Protect IGBTs By Sensing Current Using Optical Isolation Amplifiers White Paper

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Isolation amplifiers, such as the ACPL-C79A, work in conjunction with shunt resistors to provide accurate current measurements in power converters even in the presence of high switching noise. When used with a resistive divider, iso-amps work as precision voltage sensors to monitor the DC bus voltage. The current and voltage information from the iso-amps are collected by the microcontroller, which uses the data to calculate the feedback values and output signals needed to provide effective control and fault management in the power converters.

Fault Protection Requirements

In an inverter, the IGBTs are the most costly components and it thus makes sense to provide as much protection as possible to protect them from damage. The Avago iso-amps provide quick sensing of fault conditions and the algorithms the microcontroller executes can prevent the fault conditions from causing the IGBTs to fail. Additionally, the optical isolation in the iso-amps prevents the fault conditions from overloading the microcontroller and causing it to fail.

However, the IGBT protection must be cost effective – the market continues to demand sufficient IGBT protection from fault conditions at a cost that does not significantly affect the total cost of the motor drive system. To meet this demand, IGBT gate drivers and current sensors with protection features have been introduced to carry out essential fault detection in addition to their driving and sensing functions. These products provide a cost-effective scheme to implement IGBT protection and eliminate the need for separate detection and feedback components. The remainder of this paper will focus on some of the fault protection functions that can be implemented with current/voltage sensors.

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