WESTCODE An IXYS Company

Distributed Gate Thyristor types R0472YS12# to R0472YS16#

(iii)

15.0 Reverse Recovery Loss

K

Factor

=

t1

t2

15.1 Determination by Measurement

From waveforms of recovery current obtained from a high frequency shunt (see Note 1, Page 5) and

reverse voltage present during recovery, an instantaneous reverse recovery loss waveform must be

constructed. Let the area under this waveform be E joules per pulse. A new heat sink temperature can

then be evaluated from the following:

( )TK (new) = TK (original ) − E ⋅ k + f ⋅ Rth(JK )

Where k=0.227 (°C/W)/s

E = Area under reverse loss waveform per pulse in joules (W.s.)

f = rated frequency Hz at the original heat sink temperature.

Rth(JK) = d.c. thermal resistance (°C/W).

The total dissipation is now given by:

W (TOT) = W (original) + E ⋅ f

15.2 Determination without Measurement

In circumstances where it is not possible to measure voltage and current conditions, or for design

purposes, the additional losses E in joules may be estimated as follows.

Let E be the value of energy per reverse cycle in joules (curves in Figure 9).

Let f be the operating frequency in Hz

( )TK (new) = TK (original ) − E ⋅ Rth ⋅ f

Where TK (new) is the required maximum heat sink temperature and

TK (original) is the heat sink temperature given with the frequency ratings.

A suitable R-C snubber network is connected across the thyristor to restrict the transient reverse voltage

to a peak value (Vrm) of 67% of the maximum grade. If a different grade is being used or Vrm is other than

67% of Grade, the reverse loss may be approximated by a pro rata adjustment of the maximum value

obtained from the curves.

NOTE 1- Reverse Recovery Loss by Measurement

This thyristor has a low reverse recovered charge and peak reverse recovery current. When measuring

the charge, care must be taken to ensure that:

(a) a.c. coupled devices such as current transformers are not affected by prior passage of high

amplitude forward current.

(b) A suitable, polarised, clipping circuit must be connected to the input of the measuring oscilloscope

to avoid overloading the internal amplifiers by the relatively high amplitude forward current signal

(c) Measurement of reverse recovery waveform should be carried out with an appropriate critically

damped snubber, connected across diode anode to cathode. The formula used for the calculation

of this snubber is shown below:

R2

=

4⋅

Vr

CS ⋅ di dt

Vr = Commutating source voltage

Where: CS = Snubber capacitance

R = Snubber resistance

Data Sheet. Types R0472YS12# to R0472YS16# AD Issue 1

Page 5 of 12

June, 2008