AN4638 Datasheet, PDF (1/23 Pages) STMicroelectronics – The two-switch forward converter

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-to-
medium 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 switch-
off energies and the V_CE(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 field-
stop 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 two-
switch 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
DocID027309 Rev 1
1/23
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