Tri-State H-Bridge with Darlington-IGBT outputs.
Fig. 1 Tri-State H-Bridge with Darlington-IGBT outputs.

Driving Darlington Transistors with Optocouplers

by Lewis Loflin

In this series I've illustrated a number of circuit combinations. In the schematic above I used a TIP120 Darlington power transistor for the high-side (Vcc) switch.

This was done because finding a high current, low resistance on p-channel MOSFET was difficult. The 2 p-channel MOSFETs I tried, the IRF9540 and IRF6930, overheated and dropped a lot of voltage. They are designed for high-voltage, low current, high frequency switching. Both had a high rds(sat) on resistance.

MOSFETs suffer from low source-gate voltage (Vgs) often limited to 20 volts. That means I can't use a 24, 36, or 48 volt motor without a complete circuit rework.

Note: the motor voltage must be separate from the 12 volt logic voltage in this example.

The TIP120 is rated 5-amps at 60-volts. While having a gain of 1000+ saturation is achieved at 20-30mA base current. That is easily supplied by a 4N25 type optocoupler. Rb is 100 Ohms.

Saturation voltage as measured across collector-emitter was 2.1V at 3.6 amps.

See Tutorial Using TIP120 and TIP125 Power Darlington Transistors.

The optocoupler not only provides voltage isolation for higher +Vcc but can produce more drive current that CMOS logic I used here.

Optocoupler driver for MJE10005 power Darlington transistor.
Fig. 2 Optocoupler driver for MJE10005 power Darlington transistor.

The MJE10005 provides much greater current capability and operates at a much higher voltage. But with a drive base current of 400mA to 2 amps no optocoupler I know of can supply that.

So I used a TIP41A transistor the optocoupler could drive then used that to drive the MJE10005.

Photo Tri-State H-Bridge.
Fig. 3 Completed circuit with MJE10005 drivers used with IBGTs.

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