MOSFET_DRIVE_CURRENT_BOOSTER

Published:2009/6/22 23:01:00 Author:Jessie | From:SeekIC

A clean and inexpensive way to eliminate the floating-gate supply is to use the voltage available at the MOSFET's drain to drive its gate. Tying the collector of Q1(a high-voltage, small-signal, 400-V NPN)to the MOSFET's drain supplies sufficient gate-drive voltage when it's needed most-when the MOSFET's drain-to-source voltage is high. Two such circuits used back-to-back form an ac relay.Using the emitter follower attached to the drain increases gate-drive current and decreases the MOSFET's turn-on time by a factor equal to the high-voltage NPN's beta. The resulting drain-to-source voltage fall times depend on the MOSFET's size and its required gate charge. The circuit that's used gives a fall time of 200 ps for an MTP10N2f5 10-A, 250-V MOSFET. With such fall times cutting switching losses, pulse-width modulation at frequencies under 100 Hz is possible.During tune-on, VDS falls rapidly until it reaches the sum of the 1N914 diode's 0.7-V drop, the collector-emitter saturation voltage of Q1, and the gate-to-source voltage required to support the load cur-rent. At that point, the diode array completes the MOSFET's turn-on, unaided by the buffer. This slows the fall of 7,s considerably when it reaches about 5 to 7 V. In high-voltage, low-frequency systems, tail-'ma of VDS is tolerable because the tail's voltage magnitude constitutes a small fraction of the switching voltage. The IN914 makes it possible for VGS to exceed VDS as the MOSFET completes turn-on.



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