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High Voltage MOSFET Switching Circuits for H-Bridge Motor Controls

by Lewis Loflin

See the video High Voltage MOSFET Switch Tutorial

This page will discuss and review MOSFET power transistor switching circuits. The emphasis is higher voltage switching circuits. I'll be using the IRF630 and IRF9630 power MOSFETs. I'll also stress opto-coupler isolation of the power circuits from the microcontroller.

See the following spec sheets:
IRF630 N-Channel MOSFET
IRF9630 N-Channel MOSFET

N-channel opto-isolated MOSFET switching circuit using IRF630
Fig. 1 N-channel opto-isolated MOSFET switching circuit using IRF630

Fig. 1 uses the N-channel IRF630 with a 4N25 type opto-coupler. Note first the 5.2K resistor and 12-volt Zener diode. This provides 12-volts to turn on Q1 when the 4N25 transistor is switched on.

The Q1 gate-source voltage is limited to 20-volts and the 4N25 transistor collector breakdown voltage is limited to about 30-volts. This safely provides 12-volts to switch on Q1. Be aware the Q1 MOSFET is a voltage operated device and will store a charge due to gate-source capacitance.

The 15K gate bleeder resistor must be present for Q1 to turn off.

When +5V is applied to the opto-coupler input the internal LED switches on the output transistor. This switches +12-volts to Q1 gate turning on Q1, creating a current path for Ids.

When the opto-coupler is turned of the 15K gate-bleeder resistor turns off Q1.

P-channel opto-isolated MOSFET switching circuit using IRF9630
Fig. 2 P-channel opto-isolated MOSFET switching circuit using IRF9630

Fig. 2 shows how to use a P-channel IRF9630 MOSFET to switch the positive side of the power supply to the light bulbs acting as a load.

The input opto-coupler and Q1 operate the same as Fig. 1. While this show connections to a separate 12-volt supply, it can use the same resistor-Zener combination of Fig. 1.

I'm using a IRF630 MOSFET for Q1 because the high voltage in the Zener-resistor voltage divider circuit. When Q1 is switched of no current flows and we have no voltage drop across the Zener - Q2 is turned of.

When Q1 is switch on current flow Ik created a 12-volt difference across the Zener thus Q2 gate-source, turning Q2 on. The Zener diode limits Vgs on Q2 to under 20-volts.

As long as we have Ik Q2 will turn on. When Q1 is turned off Ik is turned off. The 15K gate bleeder resistor across the Zener discharges the Q2 gate turning off Q2.

New April 2018: