Applications Engineering Staff


0-02 Fair Lawn Avenue - Fair Lawn, NJ 07410


Featuring Pulse Width Modulation: high efficiency, low cost, single power supply operation, easily adjustable IB1, and easily adjustable IB2.


In all power conversion applications using high-current power transistors, satisfactory circuit operation depends on good drive circuit design and implementation. With all switching power supplies, it is important to minimize power stage switching losses. Good drive circuit design not only reduces switching losses but improves circuit reliability.

An optimum bipolar drive circuit should provide:

  1. High di/dt forward base drive (IBl) so as to turn on the power semiconductor devices quickly.
  2. Adequate IB1 current to keep the power semiconductor devices in saturation so as to minimize saturation (VCEsat) losses. This is especially important when Darlington devices are used because of their saturation voltage.
  3. Controllable di/dt reverse current drive (IB2) to sweep out stored charge in the base, thus speeding up device turn-off.

The drive circuit described here 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.

Theory Of Operation

In Figure 1, FET driver transistors Ql and Q2 receive a pulse width modulated signal from the collectors of the NPN bipolar output transistors in PWM chip IC1. Transistors Ql and Q2 are operated in parallel. This provides for greater circuit reliability, lower junction temperature, and lower driver "on" resistance. With transistors Ql and Q2 "on", forward current Ibl flows through R2, CR1, Cl, R2, and CR2 before turning on Darlington Q4. Capacitor Cl serves two purposes. During turn-on, it provides overshoot for IBl and during turnoff, the energy stored in Cl is used for reverse current (IB2) for fast turnoff for Darlington Q4.

As the gates of transistors Ql and Q2 go high and they are turning off, transistor Q3 is turning on. Reverse current IB2 then starts to flow through R3, Q3, and Darlington Q4. The reverse current IB2, or IB(off), is used to quickly remove the stored charge carriers from the base regions of Darlington Q4 for fast and reliable turn-off.

Using load-line analysis, forward and reverse drive can be optimized by adjusting turn-on and turn-off load-locus for minimum switching area. Integrated circuit IC1, and SG3524, is used as an open loop PWM drive oscillator in this circuit. Other PWM integrated circuits, as well as a variety of other circuits, could be used as drive oscillators with this drive circuit.

Figure 1

MT-6010 Switching Characteristics

The MT-6010 is susceptible to ISB turn-on second-breakdown, as well as ESB turn-off second-breakdown. Forced gains above 100 are not recommended. Care must be taken not to exceed the maximum VBE during device turn-on and VBER during device turn-off. ISB failure is much more likely to occur when high di/dt collector current is present during turn-on. Di/dt of equal to, or less than, 10 amps/usec should necessitate fast IB1 forward current (>8 amps/usec) and forced gain (hfe) not to exceed 100 at 300 to 500 amperes. Low forced gain may make Mosfet drive circuitry an absolute necessity depending on the switching speed that is needed from the MT-6010 for the system requirements.

MT-6010 Switching Data


1. Base 1 to Base 2 resistance = 100 (R7)

1. 2 Base 2 to emitter resistance = 10 (R8)

Design Hints

  1. Capacitors used in base drive circuitry should have specified AC ripple current ratings such as polypropelene, polysulfone, AG oils, and other types.
  2. Fast recovery diodes (35 - 200 ns.) should be used.
  3. Non-inductive resistors should be used.
  4. Forward current IB1 can be adjusted by changing R1 and R2.
  5. Reverse current IB2 can be adjusted by changing Cl, R3, and L1.
  6. Changing L1 will vary the di/dt of IB2.
  7. All FETS may need 15 to 18-volt zeners from gate-to-source to protect against voltage spikes, depending on the application.
  8. All FETS may need 35-volt transient suppressors with series diodes from each Mosfet's drain-to-source, depending on the application.
  9. Special attention should be paid to short lead lengths in the IB2 (reverse current) circuit path, so as to maximize IB2 di/dt.
  10. Some minimum resistance is needed in the turn-off circuit path so as to keep circuit 'Q' to a low enough value. This keeps reverse VBE and IB2 'ringing' to an acceptable level. Resistance needed with the MT-6010 ranges from approximately 0.2 ohms to 2 ohms.

Electrical Parts List

Circuit Layout Guidelines

The following checklist should be reviewed. The items that are free of added cost should be used whenever applicable. The other techniques should be used if more noise reduction is necessary.

Noise Reduction Checklist

A. Suppressing Noise at Source:

B. Eliminating Noise Coupling:

Consider using the following devices for breaking ground loops:

C. Reducing Noise at Receiver:

*(Essentially free of added cost)


  1. Blicher, Adolph, Field Effect and Bipolar Power Transistor Physics, New York, Academic Press, Inc., 1981.
  2. Ott, Henry W., Noise Reduction Techniques in Electronic Systems, New York, John Wiley and Sons, 1976.
  3. Peter, Jean Marie, The Power Transistor in Its Environment, Canoga Park, California, Thomson - CSF, 1979.
  4. Pressman, Abraham I., Switching and Linear Power Supply, Power Converter Design, Rochelle, New Jersey, Hayden Book Company, 1977.

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