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| Design Idea DI-63 (R) TinySwitch-II 4.8 W CV/CC Charger with Output Cable-Drop Compensation Application Charger Device TNY266P Power Output 4.8 W Input Voltage 85-265 VAC Output Voltage 6 V, 800 mA 7% Topology Flyback Design Highlights * * * * * Universal input, no-load consumption < 300 mW Meets CISPR-22 Class B EMI without a Y capacitor Extremely simple circuit, requires only 32 components! Circuit has output cable voltage drop compensation Ultra-low Leakage current: < 5 A at 265 VAC input winding, forward biasing D7, charging C7 and developing/ maintaining the output voltage across C7. The VBE of Q1 and the VZ of VR1 determine the CV set point. The voltage across R4, R5 and the U2-LED determine the CC set point. Resistors R7, R10 and diode D6 compensate for output-cable voltage-drop. The CV portion of the output VI curve is flat, at the end of the cable, because a current sense resistor (R10) equal to the DC resistance of the output cable is in the voltage feedback loop. Diode D6 implements temperature compensation for the cable drop. The combination of TinySwitch-II frequency jitter, the output diode snubber (R6 and C5), the T1 shield windings and careful primary clamp circuit component selection enable compliance with CISPR-22, Class B conducted EMI limits, without a Y-1 Safety capacitor. Eliminating the Y capacitor gives this circuit very low (< 5 A) AC leakage current. This circuit is suitable for portable electronics chargers. Operation Fusible resistor RF1 provides short-circuit fault protection and limits start-up inrush current. Inductors L1 and L2 and capacitors C1 and C2 form a low-cost pi () filter that attenuates conducted EMI. Transformer (T1) has two shield windings (1-3 and 1-open) which reduce the generation of EMI noise. Winding phasing and D7 orientation let no secondary winding current flow when the MOSFET in U1 is ON, so primary winding current stores energy in the core of T1. When the MOSFET in U1 turns OFF, the energy stored in T1 drives current out of the secondary D1-D4 1N4005 x 4 L1 1.0 mH R2 200 k C3 1.0 nF 500 V T1 EE13 LP = 1.25 mH 1 10 2 1 C5 1 nF R6 R7 D6 10 500 V 120 1N4148 L3 Ferrite Bead 6 V, 0.8 A L 85-265 VAC N RF1 8.2 1W Fusible C2 6.8 F 400 V D7 SB260 9 R10 0.24 C6 680 F 10 V Q1 2N3906 R8 160 R9 330 C7 100 F 10 V D5 1N4007G R3 100 C8 6.8 F 400 V 1 3 TinySwitch-II U1 D EN TNY266P BP L2 Ferrite Bead S C4 0.1 F 50 V U2 PC817D R5 1.35 2.0 W R4 300 VR1 BZX79B5V1 RTN PI-3704-091903 Figure 1. TinySwitch-II Based Charger/Adapter Circuit Diagram. DI-64 www.powerint.com September 2003 DI-63 * Secondary feedback bias current and output tolerances are minimized by using a low current part for VR1 * Primary clamp losses are minimized by keeping the voltage reflected across the transformer (VOR) low * Picking wire sizes that fill each winding layer produce transformers with the lowest leakage inductance * Putting the floating shield winding between the primary and secondary windings reduces EMI noise * A high gain opto-coupler (CTR = 200-400) keeps the CC portion of the output curve more vertical * R10 should equal the output cable resistance value 6 Output Voltage (V) 5 4 3 2 1 0 85 VAC 265 VAC Low Limit High Limit 0 100 200 300 400 500 600 700 800 900 Output Current (A) PI-3705-091703 300 250 Figure 3. 6.0 VDC, 800 mA CV/CC Curve. Input Power (mW) 200 150 100 50 0 85 115 145 175 205 235 265 TRANSFORMER PARAMETERS Core Material Bobbin Winding Order (pin numbers) Primary Inductance EE16, TDK PC40, or equivalent AL of 127.5 nH/T2 EE16, 10 pin Core shield (1-3), tape Primary (1-2), tape Shield (1-floating), tape Secondary (10-9), tape 1.25 mH 10% Input Voltage (VAC) Primary Resonant Frequency 500 kHz (minimum) Leakage Inductance 35 H (maximum) Figure 2. No-Load Input Power vs. Line Voltage. Table 1. Transformer Construction Information. A 9/03 www.powerint.com PI-3706-091803 Key Design Points 7 |
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