AN4130 datasheet(22/41 Pages) STMICROELECTRONICS

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Design guidance

AN4130

22/41

Doc ID 023315 Rev 1

4.4

Diode selection (D3)

4.4.1

Speed

D3 must be a fast-recovery part, but because of the transition-mode topology the recovery

requirements are modest. Current in the diode reverses slowly, and the diode is completely

turned off well before the FET turns on. Parts with Trr up to about 70 ns are suitable.

4.4.2

Reverse voltage

The diode must support the reflected line voltage and output voltage plus a small spike from

leakage inductance. A standard 200 V fast-recovery diode was used. A high-voltage

Schottky diode would also work, with a slight gain in efficiency and slightly increased cost.

4.4.3

Current rating

This is a low-stress application for the diode. The 1-amp rating of ST’s STTH102A is

probably too much, but the part is inexpensive, and it works well.

4.5

Snubber capacitor selection (C10)

The current at FET turn-off continues to flow in the leakage inductance of the transformer,

resulting in a primary-side voltage spike. Common practice is to use an RCD clamp or an

RC snubber to dissipate this energy as heat.

The snubber can be moved to the secondary side of the transformer if leakage inductance is

low. The primary voltage can be caught on-the-rise by an R-C network placed across the

secondary winding or the diode. This avoids the need for high-voltage diodes and capacitors

on the primary side.

Experiments with relatively low values of capacitor and resistor determined that for a narrow

range of capacitor values the primary overshoot at FET turn-off was greatly reduced. It was

also discovered that the resistor is not needed if the secondary side capacitor is properly

selected. Criteria for selection have not been determined.

The 800 V rating of the HVLED815PF's FET doesn't hurt either. It's certainly too much for a

120 V line.

4.6

Transformer design (T1)

4.6.1

Operating frequency

Higher operating frequency reduces the size of the transformer. Operating frequency can be

increased up to the point where EMI filtering requirements become the limiting factor. An

operating frequency just below 150 kHz puts the second harmonic inside the conducted EMI

band, but the harmonics are smaller and easier to filter than the fundamental. Placing the

fundamental at 120-135 kHz at the nominal line voltage peak is a good compromise,

considering component tolerances.

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