MOSFET refresher

[eaton]

Hi,
Anybody got a site for a quick refresher on using MOSFET's as switches. It's been awhile since I have used them. I'm trying to drive some higher current devices (300 to 900 mA) and want to use the PIC as directly as possible.( no BJT's and the associated resistors).

Thanks,
Eaton

[Chas]

You could start here:

[url]
http://www.irf.com/technical-info/appnotes.htm
[/url]

IR has many devices that would fit you application (from what little we know). I driven MOSFETs directly in the past, but I still feel most comfortable with a series resistor between the PIC port pin and the gate of the MOSFET.

With an N-channel device, you can connect the source to ground, the gate to the pic via a series resistor (on the order of a few tens of ohms) and the load between the drain of the MOSFET and V+. You can use a P-channel device, but you must flip the circuit over: source to V+, and load between the drain and ground.

As an aside, have you ever looked at the ULN2803 series of drivers? TI has some as well as Allegro and others. They take a TTL/CMOS level input and can drive an inductive load. There's an integral catch diode for each output.

[pat]

Just remember MOSFETs are very susceptible to ESD. Also if you're using them in an automotive environment, they must be protected against load dump using a transil (tvs).

You can read about different MOSFET failure modes in Motorola AN929 http://www.onsemiconductor.com/pub/Collateral/AN929-D.PDF

I agree with Chas, you should drive logic level MOSFETs with a resistor (eg 10k), if you're switching at a low freq, this will not affect gate charge time significantly. If the FET is damaged and dies, you can get charge transfer back through the gate which could take out your PIC (this happened to me).

Even when protected against load dump, I've still had my logic FETs fail from ESD. I've found them to be very fragile. I would strongly recommend using a self protected logic MOSFET http://www.google.com.au/search?hl=en&q=self+protected+mosfet&btnG=Search&meta=

I have used the Onsemi NID5001N with NO failures. For your lower current there is the Onsemi NIF5002. These are protected against every nasty you can think of: ESD, load dump, over current, over temperature. I have deliberatly shorted the drain to +12V and it never failed.
Onsemi have a range of self protected MOSFETs, see
http://www.onsemi.com/site/products/parts/0,4454,819,00.html

You could also have a look at Zetex BSP75G self protected logic level MOSFET.

These devices are great, you don't have to add any extra protection, and if you're driving an inductive load (realy/solenoid) you don't even need to add a flyback (freewheeling) clamping diode.

Hope this helps.

Patrick

[eaton]

Thanks for the help.

[joe]

From the typical configuration of mosfet,
source->gnd
gate-> resistor -> PIC
drain-> load -> vcc

i also note that the load eg. solenoid is always placed in parallel with a diode. Why is that so?

For the resistor used in gate, is it for prevention of current leak to PIC ?

I would like to know more about the rational of placing such devices there.. is there a good place to learn all this??

[newguy]

The diode is called a flyback diode. Note that it points "up" - anode to the drain of the mosfet, cathode to the + supply voltage.

The inductive load of the solenoid can be thought of as a water wheel. By water wheel, imagine the paddle wheel from an old paddle wheeler boat enclosed in a housing.

If the water wheel is stopped, and you try to go from no water flow to some non-zero flow, then the wheel cannot oblige. It takes some time for it to spin up to speed. In the same manner, you cannot instantaneously stop the flow through the wheel once it is established. If you do, the wheel will do everything in its power to keep the flow the same. If this means destroying a valve or itself, then it will do just that. The energy to do these things comes from the motion of the wheel.

An inductor behaves the same, only the energy is stored in the magnetic field created by the current flow through it.

The flyback diode is in place to give this "back emf" a safe discharge path once you try to interrupt the flow through the inductor.

The resistor between the mosfet's gate and the pic is simply there for protection. Protection in case the mosfet fails, and the gate somehow gets shorted to the source or to the drain. If either of these two events happens, then the pic is protected if it tries to pull that pin high or low. With the series resistor, no real harm can be done.

As to your last question, the only real place to learn all this is through experience. I'm an engineer, actually a professor, and to tell you the truth, not even universities teach this kind of stuff. If you're lucky enough to have an experienced technologist or engineer to ask these types of questions, then that's the best place to learn. If not, this forum is an awesome resource. Ask any question you like - chances are you'll get several good answers.

[joe]

thanks for the reply..

i got another question about the mosfet. My solenoid needs a 600mA at 3V and i found AFN1332N that can provide the 600mA. But the Vgate must be 2V.

The IO port used to turn on Vg is a cmos output pin [PIC12F629] thus should i use a voltage divider to get 2V ? and which range of the cmos output voltage should i use?(5V or 3.5V) as they varies widely, one configuration will not work for another..

[newguy]

Sorry - tried searching for "AFN1332N" and couldn't get any matches.

If the maximum Vgate is only 2V, I'd be really surprised. Maybe the *minimum* Vgate to turn it on is 2V?

In any case, there are lots of mosfets that will do what you want. Just go to www.digikey.com and search for "enhancement mosfet". You'll be able to narrow the search and find something that will work for you.