Configuration Using LNK520: (a) Schematic Outline; (b) Typical Output Characteristic Envelope. To achieve this goal, the minimum voltage feedback threshold should be set at VO(MAX). This will ensure that the voltage at the CC to CV transition point of the inherent characteristic will always occur below the voltage feedback threshold. However, the output voltage tolerance is then increased, since the inherent CV characteristic tolerance below VO(MAX) is added to the tolerance of the optocoupler feedback circuit. The LNK520 can also be used in the high-side configuration as shown in Figure 9(a). This configuration provides a very low component count solution with an approximate CV/CC power supply output characteristic. A typical output characteristic envelope is shown in Figure 9(b). This configuration is ideal for very low cost charger and adapter applications where output CC tolerance is loose or unspecified. Typical applications include low cost chargers and adapters where direct replacement for a linear transformer is required. In applications with a high voltage DC input voltage, the circuit is further simplified with the removal of input rectifiers, EMI filter choke and input capacitors. Typical applications of this type include auxiliary supplies in domestic appliances and industrial applications. In the high-side configuration, the CONTROL pin receives feedback current through R1 generated by the voltage across C2. To a first order, this voltage is proportional to VOUT since VOUT is reflected to the primary and appears across C2 during the off time of the LNK520 switching cycle. The output CV regulation is therefore determined by how well the voltage across C2 tracks the output voltage. This tracking is influenced by the value of the transformer leakage inductance, which introduces an error. This error, which is partially filtered by R2 and C2, causes a slope in the output CV regulation characteristic. The LNK520 is optimized for use with a bias winding where tracking of feedback voltage and output voltage is typically better than it is in the high-side configuration of Figure 9 (a). As a consequence, the increased leakage error in the high-side configuration causes the output current to increase with falling output voltage, as indicated by the output CC characteristic envelope in Figure 9 (b). In this high-side configuration, the SOURCE pins and circuit board traces form a switching node. Extra care should be taken to optimize EMI performance. The LNK520 internal MOSFET switching characteristics have been designed to significantly reduce EMI, particularly in the radiated spectrum (>30 MHz). However, the SOURCE trace area should be minimized and EMI filter components should be distanced from the SOURCE node whenever possible. In embedded applications where a high voltage DC input voltage is available, system level EMI filtering is typically located away from the power supply and circuit board layout is less critical. Applications Example The circuit shown in Figure 10 shows a typical implementation of an approximate constant voltage / constant current (CV/CC) charger using LinkSwitch in the low-side configuration. This design delivers 2.75 W with nominal peak power point voltage of 5.5 V and a current of 500 mA (Figure 11). Efficiency is greater than 65% over an input range of 85 VAC to 265 VAC. The bridge rectifier, D1-D4, rectifies the AC input. The rectified AC is smoothed by C1 and C2, with inductor L1 forming a pi-filter to filter differential mode conducted EMI. Resistor RF1 is a fusible, flameproof type providing protection from primary-side short circuits and line surges and provides additional differential EMI filtering. The switching frequency of 42 kHz allows such a simple EMI filter to be used without the need for a Y capacitor while still meeting international EMI standards. When power is applied, high voltage DC appears at the DRAIN pin of LinkSwitch (U1). The CONTROL pin capacitor C5 is then charged through a switched high voltage current source connected internally between the DRAIN and CONTROL pins. When the CONTROL pin reaches approximately 5.6 V relative to the SOURCE pin, the internal current source is turned off. The internal control circuitry is activated and the high voltage MOSFET starts to switch, using the energy in C5 to power the IC. 6 E 2/05 LNK520 5.5 V, 500 mA D1 D2 1N4005 1N4005 RF1 8.2 L 2W 85 - 265 VAC N D L1 1 mH 0.15 A R1 390 k 1/4 W R2 100 1 C4 330 pF 1 kV 7 D7 8T 11DQ06 C6 330 F 16 V 2 100T D5 1N4007GP 6 J1 5 RTN C1 4.7 F 400 V C2 4.7 F 400 V 26T 4 T1 EE16 LP = 2.52 mH D6B 1N4937 R3 15 C3 1 F 50 V LinkSwitch U1 LNK520P C D3 1N4005 D4 1N4005 S C5 220 nF R4 6.81 k 1%, 1/4 W PI-3723-111303 Figure 10. 2.75 W Constant Voltage/Constant Current (CV/CC) Charger Using LinkSwitch. 85 VAC 9 8 Output Voltage (V) 115 VAC 190 VAC 230 VAC 265 VAC Limits 7 6 5 4 3 2 1 0 0 100 200 300 400 500 600 700 Output Current (mA) Figure 11. Measured Output Characteristic of the Circuit in Figure 10. The secondary of the transformer is rectified and filtered by D7 and C6 to provide the DC output to the load. LinkSwitch dramatically simplifies the secondary side by controlling both the constant voltage and constant current regions entirely from the primary side. This is achieved by monitoring the primaryside bias voltage. Diode D5, C4, R1 and R2 form the primary clamp network. This limits the peak DRAIN voltage due to leakage inductance. Resistor R2 allows the use of a slow, low cost rectifier diode by limiting the reverse current through D5 when U1 turns on. The selection of a slow diode improves radiated EMI and also improves CV regulation, especially at no-load. E 2/05 PI-3718-092503 10 7 LNK520 The output during CV operation is equal to the primary-side bias voltage multiplied by the turns ratio. The bias voltage, in turn, is the sum of the CONTROL pin voltage (approximately 5.7 V), the voltage across the bias feedback resistor R4 and the forward voltage of D6B. Resistor R3 can be neglected as proportionally the voltage drop across this resistance is small. In CV operation, the voltage across R4 is equal to the CONTROL pin current, IDCT (2.15 mA) multiplied by the value of R4. As the output load is decreased, the output and therefore bias voltage increase resulting in increased current into the CONTROL pin. As the current into the CONTROL pin exceeds IDCS (~2 mA), the duty cycle begins to reduce, maintaining regulation of the output, reaching 30% at a CONTROL pin current of 2.15 mA. Under light or no-load conditions, when the duty cycle reaches approximately 4%, the switching frequency is reduced from 44 kHz to 29 kHz to lower light and no-load input power. As the output load is increased, the peak power point (defined by 0.5 * LP * ILIM2 * f) is exceeded. The output voltage and therefore primary-side bias voltage reduce. The reduction in the bias voltage results in a proportional reduction of CONTROL pin current, which lowers the internal LinkSwitch current limit (current limit control). Constant current (CC) operation controls secondary-side output current by reducing the primary-side current limit. The current limit reduction characteristic has been optimized to maintain an approximate constant output current as the output voltage and therefore, bias voltage is reduced. If the load is increased further and the CONTROL pin current falls below approximately 0.8 mA, the CONTROL pin capacitor C5 will discharge and LinkSwitch will enter auto-restart operation. Current limit control removes the need for any secondaryside current sensing components (sense resistor, transistor, optocoupler and associated components). Removing the secondary sense circuit dramatically improves efficiency, giving the associated benefit of reduced enclosure size. 2. Design is a discontinuous mode flyback converter with nominal primary inductance value and a VOR in the range 40 V to 80 V. Continuous mode designs can result in loop instability and are therefore not recommended. 3. A secondary output of 5 V with a Schottky rectifier diode. 4. Assumed efficiency of 65%. 5. The part is board mounted with SOURCE pins soldered to sufficient area of copper to keep the die temperature at or below 100 C. 6. An output cable with a total resistance of 0.2 . In addition to the thermal environment (sealed enclosure, ventilated, open frame, etc.), the maximum power capability of LinkSwitch in a given application depends on transformer core size, efficiency, primary inductance tolerance, minimum specified input voltage, input storage capacitance, output voltage, output diode forward drop, etc., and can be different from the values shown in Table 1. Transformer Design To provide an approximately CV/CC output, the transformer should be designed to be discontinuous; all the energy stored in the transformer is transferred to the secondary during the MOSFET off time. Energy transfer in discontinuous mode is independent of line voltage. The peak power point prior to entering constant current operation is defined by the maximum power transferred by the transformer. The power transferred is given by the expression P = 0.5 * LP * I2 * f, where LP is the primary inductance, I2 is the primary peak current squared and f is the switching frequency. To simplify analysis, the data sheet parameter table specifies an I2f coefficient. This is the product of current limit squared and switching frequency normalized to the feedback parameter IDCT. This provides a single term that specifies the variation of the peak power point in the power supply due to LinkSwitch. As primary inductance tolerance is part of the expression that determines the peak output power point (start of the CC characteristic) this parameter should be well controlled. For an estimated overall constant current tolerance of 24%, the primary inductance tolerance should be 7.5% or better. This is achievable using standard low cost, center leg gapping techniques where the gap size is typically 0.08 mm or larger. Smaller gap sizes are possible but require non-standard, tighter ferrite AL tolerances. Other gapping techniques such as film gapping allow tighter tolerances (7% or better) with associated improvements in the tolerance of the peak power point. Please consult your transformer vendor for guidance. Key Application Considerations Design Output Power Table 1 (front page) shows the maximum continuous output power that can be obtained under the following conditions: 1. The minimum DC input bus voltage is 90 V or higher. This corresponds to a filter capacitor of 3 F/W for universal input and 1 F/W for 230 VAC or 115 VAC input with doubler input stage. 8 E 2/05 LNK520 Core gaps should be uniform. Uneven core gapping, especially with small gap sizes, may cause variation in the primary inductance with flux density (partial saturation) and make the constant current region non-linear. To verify uniform gapping, it is recommended that the primary current wave-shape be examined while feeding the supply from a DC source. The gradient is defined as di/dt = V/L and should remain constant throughout the MOSFET on time. Any change in gradient of the current ramp is an indication of uneven gapping. Measurements made using a LCR bridge should not be solely relied upon; typically these instruments only measure at currents of a few milliamps. This is insufficient to generate high enough flux densities in the core to show uneven gapping. For a typical EE16 or EE13 core using center leg gapping, a 0.08 mm gap allows a primary inductance tolerance of 10% to be maintained in standard high volume production. This allows the EE13 to be used in designs up to 2.75 W with less than 300 mW no-load consumption. Using outer leg film gapping reduces inductance tolerance to 7% or better, allowing designs up to 3 W. Using the larger EE16 allows for a 3 W output with center leg gapping. The EE13 core size may be attractive in designs were space is limited or if there is a cost advantage over the EE16. The transformer turns ratio should be selected to give a VOR (output voltage reflected through secondary to primary turns ratio) of 40 V to 80 V. Higher VOR increases the output power capability of LinkSwitch but also increases no-load power consumption. This allows even higher values to be used in designs where no-load power is not a concern. However care should be taken to ensure that the maximum temperature rise of the device is acceptable at the upper limit of the output characteristic when used in a charger application. In all cases, discontinuous mode operation should be maintained and note that the linearity of the CC region of the power supply output characteristic is influenced by the bias voltage. If this is an important aspect of the application, the output characteristic should be checked before finalizing the design. Output Characteristic Variation Both the device tolerance and external circuit govern the overall tolerance of the LinkSwitch power supply output characteristic. Estimated peak power point tolerances for a LNK520, 2.75 W design are 10% for voltage and 24% for current limit for overall variation in high volume manufacturing. This includes device and transformer tolerances (7.5% assumed) and line variation. Lower power designs may have poorer constant current linearity. As the output load reduces from the peak power point, the output voltage will tend to rise due to tracking errors compared to the load terminals. Sources of these errors include the output cable drop, output diode forward voltage and leakage inductance, which is the dominant cause. As the load reduces, the primary operating peak current reduces, together with the leakage inductance energy, which reduces the peak charging of the clamp capacitor. At very light or no-load, typically less than 2 mA of output current, the output voltage rises due to leakage inductance peak charging of the secondary. This voltage rise can be reduced with a small preload with little change to no-load power consumption. The output voltage load variation can be improved across the whole load range by adding an optocoupler and secondary reference (Figure 6). The secondary reference is designed to only provide feedback above the normal peak power point voltage to maintain the correct constant current characteristic. Component Selection The schematic shown in Figure 10 outlines the key components needed for a LinkSwitch supply. Clamp diode - D5 Diode D5 can be an ultra-fast (trr < 50 ns), a fast (trr < 250 ns) or standard recovery diode with a voltage rating of 600 V or higher. A standard recovery diode is recommended as it improves the CV characteristic, but should be a glass-passivated type (1N400xGP) to ensure a defined reverse recovery time. Clamp Capacitor - C4 Capacitor C4 should be in the range of 100 pF to 1000 pF, 500 V capacitor. A low cost ceramic disc is recommended. The tolerance of this part has a very minor effect on the output characteristic so any of the standard 5%, 10% or 20% tolerances are acceptable. 330 pF is a good initial value, iterated with R1. Clamp Resistor - R1 The value of R1 is selected to be the highest value that still provides adequate margin to the DRAIN BVDSS rating at high line. As a general rule, the value of C4 should be minimized and R1 maximized. CONTROL Pin Capacitor - C5 Capacitor C5 is used during start-up to power LinkSwitch and sets the auto-restart frequency. For designs that have a battery load, this component should have a value of 0.22 F and for resistive loads a value of 1 F. This ensures there is sufficient time during start-up for the output voltage to reach regulation. Any capacitor type is acceptable with a voltage rating of 10 V or above. Bias Capacitor - C3 Capacitor C3 should be a 1 F, 50 V electrolytic type. The voltage rating is consistent with the 20 V to 30 V seen across the bias winding. Lower values give poorer regulation. E 2/05 9 LNK520 Feedback Resistor - R4 The value of R4 is selected to give a feedback current into the CONTROL pin of approximately 2.15 mA at the peak output power point of the supply. The actual value depends on the bias voltage, typically in the range 20 V to 35 V, selected during design. Higher values for the bias voltage will increase no-load power consumption. Any 1%, 0.25 W resistor is suitable. Output Diode - D7 PN fast, PN ultra-fast or Schottky diodes can be used depending on the efficiency target for the supply, Schottky diodes giving higher efficiency than PN diodes. The diode voltage rating should be sufficient to withstand the output voltage plus the input voltage transformed through the turns ratio (a typical VOR of 50 V requires a diode PIV of 50 V). Slow recovery diodes are not recommended (1N400X types). Output Capacitor - C6 Capacitor C6 should be selected such that its voltage and ripple current specifications are not exceeded. Selecting a capacitor with low equivalent series resistance (ESR) will reduce peak-peak output ripple and improve overall supply operating efficiency. LinkSwitch Layout considerations Primary Side Connections The copper area connected to SOURCE should be maximized to minimize temperature rise of the LinkSwitch device. The CONTROL pin capacitor C5 should be located as close as possible to the SOURCE and CONTROL pins. To minimize EMI coupling from the switching DRAIN node on the primary to both the secondary and AC input, the LinkSwitch should be positioned away from the secondary of the transformer and AC input. The length and copper area of all PCB traces connecting to the switching DRAIN node should be kept to an absolute minimum to limit EMI radiation. Y capacitor If a Y capacitor is required, it should be connected close to the transformer secondary output return pin(s) and the primary bulk capacitor positive terminal. Such placement will maximize the EMI benefit of the Y capacitor and avoid problems in commonmode surge testing. Quick Design Checklist As with any power supply design, all LinkSwitch designs should be verified on the bench to make sure that component specifications are not exceeded under worst-case conditions. Performing the following minimum set of tests is strongly recommended: 1. Maximum drain voltage - Verify that VDS does not exceed 675 V at highest input voltage and peak output power. 2. Maximum drain current - At maximum ambient temperature, maximum input voltage and peak output power, verify drain current waveforms at start-up for any signs of transformer saturation and excessive leading edge current spikes. LinkSwitch has a minimum leading edge blanking time of 200 ns to prevent premature termination of the on-cycle. Verify that the leading edge current spike event is below current limit at the end of the 200 ns blanking period. 3. Thermal check - At peak output power, minimum input voltage and maximum ambient temperature, verify that the temperature specifications are not exceeded for LinkSwitch, transformer, output diode and output capacitors. Enough thermal margin should be allowed for part-to-part variation of the RDS(ON) of LinkSwitch as specified in the data sheet. Under low line, peak power, a maximum LinkSwitch SOURCE pin temperature of 100 C is recommended to allow for these variations. 4. Centered output characteristic - Using a transformer with nominal primary inductance and at an input voltage midway between low and high line, verify that the peak power point occurs at ~4% above the desired nominal output current, with the correct output voltage. If this does not occur, then the design should be refined (increase LP) to ensure the overall tolerance limits are met. Selecting Between LNK500 and LNK520 The LNK500 and LNK520 differ in the circuit location of the LinkSwitch device. The LNK500 is designed for high-side operation and the LNK520 is designed for low-side operation. The LNK520 can, however, be used in the high-side configuration in certain applications. Refer to Figure 9 and supporting description. Table 2 summarizes the considerations for selecting which device to use. Design Tools Up to date information on design tools can be found at the Power Integrations Web site: www.powerint.com. 10 E 2/05 LNK520 LinkSwitch D S S Input Filter Capacitor + HV DC Input S C Y1Capacitor S S - Transformer Output Capacitor DC Out + PI-3732-103003 Figure 12. Recommended Circuit Board Layout for LinkSwitch using LNK520. E 2/05 11 LNK520 Family LNK500 * Lowest cost CV/CC implementation * Source is connected to the switching node - simple circuit configuration & low component count * Fast switching speeds minimize losses for best efficiency * Source PCB copper heatsink connected to switching node - size should be minimized to limit noise * No bias winding required - simplest circuit configuration * Perfect for linear replacement in applications where additional system EMI shielding or filtering exists Summary LNK520 * Very low cost CV/CC implementation * Source connected to quiet low-side primary return - easy layout & low noise (low-side configuration only) * Optimized switching speed - reduces radiated EMI by up to 5 dB (Figure 13) * Source PCB copper heatsink connected to primary return - area can be maximized for higher power without noise (low-side configuration only) * Bias winding required - allows higher VOR, increasing power capability (low-side configuration only) * Perfect for systems where no additional filtering or shielding exists Considerations The LNK500 is recommended for cost The LNK520 is recommended for both sensitive applications in larger systems with stand-alone charger and adapter existing EMI filtering (e.g. white goods). applications, and larger systems where EMI reduction is required (e.g. emergency lighting). Table 2. Comparison of LNK500 and LNK520. Amplitude 10 dBV/Div QP: LNK500 QP: LNK520 AV: LNK500 AV: LNK520 30.0 100.0 200.0 Frequency (MHz) Figure 13. Comparison of LNK520 and LNK500 Showing an Approximate 5 dBV Reduction in Radiated EMI. 12 E 2/05 PI-3861-031804 LNK520 ABSOLUTE MAXIMUM RATINGS(1,4) DRAIN Voltage .................................................. -0.3 V to 700 V DRAIN Peak Current......................................400 mA CONTROL Voltage ................................................ -0.3 V to 9 V CONTROL Current (not to exceed 9 V)............100 mA Storage Temperature .......................................... -65 C to 150 C Operating Junction Temperature(2) ..................... -40 C to 150 C Lead Temperature(3) ........................................................260 C Notes: 1. All voltages referenced to SOURCE, TA = 25 C. 2. Normally limited by internal circuitry. 3. 1/16 in. from case for 5 seconds. 4. Maximum ratings specified may be applied, one at a time, without causing permanent damage to the product. Exposure to Absolute Maximum Rating conditions for extended periods of time may affect product reliability. THERMAL IMPEDANCE Thermal Impedance: P or G Package: Notes: (JA) ........................... 70 C/W(2); 60 C/W(3) 1. Measured on pin 2 (SOURCE) close to plastic interface. (JC)(1) ............................................... 11 C/W 2. Soldered to 0.36 sq. in. (232 mm2), 2 oz. (610 g/m2) copper clad. 3. Soldered to 1 sq. in. (645 mm2), 2 oz. (610 g/m2) copper clad. Conditions Parameter Symbol SOURCE = 0 V; TJ = -40 to 125 C See Figure 14 (Unless Otherwise Specified) Min Typ Max Units CONTROL FUNCTIONS Switching Frequency Low Switching Frequency Duty Cycle at Low Switching Frequency Low Frequency Duty Cycle Range Maximum Duty Cycle PWM Gain CONTROL Pin Current at 30% Duty Cycle CONTROL Pin Voltage Dynamic Impedance fOSC fOSC(LOW) DCLF DC(RANGE) DCMAX DCREG IDCT VC(IDCT) ZC IC = IDCT, TJ = 25 C Duty Cycle = DCLF TJ = 25 C Frequency Switching from fOSC to fOSC(LOW), TJ = 25 C Frequency = fOSC(LOW), TJ = 25 C IC = 1.5 mA IC = IDCT, TJ = 25 C TJ = 25 C See Figure 4 IC = IDCT IC = IDCT, TJ = 25 C 34.5 24 2.7 2.0 74 -0.37 2.06 5.5 60 42 30 4.1 3.5 77 -0.27 2.15 5.75 90 49.5 36 5.5 5.0 80 -0.17 2.25 6 120 kHz kHz % % % %/A mA V E 2/05 13 LNK520 Conditions Parameter Symbol SOURCE = 0 V; TJ = -40 to 125 C See Figure 14 (Unless Otherwise Specified) VC = 0 V VC = 5.15 V Min Typ Max Units SHUTDOWN/AUTO-RESTART CONTROL Pin Charging Current Control/Supply/ Discharge Current IC(CH) ICD1 ICD2 TJ = 25 C -4.5 -2.5 0.68 0.5 -3.25 -1.8 0.75 0.6 5.6 0.9 Short Circuit Applied at Power Supply Output S2 Open C1 = 0.22 F (See Figure 14) TJ = 25 C di/dt = 90 mA/s See Note B TJ = 25 C di/dt = 90 mA/s See Notes B, C IC = ICD1, TJ = 25 C 1.5 IC = IDCT, TJ = 25 C TJ = 25 C IC = IDCT 125 200 8 300 -2 -1.0 0.82 0.7 mA mA V V % Hz TJ = 25 C Output MOSFET Enabled TJ = 25 C Output MOSFET Disabled Auto-Restart VC(AR) Threshold Voltage Auto-Restart VC(AR)hyst Hysteresis Voltage Auto-Restart Duty DC(AR) Cycle Auto-Restart f(AR) Frequency CIRCUIT PROTECTION Self-Protection Current Limit I2 f Coefficient Current Limit at Auto-Restart Power Up Reset Threshold Voltage Leading Edge Blanking Time Current Limit Delay Thermal Shutdown Temperature Thermal Shutdown Hysteresis ILIM I2 f ILIM(AR) VC(RESET) tLEB tIL(D) 228 2412 254 2710 165 2.75 300 100 135 70 280 3008 mA A2Hz mA 4.0 V ns ns C C 14 E 2/05 LNK520 Conditions Parameter OUTPUT ON-State Resistance OFF-State Drain Leakage Current Breakdown Voltage DRAIN Supply Voltage NOTES: A. For specifications with negative values, a negative temperature coefficient corresponds to an increase in magnitude with increasing temperature, and a positive temperature coefficient corresponds to a decrease in magnitude with increasing temperature. B. IC is increased gradually to obtain maximum current limit at di/dt of 90 mA/s. Increasing IC further would terminate the cycle through duty cycle control. C. This parameter is normalized to IDCT to correlate to power supply output current (it is multiplied by IDCT(nominal)/IDCT). D. Breakdown voltage may be checked against minimum BVDSS specification by ramping the DRAIN pin voltage up to but not exceeding minimum BVDSS. E. It is possible to start up and operate LinkSwitch at DRAIN voltages well below 36 V. However, the CONTROL pin charging current is reduced, which affects start-up time, auto-restart frequency, and auto-restart duty cycle. Refer to the characteristic graph on CONTROL pin charge current (IC) vs. DRAIN voltage (Figure 16) for low voltage operation characteristics. RDS(ON) IDSS BVDSS ID = 25 mA TJ = 25 C TJ = 100 C 28 42 32 48 50 700 36 50 A V V Symbol SOURCE = 0 V; TJ = -40 to 125 C See Figure 14 (Unless Otherwise Specified) Min Typ Max Units VC = 6.2 V VD = 560 V, TA = 125 C See Note D VC = 6.2 V, TA = 25 C See Note E E 2/05 15 LNK520 750 S1 10 k S2 LinkSwitch D S S S C S S 40 V 40 V C1 0.22 F PI-2894-031004 Figure 14. LinkSwitch General Test Circuit. t2 HV DRAIN VOLTAGE 0V t1 90% 90% t D= 1 t2 10% PI-2048-050798 Figure 15. Duty Cycle Measurement. Typical Performance Characteristics VC = 5.15 V PI-3758-111303 CONTROL Pin Current (mA) CONTROL Pin Charging Current (mA) 1.6 1.2 100 80 60 40 20 0 0.8 0.4 0 0 20 40 60 80 100 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 DRAIN Voltage (V) Figure 16. IC vs. DRAIN Voltage. CONTROL Pin Voltage (V) Figure 17. CONTROL Pin I-V Characteristic. 16 E 2/05 PI-2895-102303 2 120 LNK520 Typical Performance Characteristics (cont.) PI-3737-101703 80 70 60 50 40 30 20 10 0 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Breakdown Voltage (Normalized to 25 C) Duty Cycle (%) 1.0 0.9 -50 -25 0 25 50 75 100 125 150 CONTROL Pin Current (mA) Figure 18. Duty Cycle vs. CONTROL Pin Current. 1.200 Junction Temperature (C) Figure 19. Breakdown Voltage vs. Temperature. 1.200 PI-3738-101703 Switching Frequency (Normalized for 25 C) 0.800 0.600 0.400 0.200 0.000 -50 0 50 100 150 Current Limit (Normalized for 25 C) 1.000 1.000 0.800 0.600 0.400 0.200 0.000 -50 -25 0 25 50 75 100 125 150 Junction Temperature (C) Figure 20. Switching Frequency vs. Temperature. PI-3740-0101703 Junction Temperature (C) Figure 21. Current Limit vs. Temperature. 1.200 I2f Coefficient (Normalized for 25 C) 1.0 0.8 0.6 0.4 0.2 0.0 IDCT (Normalized for 25 C) 1.000 0.800 0.600 0.400 0.200 0.000 -50 -20 0 25 50 -50 -25 0 25 50 75 100 125 150 75 100 125 150 Junction Temperature (C) Figure 22. I2f Coefficient vs. Temperature. Junction Temperature (C) Figure 23. IDCT vs. Temperature. E 2/05 PI-3741-101703 1.2 PI-3739-101703 PI-2213-012301 90 1.1 17 LNK520 Typical Performance Characteristics (cont.) PI-2899-062802 PWM Gain (Normalized for 25 C) 1 0.8 0.6 0.4 0.2 0 -50 0 50 100 250 TCASE=25 C TCASE=100 C Drain Current (mA) 200 150 100 50 0 0 2 4 6 8 10 150 Temperature (C) Figure 24. PWM Gain vs. Temperature. Drain Voltage (V) Figure 25. Output Characteristics (DRAIN Current vs. DRAIN Voltage. PART ORDERING INFORMATION LinkSwitch Product Family Series Number Package Identifier G P Plastic Surface Mount DIP Plastic DIP Lead Finish Blank Standard (Sn Pb) N Pure Matte Tin (Pb-Free) Tape & Reel and Other Options Blank Standard Configurations LNK 520 G N - TL TL Tape & Reel, 1 k pcs minimum, G package only 18 E 2/05 PI-2222-031401 1.2 300 LNK520 DIP-8B D S .004 (.10) -E.137 (3.48) MINIMUM .240 (6.10) .260 (6.60) Pin 1 -D- .367 (9.32) .387 (9.83) .057 (1.45) .068 (1.73) (NOTE 6) .015 (.38) MINIMUM Notes: 1. Package dimensions conform to JEDEC specification MS-001-AB (Issue B 7/85) for standard dual-in-line (DIP) package with .300 inch row spacing. 2. Controlling dimensions are inches. Millimeter sizes are shown in parentheses. 3. Dimensions shown do not include mold flash or other protrusions. Mold flash or protrusions shall not exceed .006 (.15) on any side. 4. Pin locations start with Pin 1, and continue counter-clockwise to Pin 8 when viewed from the top. The notch and/or dimple are aids in locating Pin 1. Pin 6 is omitted. 5. Minimum metal to metal spacing at the package body for the omitted lead location is .137 inch (3.48 mm). 6. Lead width measured at package body. 7. Lead spacing measured with the leads constrained to be perpendicular to plane T. .125 (3.18) .145 (3.68) -TSEATING PLANE .120 (3.05) .140 (3.56) .048 (1.22) .053 (1.35) .014 (.36) .022 (.56) T E D S .010 (.25) M .008 (.20) .015 (.38) .300 (7.62) BSC (NOTE 7) .300 (7.62) .390 (9.91) .100 (2.54) BSC P08B PI-2551-121504 SMD-8B D S .004 (.10) -E.137 (3.48) MINIMUM .240 (6.10) .260 (6.60) .372 (9.45) .388 (9.86) E S .010 (.25) Pin 1 .100 (2.54) (BSC) -D- .367 (9.32) .387 (9.83) .057 (1.45) .068 (1.73) (NOTE 5) Notes: 1. Controlling dimensions are inches. Millimeter sizes are shown in parentheses. 2. Dimensions shown do not include mold flash or other protrusions. Mold flash or protrusions shall not exceed .006 (.15) on any side. .420 3. Pin locations start with Pin 1, and continue counter-clock.046 .060 .060 .046 wise to Pin 8 when viewed from the top. Pin 6 is omitted. 4. Minimum metal to metal .080 spacing at the package body Pin 1 for the omitted lead location is .137 inch (3.48 mm). .086 5. Lead width measured at .186 package body. .286 6. D and E are referenced Solder Pad Dimensions datums on the package body. .125 (3.18) .145 (3.68) .032 (.81) .037 (.94) .048 (1.22) .053 (1.35) .004 (.10) .009 (.23) .004 (.10) .012 (.30) .036 (0.91) .044 (1.12) 0- 8 G08B PI-2546-121504 E 2/05 19 LNK520 Revision Notes C D E 1) Released Final Data Sheet. 1) Added lead-free ordering information. 1) Minor descriptive change and formatting correction. Date 3/04 12/04 2/05 For the latest updates, visit our website: www.powerint.com Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. PATENT INFORMATION The products and applications illustrated herein (including transformer construction and circuits external to the products) may be covered by one or more U.S. and foreign patents, or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations patents may be found at www.powerint.com. Power Integrations grants its customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm. LIFE SUPPORT POLICY POWER INTEGRATIONS PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF POWER INTEGRATIONS. As used herein: 1. A Life support device or system is one which, (i) is intended for surgical implant into the body, or (ii) supports or sustains life, and (iii) whose failure to perform, when properly used in accordance with instructions for use, can be reasonably expected to result in significant injury or death to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, EcoSmart, PI Expert and PI FACTS are trademarks of Power Integrations, Inc. Other trademarks are property of their respective companies. (c)Copyright 2005, Power Integrations, Inc. Power Integrations Worldwide Sales Support Locations WORLD HEADQUARTERS 5245 Hellyer Avenue San Jose, CA 95138, USA. Main: +1-408-414-9200 Customer Service: Phone: +1-408-414-9665 Fax: +1-408-414-9765 e-mail: usasales@powerint.com CHINA (SHANGHAI) Rm 807-808A, Pacheer Commercial Centre, 555 Nanjing Rd. West Shanghai, P.R.C. 200041 Phone: +86-21-6215-5548 Fax: +86-21-6215-2468 e-mail: chinasales@powerint.com CHINA (SHENZHEN) Rm 2206-2207, Block A, Electronics Science & Technology Bldg. 2070 Shennan Zhong Rd. Shenzhen, Guangdong, China, 518031 Phone: +86-755-8379-3243 Fax: +86-755-8379-5828 e-mail: chinasales@powerint.com GERMANY Rueckertstrasse 3 D-80336, Munich Germany Phone: +49-89-5527-3910 Fax: +49-89-5527-3920 e-mail: eurosales@powerint.com INDIA 261/A, Ground Floor 7th Main, 17th Cross, Sadashivanagar Bangalore, India 560080 Phone: +91-80-5113-8020 Fax: +91-80-5113-8023 e-mail: indiasales@powerint.com ITALY Via Vittorio Veneto 12 20091 Bresso MI Italy Phone: +39-028-928-6000 Fax: +39-028-928-6009 e-mail: eurosales@powerint.com JAPAN Keihin Tatemono 1st Bldg 2-12-20 Shin-Yokohama, Kohoku-ku, Yokohama-shi, Kanagawa ken, Japan 222-0033 Phone: +81-45-471-1021 Fax: +81-45-471-3717 e-mail: japansales@powerint.com KOREA RM 602, 6FL Korea City Air Terminal B/D, 159-6 Samsung-Dong, Kangnam-Gu, Seoul, 135-728, Korea Phone: +82-2-2016-6610 Fax: +82-2-2016-6630 e-mail: koreasales@powerint.com SINGAPORE 51 Newton Road, #15-08/10 Goldhill Plaza, Singapore, 308900 Phone: +65-6358-2160 Fax: +65-6358-2015 e-mail: singaporesales@powerint.com TAIWAN 5F, No. 318, Nei Hu Rd., Sec. 1 Nei Hu Dist. Taipei, Taiwan 114, R.O.C. Phone: +886-2-2659-4570 Fax: +886-2-2659-4550 e-mail: taiwansales@powerint.com EUROPE HQ 1st Floor, St. Jamess House East Street, Farnham Surrey, GU9 7TJ United Kingdom Phone: +44 (0) 1252-730-140 Fax: +44 (0) 1252-727-689 e-mail: eurosales@powerint.com APPLICATIONS HOTLINE World Wide +1-408-414-9660 APPLICATIONS FAX World Wide +1-408-414-9760 20 E 2/05 |
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