6.1 SCR Resonant Topologies Introduction

The silicon controlled rectifier (SCR) has been used in DC/AC inverters and DC/DC power supplies for over 25 years.1 ^,2 It is available with higher voltage and current ratings at lower cost than bipolar (single or Darlington types) or MOSFET transistors. Because of the higher voltage-current ratings, it is used primarily for supplies of over 1000 W.

It will be recalled that an SCR is a solid-state switch which is easily turned on by a narrow pulse at its input terminal (gate) and latches and stays on after the input is removed. The literature is replete with schemes for turning an SCR off ( commutating it). Essentially all these schemes involve reducing its on current to zero by diverting it to an alternate path for a certain minimum time t _q . SCR turnoff will be discussed below.

Earlier DC/AC or DC/DC SCR supplies could not operate reliably at switching frequencies over about 8 to 10 kHz. This was because even the fastest inverter-type SCRs available then did not have a reliable high-impedance open circuit at their output terminals until about 10 to 20 ?s after they had been turned off and their currents had dropped to zero. Thus they could not be subjected to high voltage until the 10 to 20 ?s after their currents had been reduced to zero.

Further, early SCRs could not tolerate a large dV/dt across their output terminals even beyond the 10 to 20 ?s...