AN 5951 - Dynex Semiconductor Ltd.

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 onstate 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 i2
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)
To calculate the energy loss as per equation
(1), detailed knowledge of the reverse
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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AN 5951
recovery current when a thyristor undergoes
turn-off. The reverse recovery phase is
characterised by the peak reverse recovery
current IRR and the recovered charge QS. QS is
given by the integral of the reverse recovery
current.
 =
  
(2)
This is the shaded area in Fig. 1. For practical
reason the datasheet value of QS is integrated
for 150µs by which time the reverse recovery
current is virtually zero.
 ≈ 
∞

  
 
(5)
where VR is assumed to be quasi constant and
equal to applied peak reverse voltage VRpeak .
And from (5),
 ≈ 0.5 ×  × 
(6)
Also,
 ≈ 0.5 ×  ×  − 
(7)
The charge QA can be approximated by the
area of a triangle formed by IRR and tA. Thus
 = 0.5 ×  × 
(8)
But
 =

(9)
/
Thus
 =
2
0.5×
 /
(10)
Substituting in (7) we get,
 ≈ 0.5 ×  ×  −
In Fig. 2 the total charge QS is divided into two
regions, QA and QRR respectively; where
 =


0 
∞

  
 
 
(3)
(4)
The reverse voltage during the time interval tA
is negligible (Fig. 1) and hence energy
contribution during the period tA can be
approximated to zero. Then from (1 and 4),
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(11)
Worked example:
Fig. 2 Triangular Approximation
 =
2
0.5×
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 VRM(line
voltage), VRpeak (controlled by the snubber
circuit) and the dI/dt. The dI/dt of the turn-off
current is usually controlled by the
commutation inductance LC.
AN 5951
and QSmin = 1357.3 x (10)0.6271 = 5751µC.
Similarly from chart of Fig. 4 the IRRmax = 275A
and IRRmin =198A. VRpeak is 2500V.
18000
QSmax = 3397.4*(di/dt) 0.5061
16000
14000
Using the equation (11),
Stored Charge, Q S - (uC)
12000
Erec(max) = 8.89J and Erec(min) = 4.74J per pulse.
10000
If the repetition frequency is say 50Hz, then
the power losses are:
8000
QSmin = 1357.3*(di/dt) 0.6271
6000
Prec(max) = 8.89 x 50 = 444.5W and
Conditions:
Tj = 125oC
VRpeak ~ 2500V
VRM ~ 1700V
snubber as appropriate to
control reverse voltages.
4000
2000
Prec(min) = 4.74 x 50 = 237W.
0
0
10
20
30
Rate of decay of on-state current, di/dt - (A/us)
Fig. 3 Stored Charge
600
IRRmax = 48.236*(di/dt) 0.7553
Reverse recovery current, IRR - (A)
500
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.
Measurement Method:
400
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
IRRmin = 29.853*(di/dt) 0.8222
200
Conditions:
Tj=125oC
VRpeak ~ 2500V
VRM ~ 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 = VRM/LC
If we assume dI/dt =10A/us, the value of Qs is
given by using the equation on the chart of
Fig. 3; QSmax = 3397.4 x (10)0.5061 = 10895µC
Fig.5 Reverse recovery waveforms
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Conclusion:
Reverse recovery voltage (V)
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:
Emeas = 17.8 J.
Using the approximation method (Eqn. 11) for
the test results thus:
Erec = 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
-50
-1000
-100
-1500
-150
-2000
-200
-2500
-250
-3000
-300
Voltage
Current
-3500
-350
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)
Test equipment readings:
QS = 15610µC integrated over 500µs
Irr = 225.1A
dI/dt = 5.5 A/µs
0
Reverse recovery current (A)
The test conditions are:
Tj = 125°C
VRpeak = 3030V
Snubber setting: 14Ω and 12µF
300000
250000
200000
150000
100000
50000
0
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.
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0.0E+00
2.0E-04
4.0E-04
Time (s)
Fig. 7 Reverse recovery instantaneous power
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