From what I understand, MOSFETs inherently have a diode between the source and drain known as a body diode. If I were to use an nMOSFET as a low side switch, would it be bad to have a diode between the load and the drain of the MOSFET? What effect would it have?

Body diode cancel

Schematic is a representation only and is not intended to produce accurate simulation results

Edit to clarify comment: The goal is to enable 2 low side switches (not at the same time) to switch between 2 rgb controllers. Originally I wanted to multiplex between the 2 controllers, but the couldn't find any with a large current rating. Without D1 and D2, the switching of controller 1 would affect current through controller 2 and vice versa. If there's a better way to switch between low side switches I am open to suggestions. (RGB Led should be LED Strip. Resistors are embedded)

Low Side Enable


2 Answers 2


It would decrease the max voltage across the load by a voltage equal to the forward voltage (Vf) of D1. One reason I can think of to do this is if whatever the load is cannot handle the full Vdd across it.

I'm not sure what you mean by "cancel the body diode". When the FET is fully enhanced, it will effectively short out the diode and current can move through the FET in both directions. When the FET is off, the body diode is just like a power rectifier; it will pass current in one direction. In your circuit it will be reverse-biased by Vdd, so there will be no electron flow with or without D1.

If you have a situation where it is possible for both positive and negative current to appear across the FET, and you want to block both directions when the FET is off, such as reverse-polarity protection of a power supply input, then D1 will do this in the circuit as shown. There are two better ways to do this.

  • 5
    \$\begingroup\$ Will you share the two better ways? \$\endgroup\$ Apr 29 at 22:12
  • \$\begingroup\$ I googled reverse polarity protection, it looks like a zener diode between from the gate to the source is one way to do it, but is typically used with pMOSFETs. @AnalogKid Is this one of the ways you were referring to? and what is the other way? \$\endgroup\$
    – superN8
    Apr 30 at 15:54
  • 1
    \$\begingroup\$ It also can be done with an n-channel MOSFET in the return leg. The zener is needed only if the gate-source voltage might exceed the datasheet max. This usually is +/-20 V, so you would not need a zener in a 12 V application. \$\endgroup\$
    – AnalogKid
    Apr 30 at 16:34

(EDIT: I thought you were asking about a diode in parallel with the body diode. My bad.)
The diode as you suggested would not "cancel the body diode" because it wouldn't be engaged to begin with: with MOSFET off and positive supply on its drain, body diode would be off.
With MOSFET on, the body diode would be shorted out and it would be off as well.
The only time the body diode is engaged or turned on is when there is a current flow in reverse direction through the MOSFET, meaning positive voltage on source electrode and negative voltage on drain for N-channel type MOSFET.
As "AnalogKid" has already pointed out, the only useful scenario for your diode is to block this reverse current, but in these situations often a diode drop/loss presents too much power loss or too much heat generation in the diode, and another MOSFET is added and turned in the opposite direction to prevent current in either direction when MOSFETs are off, and to have a significantly lower voltage drop and power loss when MOSFETs are on, as in the circuit below:


simulate this circuit – Schematic created using CircuitLab

(Thanks to the user "AnalogKid" who pointed out that MOSFETs in this type of arrangement have sources tied together, which allows using a common driver for both.)

(OLD ANSWER below, assuming a diode parallel to body diode:)

Yes, you can have it and it's frequently done.

Most of the time, the body diode is actually a positive addition which protects the MOSFET in some cases or helps its function in other case (for example, using it in synchronized rectifiers as a rectifier diode for a brief moment until MOSFET turns on and reduces the voltage drop).
However, it is usually slower than the ultrafast or Schottky diodes you can get, and it also has a higher forward voltage drop.
If your MOSFET is doing some very high speed switching, its body diode's typically long reverse recovery time can waste too much power at MOSFET turn-off as it would let a significant amount of current flow through it backwards, which not only wastes more energy and produces more heat because such current is multiplied with the larger Vds at that moment, but it also risks destroying the MOSFET.

When you add a faster diode with a lower voltage drop, the body diode is not even engaged.

There are MOSFETs whose body diode is intentionally made faster and having a lower voltage drop or a separate diode die is added/integrated with the MOSFET.

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    \$\begingroup\$ The body diode does not protect the device against inductive kickback in the case of a single device - it is in the wrong place. It can be useful when arranged as a half-bridge with another device. \$\endgroup\$ Apr 29 at 21:58
  • \$\begingroup\$ Exactly. You're right. \$\endgroup\$ Apr 29 at 22:09
  • \$\begingroup\$ Is there a similar circuit i could apply only on the low side? Ultimately my goal is to swich between 2 low side switches to control a comman anode rgb led, to control them separately I wont be able to affect the high side of the load \$\endgroup\$
    – superN8
    Apr 29 at 22:31
  • \$\begingroup\$ I need more detail in order to tell you. It depends on what exactly are you trying to accomplish. I don't understand your question. \$\endgroup\$ Apr 29 at 22:48
  • \$\begingroup\$ I edited the question to respond since I am not sure if I can add pictures in comments \$\endgroup\$
    – superN8
    Apr 30 at 0:10

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