Author Topic: Selecting MOSFET  (Read 6058 times)

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Online Zero999

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Re: Selecting MOSFET
« Reply #25 on: January 17, 2019, 11:43:06 am »
Another thing you could consider is a MOSFET driver IC such as the TC4423, which will do the level shifting for you. As you're driving a P-channel MOSFET, you'll want an inverting driver, if you want the MOSFET to turn on, when the output is high. There are plenty of other MOSFET driver ICs available.
http://ww1.microchip.com/downloads/en/devicedoc/21421d.pdf
 

Offline spec

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Re: Selecting MOSFET
« Reply #26 on: January 17, 2019, 02:03:56 pm »
+ permal

I meant to say that you can cater for different voltages just by adding another resistor. But it does mean that you would have to change the resistor to suit the voltage range that you are using, as shown in the attached schematic below:
« Last Edit: January 17, 2019, 02:06:14 pm by spec »
 

Offline sairfan1

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Re: Selecting MOSFET
« Reply #27 on: January 17, 2019, 04:28:39 pm »
link at 'great help' reaches at page not found, can you please update it, thanks
 

Offline exe

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Re: Selecting MOSFET
« Reply #28 on: January 17, 2019, 04:32:25 pm »
schematic below

Just in case, the first mosfet doesn't need to be that beefy. Although, it may not make sense to have a separate stock of less powerful fets...
 

Offline spec

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Re: Selecting MOSFET
« Reply #29 on: January 17, 2019, 05:47:44 pm »
schematic below

Just in case, the first mosfet doesn't need to be that beefy. Although, it may not make sense to have a separate stock of less powerful fets...
Yes, that is the philosophy. If you ever needed a low side power switch, for example, you could simply use the NMOSFET.
 

Offline permalTopic starter

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Re: Selecting MOSFET
« Reply #30 on: January 17, 2019, 07:31:09 pm »
+ permal

I meant to say that you can cater for different voltages just by adding another resistor. But it does mean that you would have to change the resistor to suit the voltage range that you are using, as shown in the attached schematic below:
Yeah, but in my case I have 5V and 12V available and I don't know before hand what will be connected so I'll have to go for the zener-variant.

Sure :)

-- great explanation by Spec here --
Thank you  :-+

Ok, so I'm now at the following schematic.

Questions:

1. I've noticed that you don't add the pull-down resistor in your example schematics, @Spec - is there a reason for that? The MCP23017 do not have built-in pull downs so I see no way of excluding them (R1605).

2. R1603 and D1601; the current limiter and LED, how should I reason about those? I'm sure they affect the circuit somehow (a part from drawing current).

3. If the wire between CTRL and R1601 is "long", would a decoupler cap do any good? Sprinkle a 10nF on it?

4. The inrush resistors - how do I calculate appropriate values for them? If they can be reduced to 10R, then they feel somewhat arbitrary to me (though I assume they are not)?

5. Am I reading the data sheet for CZRQR52C6V8-HF correctly, in that its foot print is SOD-923F, equal to 0402 (1005 metric)?
« Last Edit: January 17, 2019, 07:46:11 pm by permal »
 

Offline spec

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Re: Selecting MOSFET
« Reply #31 on: January 17, 2019, 09:01:25 pm »
in my case I have 5V and 12V available and I don't know before hand what will be connected so I'll have to go for the zener-variant.
That is probably the best move all round. You then have a universal switch.

1. I've noticed that you don't add the pull-down resistor in your example schematics, @Spec - is there a reason for that? The MCP23017 do not have built-in pull downs so I see no way of excluding them (R1605).
Nothing to do with me mate- you are the systems engineer on this. :)

2. R1603 and D1601; the current limiter and LED, how should I reason about those? I'm sure they affect the circuit somehow (a part from drawing current).
Not sure what your intention is with the resistor and LED.

3. If the wire between CTRL and R1601 is "long", would a decoupler cap do any good? Sprinkle a 10nF on it?
Yes, decoupling is always a good thing. 100nF X7R are the standard decouplers. But as long as the gate stopper is wired OK, there should not be a problem with long wires.

4. The inrush resistors - how do I calculate appropriate values for them? If they can be reduced to 10R, then they feel somewhat arbitrary to me (though I assume they are not)?
They are not inrush current limiters, they are gate stoppers. 50R to 100R is a good value for medium speed work and lower values for high speed work. The gate stoppers play no part in the basic function of the circuit. They are normally only there to inhibit oscillation. But with the zener diode limiters they also limit the current when the zener diodes are conducting. So on one hand you would like to make them 100R say, but on the other hand you want to make them as large as possible to limit the current. So 470R is a compromise.

5. Am I reading the data sheet for CZRQR52C6V8-HF correctly, in that its foot print is SOD-923F, equal to 0402 (1005 metric)?
Afraid you are asking the wrong person. Perhaps another member can advise.
« Last Edit: January 17, 2019, 09:13:37 pm by spec »
 
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Offline permalTopic starter

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Re: Selecting MOSFET
« Reply #32 on: January 18, 2019, 07:17:53 am »
They are not inrush current limiters, they are gate stoppers. 50R to 100R is a good value for medium speed work and lower values for high speed work. The gate stoppers play no part in the basic function of the circuit. They are normally only there to inhibit oscillation. But with the zener diode limiters they also limit the current when the zener diodes are conducting. So on one hand you would like to make them 100R say, but on the other hand you want to make them as large as possible to limit the current. So 470R is a compromise.
Hm, I'm sure I read somewhere they were called inrush resistors, but based on your explanation stop resistors makes sense so we'll go with that :)

Nothing to do with me mate- you are the systems engineer on this.
Just thought I'd ask, since you provide such a complete circuit otherwise but let the gate float on that mosfet.

Not sure what your intention is with the resistor and LED.
Just as an indicator showing the state of the switch.

Yes, decoupling is always a good thing. 100nF X7R are the standard decouplers. But as long as the gate stopper is wired OK, there should not be a problem with long wires.
So, "sprinkling caps" is really a thing then? As long as it's not A/C, put them everywhere?
« Last Edit: January 18, 2019, 07:03:16 pm by permal »
 

Offline permalTopic starter

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Re: Selecting MOSFET
« Reply #33 on: February 06, 2019, 08:10:39 pm »
Finally got the parts and PCB in hand. Success! :)
 


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