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To achieve optimum performance in switching power supplies, switching losses in the power stage must be minimized. Switching losses in bipolar-based designs depend to a large extent on the quality of the drive circuit's design and implementation. A good bipolar drive circuit should provide the following:

The drive circuit illustrated in the diagram provides adjustable forward and reverse current with a single power supply. It is simple, efficient and can be modified to drive a variety of power semiconductors. Operation is straightforward. The FET drivers, transistors Ql and Q2, are operated in parallel to provide greater circuit reliability, lower junction temperature, and lower conduction losses in the driver due to on-resistance. They are gated on and off by a pulse-width modulated (PWM) signal from the collectors of the NPNbipolar output transistors in the PWM controller chip, IC1.

With transistors Ql and Q2 turned on, forward current flows through capacitor Cl before turning on Q4, the power Darlington being driven. Capacitor Cl serves two purposes. During turn-on, it provides overshoot for the forward current; during turn-off, energy stored in Cl provides additional reverse current to increase the speed with which the power Darlington being driven turns off.

As the gates of transistors Ql and Q2 go high, turning them off, Q3 is turning on. Reverse current then starts to flow through R3 and Q3 from the base of the power Darlington being driven. Using load-line analysis, forward and reverse drive can be optimized by adjusting turn-on and turn-off load-locus for minimum switching area.

The PWM controller chip is used as an open-loop PWM drive oscillator in this circuit. Although an SG3524 was used in the prototype circuit, other PWM, controllers as well as a variety of other circuits, can be used as the drive oscillator. Forward base-drive current can be adjusted by changing the values of capacitor Cl, resistor R3, and inductor L1. Changing LI however, changes the reverse current's di/dt.

Design Tips

  1. Capacitors used in base drive circuitry should be able to carry high values of ac ripple current. Polypropylene, polysulfone and ac oil-filled types are good choices.
  2. Use fast-recovery diodes with recovery times of under 200 nanoseconds.
  3. All the FETs used in the driver may require zener diodes rated at between 15 and 18 volts connected between their gate and source to protect them against voltage spikes. However, care should be taken to ensure that the drive voltage never exceeds the zener diode voltage in order to prevent damage to them.
  4. All the FETs used in the drive may require 35-volt transient suppressers connected between drain and source.
  5. Special care should be taken to keep lead lengths in the reverse-current circuit path as short as possible to minimize series inductance and resistance in order to maximize reverse-current di/dt.
  6. Some minimum resistance is needed in the turn-off circuit path to keep circuit "Q" low enough to prevent the reverse current and the base-to-emitter voltage of the power Darlington being driven from the ringing excessively. The minimum resistance value required varies from one power Darlington to another, but for large devices the typical range is 0.2 to 2.0 ohms.



A. Blicher, "Field Effect" and Bipolar Power Transistor Physics", Academic Press, Inc., New York, 1981.

H.W. Ott, 'Noise Reduction Techniques in Electronic Systems", John Wiley and Sons, New York, 1976.

J.M. Peter, "The Power Transistor in Its Environment", Thomson-CSF, Canoga Park, CA, 1979.

A.I. Pressman, 'Switching and Linear Power Supply, Power Converter Design", Havden Book Company, Rochelle, NJ, 1977.

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