Protect IGBTs by Sensing Current Using Optical Isolation Amplifiers

  
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Insulated-gate bipolar transistors (IGBTs) require full protection to avoid damage and failures resulting from conditions such as short circuits, overloads and overvoltages. The protection is key to ensuring safe and stable power-converter operations in applications such as motor drives and solar and wind power generation systems. To detect over-current and overload conditions, isolation amplifiers
Protect IGBTs by Sensing Current Using Optical Isolation Amplifiers - schematic

featuring fast response or fast fault feedback can be used on the output phases and the DC bus. Figure 1a shows a typical block diagram of a power converter in an AC motor drive. It consists of an inverter that converts the DC bus voltage to AC power at a variable frequency to drive the motor. IGBTs are expensive power switches that form the heart of the inverter. These power devices must operate at a high frequency and must be able to withstand high voltages. Isolation amplifiers (iso-amps), such as the ACPL-C79A 1 shown in Figure 1b, 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. 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...



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