Three basic rules and the heat generator

Three basic rules

Thyristor controls, more commonly called SCR controls, are widely used for control of electric heat and electric motors. For the most part they are replacing conventional electromechanical controls such as contactors, autotransformers and starters because of their vastly improved performance and reliability. However, there are three basic rules which must not be overlooked in the application of SCR controls:

1. Positive, nondestructive overvoltage protection must be provided.
2. Load fault overcurrent protection must be provided.
3. Heat transfer of the nominal 2 watts per amp per phase (generated by all silicon power semiconductors) must be provided.

The following addresses the last requirement in some detail.

Heat generator

It is not widely understood that all silicon semiconductors have a forward voltage drop in the conduction direction which, for power silicon, typically ranges from 1 volt to more than 2 volts. This voltage drop multiplied by the RMS current flowing through the device results in watts, sometimes kilowatts, of heat at the semiconductor mounting point.

If this heat is concentrated at the semiconductor and not allowed to dissipate, the temperature of the device will very rapidly rise to levels far greater than an SCR’s 125 °C maximum rating* (for a diode, this temperature is typically 200 °C), causing instant destruction.

The proper international nomenclature for this forward conduction drop is:

Diodes

V_FM = maximum peak forward voltage drop at rated current.

SCRs

V_TM = maximum on-state voltage drop at rated current.

Referring to the typical AC switch or DC bridge arrangements of various solid state power circuits (see Payne Engineering Application Note 8-18), it is quite evident that the equipment designer must provide a heat transfer mechanism to dissipate literally thousands of watts in any given control assembly. A 600 HP three phase motor starter, for example, would generate over 3600 watts of heat to be dissipated, while a much simpler 3 kW heat seal control would need to rid itself of only 30 watts or so.

* Typically, 125 °C is a true maximum SCR temperature with allowed current at 0 amperes.