AN2025 datasheet(4/11 Pages) STMICROELECTRONICS

4 / 11 page

AN2025 - APPLICATION NOTE

4/11

The first phase corresponds to the charging of the junction capacitance of the diode, Cj.

The voltage across the diode VD decreases until it reaches the clamping voltage of the diode -VClamp (see

figure 7). As was explained above, when the total current is equal to IPEAK, VD is equal to -VS.

Once the current has reached IPEAK, it then increases to reach the value I1 corresponding to VD = -VClamp

(see figure 7).

Phase 2: t

∈ [t1, t2]

At t = t1: iT = I1

vD = VClamp

During this phase, the diode works in the ava-

lanche region. Consequently, the voltage across

the diode is equivalent to a voltage generator

equal to VClamp.

The total current increases linearly with a slope

equal to:

(see figure 7)

After t2, the voltage across the diode increases towards -VS (see figure 7).

These considerations show that:

I1 < IPEAK

III.2.3. Estimation of the maximum avalanche peak power: PPEAK_AV

The figure 8 shows in blue color the total current IT (diode + snubber) and in black line the real avalanche

current waveforms during the switching-off of the diode.

Figure 8: Total current and avalanche current waveforms when the diode works in the avalanche area

--------

VClampVs

----------------------------

IPEAK

IAR

tp < 1µs

iAVALANCHE

VClamp

Phase2

diT

V-V

Clamp

VClamp

iAVALANCHE

AN2025 Datasheet(PDF) 4 Page - STMicroelectronics