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UC1852 UC2852 UC3852
High Power-Factor Preregulator
FEATURES * Low-Cost Power Factor
Correction
DESCRIPTION
The UC1852 provides a low-cost solution to active power-factor correction (PFC) for systems that would otherwise draw high peak current pulses from AC power lines. This circuit implements zero-current switched boost conversion, producing sinusoidal input currents with a minimum of external components, while keeping peak current substantially below that of fully-discontinuous converters. The UC1852 provides controlled switch on-time to regulate the output bulk DC voltage, an off-time defined by the boost inductor, and a zero-current sensing circuit to reactivate the switch cycle. Even though switching frequency varies with both load and instantaneous line voltage, it can be maintained within a reasonable range to minimize noise generation. While allowing higher peak switch currents than continuous PFCs such as the UC1854, this device offers less external circuitry and smaller inductors, yet better performance and easier line-noise filtering than discontinuous current PFCs with no sacrifice in complexity or cost. The ability to obtain a power factor in excess of 0.99 makes the UC1852 an optimum choice for low-cost applications in the 50 to 500 watt power range. Protection features of these devices include under-voltage lockout, output clamping, peak-current limiting, and maximum-frequency clamping. The UC1852 family is available in 8-pin plastic and ceramic dual in-line packages, and in the 8-pin small outline IC package (SOIC). The UC1852 is specified for operation from -55C to +125C, the UC2852 is specified for operation from -40C to +85C, and the UC3852 is specified for operation from 0C to +70C.
* Power Factor Greater Than 0.99 * Few External Parts Required * Controlled On-Time Boost PWM * Zero-Current Switching * Limited Peak Current * Min and Max Frequency Limits * Starting Current Less Than 1mA * High-Current FET Drive Output * Under-Voltage Lockout
TYPICAL APPLICATION
UDG-92001
10/94
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (Low-impedance Source)..........................30.0V Supply Current (High-impedance Source) ......................30.0mA OUT Current, Peak .............................................................1.0A OUT Energy, Capacitive Load............................................ 5.0J Input Voltage, ISNS ............................................................5.0V Input Voltage, VFB............................................. -0.3V to +10.0V COMP Current ...............................................................10.0mA ISET Current..................................................................-10.0mA Power Dissipation at Ta25C (Note 3) ..............................1.0W Storage Temperature ....................................... -65C to +150C Lead Temperature (Soldering, 10 Seconds) ................... +300C Note 1: All voltages with respect to GND (Pin 1). Note 2: All currents are positive into the specified terminal. Note 3: Refers to DIL-8 Package. Consult Packaging Section of Unitrode Integrated Circuits databook for thermal limitations and considerations of package.
CONNECTION DIAGRAM
UC1852 UC2852 UC3852
DIL-8 (TOP VIEW) J or N Package
SOIC-8 (TOP VIEW) D Package
UDG-92002
ELECTRICAL CHARACTERISTICS Unless otherwise stated, VCC=24V, ISET=50k to GND, RAMP=1nF to GND, ISNS=
-0.1V, VFB connected to COMP, no load on OUT, -55CUC1852, -40C2
DETAILED BLOCK DIAGRAM
UC1852 UC2852 UC3852
UDG-92003
PIN DESCRIPTIONS
COMP: COMP is the output of the error amplifier and the input of the PWM comparator. To limit PWM on-time, this pin is clamped to approximately 10V. To implement soft start, the COMP pin can be pulled low and ramped up with a PNP transistor, a capacitor, and a resistor. GND: Ground for all functions is through this pin. ISET: The dominant function is of this pin is to program RAMP charging current. RAMP charging current is approximately 5V divided by the external resistor placed from ISET to ground. Resistors in the range of 10k to 50k are recommended, producing currents in the range of 100A to 500A. A second function of ISET is as reference output. The ISET pin is normally regulated to 5V 10%. It is critical that this pin only see the loading of the RAMP programming resistor, but a high input-impedance comparator or amplifier may be connected to this pin or to a tap on the RAMP programming resistor if required. The third function of the ISET pin is as a FAULT output. In the event of an over-current fault, the ISET pin is forced to approximately 9V by the fault comparator. This can be used to trip an external protection circuit which can disable the load or start a fault restart cycle. ISNS: This input to the zero and over current comparators is specially built to allow operation over a 5V dynamic range. In noisy systems or systems with very high Q inductors, it is desirable to filter the signal entering the ISNS input to prevent premature restart or fault cycles. For best accuracy, ISNS should be connected to a current sense resistor through no more than 200 ohms. OUT: The output of a high-current power driver capable of driving the gate of a power MOSFET with peak currents exceeding 500mA. To prevent damage to the power MOSFET, the OUT pin is internally driven by a 12V supply. However, lead inductance between the OUT pin and the load can cause overshoot and ringing. External current boost transistors will increase this overshoot and ringing. If there is any significant distance between the IC and the MOSFET, external clamp diodes and/or series damping resistors may be required. OUT is actively held low when the VCC is below the UVLO threshold. RAMP: A controlled on-time PWM requires a timer whose time can be modulated by an external voltage. The timer current is programmed by a resistor from ISET to GND. A capacitor from RAMP to GND sets the on time in conjunction with the voltage on COMP. Recommended values for the timer capacitors are between 100pF and 1nF. VCC: VCC is the logic and control power connection for this device. VCC current is the sum of active device supply current and the average OUT current. Knowing the maximum operating frequency and the MOSFET gate charge (Qg), average OUT current can be estimated by: IOUT = Q g x F To prevent noise problems, bypass VCC to GND with both a ceramic and an electrolytic capacitor. VFB: VFB is the error amplifier inverting input. This input serves as both the voltage sense input to the error amplifier 3
TYPICAL CHARACTERISTICS
UC1852 UC2852 UC3852
Error Amplifier Gain and Phase 100 80 60 Gain (dB) 40 Phase (deg) 20 0 -20 10 100 1000 10000
UDG-92005
Max Frequency vs. Rset and Ct 1000 100pF 500pF 1nF 3nF
Phase
Gain
Maximum Frequency, 100 kHz
10 1 10 Rset, k
UDG-92006
100
Frequency, kHz
Max On-Time vs. Rset and Ct 1000
200
OUT Rise and Fall Time
Fall Time 150
100
Maximum On-Time s
10
10nF
ns 100
3nF 1nF
50 Rise Time
300pF 1 1 100pF
0
10 Rset, k
100
UDG-92007
0
5 Load Capacitance, nF
10
UDG-92008
4
APPLICATION INFORMATION: A 100 Watt Power Factor Preregulator
UC1852 UC2852 UC3852
UDG-92004
This circuit demonstrates a complete power factor preregulator based on the UC3852. This preregulator will supply up to 100 watts at 400VDC and exhibit power factor greater than 0.995 with less than 10% total harmonic distortion. Operating input range is 90V to 160V RMS at 50Hz to 60Hz. This design is intentionally simple, yet fully functional. The UC3852 can also be used in designs featuring soft start, over-voltage protection, wide power-line voltage operation, and fault latching. For more information on applying the UC3852, refer to Unitrode Application Note U-132. PARTS LIST
C1 C2 C3 C4 C5 C6 D1 D2 D3 IC1 0.47F/250VAC X2 Class Polyester 1nF/16V Ceramic 68F/35V Aluminum Electrolytic 180pF/16V Ceramic 0.1F/16V Polyester or Ceramic 82F/450V Aluminum Electrolytic 2A/500V Bridge Rectifier (Collmer KBPC106 or Powertex MB11A02V60) 100mA/50V Switching Diode (1N4148) 2A/500V 250ns Recovery-Time Rectifier (Motorola MR856) UC3852N Power Factor Controller IC R1 R2 R3 R4 R5 R6 R7 R8 Q1 L1 IRF830 4.5A/500V 1.5 Power FET 680H (Renco RL3792 with 10 Turn 24 AWG Secondary) 150k, 14W 0.2, 12W Carbon Composition 10, 14W 13.3k, 14W 1M, 14W 20k, 14W 200k, 12W 200k, 12W
UNITRODE INTEGRATED CIRCUITS 7 CONTINENTAL BLVD. * MERRIMACK, NH 03054 TEL. (603) 424-2410 * FAX (603) 424-3460
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Copyright (c) 1999, Texas Instruments Incorporated

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