MOSFET explanation please

[bigfarmerdave]

I'm putting together a project in which I need a MOSET to switch power on and off to a 12 volt DC solenoid. I am just looking for a plain old N-channel power MOSFET. While looking, I also came across a "high side power MOSFET switch" but am not sure what the differences are between that and a plain old power MOSFET. Can anyone give me a brief explanation? In addition, can anyone point me toward an online reference of MOSFETS that I could use as a bit of an educational guide? Thanks for the help guys.

 

[RussianKid]

well, Wikipedia: https://en.wikipedia.org/wiki/Mosfet

however what you're describing just sounds like a normal MOSFET, albeit one with advertised high power (voltage?).

If you were to provide a link, then we could clarify whether what you encountered is anything out of the ordinary.

 

[Analog]

I'm putting together a project in which I need a MOSET to switch power on and off to a 12 volt DC solenoid. I am just looking for a plain old N-channel power MOSFET. While looking, I also came across a "high side power MOSFET switch" but am not sure what the differences are between that and a plain old power MOSFET. Can anyone give me a brief explanation? In addition, can anyone point me toward an online reference of MOSFETS that I could use as a bit of an educational guide? Thanks for the help guys.

A highside switch is a mosfet that switches the 'highside', or positive side of the power supply. Your typical N-channel is a low side switch, meaning the load goes on the drain side to +Vdd.

Highside switches are great for automotive solutions, because the return can simply be the chassis, which decreases the wire count by one.

 

[jpanhalt]

Can you give the part number for the high-side switch? It may include a high-side drive for an N-channel, or maybe it's a P-channel. It sounds like you have the right idea, though.

John

 

[bigfarmerdave]

The part number of the high side switch that I came across is a International Rectifier, part number IPS5551T. Looking at a IPS0551T, what are the differences between the two? Are they both considered high side switches?

 

[jpanhalt]

That is an interesting device. It includes circuitry, including a charge pump, for a high-side, N-channel mosfet drive and mosfet in one package. Remember the on/off signal is referenced to Vcc, not to the source (ground).

It looks like a fun device to experiment with. John

Edit: Just realized your question has another part. The IPS0551 logic signal is referenced to its source (ground) like a conventional N-channel mosfet. It seems to offer protection from overcurrent and a gate drive, but is not specifically intended for high-side switching. You will note that the application example has it on the low-side of the load. The gate drive does not include a charge pump that would enable high-side switching.

 

[bigfarmerdave]

So the IPS5551T MOSFET has to see a voltage at the gate (VCC-Vin) that is larger than the specified "high" threshold voltage in order to turn on correct? How do you know or set the "upper" threshold voltage to turn the MOSFET on? Then to turn the MOSFET off, VCC-Vin must be less than the "lower" threshold voltage? Once again, how do we know what the "low" threshold voltage is?

With the IPS0551T, the gate voltage has to be larger than the source correct? Looking at the data sheet, am I reading it correctly that a 5 volt input at the gate, would turn the IPS0551T on?

One more question came to mind when reading your last sentence. Does a MOSFET have to have a charge pump in order to do high side switching then?

P.S. Thanks for the previous explanation! Still trying to catch on to this stuff.

 

[jpanhalt]

So the IPS5551T MOSFET has to see a voltage at the gate (VCC-Vin) that is larger than the specified "high" threshold voltage in order to turn on correct? How do you know or set the "upper" threshold voltage to turn the MOSFET on? Then to turn the MOSFET off, VCC-Vin must be less than the "lower" threshold voltage? Once again, how do we know what the "low" threshold voltage is?

That is saying that your logic signal has to go nearly to, but does not have to be greater than Vcc. That is often much easier to do than to exceed Vcc (see charge pump below).

With the IPS0551T, the gate voltage has to be larger than the source correct? Looking at the data sheet, am I reading it correctly that a 5 volt input at the gate, would turn the IPS0551T on?

Yes, in a low-side configuration. If the IPS0551 is high side, you get the same problems as with a simple high-side, N-channel mosfet.

One more question came to mind when reading your last sentence. Does a MOSFET have to have a charge pump in order to do high side switching then?

There is no requirement for a charge pump per se, but when the source is at or near the top rail voltage (i.e., Vcc), which happens in a high-side configuration, you need a way to get the gate voltage even higher to turn the mosfet on. A charge pump is a cheap and easy way to do that.

John