Design Alternatives for a 500 Ampere 20 Kilohertz Transistor Switch

Low frequency hybrid Darlington modules are commercially available up to 300 amperes and have been proposed at the 500 ampere level. Modules operating at higher frequencies would be appealing in applications such as uninterruptible power supplies (UPS), DC-DC converters, and switching regulators. Analysis of the power losses involved in operation at high current and ultrasonic frequencies shows that substantial switching and conduction losses must be dissipated by a 500 ampere switch. A hypothetical 500 ampere hybrid module operating at 20 kHz is projected and values are estimated for its switching losses and thermal characteristics. Analysis of this hypothetical single side cooled module shows that the heatsink required for operation in a practical industrial ambient is not realizable with available heatsink technology. A practical approach is presented to realize a 500 ampere, 450 volt, 20 kHz switch using a minimum number of paralleled transistors on separate heatsinks. Sample calculations of power losses and thermal performance are presented. Guidelines are presented for selection of transistors to ensure sharing of collector current. Current sharing results are presented for an actual switch, designed according to the guidelines.