DocID027309 Rev 1

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AN4638

Two-switch forward converter general concepts

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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 continuous-

conduction 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

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.