Distributed Gate Thyristor types R1045NC28L & R1045NC32L
15.0 Reverse Recovery Loss
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:
TSINK(new) TSINK(original) E k f RthJ Hs
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(J-Hs) = 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
TSINKnew TSINKoriginal E Rth f
Where TSINK (new) is the required maximum heat sink temperature and
TSINK (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
Vr = Commutating source voltage
Where: CS = Snubber capacitance
R = Snubber resistance
Data Sheet. Types R1045NC28L & R1045NC32L Issue A1
Page 5 of 13