Application Hint 11 Micrel Application Hint 11 500kHz 30W Off-Line Switching Power Supply by George Hall Q1's drain voltage to rise above C1's voltage. IC1 voltage is now supplied from the low impedance winding of T1 (pins 3 and 4). The output voltage rises until IC2's reference voltage reaches approximately 2.5V where it begins drawing current through the diode of optocoupler IC3. IC3's detector transistor conducts, raising the voltage on pin 2 of IC1. When the output voltage equals 5V, pin 2 of IC1 will be 2.5V and current mode PWM operation will regulate the output precisely over varying line and load changes. R14 and C11 provide stability compensation for IC2. Q1 Protection Components D2, C3, and R9 protect Q1 from avalanche breakdown and possible destruction by clamping the leakage inductance spike to a safe level. C6 and R10 suppress parasitic oscillations from D6. Circuit Description Line Input Alternating line voltage is rectified by D1 and filtered by C1 to provide a dc bus voltage for the main transformer T1 and MIC38HC43 controller IC1. Thermistor RT1 limits the in-rush current to C1, protecting D1, and reducing the chance of an unacceptable momentary voltage drop on the ac input line during turn-on. PWM Operation Resistors R1 and R2 charge C2 until its voltage exceeds the UVLO (undervoltage lockout) of IC1 which causes output drive to be applied to Q1. This lowers Q1's drain voltage and charges T1's primary until the current sense voltage at pin 3 of IC1 exceeds 1V. IC1 then removes drive from Q1. With Q1 off, T1 discharges into both the output (T1 pins 7,8 and 5,6) and tertiary (T1 pins 3 and 4) circuits and causes 4 +5V 1 F1 Line D1 3N250 RT1 10, 1A 2A EMI FILTER 85--264Vac R12 4.99k 1% R14 47k C1 100F 400V Neutral 2 C6 1000pF R10 R13 4.99k 1% 10 1/2W VREF R1 330k 5 1/2 IC3 4N35 4 R4 1.2k IC1 MIC38HC43 1 2 R3 1.2k R2 330k 3 4 COMP VREF FB VDD IS VOUT RT/CT GND C3 3300pF 200V 1 R9 20k 2W 2 D2 MUR160 8 R8 33 7 D3 MUR115 3 6 5 D4 16V 4 CT1 D5 MBR030 10H D6 MBR1045 7,8 C7, C8, C9 3 x 1200F 10V 5,6 C10 470F 10V R11 200 T1 (LP = 40H) Symbol CT1 T1 L1 Part Number ETS92420 ETS92423 ETS92422 R7 6.8 Q1 IRF830 Note 2: All resistors 1/4W unless otherwise noted. 500kHz 30W Off-Line Switching Power Supply 1997 VOUT +5V, 6A L1 Note 1: Magnetic components are available from Energy Transformation Systems, Inc., tel: (415) 324-4949 R5 2.2k C4 C5 0.1F 1000pF C11 1000pF IC2 TL431A 75% Efficiency at 115vac C2 22F 25V (8 pin) R6 22 1/2 IC3 4N35 R15 1k 4-191 C12 0.1F Return Application Hint 11 Micrel EMI Filter Electromagnetic interference feedback into the ac input line from the operation of switched mode power supplies requires EMI filtering to comply with national and international standards. Use these standards to determine the acceptable levels of line conducted emissions for the specific application and location. EMI filtering may be simplified by procuring several packaged EMI filters from a reputable source. Select the appropriate filter by EMI measurement. Include the selected filter in the final design or substitute the individual components (from the filter's parts list). Printed circuit board layout and component placement will affect conducted emissions. If you are not qualified in the this area consult an expert. Circuit Layout Care should be taken when designing high frequency converters to avoid capacitive and inductive coupling of the switching waveform into high impedance circuitry such as the error amplifier, oscillator, and current sense amplifier. Avoid long printed circuit traces and component lead lengths. Locate oscillator and compensation circuitry near the IC. Use high frequency decoupling capacitors on VREF and, if necessary, on VDD. Return high di/dt currents directly to the source and use large area ground planes where possible. Safety Always proceed with caution when working on off-line supplies as lethal voltages are present. Never work on the supply without someone nearby who is aware of the hazards and can take steps to avoid serious injury to yourself in the event of an accident. Block Diagram Pin Configurations 7(11) VD (Output) 7(12) VDD (Ckt) VREF 5V REF COMP 1 FEEDBACK/EA 2 ISENSE 3 35V UVLO 8-pin DIP 6(10) Output T S R Q COMP 1 NC 2 Q FEEDBACK/EA 3 NC 4 2.5V 5 GND * OSC 4(7) 2(3) Feedback /EA 6 OUTPUT RT /C T 4 8(14) RT/CT 8 VREF 7 VDD ISENSE 5 2R - + + R 1V NC 6 - RT /C T 7 1(1) 5(9) Comp Analog Ground 3(5) I Sense 5(8) Power Ground * MICx8C42, 43 / MICx8HC42, 43 MICx8C44, 45 / MICx8HC44, 45 14 VREF 13 NC 12 VDD 11 VD 10 OUTPUT 9 ANALOG GND 8 POWER GND 14-pin DIP (suffix -1, full featured parts) Also available in 8-pin and 14-pin SOIC packages. 4-192 1997