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Topic: Energizing a mosfet (Read 3599 times) previous topic - next topic

smeezekitty

I am fairly new to working with MOSFETs since I normally use BJT but I need to use a mosfet because I need low on resistance.
Using a P-Channel mosfet with the source connected to +15v, is bringing the gate down to 0v sufficient to turn the mosfet fully on?

Thanks in advance.
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Grumpy_Mike

Yes but taking it up to 5V is not enough to turn it off.

smeezekitty


Yes but taking it up to 5V is not enough to turn it off.

I was thinking of using a pullup resistor to +15 and use an 2n2222 driven with a MCU pin (with base resistor of course) to pull it down. Would this work?
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
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Grumpy_Mike

No it will fry your arduino's output pin. You need to pull it up to 12V with a resistor and pull that resistor down with a transistor.

smeezekitty


No it will fry your arduino's output pin. You need to pull it up to 12V with a resistor and pull that resistor down with a transistor.

You must have misunderstood me.
I attached a schematic.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

Docedison

Yes absolutely But 15 V enhancement on a mosfet gate is close to the limit, You could put a 5V Zener from gate to collector in series with the existing resistor, cathode to the gate and pull up for a cheap level shifter a 1/4 w 5V1 zener would work well. The Problem is the diaellectric used fir the gate... It will 'Punch Through' at about 18 to 20 Volts and -10V is more than enough enhancement to fully turn on the gate... IMO but as always check the data sheet for max ratings. The Gate is very sensitive to electrical damage... of any kind.

Doc
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smeezekitty


Yes absolutely But 15 V enhancement on a mosfet gate is close to the limit, You could put a 5V Zener from gate to collector in series with the existing resistor, cathode to the gate and pull up for a cheap level shifter a 1/4 w 5V1 zener would work well. The Problem is the diaellectric used fir the gate... It will 'Punch Through' at about 18 to 20 Volts and -10V is more than enough enhancement to fully turn on the gate... IMO but as always check the data sheet for max ratings. The Gate is very sensitive to electrical damage... of any kind.

Doc

The mosfet I am probably going to use is here http://www.fairchildsemi.com/ds/FQ/FQP7P06.pdf
It shows the gate-source voltage as +25 so 15v should be ok. The 15v is going to be regulated with a voltage regulator.
I suppose I could use a zener dropper although it seems a bit redundant but as I said before I do not really understand mosfets.
I may have to put a 20v or so zener across the 15v line so it does not go to high from a spike of driving an inductive load.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

Docedison

Go look up a 1.5KE18, it's a Tranzorb, use it on the +15V source and if you are still concerned put a 18 V 1/4 W zener from gate to ground and you will be reasonably well protected. A silicon diode like a 1N5408 is a good idea as well for neg spike control. From +15V to ground, the anode, cathode to +15V... Otherwise you show a fair understanding of how to use a Mosfet.

Doc
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dc42


The mosfet I am probably going to use is here http://www.fairchildsemi.com/ds/FQ/FQP7P06.pdf
It shows the gate-source voltage as +25 so 15v should be ok. The 15v is going to be regulated with a voltage regulator.


Yes, you can drive it as you were intending to, since 15v is well below the Vgs rating of 25V.

If you do want to drop the gate voltage a little, then an easier way than using a zener is to replace the single pullup resistor by a voltage divider, e.g. 470 ohms between mosfet gate and 2n2222 collector, and 1K between mosfet gate and +15v (giving 10v Vgs instead of 15v).

If the load is highly inductive, don't forget to include a flyback diode in parallel with it.
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Docedison

Mine was an old school engineering text book idea, yours is much better, I hadn't even considered it that way. I have always had a couple of drawers of zeners when working, material like that encourages laziness, now I make do with TIL431's. And occasionally very good advice from others.

Doc
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MarkT


Go look up a 1.5KE18, it's a Tranzorb, use it on the +15V source and if you are still concerned put a 18 V 1/4 W zener from gate to ground and you will be reasonably well protected. A silicon diode like a 1N5408 is a good idea as well for neg spike control. From +15V to ground, the anode, cathode to +15V... Otherwise you show a fair understanding of how to use a Mosfet.

Doc

If you put the zener between gate and source (cathode to  source for p-channel) you'll protect the gate from overvoltage and from going negative (not that it minds going negative).   For source/drain overvoltage a TVS diode would be useful, but do we know if that's an issue even?
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smeezekitty

The load is mostly capacitive but somewhat inductive and resistive as well (nice mish-mash).

Go look up a 1.5KE18, it's a Tranzorb, use it on the +15V source and if you are still concerned put a 18 V 1/4 W zener from gate to ground and you will be reasonably well protected. A silicon diode like a 1N5408 is a good idea as well for neg spike control. From +15V to ground, the anode, cathode to +15V... Otherwise you show a fair understanding of how to use a Mosfet.

Doc

I will probably go this route.
Quote

Yes, you can drive it as you were intending to, since 15v is well below the Vgs rating of 25V.

If you do want to drop the gate voltage a little, then an easier way than using a zener is to replace the single pullup resistor by a voltage divider, e.g. 470 ohms between mosfet gate and 2n2222 collector, and 1K between mosfet gate and +15v (giving 10v Vgs instead of 15v).

Wouldn't the gate still rise to +15v when the NPN transistor is turned off?
Quote

If the load is highly inductive, don't forget to include a flyback diode in parallel with it.

Of course. But this will not help on positive spikes.
Quote


If you put the zener between gate and source (cathode to  source for p-channel) you'll protect the gate from overvoltage and from going negative (not that it minds going negative).
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OK
Quote
For source/drain overvoltage a TVS diode would be useful, but do we know if that's an issue even?

I doubt that it will exceed 60v but it can be hard to say.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

Grumpy_Mike

Quote
The load is mostly capacitive but somewhat inductive and resistive as well (nice mish-mash).

No you can't say that.
If you have a capacitive load and an inductive load the two cancel out according to the values of C and L and you end up with either a capacitive or an inductive load. In the case where they both are equal, that is a resonance point.

smeezekitty


Quote
The load is mostly capacitive but somewhat inductive and resistive as well (nice mish-mash).

No you can't say that.
If you have a capacitive load and an inductive load the two cancel out according to the values of C and L and you end up with either a capacitive or an inductive load. In the case where they both are equal, that is a resonance point.

I am still going to use plenty of protection since diodes are only a few cents.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

smeezekitty

Now off to solving the next hurdle: current sensing.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

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