AN1453 datasheet(5/9 Pages) STMICROELECTRONICS

5 / 9 page

APPLICATION NOTE

5/9

Without a snubber, in this example the diode is

repeatedly in conduction because the oscillation is

very strong. Furthermore, the voltage is close to

the breakdown voltage. This means that the

system is no longer reliable and a snubber circuit is

necessary.

On these 2 oscillograms, we can see that the value

of the maximum reverse current (IRM) is defined

when the reverse voltage rises (typical behaviour

of a bipolar diode). At this time the voltage is not

fixed by the diode.

The curve Qrr,IRR versus dIF/dt and Tj is given in

the datasheet. For example in Fig. 12, the

evolution of IRM versus dIF/dt for a STPR1620CT

can be observed.

It can be also noticed, that the parameter IRM

significantly increases with the temperature.

In continuous mode the dIF/dt (few hundred A/µs)

is fixed by the leakage inductance and the reverse

voltage (VR):

with V

out

⋅

It is many time higher than in discontinuous mode

(lower than 1A/µs):

withL

out

〉〉

(LS: Secondary inductance)

Thus, with this curve we can see that, in

continuous mode (high dIF/dt), the bipolar diode

must evacuate a non-negligible charge, which

means a higher IRM. This is verified on oscillogram

Fig. 11.

With this value of IRM, an equivalent model at t0

with a snubber circuit can be established:

Where:

Vs: secondary voltage

=⋅

Lf: leakage inductance of the transformer

Cj: junction capacitance

CQrrb:

equivalent

capacitance

modeling

the

reverse charge, necessary for the establishment of

the potential barrier, which supports the reverse

voltage.

Vout: output voltage

With the following initial conditions at t=t0:

and V

LRM

bipolar

=≈

The equivalent schematic can be used to define

DRmax

NB:

1) Without snubber, there is a L f, C circuit

( C=C j +C Qrrb) which lead to a second order

differential equation:

0 and

1 / L

+⋅

⋅

ωω

with initial conditions at t=t0:

and V

LRM

In this equation, an approximation is made with C

constant, because in reality Cj and CQrrb vary with

the voltage applied.

The solution of the differential equation gives us:

max

⋅













Therefore we can see that the VRmax depends the

leakage inductance (Lf) and on recovery charge

(IRM). Thus, V

Rmax

is very dependent on the

temperature.

STPR1620CG/CT

100

200

500

dIF/dt(A/µs)

IRM(A)

IF=IF(av) 90% confidence

Tj=125°C

Fig. 12: Peak reverse recovery current versus

dIF/dt (per diode)

Qrrb

I=I

L f

V= V + V

o u t

Fig. 13: Equivalent model at t0 for a bipolar diode

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