Author Topic: MOSFET HEXFET Help please  (Read 1441 times)

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Offline andybarrett1Topic starter

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MOSFET HEXFET Help please
« on: March 19, 2019, 08:58:52 am »
Hi All .... Looking for a little Help

I am using a similar  circuit as below to switch a High Power Led from a PIC Processor using a IRL520 HEXFET

All is working fine. However I would like the LED  to be switched from the positive side so effectively one side of the LED is 0v and not 12v as in the picture. if it was a BJT it would be PNP not NPN am looking for?

Is there a P-Channel of the IRL520 I have missed..... Do I need to do something else maybe?

Thanks for reading

Andy

« Last Edit: March 19, 2019, 09:00:56 am by andybarrett1 »
 

Offline RES

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Re: MOSFET HEXFET Help please
« Reply #1 on: March 19, 2019, 09:20:16 am »
IRF9520

Offline Psi

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Re: MOSFET HEXFET Help please
« Reply #2 on: March 19, 2019, 09:22:47 am »
Yes, you need a P-Channel fet.

Not all N-Channel fets have a matching P-Channel version.

You will need to look at the specs you need. max voltage/current gate threshold etc. and select a P-Channel fet which meets your requirements.

Be aware that the resistor will then go to VCC instead of GND and your logic will be inverted.
« Last Edit: March 20, 2019, 12:17:00 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 
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Online Ian.M

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Re: MOSFET HEXFET Help please
« Reply #3 on: March 19, 2019, 12:14:18 pm »
... and for supply voltages greater than the MCU's Vcc, you'll also need a level shifter.  e.g. with a 12V supply, you'll need to take the gate between +12V for Off and at least 2x the Vgs threshold voltage negative of that for On.  As the supply voltage is significantly less than the max Vgs rating, you can pull it all the way down to 0V, so a simple discrete small signal N-MOSFET will do for the level shifter if you aren't PWMing the load at a high frequency.   It adds another stage of inversion so you are back to logic high for On.  N.B. The load turn-off time is then determined by how fast the gate pullup resistor can discharge the gate - you have to tradeoff fast turnoff v wasted power when on.

With higher supply voltages, pulling the gate of a high side MOSFET all the way down to 0V could exceed its max Vgs rating causing gate oxide breakdown so your level shifter would need to be considerably more complex to limit or clamp the Vgs applied to the MOSFET.
 
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Offline Peabody

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Re: MOSFET HEXFET Help please
« Reply #4 on: March 19, 2019, 01:24:31 pm »
My understanding is that basic stamps are 5V.  So it seems to me that the circuit is already the way it needs to be for controlling a 12V source with a 5V processor I/O pin.  If you invert everything, wouldn't you have the P-channel gate pullup resistor connected to 12V?  That's not going to be good for the I/O pin.  Could someone post a schematic of how this would look using a P-channel?
 
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Offline andybarrett1Topic starter

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Re: MOSFET HEXFET Help please
« Reply #5 on: March 19, 2019, 02:25:54 pm »
Thank you all for your help so far.... As Peabody as mentioned inverting everything might give me more issues yet to be explored.

Looking forward to seeing what others have to contribute ?

BR
Andy
 

Online Ian.M

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Re: MOSFET HEXFET Help please
« Reply #6 on: March 19, 2019, 02:35:46 pm »
@Peabody:  Here's *ONE* way to do it - there are many others but they all involve an extra level-shifting stage between the MCU and the power PMOSFET.
 
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Offline andybarrett1Topic starter

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Re: MOSFET HEXFET Help please
« Reply #7 on: March 19, 2019, 04:17:16 pm »
Ian

That is more or less what I was looking at doing but I was using a small BJT in place of the 7000 Device.

Maybe there is a good reason to use the Ntype  |O... Still helps the learning process :-)

Andy
 

Offline Peabody

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Re: MOSFET HEXFET Help please
« Reply #8 on: March 19, 2019, 04:21:28 pm »
Ian.M, it looks like that would work fine.  But it's a good bit more complicated that what the OP has now.  We don't know why he wants to switch things around, but for now I'm going to stick with my contention that he already has the simplest, best circuit.  If the load was also tied to 5V, then it could be done either way with no problem.  But I think the 12V pretty much dictates which way to do it.  So Andy, why do you need to change what you have?  The LED won't care what it's tied to.  It only cares about the current flowing through it.
 

Offline Psi

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Re: MOSFET HEXFET Help please
« Reply #9 on: March 20, 2019, 12:22:08 pm »
For loads under 2A i sometimes just go for 2A digital optocoupler IC's or a gate driver IC's.
They increase the power output of the micro pin but keep its ability to source and sink.
Therefore you can use them to drive to GND or to VCC.

Sometimes the fact that you get isolation is handy too.
And because the IC has its own power input you get the option to do level shifting if needed,
« Last Edit: March 20, 2019, 12:24:46 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline andybarrett1Topic starter

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Re: MOSFET HEXFET Help please
« Reply #10 on: March 20, 2019, 01:36:28 pm »
HI

The reason I was looking to invert the Output was / is I have four of these outputs on the one Pic. So usually in the field we would use 0v as a common then connect the four positive switching voltages.

The hard bit is remembering that 12v is common to all four LEDs

BR
Andy
 

Online Zero999

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Re: MOSFET HEXFET Help please
« Reply #11 on: March 21, 2019, 11:36:30 am »
For loads under 2A i sometimes just go for 2A digital optocoupler IC's or a gate driver IC's.
They increase the power output of the micro pin but keep its ability to source and sink.
Therefore you can use them to drive to GND or to VCC.

Sometimes the fact that you get isolation is handy too.
And because the IC has its own power input you get the option to do level shifting if needed,
Be careful, MOSFET driver ICs are not normally specified for continuous operation at the full current. You might need to work out the power dissipation to determine what current can be safely switched continuously.

HI

The reason I was looking to invert the Output was / is I have four of these outputs on the one Pic. So usually in the field we would use 0v as a common then connect the four positive switching voltages.

The hard bit is remembering that 12v is common to all four LEDs

BR
Andy
What's the current rating of the LEDs? How about using an IC?
The TBD62783 has eight channels. If you only need four, then two channels can be connected in parallel to reduce the voltage drop and power dissipation.
 
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