EPR-33 Datasheet, PDF (7/32 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP 33 - 45 W LCD Monitor External Power Supply using TOP247Y (TOPSwitch-GX)

28-Mar-2003
EPR-33 – 45 W External Adapter Supply
4 Circuit Description
The schematic in Figure 2 shows an off-line flyback converter using the TOP247Y. The
circuit is designed for 90 VAC to 265 VAC input and 12 V, 3.75 A output.
4.1 Input EMI Filtering
Capacitor CX1 and the L1 leakage inductance filter differential-mode conducted EMI.
Inductor L1 and CY1 filter common-mode conducted EMI. Inductor L4 damps line-cord
resonance.
4.2 TOPSwitch-GX Primary
Rectifier bridge BR1 and C1 provide a high voltage DC BUS for the primary circuitry. C2
bypasses the high voltage DC rail. Resistor R11 provides damping that reduces mid-
frequency conducted EMI. The DC rail is applied to the primary winding of T1. The other
side of the transformer primary is driven by the integrated MOSFET in U1. Diode D1 and
VR1 clamp leakage spikes generated when the MOSFET in U1 switches off. Capacitor
C3 reduces the operating temperature of VR1 by bypassing the leading edge of the
primary leakage spike away from VR1. Resistor R2 provides damping to reduce drain
ringing. Resistor R1 sets the low-line turn-on threshold to approximately 69 VAC, and
also sets the overvoltage shutdown level to approximately 320 VAC. R3 sets the U1
current limit to approximately 55% of its nominal value. This limits the output power
delivered during fault conditions. C4 bypasses the U1 CONTROL pin. C5 has 3 functions.
It provides the energy required by U1 during startup, sets the auto-restart frequency
during fault conditions, and also acts to roll off the gain of U1 as a function of frequency.
R4 adds a zero to stabilize the power supply control loop. Diode D2 and C6 provide
rectified and filtered bias power for U2 and U1.
4.3 Output Rectification
The output of T1 is rectified and filtered by D3, C7, and C8. Inductor L2, C9, and C10
provide additional high frequency filtering. Resistor R12 and C13 provide snubbing for
D3. Choosing the proper snubber values is important for low zero-load power
consumption and for high frequency EMI suppression. The snubber components were
chosen so that the turn-on voltage spike at the D3 anode is slightly under-damped.
Increasing C13 and reducing R12 will improve damping and high frequency EMI, at the
cost of higher zero-load power consumption.
4.4 Output Feedback
Resistors R8 and R9 divide down the supply output voltage and apply it to the reference
pin of error amplifier U3. Shunt regulator U3 drives optocoupler U2 through resistor R6 to
provide feedback information to the U1 CONTROL pin. The optocoupler output also
provides power to U1 during normal operating conditions. Diode D4 and C11 apply drive
to the optocoupler during supply startup to eliminate output voltage overshoot. Diode D4
isolates C11 from the supply feedback loop after startup. Resistor R5 discharges C11
when the supply is off.
Page 7 of 32
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