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Protection Mosfet - Drain Current Limit with Vgs

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mic5

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I am driving a relay with Mosfet driver. Mosfet has internal current and and temperature limiting circuit. Maximum drain current Id = 2A. As per the curve Vgs Vs Id, Can I set the drain current limit as 300mA with the Vgs voltage?

What are the factors I have to look into?
 
Put a diode across the relay.

Because the gate voltage is low 2.5V or 3.3, use a logic level MOSFET.

What MOSFET are you using?
 
Mosfet part : NIF5002N. Fly back diode is given across the relay coil.
 
The NIF5002 is a nice part. I don't see that you need current limit or temperature shutdown.
2.5V is a little low for gate voltage.
 
You can't adjust the current limit. It's fixed by the design.
 
With 28 volts applied to a 300 ohm relay, the most current the mosfet should ever see is about 95mA. Anything more will be due to a short circuit in the relay or wiring.
 
Anything more will be due to a short circuit in the relay or wiring.

For this purpose only the MOSFET is selected with current limit. Anyhow the trace for the drain should have at least 2A capability. If the current limit is reduced, the PCB design will be easy.
 
Is it fixed by the chip manufacturer? What about the plot Vgs vs Id?

To try and put it more clearly than has already been stated, the relay current is limited by ohms law. 28V divided by 300 ohms = 93 milliamps. You cannot draw more than 93 milliamps through the relay, no matter what MOSFET you use -- much less 300 mA.

Besides that, you cannot precisely control the FET current using the gate voltage as the FET current varies considerably with temperature variation and from one FET to another FET. You can try to use worst case specs to set a current limit, but then you might not be assured to fully energize the relay under normal conditions.
 
In case of short circuit the current will be 2A. If it is limited to 300mA, the PCB trace width can be reduced to considerable amount.

For 2A trace width required is 100mils (approx.), but 300mA requires only 10mils(approx).
 
In case of short circuit the current will be 2A. If it is limited to 300mA, the PCB trace width can be reduced to considerable amount.

For 2A trace width required is 100mils (approx.), but 300mA requires only 10mils(approx).

It is unusual to design for a short circuit other than from components failing/shorting, except in limited cases such as a power souce where the output is user accessible. If the FET shorts, you're still okay. A shorted relay coil is improbable. Is there something going on that isn't obvious to reasonably expect the possibility of a shorted load, or some other short? If so, the usual course is to prevent that possibility altogether (within reason) or to provide a protective device suited for that purpose or to provide for the short circuit current to exist without harm to anything/anyone. Trying to protect the trace against excessive current using the FET gate voltage, especially when the normal load current is so close the current limit, is not usual and, personally, I don't recommend it. You can see from the datasheet that with a 2.5V gate voltage, the drain current can vary from 50ma to 400ma over the FETs operating temp range, and those are typical values given, not limiting values.
 
and; if you use a transistor (MOSFET) in current limit mode you will wind up with a very hot part. 28V*300mA=8.4watts. very hot!
Last time I used a 'poly fuse'. It heals its self after the short is removed. The fuse is slow to react but will save silicon.
 
In case of short circuit the current will be 2A. If it is limited to 300mA, the PCB trace width can be reduced to considerable amount.

For 2A trace width required is 100mils (approx.), but 300mA requires only 10mils(approx).

Read the Current Limit spec under the Self Protection Characteristics of the NIF5002 data sheet.

The 2 amp rating of the part is the minimum that it will pass under it's worst case operating condition. (min value at 150C)

But the limit could be as high as 6.3 Amps. (max value at 25C) If you are trying to protect the PCB traces, that is the number you need to use, unless the current is inherently limited somewhere else.

Looking across the min / max with all else being the same, the ratio is a bit more than 1:2. If you need tighter numbers than that, you may need to find a different way.
 
I suggest you use a logic-level MOSFET, but put a resistor of about 5 ohms in series with the Source of the MOSFET. That will limit the current to around 300mA, depending on the gate threshold voltage of the MOSFET, and the gate voltage that you are supplying. It will limit the dissipation in the MOSFET to around 10 W. That could still be too much and a MOSFET with thermal protection such as the NIF5002 could be a good idea in conjunction with a current limit resistor.
 
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