In today’s power converter, the commutation
speed of the transistor and the operating
frequenciesare higherandhigher.
Fast diodes used for freewheel, snubber, and
rectifier functions become one of the main causes
of the power losses. In the range of 600V-1200V,
SGS THOMSON has developed a new family of
ultrafastdiodes.
Taking into account these new constraints which
are different from one application to another,
SGS-THOMSONproposes twoseries :
TURBOSWITCH ”A” and TURBOSWITCH ”B”.
The specific characteristics of these two series
offer to the designer a doublechoice, allowing him
to use the best diodein thisapplication.
I. INTRODUCTION
The choice of the optimum diode for a given
applicationdependsontheestimationofthepower
lossesgenerated by the diode. This note explains
how to calculate the different losses with
information given in the datasheet and shows the
difference between TURBOSWITCH”A” and
TURBOSWITCH”B” and their respective
advantages.
II. LOSSESCALCULATION
II.1 Conduction losses
Conductionlosses are estimated with the classical
formula :
Pcon = Vto IF(AV) +rdI2F(RMS)
Vto : Thresholdvoltage(Fig.1)
rd : Dynamical resistance (Fig.1)
IF(AV) : Averagecurrent
IF(RMS) : RMS current
II.2. Turn on losses
When the diode turns ON, the voltage across the
diode increases to VFP (Peak forward voltage)
before it decreases to 1.1 VFat the time tFR
(Forward recovery time) (Fig.2).
APPLICATION NOTE
B. Rivet
AN601/0193
NEW HIGH VOLTAGE ULTRA-FAST DIODES :
THE TURBOSWITCHTM A and B SERIES
Fig.2 :Turn ONwaveforms
Fig.1 : Approximation ofthe forwardcharacteristic
IFIF
VFVF
Vto
rd
IF
VFP
tFP
VF
VF
1.1
t
t
(dIF/dt)ON
TM : TURBOSWITCH is a trademark of
SGS-THOMSON MIcroelectronics 1/5
Turn ON losses can be approximated by the
followingformula :
Pon = 0.4 (VFP -V
F)xt
FR xI
Fxf
Where f is theoperating frequency:
VFP andtFR dependon(dIF/dt)ONandIF. Curves in
the datasheetgivingVFP andtFR versus(dIF/dt)ON
allow the estimation of Ponfor eachapplication.
Example:
IF=8A
(dIF/dt)ON = 64A/µs
f = 100kHz
With an STTA806D
(TURBOSWITCHA, 8 A / 600 V / TO220AC)
in theseconditions
VFP(max) = 10V
tFR(max) = 500ns
Pon = 1.4 W
II.3. Turn-on losses
Turn-off losses are studied in the case of a
freewheel function where the switch is a MOS
transistor(Fig.3).
IL=Load current
IRM = Maximum reverse recoverycurrent of the
freewheeldiode D
The typical waveform of the current and the
voltage when the transistor switches ON and the
diode switches OFF is shown in Fig.4 in the case
where the stray inductanceis low (< 50 nH ).
The turn-OFFlossesinthediode canbecalculated
by :
P
OFF
=
Vaa
.
I
RM
2.
S
.
f
6(
dI
F
/
dt
)
OFF
II.4. Transistor lossesdue to the diode
When the diode switches OFF, the recovery
current flows in the transistor which induces
turn-ON losses in the transistor. The turn-ON
losses in the transistor due to the diode can be
estimatedby :
P
ON
(
tr
)=
Vaa
.
I
RM
2(3+2
S
)
f
6(
dI
F
/dt
)
OFF
+
Vaa
.
I
RM
.
I
L
(
S
+2)
f
2(
dI
F
/dt
)
OFF
Turn-ON losses in the transistor are generally
much higherthanturn-OFFlosses in thediode.
These two formulasinclude IRM and S parameters
which characterize the turn ON behaviour. These
parametersdepend on the (dIF/dt)OFF.
Fig.3 : Basiccircuit with freewheeldiode
Vaa
D
II
L+RM
I
RM
IL
tr
Fig.4 : Current and voltage waveforms of a free-
wheeldiode at turn-OFF andthe associated trans-
istor at turn-ON
V
I
Vaa
Vaa
II
L+ RM
I
RM
TRANSISTOR
DIODE
S=tb/ta
(dIF/dt) OFF
ta tb
IL
APPLICATION NOTE
2/5
In the datasheet, curves giving IRM and S versus
(dIF/dt)OFF allow to calculate these losses for a
given application.
Example:
Vaa = 400V
f = 30kHz
IL = 12A
(dIF/dt)OFF = -500A/µs
Tj = 125°C
witha STTA12060
(TURBOSWITCH”A”, 12 A /600 V / TO220AC)
we find:
Poff = 0.43W
Pon(tr) = 9.5 W
III. COMPARISON BETWEEN TURBOSWITCH
”A” AND TURBOSWITCH ”B”
III.1. Differencebetween characteristics
The design of a fast rectifier is known to be the
resultofa trade-offfora givenreversevoltage,and
the compromise can be explained in the fig.5.
For the diode of the family ”A”, the compromise VF
-I
RM hasbeenchosen to reduce the totallosses in
both the diode and the companion transistor in a
freewheel configuration.
On the other hand, the compromise of the family
”B” has been chosen to minimize the conduction
losses.
Table inFig.6summarizes themaincharacteristics
of a STTA806D
(TURBOSWITCH”A”, 8 A / 600V / TO220AC)
and a STTB806D
(TURBOWSITCH”B”, 8 A / 600V/ TO220AC)
Data in this tableshow thatconduction losses and
switch-ON losses will be lower in a
TURBOSWITCH ”B” while switch-OFF losses will
be lower in a TURBOSWICH ”A”.
The oscillogram inFig.7 shows thecurrent in a
STTA806D andin a STTB806Dwhen the
diodes switch-OFF in the following conditions :
VR= 350V
(dIF/dt)OFF = -300 A/µs
Tj = 100°C
IF=12A
This oscillogram shows that the IRM value is
approximatelytwo timeslower with a STTA1206D,
and that STTB1206D is a very soft diode.
III.2. Applicationexamples
Example 1 : In this example, a comparison of the
loss differences is done in a freewheel application
where the current in the diode is rectangular.The
main parametersare :
Peakcurrent
IM=12A
Vaa = 400V
Fig.5 : Compromisebetween IRM and VFfor a gi-
ven reversevoltage
INCREASING
THE SPEED
OF A RECTIFIER
IRM SWITCHING LOSSES
VFCONDUCTION LOSSES
TYPE (dIF/dt)OFF
= 500A/µs
IF=8A
Tj =125°C
IF=8A
Tj =
125°C
(dIF/dt)ON
=64A/µs
Tj= 25°C
IRM SV
FV
FP tFR
typ typ max max max
STTA806D 14 A 0.45 1.5 V 10 V 500 ns
STTB806D 28 A 0.79 1.3 V 8 V 500 ns
Fig.6 : Main characteristicsof a STTA806Dand a
STTB806D
Fig.7 : Switch-OFFoscillogram of STTA806Dand
STTB806D
APPLICATIONNOTE
3/5
Dutycycle:δ = 0.6
(δIF/dt)ON= 200 A/µs
(dIF/dt)OFF=500 A/µs
Tj=125°C
f = 30 kHz
In these conditions the reverse recovery
characteristics of the diodes are given in fig.7 :
The losses of the table fig.8 are calculatedby
using relationsgiven in part2.
In this type of application,the TURBOSWITCH ”A”
is obviouslythe betterchoice.
Example2 : In this example, the diode is used
as a rectifierdiode with the following conditions:
IF=12A
(dIF/dt)ON= (dIF/dt)OFF= 100 A/µs
Vaa= 350V
Tj =125°C
δ=0.8
f = 20 kHz
The estimatedlosses are summarized in the table
fig.9
In thisapplication,we haveto takeinto accountthe
leakage inductance and the fact that a very soft
diode is requiredto limit the overvoltage.The total
losses are 10% lower with the STTB1206D,
therefore the TURBOSWITCH ”B” is the best
choice.
IV. CONCLUSION
This note shows how to calculate the different
losses due to the diodes in basic power switching
circuits. These calculations can be doneby using
the parameters given in the datasheet of the
TURBOSWITCH ”A” and theTURBOSWITCH ”B”.
In most of cases, it is easy to choosebetween the
”A” typeand the ”B” type.
The ”A” type is very efficient in freewheel diode
applications with high frequencies (f > 10 kHz).
The ”B” type is better when conductionlosses are
predominant like in the case of the power factor
corrector circuit in discontinuous mode (low
(diF/dt)OFF), or for applications where very high
soft recovery behaviour is required (commutation
with seriesinductances,for example).
TYPE IRM S
STTA1206D 16 A 0.42
STTB1206D 30 A 0.90
Fig.7 : Reverse recovery characteristics of
STTA1206D and STTB1206D with the conditions
of the example 1
TYPE Con-
duction
losses
Switch
ON
losses
Switch
OFF
losses
Tran-
sistor
losses
Total
losses
STTA1206D 9 W 0.1 W 0.43 W 9.5 W 19 W
STTB1206D 7.8 W 0.07 W 3.2 W 29.8 W 40.9 W
Fig.8 : Comparison between STTB1206D and
STTA1206Din afreewheel diode function
TYPE Conduc-
tion
losses
Switch
ON
losses
Switch
OFF
losses
Total
losses
STTA1206D 14.4 W negligible 0.2 W 14.6 W
STTB1206D 12.4 W negligible 1 W 13.4 W
Fig.9 : Comparison between STTA1206D an
STTB1206Din arectifier function
APPLICATION NOTE
4/5
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no re-
sponsability for the consequences of use of such informationnor for any infringement of patents or otherrights of thirdpar-
ties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of
SGS-THOMSON Microelectronics. Specifications mentioned in this publicationare subject to change without notice. This
publication supersedes and replaces all information previously supplied.
SGS-THOMSON Microelectronics products are notauthorized for useas critical components in life support devices or sys-
tems without the express written approval of SGS-THOMSON Microelectronics.
1993 SGS-THOMSON Microelectronics- Printed in France - All rights reserved.
SGS-THOMSON MicroelectronicsGroup of Companies
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APPLICATIONNOTE
5/5
