DER-120
22 W Triple Output Flyback – DPA424R
October 26, 2005
4 Circuit Description
4.1 Input EMI filtering
Figure 1 shows a single-ended Flyback converter using the DPA424R. The circuit is designed
for an input range of 28 V to 60 V providing three isolated outputs at +3.3 V, +9 V and +24 V at
2 A, 0.6 A and 0.4 A respectively. C10 and C14 bypass the DC rail. 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. VR1 clamps the U1 drain leakage spikes. C1 and R3 dampen the drain-source
voltage ringing.
R9 is used to set the low line turn-on threshold to approximately 23 V, and also sets the over-
voltage shutdown level to approximately 61 V. C4 bypasses the U1 control pin, and provides
the peak current necessary for driving the DPA-Switch internal MOSFET. C5 has three
functions. It provides the energy required by U1 during startup, sets the auto-restart frequency
during fault conditions, and also reduces the gain of U1 as a function of frequency. R1 adds a
zero to stabilize the power supply control loop.
4.2 Output Rectification
The output of T1 is rectified and filtered by D3 (D7, D2) and C2 (C6, C12). L6 and C8 provide
additional high frequency filtering.
An auxiliary Flyback winding on T1 powers U1 during normal operation. This winding delivers
energy during the off time of U1, with an output voltage proportional to the supply output voltage.
The turns-ratio of T1 sets the output voltage of the auxiliary winding to approximately 12 V. D4
and C8 rectify and filter the auxiliary winding output.
4.3 Output Feedback
R6 and R7 divide down the supply output voltage and apply it to the reference pin of error
amplifier U3. C9 and R8 roll off the gain of U3. U3 drives the opto-coupler LED of U2 through
resistor R10 to provide feedback information to the CONTROL pin of U1. The opto-coupler
phototransistor output also provides power to U1 during normal operating conditions. R2
provides bias current to U3.
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