EPR-34 Datasheet, PDF (7/28 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP 34 - Single Output 30 W AC-DC Power Supply

10-Feb-2004
EPR-34 – 12 V, 30 W, Universal Input
4 Circuit Description
The schematic in Figure 2 shows an off-line flyback converter using the TOP245Y. The
circuit is designed for 85 VAC to 265 VAC input and provides an isolated 12 V, 2.5 A
output.
4.1 Input EMI Filtering
Capacitor CX1 and the leakage inductance of L1 filter differential mode conducted EMI.
Inductor L1 and CY1 filter common mode conducted EMI.
4.2 TOPSwitch Primary
Rectifier bridge BR1 and C1 provide a high voltage DC supply rail for the primary
circuitry. 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 C2 reduces
the operating temperature of VR1 by bypassing the leading edge of the primary leakage
spike away from VR1. Resistor R3 provides damping to reduce drain ringing improving
EMI. Resistor R1 sets the low-line turn-on threshold to approximately 69 VAC and sets
the overvoltage shutdown level to approximately 320 VAC. Resistor R4 sets the U1
current limit to approximately 70% of its nominal value. Resistor R2 reduces the U1
current limit as a function of line voltage so that maximum overload power is relatively
constant (<50 W) over the entire input voltage range. This limits the output power
delivered during fault conditions. Capacitor C4 bypasses the U1 CONTROL pin while C3
has three 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. Resistor R5 adds a zero to the control loop to stabilize the power
supply. Diode D2 and capacitor C5 provide rectified and filtered bias power for U2 and
U1.
4.3 Output Rectification
The secondary of T1 is rectified and filtered by D3, C6, and C7. Inductor L2 and C8
provide additional high frequency filtering. Resistor R11 and C11 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 C11 and reducing R11 will improve damping and high frequency EMI, at the
cost of higher zero load power consumption.
4.4 Output Feedback
Resistors R9 and R10 divide down the supply output voltage and apply it to the reference
pin of error amplifier U3. Shunt regulator U3 drives the LED of optocoupler U2 through
resistor R6 to provide feedback information to the U1 CONTROL pin. The optocoupler
output also provides power to U1 from the bias winding during normal operating
conditions. Diode D4 and capacitor C10 apply drive to the optocoupler during supply
Page 7 of 28
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