# Flyback diode and MOSFET rating vs DC motor stale current

I'm trying to determine a suitable flyback diode for a DC motor controlled by a N-channel MOSFET. The specs the motor read:

Rated current: 2 A
200 mA no-load current draw @ 12 V
12 A stall current @ 12 V
No load speed of 5,500 RPM


The MOSTFET I have is IRF520.

Should the flyback diode have a "Maximum average forward rectified current" above 2A, 200mA or 12A?

(EDIT: The motor will only be spinning in one direction, so there is no H-bridge.)

• Will you be applying PWM? Commented Oct 31, 2020 at 23:12
• @BruceAbbott Would like to know the answer for both with and without PWM if it's different. Commented Oct 31, 2020 at 23:14

At turnoff the flyback diode will momentarily carry the same current as your motor is consuming. This current will then reduce quickly depending on the inductance and resistance of the motor.

You will find a rating in the diode data sheets for non repetitive surge current normally about a 10ms rating. The inductive spike at turnoff should not last anywhere near this long so that so as long as the surge rating exceeds your maximum motor current (12A) you will be good to go.

The maximum average current of the diode is unimportant unless you are repeatedly turning off the motor rapidly say for PWM. In this case use the average current the motor is taking.

The IRF520 is not rated at 12A which is the current you will get at startup or if you stall the motor. You should find a mosfet rated at least at 15A.

• Okay, so the 12A is covered by the peak current that the diode can handle. But, which current of the motor should be used in determining the max average current of the diode? The rated current (2A) or the no-load current (200mA)? Commented Oct 31, 2020 at 23:05
• Answer edited to address this. If you are using the mosfet just to turn the motor on and off infrequently you don't need to worry about this.
– RoyC
Commented Oct 31, 2020 at 23:16
• After your edit, if I understand correctly, diode peak current > "motor stall current". If using PWM, diode average current > motor rated current. If not using PWM, diode average current must be sufficiently larger than motor no-load current. I hope I got that right. Commented Oct 31, 2020 at 23:24
• Nearly, if you are not using PWM the only rating that matters is the diode peak current > motor stall current, if you are using PWM diode average current > motor average current. In the PWM case the actual diode current will be less than the motor average current but that would be the safe level for the diode average current.
– RoyC
Commented Oct 31, 2020 at 23:31

200 mA, 2A or 12 A?

When the motor open drain/collector turns off, the motor coasts to a stop and the current at the time of switching is all that needs to be sustained for a brief time constant T=L/Rdiode thus V^2/Rd=Pd is minimal and <=1A rated will do. This of course must include start/stopping really fast at NEAR 0 RPM, which is unlikely but then <12A RULES APPLY/

When the motor is turned off by a push-pull complementary driver or Half-Bridge, the switch acts as a short circuit brake and thus must sustain the locked rotor current at max RPM which is the same as start current and works in the opposite direction with de-acceleration for the time it takes to stop.

• so the heat rise is greater but minimal and SOA curves and heatsink thermal rise rules apply but 12A peak current must be less than the absolute maximum.

• In most cases the 1N4001 or equivalent is suitable here , reversed across the switch so the current loop is the same and ought to be minimum area for low EMI otherwise put directly across coil of the unidirectional motor.

Should the flyback diode have a "Maximum average forward rectified current" above 2A, 200mA or 12A?

If PWM is applied using a single FET (not 'half-bridge') the diode recirculates current through the motor during the 'OFF' portion of the PWM cycle. Therefore it has to handle a peak current equal to the motor current. But how much will that be, and what about the average current?

As PWM ratio is reduced the ratio of motor current to power supply current increases proportionally. If the motor draws 2 A from the power supply at 100% PWM then at 99% PWM the diode must pass ~2 A, but only 1% of the time. At 50% PWM an average power supply current of 2 A corresponds to a motor current of 4 A. Half the time this current comes from the power supply, and the other half of the time it goes through the diode. In your case this means the diode would need to be rated for at least 2A average current.

However if the torque load reduces significantly at lower rpm then the motor current will also reduce, and the diode can have a lower average current rating. When driving a propeller or fan (which absorbs power proportional to rpm cubed) a maximum average diode current of ~0.25 A would occur at ~85% PWM.

In most applications (where lower rpm = reduced load) an average diode current rating equal to the rated motor current is more than sufficient.

If the motor is used in a basic on/off application (no PWM) then the diode only has to handle one current spike each time the motor is turned off. In this case a peak rating of 2A should be sufficient, and a 200 mA diode would probably be OK.

The MOSTFET I have is IRF520.

The IRF520 is rated for 6.5 A absolute maximum average current at 100 °C, which sounds like plenty. However at 2 A the typical voltage drop is ~1V (with 10V Gate drive), so it will dissipate ~2 W and need a heat sink. A FET rated for higher current should have lower loss and could possibly be used without a heat sink. To minimize heating and maximize voltage at the motor I usually aim for a voltage drop of 0.1 V or less, which requires RDSon <=50 mΩ at 2 A.