AN 5951
Estimation of turn-off losses
in a thyristor due to reverse
recovery
Application Note
AN5951-3 January 2010 LN27002
Authors: Dinesh Chamund; Colin Rout
Introduction:
The total power losses in a thyristor are
comprised of off-state losses, switching losses
and conduction losses. The off-state losses
are the steady state losses as a result of
blocking voltage and current (leakage
current). The switching losses are the dynamic
losses encountered during the turn-on and
the reverse recovery phases of the thyristor.
The conduction losses are the steady state on-
state losses during the conduction phase of
the thyristor. In the majority of the phase
control thyristor applications the conduction
losses are the dominant power losses
compared to others. Therefore it is often
sufficient to design thermal circuit using just
the conduction losses with some safety
margin. To help towards this process Dynex i
2
phase control thyristor datasheets give charts
of power dissipation under the commonly
encountered waveforms such as sine wave
and the rectangular wave for different
conduction angles.
The switching power losses are the function
of the repetition frequency and the
commutating di/dt. Therefore these losses
become significant at higher frequencies and
for high di/dt. For high voltage applications
the contribution made by the reverse
recovery losses can no longer be ignored. The
reverse recovery energy is given by:
=
×
()
(1)
recovery current and voltage waveforms is
required. This is usually acquired through
actual measurements in the real circuit.
However for initial design purposes and
dimensioning of the device, a quick method of
estimating the recovery losses is desirable. In
this Application Note a method of estimating
power losses due to reverse recovery is
outlined.
Fig. 1 Thyristor Turn-off waveforms
Approximation of reverse recovery
waveforms:
Fig. 1 shows the current and voltage
waveforms observed during the turn-off
phase of a thyristor. The charge stored during
the conduction phase is extracted as reverse
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To calculate the energy loss as per equation
(1), detailed knowledge of the reverse
AN 5951
recovery current when a thyristor undergoes
turn-off. The reverse recovery phase is
characterised by the peak reverse recovery
current I
RR
and the recovered charge Q
S
. Q
S
is
given by the integral of the reverse recovery
current.
=
(2)
≈
∞
(5)
where V
R
is assumed to be quasi constant and
equal to applied peak reverse voltage V
Rpeak
.
And from (5),
≈ 0.5 ×
×
Also,
≈ 0.5 ×
×
−
(7)
(6)
This is the shaded area in Fig. 1. For practical
reason the datasheet value of Q
S
is integrated
for 150µs by which time the reverse recovery
current is virtually zero.
The charge Q
A
can be approximated by the
area of a triangle formed by I
RR
and t
A
. Thus
= 0.5 ×
×
But
=
Thus
=
2
0.5×
/
(8)
/
(9)
(10)
Substituting in (7) we get,
≈ 0.5 ×
×
−
2
0.5×
(11)
Fig. 2 Triangular Approximation
Worked example:
For illustration purpose, thyristor part number
DCR3030V42 is chosen and the charts of
stored charge and reverse recovery current
from the datasheets are reproduced in the
Fig. 3 and Fig. 4 respectively.
The calculation begins with known
parameters of the circuit the V
RM
(line
voltage), V
Rpeak
(controlled by the snubber
circuit) and the dI/dt. The dI/dt of the turn-off
current is usually controlled by the
commutation inductance L
C
.
In Fig. 2 the total charge Q
S
is divided into two
regions, Q
A
and Q
RR
respectively; where
=
=
0
∞
(3)
(4)
The reverse voltage during the time interval t
A
is negligible (Fig. 1) and hence energy
contribution during the period t
A
can be
approximated to zero. Then from (1 and 4),
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AN 5951
18000
16000
14000
Q
Smax
= 3397.4*(di/dt)
0.5061
and Q
Smin
= 1357.3 x (10)
0.6271
= 5751µC.
Similarly from chart of Fig. 4 the I
RRmax
= 275A
and I
RRmin
=198A. V
Rpeak
is 2500V.
Using the equation (11),
12000
Stored Charge, Q
S
- (uC)
10000
E
rec(max)
= 8.89J and E
rec(min)
= 4.74J per pulse.
If the repetition frequency is say 50Hz, then
the power losses are:
P
rec(max)
= 8.89 x 50 = 444.5W and
P
rec(min)
= 4.74 x 50 = 237W.
30
8000
Q
Smin
= 1357.3*(di/dt)
0.6271
6000
4000
2000
0
0
10
20
Rate of decay of on-state current, di/dt - (A/us)
Conditions:
Tj = 125
o
C
V
Rpeak
~ 2500V
V
RM
~ 1700V
snubber as appropriate to
control reverse voltages.
Fig. 3 Stored Charge
600
I
RRmax
= 48.236*(di/dt)
0.7553
500
Reverse recovery current, I
RR
- (A)
It should be noted that the minimum recovery
losses correspond to the maximum
conduction losses and vice a versa. Ideally
both the conditions should be calculated and
the worst case value should be used to design
the thermal circuit (heat sink etc). Using both
the maximum conduction losses and
maximum recovery losses will lead to over
dimensioning of the heatsink.
400
Measurement Method:
In this method the reverse recovery energy is
determined by the measurement of the
reverse recovery current and voltage using
stored charge test equipment. The thyristor
part tested was DCR2400B85. Fig.5 shows the
oscillogram of the measured waveforms.
300
I
RRmin
= 29.853*(di/dt)
0.8222
200
Conditions:
Tj=125
o
C
V
Rpeak
~ 2500V
V
RM
~ 1700V
snubber as approriate to control
reverse voltages
100
0
0
10
20
30
Rate of decay of on-state current, di/dt - (A/us)
Fig. 4 Reverse Recovery Current
Thus
dI/dt = V
RM
/L
C
If we assume dI/dt =10A/us, the value of Qs is
given by using the equation on the chart of
Fig. 3; Q
Smax
= 3397.4 x (10)
0.5061
= 10895µC
Fig.5 Reverse recovery waveforms
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The test conditions are:
T
j
= 125°C
V
Rpeak
= 3030V
Snubber setting: 14Ω and 12µF
Test equipment readings:
Q
S
= 15610µC integrated over 500µs
I
rr
= 225.1A
dI/dt = 5.5 A/µs
Fig. 6 shows the digitised reverse recovery
current and voltage waveforms plotted in an
Excel chart. The Excel spreadsheet is used to
multiply the digitised voltage and current
waveforms to obtain the instantaneous power
waveform as shown in Fig.7. Finally
integrating this power waveform gives the
energy per pulse. Again numerical integration
was performed within the spreadsheet using
the trapezium rule. The result of this
integration gave the measured value of the
reverse recovery energy:
E
meas
= 17.8 J.
Using the approximation method (Eqn. 11) for
the test results thus:
E
rec
= 0.5x3030x(15610-(0.5x(225.1)
2
/5.5))
= 16.7 J.
The approximation result is within 10% of the
measured value.
Using the datasheet curves for DCR2400B85,
the maximum and minimum values of
recovery energy per pulse are 21.7 J and 15.9
J respectively.
0
-500
-1000
-1500
-2000
-2500
-3000
-3500
0
-50
-100
-150
-200
-250
-300
-350
Reverse recovery voltage (V)
Voltage
Current
0.0E+00 1.0E-04 2.0E-04 3.0E-04 4.0E-04
Time (s)
Fig. 6 Reverse recovery current and voltage
400000
350000
Reverse recovery power (W)
300000
250000
200000
150000
100000
50000
Conclusion:
A method for estimating reverse recovery
losses in a thyristor using datasheet curves is
presented and verified with actual
measurement. The approximated value lies
within 10% of the measured value.
0
0.0E+00
2.0E-04
Time (s)
4.0E-04
Fig. 7 Reverse recovery instantaneous power
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Reverse recovery current (A)
AN 5951
HEADQUARTERS OPERATIONS
CUSTOMER SERVICE
DYNEX SEMICONDUCTOR LIMITED
Doddington Road, Lincoln, Lincolnshire, LN6 3LF
United Kingdom.
Phone: +44 (0) 1522 500500
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Phone: +(0) 1522 502753 / 502901
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e-mail: power_solutions@dynexsemi.com
© Dynex Semiconductor 2003 TECHNICAL
DOCUMENTATION-NOT FOR RESALE.
PRODUCED IN UNITED KINGDOM
Web: http://www.dynexsemi.com
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