Transient Processes in a MOSFET

For a MOSFET, three transient processes can be identified. The first is the turn-on transient process associated with the MOSFET and the second is its turn-off process and the third is the turn-off transient process associated with the body-diode of the MOSFET. The turn-on transient of the body diode is relatively fast and can be ignored.

For a fast MOSFET, the turn-on transient process is characterized by two time periods, one is the turn-on time delay and the second is the rise time. During the turn-on time delay, the gate-to-source voltage builds up to its threshold value and during the rise time, the device current rises to about 90% of its final value. There is a further delay before the drain-to-source voltage becomes equal to its conduction drop. The delay time can be reduced to some extent by a stiff gate drive signal. When the MOSFET is turned off, there is a delay period corresponding to the period in which the gate voltage reduces to its threshold level. Then the cross-over period that includes a delay period and a fall period. During the delay period, the drain-to-source voltage rises from its conduction value to its blocking value. After this delay period has elapsed, the fall period follows during which the current through the MOSFET decreases. For a fast MOSFET, the total turn-on transient lasts for about 50 ns, whereas the turn-off process lasts for about 100 ns.

The body diode is comparatively slow. It turns on quite fast, but its turn-off process is quite slow, of the order of 500 ns. Hence when the MOSFET is to be operated at high frequency, it is preferable to use an external diode. In such a case, an additional diode may be required, that has to be connected in series with the MOSFET.

A circuit that can be used for simulating the transient processes in a MOSFET is shown in Fig. 3. Here Q1 is turned on and off, whereas Q2 remains off. When Q1 is turned off, the load current is diverted through diode D2, the body diode of Q2 . When Q1 is turned on, both the turn-on transient of Q1 and the reverse recovery transient of D2 occur. The simulation that is displayed as applet 3 is again quite approximate. The purpose is to illustrate how MOSFET functions as a switch. An air-cored inductor, labeled L3 in Fig.3, is necessary to be used to reduce the reverse recovery current of the diode. The time constant due L3 and R3 should be much less compared with the cycle period corresponding to switching frequency. The turn-on process of the MOSFET is quite slow, due to the slow turn-off of the body diode D2. In applications where the MOSFET has to be switched on and off at higher frequency of the order of 20 kHz and above, it is the practice to bypass the body diode by an external diode. In this application, such a technique is unnecessary because the body diode does not have to conduct at all.

click here to open the applet

APPLET FOR TRANSIENT PROCESSES IN A MOSFET

The losses in a FET are due to three factors:

1. Conduction losses due to on-state voltage,
2. Turn-on losses and
3. Turn-off losses.

The fourth applet simulates the step-down SMPS with PWM control in the open loop. The aim of this simulation is to estimate the losses in the switch and the diode and to show how the modifications to the power circuit lead to reliable operation, whether the conduction is continuous or discontinuous.

 
TO THE TOP