Electric motors are used in elevators, food processing equipment, factory automation, robots, cranes … the list goes on. AC induction motors are common in such applications, and invariably, the insulated gate bipolar transistors (IGBTs) used in the power stages to drive them. The typical bus voltage is 200 VDC to 1,000 VDC. The IGBTs are electronically commutated to achieve the sinusoidal currents that AC induction motors require.
Protecting humans who operate heavy machinery from electric shock is of primary importance when designing motor drives, followed by efficiency, size and cost. Although IGBTs can handle the high voltages and currents needed to drive motors, they do not provide safety isolation to protect against shock. The important task of providing safety isolation in the system is entrusted to the gate drivers that drive the IGBTs.
Opto isolated gate drivers have been used with great success to drive IGBTs and provide galvanic safety isolation. The input stage of an opto isolated gate driver contains a single aluminum gallium arsenide (AlGaAs) LED. The output stage consists of a photo detector and amplifier, followed by pullup and pulldown transistors to drive the output. A thick layer of transparent silicone in the final package separating the input and output stages provides the safety isolation. Simplicity of the current-driven input stage, good noise immunity and safety isolation are the primary reasons that motor-drive manufacturers have adopted opto isolated gate drivers in virtually all of their designs.