EPR-21 – 5 V, 30 W DC-DC Converter

19-Nov-02

4 Circuit Description

4.1 Primary Circuitry

The schematic in Figure 2 shows a single-ended forward converter using the DPA424R.

The circuit is designed for 36 V to 72 V input range and 5 V, 6 A output. C1 and L1

provide input filtering. C2 and C3 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. C8 reduces the amplitude of leakage spikes generated when the

MOSFET in U1 switches off. VR1 clamps the U1 drain voltage to a safe value during fault

conditions, but is inactive during normal operation.

R1 is used to set the low line turn-on threshold to approximately 33 V, and also sets the

overvoltage shutdown level to approximately 88 V. R3 sets the U1 current limit to

approximately 85% of its nominal value, limiting the output power delivered during fault

conditions. C5 bypasses the U1 CONTROL pin, and provides the peak current necessary

for driving the DPA-Switch internal MOSFET. C6 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. R4 adds a zero

to stabilize the power supply control loop.

4.2 Output Rectification

The output of T1 is rectified and filtered by D2, L2, C10, and C11. This is the lowest cost

rectification scheme. DPA-Switch is however fully compatible with simple self-driven

synchronous rectification schemes as described in the DPA-Switch datasheet. C12

provides additional high frequency filtering, and is located close to the output terminals of

the supply. C7 provides bypassing for high frequency common-mode noise coupled from

the primary to the secondary of T1. R14 damps primary-to-secondary circulating current,

which would otherwise appear imposed on the output ripple voltage.

C9 and R5 provide some snubbing to D2, but their main purpose is to reset T1 during the

U1 off time. During the U1 off time, magnetizing energy stored in the T1 primary during

on time is coupled to the secondary, and charges C9. The voltage on C9 is a 1/2 sinusoid

with a period determined by C9 and the reflected primary inductance of T1. C9 is sized

such that the voltage generated during the off time is sufficient to reset T1 before the next

switching cycle occurs. R5 provides damping for C9 to prevent high frequency ringing

during switching transitions.

U1 is powered during normal operation by an auxiliary flyback winding on L2. This

winding delivers energy during the off time of U1, with an output voltage proportional to

the supply output voltage. The turns ratio of L2 sets the output voltage of the auxiliary

winding to approximately 12 V. D1 and C4 rectify and filter the auxiliary winding output.

Page 6 of 32

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