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MC33154P Datasheet(PDF) 10 Page - Motorola, Inc |
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MC33154P Datasheet(HTML) 10 Page - Motorola, Inc |
10 / 12 page MC33154 10 MOTOROLA ANALOG IC DEVICE DATA properly terminated sense IGBT is very low, normally less than 100 mV. The sense IGBT approach requires a blanking time to prevent false tripping during turn–on. The sense IGBT also requires that the sense signal is ignored while the gate is low. This is because the mirror normally produces large transient voltages during both turn–on and turn–off due to the collector to mirror capacitance. A low resistance current shunt may also be used to sense the emitter current. A very low resistance shunt (5.0 m Ω to 50 m Ω) must be used with high current IGBTs. The output voltage of a current shunt is also very low. When the output is an actual short circuit the inductance will be very low. Since the blanking circuit provides a fixed minimum on–time the peak current under a short circuit may be very high. A short circuit discern function may be implemented using a second comparator with a higher trip voltage. This circuit can distinguish between an overcurrent and a shorted output condition. Under an actual short circuit the die temperature may get very hot. When a short circuit is detected the transistor should be turned–off for several milliseconds to cool down before the device is turned back on. The sense circuit is very similar to the Desaturation circuit. The MC33154 uses a combination circuit that provides protection for both Short Circuit capable IGBTs and Sense IGBTs. APPLICATION EXAMPLES The simplest gate drive circuit using the MC33154 is shown in Figure 30. The optoisolator requires a pull up resistor. This resistor value should be set to bias the output transistor at the desired current. A decoupling capacitor should be placed close to the IC to minimize switching noise. A bootstrap diode may be used to for a floating supply. If the protection features are not used, then both the desaturation input and the current sense input should be grounded. When used with a single supply the Kelvin Gnd and VEE pins should be connected. Separate resistors are recommended for turn–on and turn–off. Figure 30. Basic Application 7 4 3 2 1 5 8 6 Fault Input Desat/ Blank Output Sense Gnd VEE VCC MC33154 18 V When used with a dual supply as shown in Figure 31, the Gnd pin should be Kelvin connected to the emitter of the IGBT. If the protection features are not used, then both the desaturation input and the current sense input should be connected to Gnd. The input optoisolator, however, should be referenced to VEE. Figure 31. Dual Supply Application 7 4 3 2 1 5 8 6 Fault Input Desat/ Blank Output Sense Gnd VEE VCC MC33154 15 V –5.0 V If Desaturation protection is desired as shown in Figure 32, a high voltage diode is connec ted to the Desaturation/Blanking pin. The blanking capacitor should be connected from the Desaturation pin to the VEE pin. If a dual supply is used the blanking capacitor should be connected to the Kelvin Gnd. Because desaturation protection is used in this example, the sense input should be tied high. The MC33154 design ANDs the output of the overcurrent comparators with the output of the desaturation comparator, allowing the circuit designer to choose either type of protection. Although the reverse voltage on collector of the IGBT is clamped to the emitter by the free wheeling diode, there is normally considerable inductance within the package itself. A small resistor in series with the diode may be used to protect the IC from reverse voltage transients. Figure 32. Desaturation Application 7 4 3 2 1 5 8 6 Fault Input Desat/ Blank Output Sense Gnd VEE VCC MC33154 18 V When using sense IGBTs or a sense resistor, as shown in Figure 33, the sense voltage is applied to the current sense input. The sense trip voltages are referenced to the Kelvin Gnd pin. The sense voltage is very small, typically about 65 mV, and sensitive to noise. Therefore, the sense and ground return conductors should be routed as a differential pair. An RC filter is useful in filtering any high frequency noise. A blanking capacitor is connected |
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