Power IC Technologies:Sense and Protection Circuit.

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Sense and Protection Circuit

Power ICs have a significant advantage over discrete devices on the power switch protection because the protection circuit can be integrated on the same chip. Figure 7.38a shows the over-temperature sense circuit. In this circuit, the junction temperature dependence of diode forward voltage drop is utilized to sense the temperature. The series diodes are located close to the power switches so that the junction temperature of the diodes responds to the temperature change of the power switches. Comparing the

Power IC Technologies-0106

Power IC Technologies-0107

forward voltage drop across the diodes with the reference voltage, the circuit senses the over temperature at the power switches and the fault signal is fed back to the gate control logic.

Figure 7.38b shows the over-current sense circuit for bipolar power transistors. The magnitude of current through the main transistor is reflected to the current through the sense transistor with the current mirror circuit configuration. Therefore the voltage drop across the resistor in the collector of the sense transistor is proportion to the current through the main transistor. Comparing the voltage drop with the reference voltage, the circuit detects the over current. Figure 7.38c shows the over-current sense circuit for IGBTs or MOSFETs, which has a similar configuration with Figure 7.38b. In this circuit, the over current is detected by the voltage drop across the resistor in the sense IGBT (MOSFET) emitter (source).

In the short-circuit protection, the collector current should be squeezed or terminated as soon as the short-circuit operation is detected. For this purpose, the protection circuit directly draws down the gate voltage with the short feedback loop. Figure 7.39 shows a typical short-circuit protection circuit for IGBTs. When a high current flows through the IGBT, the voltage drop across the emitter resistor of the sense IGBT directly drives the npn transistor. The transistor draws down the gate voltage so that the collector current is reduced to the level of safe turn-off operation.

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