Author Topic: MOSFET information  (Read 845 times)

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Offline danners430

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MOSFET information
« on: May 13, 2018, 06:47:33 pm »
Hey guys,

I'm designing a digital switching circuit for model railway solenoid point motors - some folks have advised me to use STP36NF06L MOSFETs, but I've no idea about how to actually select the correct device. Does anyone have any advice?

Also, can you use MOSFETs both high and low side, so I can effectively create a matrix of outputs to allow me to conserve microcontroller outputs?

Cheers guys!

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Offline Zero999

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Re: MOSFET information
« Reply #1 on: May 13, 2018, 08:07:52 pm »
Please provide more information?

IF N-MOSFETs are used on the high side, then their gate voltages need to be raised above the load voltage to turn on fully. For example, if the load is 12V and the MOSFET requires 5V, between the gate and source, to pass the required current, with a low on resistance, the total gate voltage will need to be 12 + 5 = 17V. If it's just given 12V, the voltage drop will be very high.
 

Offline danners430

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Re: MOSFET information
« Reply #2 on: May 13, 2018, 08:11:34 pm »
Thanks for the reply,

The solenoids run off a capacitor discharge unit, which is charged using a 16V AC supply. Presumably my first step should therefore be to measure the potential when the capacitors are charged?

Also, presumably you'd use P-channel devices on the high side...? Or am I just assuming that from transistor theory?

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Offline janoc

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Re: MOSFET information
« Reply #3 on: May 13, 2018, 08:27:58 pm »
It would be good to know how your solenoids actually work. From my time playing with model trains, the points were controlled by two coils pulling at the same metal slug (each in one direction) and that was moving the points. All controlled by momentarily connecting one  solenoid to 16V DC (not AC) and with no capacitors. In fact, adding a capacitor tended to destroy the mechanism because the very thin magnet wire of the solenoids has melted. So there were 3 wires coming out of it - one common and one for each of the coils. I guess your system is different.

If you want to control something like that with a MOSFET, you need to know how much current the solenoid actually takes when energized - that you use to dimension the MOSFET.

The other part you need is that the MOSFET has to be a logic level one, which is able to conduct full current even with a low voltage on the gate. Normal, non-logic level FETs need like 8-10V on the gate to turn on, so that wouldn't work if you tried to control it from a microcontroller directly, without a driver.

Re high side switch - sure, you can use a P-FET too, that will work. If you wanted to use an N-FET for the high side switch as well, there are tricks such as bootstrapping or using a gate driver with a built-in charge pump to elevate the gate voltage. However, I think that would be needlessly complicated for your use case.

Finally, unless you need really high currents I would also consider using ready-made ICs - there are plenty of ICs on the market that have half-bridge or a full H-bridge transistor configuration inside (even several), including any required drivers, protection and control logic. That will likely be cheaper than trying to do it with discrete components and certainly smaller.

« Last Edit: May 13, 2018, 08:31:10 pm by janoc »
 

Offline danners430

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Re: MOSFET information
« Reply #4 on: May 13, 2018, 08:34:54 pm »
Hmm... That's a thought, but I'll go through each of the points individually to try and give everyone the best understanding possible :-)

The solenoid motors I'm using are Peco PL-11 or PL-10 motors. Peco themselves don't publish any technical details about the solenoids, which doesn't help much, but they do recommend the use of a CDU, in my case a Gaugemaster CDU. Again, they don't publish any details either, so any info I need I would have to obtain using my cheap and nasty multimeter :-(
The CDU sits between the 12V DC / 16V AC input and the motors, and also acts as short circuit prevention, as it automatically shuts off after delivering it's pulse.

Are logic level MOSFETs a thing? Sorry if it's an obvious question, but my experience thus far has been with simple bipolar transistors, and I suspect they would go pop if I used them for this...

Finally, what sort of currents would i be looking at before I had to abandon ICs? And what particular type of IC would I be looking for? I'm custom designing PCBs for this anyway, so unless they come in a package that isn't hand-solderable, anything should work in theory :-)

Cheers for the help thus far!!

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Offline janoc

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Re: MOSFET information
« Reply #5 on: May 13, 2018, 09:23:22 pm »
Hmm... That's a thought, but I'll go through each of the points individually to try and give everyone the best understanding possible :-)

The solenoid motors I'm using are Peco PL-11 or PL-10 motors. Peco themselves don't publish any technical details about the solenoids, which doesn't help much, but they do recommend the use of a CDU, in my case a Gaugemaster CDU. Again, they don't publish any details either, so any info I need I would have to obtain using my cheap and nasty multimeter :-(
The CDU sits between the 12V DC / 16V AC input and the motors, and also acts as short circuit prevention, as it automatically shuts off after delivering it's pulse.

Ah ok. From what I can see, the "CDU" is just 2 capacitors and rectifier diode sold for a rather ridiculous price. OK. Your "motors" seem to be using pretty much the same principle as what I have described, even though they look a bit different from what I have used back then.


Are logic level MOSFETs a thing? Sorry if it's an obvious question, but my experience thus far has been with simple bipolar transistors, and I suspect they would go pop if I used them for this...

Logic level MOSFET just means that the MOSFET will start conducting at a lower gate voltage than a normal one. Normal ones need about 5-10V gate voltage (depending on type), logic level ones anywhere from 0.5-4V (again depending on type). They are called "logic level" because this allows them to be controlled directly by digital logic (e.g. a microcontroller).  E.g. that STP36NF06L is a logic level FET.

BTW, a normal bipolar transistor would work just fine as well. You just may need to use a darlington pair (e.g. something like TIP120) because the microcontroller may not have enough current to drive a power transistor otherwise. The disadvantage is lower max. current you can switch than MOSFETs and higher losses, the advantage is simplicity.

Finally, what sort of currents would i be looking at before I had to abandon ICs? And what particular type of IC would I be looking for? I'm custom designing PCBs for this anyway, so unless they come in a package that isn't hand-solderable, anything should work in theory :-)

Depends - you can get ready to go bridge ICs even for 10s of amperes. Given that your solenoid is only driven by a short pulse every once in a while (so things have time to cool down), you can use an IC even for fairly large currents.

But I have just now realized - you want to build a matrix, not to switch polarity through the coil (that's what a bridge would be good for). So for that you can't use a bridge IC but there are some transistor arrays/drivers, e.g. (bipolar):

NPN:
https://www.digikey.com/catalog/en/partgroup/npn-transistor-arrays/31744

PNP:
https://www.digikey.com/catalog/en/partgroup/pnp-transistor-arrays/41969

MOSFET variants supposedly exist too but I haven't found them :(
 

Offline danners430

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Re: MOSFET information
« Reply #6 on: May 13, 2018, 09:42:18 pm »
That's amazing, cheers for the invaluable help!!

I'll spend some time trawling the RS Components catalogue tomorrow to get a suitable set of FETs that I can hopefully find cheaper elsewhere ( [emoji12] ), then get a couple and do some experiments :-)

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Offline janoc

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Re: MOSFET information
« Reply #7 on: May 14, 2018, 09:49:44 am »
If you want to save yourself a lot of trouble, don't "buy cheaper" from places like eBay or AliExpress. You will pay 2x because the ones you will receive will be invariably fakes and then you will need to order the genuine part from RS or Farnell anyway.

Unfortunately things like MOSFETs are one of the most counterfeited items on the planet because they are so easy to fake - just grab a cheap Chinese part with somewhat similar specs and relabel it as a much more expensive Western part.
 

Offline danners430

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Re: MOSFET information
« Reply #8 on: May 14, 2018, 09:51:30 am »
Nah, i don't buy stuff off eBay or Ali etc.

I was referring to places like Rapid Electronics, who are generally cheaper than Farnell or RS in my experience :-)

eBay is perfect for components that you want to test for larger projects, like breadboarding etc. But not for permanent installations or big projects :-)

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