Distributed Gate Thyristor types R0878LC18x to R0878LC21x
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 ⋅ Rth(J −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
( )TSINK (new) = TSINK (original ) − 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. Type R0878LC18x to R0878LC21x Issue 3
Page 5 of 12