High-power DC power supplies: there's no need to risk the device failures sometimes encountered when using voltage-fed converters.(POWER SUPPLIES/SOURCES)

Switching power supplies in the tens of kilowatt power range have been slowly replacing traditional silicon-controlled rectifier (SCR)-based topologies over the past several decades. The advantages and disadvantages are well known.

High-frequency operation of switching power supplies enables magnetic components to be reduced in size and weight and allows faster response times to line and load perturbations. On the downside, the demands placed on switching devices tend to make high-power switching power supplies less reliable than their SCR-based counterpart.

Numerous power-circuit topologies currently are being deployed for high-power switch-mode applications. The most common configurations consist of three power conversion stages:

* An AC-to-DC converter that converts the three-phase incoming mains to a DC voltage.

* A DC-to-AC inverter that converts the voltage on the DC bus to a high-frequency AC voltage.

* A secondary AC-to-DC converter that converts the high-frequency AC voltage to DC voltage.

[FIGURE 1 OMITTED]

The two AC-to-DC converters are very similar in function except for the operating frequencies. The converters consist primarily of rectifiers, low-pass filters, and snubbers. The snubbers limit switching transient voltages and absorb energy stored from parasitic components.

The second stage, the DC-to-AC converter, generates a high-frequency voltage that generally drives a transformer at 20 kHz or above. The transformer is required for ohmic isolation and production of an output voltage as determined by the transformer turns ratio. The DC-to-AC converter is the most complex stage, and there are numerous power-processing topologies presently in production.

Most high-power DC-to-AC converters use an H-bridge configuration (four power devices) for exciting the high-frequency transformer. The H-bridge is controlled with pulse-width modulation (PWM) or other modulation strategies to produce a voltage of limited pulse width or amplitude. …

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