Author Topic: Saturating a MOSFET with a constant current sink driver  (Read 5183 times)

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Offline Wimberleytech

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Re: Saturating a MOSFET with a constant current sink driver
« Reply #25 on: January 07, 2020, 10:32:19 pm »

So apart from my irresponsible description of voltage (thanks I'll make sure to correct it in future), its just that I assumed that source will be at 5V after saturing the MOSFET given the load. In a previous case, does that mean if the MOSFET was on and 10A of current started flowing from drain to source, the voltage across drain and source would be ~3V?

Thank you for continuing to help.

Didn't mean to be pejorative.
Think about this.  Imagine an NPN BJT with the collector at 5 volts and the base at 5 volts.  With some amount of current flowing collector to emitter, will the emitter voltage ever be five volts?  MOSFETS are a different beast, but similar in this way. 
If the gate and drain of an NMOS transistor are connected to five volts, the source can never rise above 5V-Vth. 

If you know how to use LTSpice, I will generate a simulation for you to examine
« Last Edit: January 08, 2020, 02:08:49 am by Wimberleytech »
 

Offline OleenickTopic starter

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Re: Saturating a MOSFET with a constant current sink driver
« Reply #26 on: January 08, 2020, 07:33:07 am »

So apart from my irresponsible description of voltage (thanks I'll make sure to correct it in future), its just that I assumed that source will be at 5V after saturing the MOSFET given the load. In a previous case, does that mean if the MOSFET was on and 10A of current started flowing from drain to source, the voltage across drain and source would be ~3V?

Thank you for continuing to help.

Didn't mean to be pejorative.

No no its fine I didn't take it that way. I'm glad you corrected me.

Think about this.  Imagine an NPN BJT with the collector at 5 volts and the base at 5 volts.  With some amount of current flowing collector to emitter, will the emitter voltage ever be five volts?  MOSFETS are a different beast, but similar in this way. 

The emitter voltage would depend on the current flowing collector to emitter. So the voltage across base and emitter would be influenced by the current flowing collector to emitter, which is governed by the transistors RDSon. I think?

If the gate and drain of an NMOS transistor are connected to five volts, the source can never rise above 5V-Vth. 

If you know how to use LTSpice, I will generate a simulation for you to examine

I do know the basics of LTSpice, but I just "simulated" it on a breadboard with a PMOS and NMOS.

Measuring voltage across source and ground (the load should be at 5V).
NMOS (load is not at 5V):
905136-0

PMOS (load is 5V):
905140-1
 

Offline Wimberleytech

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Re: Saturating a MOSFET with a constant current sink driver
« Reply #27 on: January 08, 2020, 02:51:19 pm »
Please draw what you are measuring...like what I show here.
 
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Offline OleenickTopic starter

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Re: Saturating a MOSFET with a constant current sink driver
« Reply #28 on: January 09, 2020, 01:56:02 am »
Please draw what you are measuring...like what I show here.

Here are the two circuits I was testing, minus the capacitor which I didn't add to the breadboard.
My voltage measurements were across the blue and red nodes on each NCH and PCH diagram.
 

Offline OleenickTopic starter

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Re: Saturating a MOSFET with a constant current sink driver
« Reply #29 on: January 12, 2020, 05:44:07 am »
I think I've finally found a nice MOSFET. If you're curious (https://au.element14.com/infineon/ipb120p04p4l03atma1/mosfet-p-ch-40v-120a-to-263-3/dp/2480803)

I've been looking more into MOSFETs, and I can see clearer now what's been described to me here.

Thanks for all the support.
 

Offline Wimberleytech

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Re: Saturating a MOSFET with a constant current sink driver
« Reply #30 on: January 12, 2020, 01:41:54 pm »
I think I've finally found a nice MOSFET. If you're curious (https://au.element14.com/infineon/ipb120p04p4l03atma1/mosfet-p-ch-40v-120a-to-263-3/dp/2480803)

I've been looking more into MOSFETs, and I can see clearer now what's been described to me here.

Thanks for all the support.

Yes, that will work nicely.
 
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