AN2025 - APPLICATION NOTE

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IV. Estimation of the average avalanche power losses

The accurate method given below allows the maximum average avalanche power to be determined, and

it can be used to optimize the choice of the diode in order to improve the converter's efficiency. The equiv-

alent circuit during the time the diode works in the avalanche is simulated. From this simulation, the real

avalanche current as well as the pulse duration can be found from which an estimation of the maximum

avalanche energy can be made. The simulation is performed using the Pspice software. The 2 steps that

comprise this method are explained using an adaptor for Notebook as a concrete example (see figure 10).

First step: Measurement

This first step consist of measuring 3 waveforms:

⇒ the total current i

⇒ the voltage across the diode V

D

⇒ the voltage across the snubber capacitor V

CS

Figure 11 shows typical waveforms corresponding

to the example.

The maximum avalanche energy in the diode corresponds to a diode with a minimum clamping voltage

Figure 11: Waveforms when the diode turns off

V

= 14V

C0

i with

delay time

T

i without

delay time

T

0V

0A

20V/div

1A/div

10ns/div

0 scope

V = m.V

S

IN

Figure 10: 70W Adaptor for Notebook, using a

STPS16H100CT in the avalanche

STPS16H100CT

I= 4A

OUT

V

= 18V

OUT