AN2025 datasheet(3/11 Pages) STMICROELECTRONICS

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AN2025 - APPLICATION NOTE

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III.2. Switching-off analysis (simple method)

III.2.1. Introduction

The figure 5 shows the equivalent circuit that can be used to simulate a secondary rectification function

when the diode turns off : LF represents the leakage inductance of the transformer. The diode is modelized

by the capacitance Cj, RS and CS are the snubber components.

The figure 6 shows the corresponding current and voltage waveforms taking into account the delay time

between current and voltage probes. When the total current (current in the diode + current in the snubber)

is at maximum (IPEAK), the voltage across the leakage inductance is zero (dIT/dt = 0). Consequently the

voltage across the diode is equal to VS.

iT = IPEAK ⇔ vD = -VS

III.2.2. Switch-off behavior when the diode works in the avalanche area

The figure 7 shows the switch-off behavior when the diode works in the avalanche area.

This characteristic is made up of 2 distinct phases.

Phase 1: t

∈ [t0, t1]

At t = t0: iT = I0

vD = 0

Figure 5: Basic equivalent circuit

Snubber

L : leakage inductance of the transformer

Figure 6: Total current (iT) and voltage (VD)

when the diode turns off

20V/div

1A/div

20ns/div

i with delay time

IPEAK

-VSPIKE

-VRRM

-VS

i without delay time

Figure 7: Switch-off behavior when the diode

works in the avalanche area

-VS

-VRRM

IPEAK

vD, iT

dIT/dt

Phase2

Phase1

-VClamp

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