Comparator u3 compares voltage drop across the

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Unformatted text preview: igh Temperature cut-off, a bridge circuit is used with the CBI module’s thermistor. Comparator U3 compares voltage drop across the thermistor to the stable voltage from the Zener diode. P1 can be used to preset the cut-off point at which the comparator’s output goes low. This is fed into the DSP as OVERTEMP signal. Short circuit protection is provided by continuously monitoring the voltage drop across a SHUNT. Note that one end of SHUNT is connected to the power supply ground GND1. The voltage picked up from this SHUNT is amplified by a low noise Op Amp and is then compared to the stable voltage from the same Zener. When short circuit occurs, the comparator output (FAULT) goes low. 1% metal film resistors are used throughout in both these circuits to ensure precision and stability. C3 and C4 help in offering low pass filtering to avoid nuisance tripping. Principle of DESAT sensing for detecting overload on an IGBT has been explained before in section 2.5 above. In the case of AC Motor Drive, each IGBT has to be protected from overload using separate DESAT sensing. Fig. (12) and Fig. (13) show the connection for each IGBT. DESAT sensing is done on the isolated side of each opto-coupler, while the resultant FAULT signal is generated on the common input side with respect to GND1. Each FAULT signal is open collector type and hence can be tied IXAN0009 together with other FAULT signals from other opto-coupler or from other comparators. The DSP will stop output drive signals when either FAULT or OVERTEMP signal goes low. When this happens, notice that IXD_414 offers a -ve bias of -5V to guarantee turn-off conditions, even in presence of electrical noise. -5V is applied to gate of each IGBT during turn-off even under normal operating conditions. After fault is cleared, the DSP can issue a RESET signal for resuming normal operation. 5.0 PRACTICAL CONSIDERATIONS When designing and building driver circuits for MOSFET/IGBT, several practical aspects have to be taken care of to avoid unpleasant voltage spikes, oscillation or ringing and false turn-on. More often than not, these are a result of improper or inade...
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This note was uploaded on 01/15/2014 for the course ECE 624 taught by Professor Staff during the Winter '08 term at Ohio State.

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