dm00152534

AN4638
Application note
Welding machines: V and HB series IGBTs
on two-switch forward converters
Anselmo Liberti, Rosario Gulino
Introduction
The two-switch forward converter (also known as asymmetrical half-bridge forward
converter) is a popular topology often used in industrial welding machines with low-tomedium power requirements. The inverter stage in this hard-switched forward topology
includes two transistor switches and two fast recovery diodes placed at each end of the
high-frequency power transformer primary winding. The output stage at the transformer
secondary side includes rectifier diodes and an LC filter operating as a buck converter,
hence the DC voltage output signal has a small switching ripple.
The new ST 600 V/650 V IGBT technologies with trench gate field-stop structures are
specifically tailored to meet the needs of this topology in single phase main, where the
working frequency is generally set to ~60kHz, exhibiting the highest performance in
comparison with the principal competition thanks to the excellent trade-off between switchoff energies and the VCE(sat) parameter.
This paper shows the test results for the field-stop high-speed technology V and HB series
IGBTs analyzed on two different 4 kW and 6 kW manual metal arc (MMA) welding platforms
with a two-switch forward topology configuration implemented for the power converter
sections. An evaluation of the thermal and electrical performance in terms of power
dissipation and switching characteristics of these IGBTs under full-load steady state
operation is provided, as well as different working conditions of the boards.
Modern DC welding machines generally use switch-mode DC/DC power supplies to output
a regulated high DC current and low DC voltage, with an isolation transformer which
provides galvanic isolation between the input and output sections. Thanks to the new fieldstop high-speed technology IGBTs, switch mode DC/DC power supplies for welding
machines can now operate at high frequencies with low losses by providing rapid rise and
fall times during the turn-on and turn-off switching transients.
Due to the high switching frequencies involved, welding equipment can be rendered lighter,
more portable and less expensive by mounting smaller high frequency transformers and
magnetic components. IGBTs must be selected not only on the basis of the maximum
current capability and blocking voltage rating, but also the low values for the on-state
saturation voltages in order to minimize the conduction power losses, particularly during
maximum working power conditions. Finally, the effective reduction of conduction and
switching losses for the power switches in the converter section leads to higher overall
welding machine system efficiency and improved performance [1, 2].
The high-power capacity and function mode of the transformer and switches render the twoswitch forward converter among the most suitable configurations for DC/DC converters
meeting the requirements of arc welding machines and suitable for use in the current supply
for MMA welding processes, as described further on.
February 2015
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www.st.com
23
Contents
AN4638
Contents
1
Two-switch forward converter general concepts . . . . . . . . . . . . . . . . . . 3
2
STMicroelectronics V and HB series IGBTs . . . . . . . . . . . . . . . . . . . . . . 5
3
Electrical specifications of welding machines . . . . . . . . . . . . . . . . . . . . 6
4
Description of test conditions and analyses . . . . . . . . . . . . . . . . . . . . . 7
5
V series IGBTs on a 4 kW welder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1
6
Overview of thermal and electrical measurements . . . . . . . . . . . . . . . . . . 9
HB series IGBTs on a 6 kW welder . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1
Overview of thermal and electrical measurements . . . . . . . . . . . . . . . . . 13
7
Electrical analysis and waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Appendix A Topology equations and device selection. . . . . . . . . . . . . . . . . . . . 20
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
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Two-switch forward converter general concepts
Two-switch forward converter general concepts
The two-switch forward converter circuit employs two power switches to energize the
primary side of a ferrite high frequency transformer and two freewheeling diodes to
discharge the magnetization inductance of the primary side winding of the transformer. At
the secondary side, transformer output voltage is initially scaled, then rectified by the diode
bridge rectifier (half-bridge or full-bridge, depending on the application requirements) and
finally filtered to provide smooth DC voltage or current. The two power switches are
simultaneously switched on and the gates are driven by square waves with variable duty
cycle based on the pulse width modulation technique. The duty cycle variation therefore
allows voltage modulation on the output of the power supply and, consequently, the welding
current modulation[3].
In welding applications, the forward converter is normally designed to operate in continuousconduction mode (CCM) for the output inductor current at a fixed switching frequency (the
output inductor lends continuous-mode operation to the current as the main IGBT switches
turn on before this output current ceases). In particular, a single-ended converter requires a
larger transformer and output inductor for welding machines above the 4 kW output level.
Figure 1 illustrates the two-switch forward converter configuration.
Figure 1. IGBTs mounted on a double switch forward topology.
The input voltage seen by the inverter stage used in this power range is the output voltage
of a power factor correction (PFC) converter, typically in the 380 V – 400 V range for single
phase welding machine inverters.
The double switch forward configuration is preferable to alternative configurations because
power switches in the off state are subject to a voltage stress equal to the DC input voltage
VDC (or Vbulk) and the transformer design is more straightforward than other isolated
topologies because no additional winding to reset the magnetic flux in the core is required.
Moreover, since the two recirculating diodes clamp the input voltage, no snubber circuit is
required and any overshoot beyond the input voltage can be managed with a proper circuit
layout to minimize stray inductances.
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Two-switch forward converter general concepts
AN4638
Other distinctive features include:
•
The clamp diodes recover the magnetizing energy stored in the core during on-time,
which is fed back to the supply; therefore, any voltage spikes generated by leakage
inductance are clamped to the DC input bulk voltage level.
•
No IGBT antiparallel co-packaged diode conduction is required under any operating
condition.
•
The maximum duty cycle is limited to 50% to guarantee full demagnetization of the
ferrite transformer during toff (transformer reset) to prevent magnetic flux saturation.
•
The core flux only operates in one quadrant of the BH loop.
•
The configuration is unable to operate in zero-voltage switching (ZVS) mode because
this limits its operation frequency.
Even if the IGBT voltage stress should, in theory, be clamped to the maximum DC input bulk
voltage level and recycled back to the input capacitor via the activation of the fast recovery
diodes, the IGBTs may actually see an additional spike during turn-off switching which is
affected by the leakage inductance energy, the switching speed, and the circuit layout.
Leakage inductance is difficult to control and often varies in production[4].
Refer to “Topology equations and device selection” in the Appendix, which details some
useful equations for the selection of the IGBTs used in testing.
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STMicroelectronics V and HB series IGBTs
STMicroelectronics V and HB series IGBTs
The 600 V/650 V trench gate field-stop technology IGBTs (V and HB series respectively)
were tested in two different single phase main welding machine platforms implementing the
two-switch forward converter configuration, as detailed below:
•
IGBTs STGW40V60DF and STGW60V60DF (TO-247 package) were two-by-two
parallel tested on a 4 kW application board, switching at ~63 kHz.
•
IGBTs STGW40H65DFB and STGW60H65DFB (TO-247 package) were two-by-two
parallel tested on a 6 kW application board, switching at ~58 kHz.
The following table shows the voltage and current ratings found in the electrical
characteristics section of the respective component datasheets.
Table 1. Voltage and current rating of IGBTs
Device
selection
Techno/
series
Trench gate
field stop
“V” series
Blocking
voltage
rating
(VCES max)
Main benefits
Key products
600 V
– very high switching
speed
– low thermal
resistance
– safe paralleling
operation
– very fast soft
40 A – 60 A
recovery antiparallel STGxnnV60DF
diode
– low saturation
voltage
– tail-less switching off
– positive VCE(sat)
temperature
coefficient
650 V
– high switching
speed
– low thermal
resistance
– safe paralleling
operation
– very fast soft
recovery antiparallel
40 A – 60 A
STGxnnH65DFB
diode
– very low saturation
voltage
– minimized tail
current
– slightly positive
VCE(sat) temperature
coefficient
Primary
switches
Q1 and Q2
Trench gate
field stop
“HB” series
Current
rating
([email protected]°C)
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Electrical specifications of welding machines
3
AN4638
Electrical specifications of welding machines
Manual metal arc (MMA) welding involves striking an arc between a covered metal
electrode and a workpiece, and is a highly popular arc welding process for small domestic
projects or light professional use.
The primary function of a welder power source is to provide sufficient output power or
current to melt the joint at the end of the electrode so as to weld the metal. To achieve this,
an arc welding machine power supply is required to control the load current/voltage over a
wide range, with a sufficiently high current (50 A – 500 A) but a relatively low voltage (10 V –
50 V and 80 V for ignition). The load range of the arc welding process is such that the
power supply is expected to operate from open-circuit (no-load condition) to short-circuit
(when the electrode is in contact with the workpiece for a short span of time). Also,
transitions occur during the striking of the arc, rapid arc length changes and metal transfer
across the arc, and the power supply must respond to these changes rapidly.
The following technical data relates to the two single-phase manual metal arc welding
machine platforms, 4 kW (MMA160) and 6 kW (MMA200), tested during the analysis
performed with the 600 V/650 V trench gate field-stop technology IGBTs, V series and HB
series respectively.
Table 2. Technical data for welding machines
MMA160
Parameter
Mains voltage
Frequency
Open circuit voltage
MMA200
Value
1 phase 230 V ± 10%
50/60 Hz
95 V
Parameter
Mains voltage
Frequency
Rated input current
Value
1 phase 230 V ± 15%
50/60 Hz
38.2 A
Electrode nominal
power at 100%
3.5 kW
Output current
adjustment
15 A – 190 A
TIG DC nominal power
at 100%
2.3 kW
Output voltage
20.6 V – 27.6 V
Electrode arc voltage
20.2 V – 26.4 V
No-load voltage
94 V
TIG arc voltage
10.2 V – 16.4 V
Duty cycle at 40 °c
20%
Max current
27 A
Efficiency
85%
Power factor
0.85
Power factor
0.73
Details regarding welding processes and arc control on the welding machines are
extensively explained in [5,6].
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4
Description of test conditions and analyses
Description of test conditions and analyses
The switching performance and temperature characteristics of the IGBT devices were
evaluated at full load steady state operation in “open board” conditions at room temperature
(25 ºC), as well as various operating conditions for the board by stepping the input/output
power level up to the maximum value, with a 220 VAC input main voltage and 50 Hz
frequency.
In both the analyses, the full load condition was reproduced by connecting the output of the
board with a resistive load composed of a series of power resistances arranged in parallel
and with a ceramic base in order to reach the total value of about 145 mΩ. During the tests,
a fan was used to maintain a constant temperature and therefore help keep total resistance
value constant. This external fan was mounted away from the welding machine to avoid any
influence on the thermal performance of the devices. Moreover, in all the tests performed,
the IGBTs were screwed to the same heatsink as per the original mounting arrangement.
In open board conditions (removing the external metal cover of the welding machine) at
ambient temperature (~25 °C), the measurements were performed with the IGBTs
connected with wires in order to allow the insertion of the probes for the scope to record the
signals. Under these conditions, particular attention was focused on the electrical behavior
of parameters like the VCE voltage across collector and emitter pins, the collector (or
emitter) current IC and the VGE voltage across gate and emitter pins during normal
operation.
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V series IGBTs on a 4 kW welder
5
AN4638
V series IGBTs on a 4 kW welder
Analysis of the V Series IGBTs was developed under the following test conditions:
•
switching frequency fixed to ~63 kHz
•
gate driving resistances R1, R2, R3, R4 = 4.7 Ω and R5, R6 = 2.2 Ω
•
power ranging from 2 kW up to maximum input power
•
output load resistance ~145 mΩ
Figure 2. 4 kW welder power converter section
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Four IGBTs were connected two by two paralleled in a “double switch forward DC-DC
converter” configuration of the 4 kW single phase welding machine board tested in “free air”
(open board without housing) conditions with ambient temperature TAMB = 25 °C (+/- 2 °C).
The signal waveforms and temperatures were acquired and measured for the two paralleled
low-side devices, Q1 and Q2.
Below are the results achieved from the analysis performed with the STGW40V60DF IGBTs
tested on a 4 kW welder. The table below lists the main parameters for these components;
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V series IGBTs on a 4 kW welder
for more details regarding these IGBTs, please refer to the relevant datasheet available at
www.st.com.
Table 3. STGW40V60DF main parameters
5.1
Device
Package
BVCES
[email protected] °C
Tj-max
VCESAT(typ)
@25 °C
STGW40V60DF
TO-247
600 V
40 A
175 °C
1.8 V
Overview of thermal and electrical measurements
The electrical measurements for the input power, input current and power factor (PF) taken
by the power meter at the input of the welder as well as the temperature and time values
achieved by the STGW40V60DF IGBTs during the electro-thermal analysis carried out on
the board are detailed in the following table and graph; the operating conditions for the
application (220 VAC, 50 Hz input) are:
•
full load steady state operation under 2 kW input power conditions;
•
full load steady state operation under 3 kW input power conditions;
•
full load steady state operation under maximum power absorbed in input conditions.
The case temperatures measurements were taken by a thermo camera once thermal
stability had been achieved by the devices running on the board. Operating times were
measured for maintained maximum input power (~4 kW), starting from a cold state
condition, until the thermal protection interrupted operation.
Table 4. Thermal and electrical measurements
Device
STGW40V60DF
Input power
(kW)
Input current
(A)
PF
Temp (°C)
2
15.4
0.58
62°
3
22.2
0.61
83°
~3.8(max)
26.3
0.66
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Time (min: s)
10 min17 s
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V series IGBTs on a 4 kW welder
AN4638
Figure 3. STGW40V60DF temperature and operating time at 220 VAC
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The measurement of the total turn-off energies (taken as the mathematical sum of the
energies related to each device of the low-side pair of tested IGBTs) are detailed in the table
and graph below, starting from 2 kW up to the maximum power level of 220 VAC input main
voltage at 50 Hz.
Table 5. STGW40V60DF total turn-off energy measurements
10/23
Device (Q1 + Q2)
EOFF @ 2 kW (µJ)
EOFF @ 3 kW (µJ)
EOFF @ max power (µJ)
STGW40V60DF
311
466
550
DocID027309 Rev 1
AN4638
V series IGBTs on a 4 kW welder
Figure 4. STGW40V60DF total turn-off energy at 220 VAC
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The STGW40V60DF IGBTs demonstrate highly satisfactory thermal behavior, achieving
83 °C approx. for 3 kW input power, while operation at maximum input power lasted
10 minutes and 17 seconds from a cold initial state, before the thermal sensor triggered
shut-down due to a breach of the maximum threshold setting (approx. 105 °C).
The overall electrical and power dissipation performance was very good for the two IGBTs
tested: capable of switching with low amounts of turn-off energies in all the different working
conditions for the application, including the worst case maximum value of ~550 µJ for turnoff switching during maximum input power operation.
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HB series IGBTs on a 6 kW welder
6
AN4638
HB series IGBTs on a 6 kW welder
Analysis of the HB Series IGBTs was developed under the following test conditions:
•
switching frequency fixed to ~58 kHz
•
gate driving resistances R1, R2, R3, R4 = 4.7 Ω
•
RC Snubber network with Rsn = 22 Ω and Csn = 1 nF
•
power ranging from 2 kW up to maximum input power
•
output load resistance ~145 mΩ
Figure 5. 6 kW welder power converter section
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Four IGBTs were connected two by two paralleled in a “double switch forward DC-DC
converter” configuration of the 6 kW single phase welder board tested without the external
metal shroud but with the internal plastic cover, at ambient temperature TAMB = 25 °C (+/2 °C). The signal waveforms and temperatures were acquired and measured for the two
paralleled high-side devices, Q1 and Q2.
Below are the results achieved from the analysis performed with the STGW60H65DFB
IGBTs tested on a 6 kW welder. The table below lists the main parameters for these
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HB series IGBTs on a 6 kW welder
components; for more details regarding these IGBTs, please refer to the relevant datasheet
available at www.st.com.
Table 6. STGW60H65DFB main parameters
6.1
Device
Package
BVCES
[email protected] °C
Tj-max
VCESAT(typ)
@25 °C
STGW60H65DFB
TO-247
650 V
60 A
175 °C
1.6 V
Overview of thermal and electrical measurements
The electrical measurements for the input power, output current and output power as well as
the temperature and time values achieved by the STGW60H65DFB IGBTs during the
electro-thermal analysis carried out on the board are detailed in the following table and
graph; the operating conditions for the application (220 VAC, 50 Hz input) are:
•
full load steady state operation at 90 A output current;
•
full load steady state operation at 130 A output current;
•
full load steady state operation at 165 A output current;
•
full load steady state operation at 200 A output current;
The case temperatures measurements were taken with thermocouples on the cases once
thermal stability was achieved by the devices running on the board. Operating times were
measured for maintained maximum output current (~200 A) until the thermal protection
interrupted operation.
Table 7. Thermal and electrical measurements
Device
Output current
(A)
Input power
(W)
Output power
(W)
Temp (°C)
90
~2683
~2233
59
130
~3864
~3172
71
165
~4949
~3997
87
200
~5883
~4624
Time (min:s)
STGW60H65DFB
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HB series IGBTs on a 6 kW welder
AN4638
Figure 6. STGW60H65DFB temperature and operating time at 220 VAC
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The measurement of the total turn-off energies (taken as the mathematical sum of the
energies related to each device of the high-side pair of tested IGBTs) are detailed in the
table and graph below, starting from 90 A up to the maximum output current of 200 A for a
220 VAC input main voltage at 50 Hz.
Table 8. STGW60H65DFB total turn-off energy measurements
14/23
Device (Q1 + Q2)
EOFF @ 90 A
(µJ)
EOFF @ 130 A
(µJ)
EOFF @ 165 A
(µJ)
EOFF @ 200 A
(µJ)
STGW60H65DFB
~586
~787
~887
~947
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HB series IGBTs on a 6 kW welder
Figure 7. STGW60H65DFB total turn-off energy at 220 VAC
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The STGW60H65DFB IGBTs demonstrate highly satisfactory thermal behavior, achieving
87 °C approx. for 165 A output current, while operation at maximum output current lasted
8 minutes and 15 seconds from a cold initial state, before the thermal sensor triggered shutdown due to a breach of the maximum threshold setting (approx. 105 °C).
The overall electrical and power dissipation performance is very good for the two IGBTs
tested: capable of switching with low amounts of turn-off energies in all the different working
conditions for the application, including the worst case maximum value of ~947 µJ for turnoff switching during maximum output current.
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Electrical analysis and waveforms
7
AN4638
Electrical analysis and waveforms
The acquired waveforms below illustrate the switching performance of the tested IGBTs.
Use the following color codes to interpret the information:
•
Q1 collector (or emitter) current: IC1 signal in yellow
•
Q2 collector (or emitter) current: IC2 signal in dark blue
•
Q1 and Q2 collector-emitter voltage: VCE signal in red
•
Q1 and Q2 gate-emitter voltage: VGE signal in green;
•
Q1 instantaneous power (VCE x IC1): power signal in orange
•
Q2 instantaneous power (VCE x IC2): power signal in pink
•
total collector current (IC1 + IC2): IC1 + IC2 signal in orange
•
total instantaneous power (VCE x (IC1+IC2)): power signal in light blue.
The figures below depict the signals for the 4 kW welder for both paralleled low-side devices
Q1 and Q2 of the STGW40V60DF IGBTs during the steady state operation under maximum
power input absorption conditions.
Figure 8. STGW40V60DF full load steady state (220 VAC & max power input)
The following figure depicts the sum total current signal for the Q1 and Q2 collector currents
during turn-off switching transients under maximum input power absorption conditions.
Figure 9. STGW40V60DF turn-off switching details (220 VAC & max power input)
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Electrical analysis and waveforms
The figures below depict the signals for the 6 kW welder for both paralleled high-side
devices Q1 and Q2 of the STGW60H65DFB IGBTs, during the steady state operation and
switch-off phases under maximum power input absorption conditions (200 A output current).
Figure 10. STGW60H65DFB full load steady state (220 VAC & max power input)
Figure 11. STGW60H65DFB Q1 turn-off switching details (220 VAC & max input power)
Figure 12. STGW60H65DFB Q2 turn-off switching details (220 VAC & max input power)
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Conclusion
8
AN4638
Conclusion
Test results for the field-stop high-speed technology V and HB series IGBTs, analyzed on
two-switch forward topology converters for welders were presented. Very good performance
in terms of power dissipation and switching characteristics were achieved in normal steadystate working conditions on 4 kW and 6 kW evaluation welding platforms, thanks to the ideal
trade-off between VCESAT and EOFF for these IGBTs.
The STGW40V60DF IGBTs showed particularly high thermal performance with around
83 °C measured under a 3 kW input power condition, while operation time was 10 min:17 s
at maximum operating power (from an initial cold state of the welder) prior to thermal sensor
triggering shut-down due to a breach of the temperature threshold limit. The
STGW60H65DFB IGBTs also achieved acceptable temperatures of around 87 °C at 165 A
output current, while the continuous operation time was 8 min:15 s at maximum output
current.
The acquired waveforms clearly demonstrate considerable overall electrical performance
for the V and HB series IGBTs, with low turn-off energies across the different working
conditions for the welding machines, not exceeding ~550 µJ and ~947 µJ respectively
during maximum operating input power and output current.
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9
Bibliography
Bibliography
1.
STMicroelectronics, L. Wuidart, “Topologies for switched mode power supplies”, 1999.
2.
R. W. Erickson, D. Maksimovic, Fundamentals of Power Electronics, Kluwer Academic
Publishers, 2nd Edition, 2003.
3.
STMicroelectronics, AN3200 “2.5 kW MMA welding machine”, September 2010.
4.
Ned Mohan, Tore Undeland, Willliams P. Robbins. “Power electronics, converters,
applications and design”. 2nd edition. John Wiley & Sons, Inc. New York 1995 (USA).
5.
J. Schupp, W. Fischer, and H. Mecke, “Welding Arc Control with Power Electronics,”
IEE Power Electronics and Variable Speed Drives Conference, pp. 443-450, 2000.
6.
Tomsic M.J, N. Crump and others “The welding handbook: Welding processes”.
Volume 2. 8th edition. American Welding Society. Miami (USA). pp 2-29, 73-80.
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Topology equations and device selection
Appendix A
AN4638
Topology equations and device selection
During the IGBT switch-off transients, an overvoltage appears across switches Q1 and Q2
due to transformer leakage inductance, which requires a blocking voltage capacity of more
than VIN,max as shown below:
Equation 1
V CES > V IN ,max
where
V IN ,max = maximum DC bus voltage
The current ratings of the primary switches are given by the following expressions in terms
of IC,max and IC,ave,on, where the collector current assumes a trapezoidal shape typical of
the CCM working operation mode:
Equation 2
∆I C
IC ,max ≥ I C ,ave ,on + -------2
where
IC ,max + I C ,min
I C ,ave ,on = ------------------------------------- = average current during transistor on time
2
∆I C = IC ,max – I C ,min = maximum peak to peak ripple on the IGBT collector current
Equation 3
P OUT
I C ,ave ,on ≥ ----------------------------------------------η ⋅ V IN ,min ⋅ D MAX
where
I C ,ave ,on = average current during transistor on time
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POUT = η ⋅ P IN = output power;
η = efficiency;
T ON
D = ----------- = duty cycle;
T
DMAX = maximum duty cycle
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P IN = input power
AN4638
Topology equations and device selection
Figure 13. Two switch forward topology
Figure 14. IGBT Collector current in CCM
Setting the following electrical conditions at maximum input power operation for the 6 kW
welder:
P OUT
PIN = -------------- = 6000 W; V IN ,max = 220 ⋅ 2;
η
T ONmax
D MAX = --------------------- = 31%; ∆I C ,max = 25% ⋅ I C ,max
T
Therefore, from the previous formulas, the maximum collector current is:
P OUT
6000
I C ,ave ,on = ----------------------------------------------- = -------------------------------------- = 62.2 A
η ⋅ V IN ,min ⋅ D MAX 220 ⋅ 2 ⋅ 0.31
∆I C
0.25 ⋅ I C ,max
IC ,max ≥ I C ,ave ,on + -------- = I C ,ave ,on + -------------------------------2
2
2 ⋅ I C ,ave ,on
I C ,max ≥ ----------------------------- ∼ 72 A
1.75
The last result is valid for a single IGBT device mounted in a two-switch forward
configuration, while for two devices connected in parallel, the value for each IGBT is:
72
I C ,max ≥ ------ = 36 A
2
which is the maximum operating collector current for a single IGBT device connected in a
paralleled configuration for a 6 kW welder.
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Revision history
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Revision history
Table 9. Document revision history
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Date
Revision
17-Feb-2015
1
Changes
Initial release.
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