reverse biased mosfet 2

[stephenjvb]

I am revising a circuit board and have a problem with part of the design. There are clear errors (beyond my question here) but I don't understand the circuit so I'm not sure how to fix it. The data sheet for the VNH3SP30-E, figure 33 (... short circuit protection) shows a N MOSFET with the circuit ground (-V - this is not a British car) connected to D and the IC ground connected to S. The gate is pulled up (connected to +Vcc) when the board is powered.

If the mosfet were installed normally and always powered I suppose it would act as a really low value resistor with a large heat sink, but that's not the circuit.

This circuit would seem to use the body diode to conduct power when the power is connected correctly. However I can find no specs on how much power the body diode will handle or the forward voltage drop for any n-mosfet. This circuit is designed for 30A current through the Mosfet. I'm surprised the body diode can handle 30A but what else could it be?

If the body diode conducts the power why even bother driving the N-MOSFET gate?

The Mosfet could be drawn incorrectly, but reversed there is no reverse voltage protection. Data sheet section 3.1 offers figure 33 as an example of reverse voltage protection.

Why power the Mosfet and why not just use use a really big diode?

I'm tempted to eliminate this altogether but I'm worried that I have my head in a bucket and this is a sublimely elegant circuit beyond my limited knowledge.

Any suggestions?

Thanks.

(The ST link to the datasheet doesn't work - I got mine from Mouser)

 

[VEBill]

See data sheet section 3.1 'Reverse battery protection'. The "N-channel MOSFET connected to the GND pin" as shown in Figure 33 is option 2.

Perhaps you'd prefer option 1.



Reference:

http://pdf1.alldatasheet.com/datasheet-pdf/view/216700/STMICROELECTRONICS/VNH3SP30-E.html

 

[MacGyverS2000]

Body diodes are often used in place of discretes due to their quite low on-resistance.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[stephenjvb]

To VE1BLL: I would use option 1 but the customer used option 2 and messed it up with both sides of the mosfet connected to logic ground, effectively eliminating the mosfet. I don't understand the circuit, the person who designed it isn't available and they need a completed (and presumably working) proto by Friday. (They didn't want it good, they wanted it Wednesday. - R. Heinlein) Problems with their circuit were rumored, maybe that's why the mosfet was shorted, or maybe it was just a mistake. At this point I don't know and I'd like to.

To MacGyverS2000: I don't see the specs on body diodes anywhere so how do you use them without frying them? If that's the point of the circuit why does the mosfet meed to be turned on?

- Thanks

 

[stephenjvb]

To VE1BLL: I would use option 1 but the customer used option 2 and messed it up with both sides of the mosfet connected to logic ground, effectively eliminating the mosfet. I don't understand the circuit, the person who designed it isn't available and they need a completed (and presumably working) proto by Friday. (They didn't want it good, they wanted it Wednesday. - R. Heinlein)Problems with their circuit were rumored, maybe that's why the mosfet was shorted, or maybe it was just a mistake. At this point I don't know and I'd like to.

To MacGyverS2000: I don't see the specs on body diodes anywhere so how do you use them without frying them? If that's the point of the circuit why does the mosfet meed to be turned on?

- Thanks

 

[zappedagain]

You want to look for parameters similar to 'diode forward voltage' and 'continuous source current (body diode)' or 'pulsed source current (body diode)'. I pulled these terms from a Zetex ZXMN6A08K datasheet.

Z

 

[stephenjvb]

I just learned that this IS an elegant circuit. I was told in college oh so long ago that mosfets were polarized devices like transistors (I think by a Professor appropriately named Looney).
This is functionally true. However if the gate is positive with respect to the source current can flow in either direction. This is mostly useless, except for this circuit. If you reverse the mosfet then when it is off the base would have to be above the source to turn it on which would be +V except for the body diode that pulls the source down to the drain. Since the gate is pulled up the mosfet is always on so the body diode doesn't fry, so long as the polarity is correct. If the polarity is wrong neither the mosfet nor the body diode conduct.

This is a GREAT low side high current low drop reverse voltage protector. It knows no peers.

 

[VEBill]

Thank you posting that.