Bi-Directional Load Switch?

[JeremyB]

I want to use a power MOSFET as a switch between a lithium ion rechargeable battery and its load. The switch will be controlled (ON/OFF) by an MCU.

My question is this: Are power MOSFETs bi-directional (allow current to flow in both directions for battery to delivery power and receive a charge) or should I be looking into something else?


Thanks,
Jeremy

 

[ZekeR]

Yes Jeremy, power MOSFETs conduct in both directions. When you apply a voltage to the gate, a conductive channel forms inside, which behaves just like a short piece of wire.

Here's the thing to be careful of: the MOSFET only turns off in one direction. For example, with a N-channel MOSFET, if you pull the gate LOW (so it's off) and apply a positive voltage to the DRAIN (w.r.t. the SOURCE), the MOSFET will not conduct. However, if you apply a positive voltage to the SOURCE w.r.t the DRAIN, the MOSFET will conduct! This is due to the "Body Diode" inside the MOSFET; you can confirm it using a multimeter set to "Diode" mode. If the MOSFET is on, the Multimeter will read 0 volts from DRAIN to SOURCE and vice versa. If the MOSFET is off, the multimeter will read "Open" when the red probe is on the DRAIN and the black probe is on the SOURCE, and if you flip the connections the multimeter will read ~0.6V.

So bottom line, the FET will conduct in both directions, but it will turn off in only one direction. So be careful. If you need it to turn on and off in both directions, then you need to place two MOSFETs back-to-back (there are nuances to be careful of, too).

 

[goldsmith]

Yes Jeremy, power MOSFETs conduct in both directions

Hi zeker
Unfortunately i'm disagree with you .
Consider that you have a power mosfet with anti parallel diode ! or a simple mosfet without it ! of course they didn't design fro large signals in both directions .
Regards
Goldsmith

- - - Updated - - -

Hi Jeremy
You can use an H bridge.
Best Wishes
Goldsmith

 

[mtwieg]

Hi zeker
Unfortunately i'm disagree with you .
Consider that you have a power mosfet with anti parallel diode ! or a simple mosfet without it ! of course they didn't design fro large signals in both directions .
Regards
Goldsmith

So long as a MOSFET is operating in its linear region, it can conduct equally in both direction with little difference. Think of synchronous rectification MOSFETs; the fact that the current is reversed doesn't make a different.

The issue with the MOSFET is that by itself it's only a two quadrant switch.

 

[goldsmith]

Hi dear mtwieg
Of course you are right , but , there are some problems that make the issue more complicated : 1- usually substrate connected to the source 2- we have a anti parallel diode with most of mosfets . isn't it ?
Best Regards
Goldsmith

 

[RCinFLA]

Unless the substrate diodes are brought out as a forth pin then reverse voltage will conduct the drain to source body diode. Yes the MOSFET can conduct in both direction but reverse Vds is clamped to 0.7v to 1.0v by forward conducting body diode.

For a bi-directional power switch put two MOSFET's in series, typically two P-ch's so you pull down on gates to turn both devices on. This is the way it is commonly done in cellphones where USB-OTG can source current or run phone/charge battery. You could use N-ch's but you would have to push the gate voltage above the switched supplies which is usually not an option.

 

[JeremyB]

Unless the substrate diodes are brought out as a forth pin then reverse voltage will conduct the drain to source body diode. Yes the MOSFET can conduct in both direction but reverse Vds is clamped to 0.7v to 1.0v by forward conducting body diode.

For a bi-directional power switch put two MOSFET's in series, typically two P-ch's so you pull down on gates to turn both devices on. This is the way it is commonly done in cellphones where USB-OTG can source current or run phone/charge battery. You could use N-ch's but you would have to push the gate voltage above the switched supplies which is usually not an option.

Is there anything that i need to be careful of when putting two power MOSFETs in series? If I use two P-channel MOSFETs, is it as simple as connecting their sources together, their gates together, and using their drains for input/output? I looked for integrated circuits that incorporate this design but have not been lucky in my search.

 

[RCinFLA]

Is there anything that i need to be careful of when putting two power MOSFETs in series? If I use two P-channel MOSFETs, is it as simple as connecting their sources together, their gates together, and using their drains for input/output? I looked for integrated circuits that incorporate this design but have not been lucky in my search.

You got it, source to source and gate to gate. Put a resistor from source to gate just to ensure turn off.

 

[JeremyB]

You got it, source to source and gate to gate. Put a resistor from source to gate just to ensure turn off.

That's great, thank you. I'm slowly getting comfortable with this idea. Here is an integrated load switch I found as a potential solution: https://www.fairchildsemi.com/ds/FD/FDC6331L.pdf
Would I run into any issues if I placed two of these back to back?

The app note (AN-1030) states:
"It is important that the output voltage be lower than the input voltage. An output voltage higher than the input voltage will be clamped via the intrinsic diode of the P-Channel MOSFET, which could cause significant current flow. This is not a problem with single energy sources, but it can become a problem with multiple energy sources, such as a charger."

This is obviously referring to using only one device. Will placing two of these devices in series solve this problem?

 

[RCinFLA]

That's great, thank you. I'm slowly getting comfortable with this idea. Here is an integrated load switch I found as a potential solution: https://www.fairchildsemi.com/ds/FD/FDC6331L.pdf
Would I run into any issues if I placed two of these back to back?

The app note (AN-1030) states:
"It is important that the output voltage be lower than the input voltage. An output voltage higher than the input voltage will be clamped via the intrinsic diode of the P-Channel MOSFET, which could cause significant current flow. This is not a problem with single energy sources, but it can become a problem with multiple energy sources, such as a charger."

This is obviously referring to using only one device. Will placing two of these devices in series solve this problem?

Yes, that comment is for one-way load switch. If you use two, tying two inputs in common, two controls in common, and use the two outputs for bi-directional operation it should be fine. Device 8 volt Vds breakdown voltage spec must be observed for maximum peak to peak input output voltage.

 

[JeremyB]

Yes, that comment is for one-way load switch. If you use two, tying two inputs in common, two controls in common, and use the two outputs for bi-directional operation it should be fine. Device 8 volt Vds breakdown voltage spec must be observed for maximum peak to peak input output voltage.

Thank you so much RCinFLA, you were extremely helpful. I think the next thing to do is buy some parts and construct the circuit. I won't know anything for sure until I try it. Thanks again!

-Jeremy