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Softstart Using P-Channel FET

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wuchy143

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Hi,

I'm using a simple FET hardware soft-start for my current 5V design. The Fet needs to supply ~200mA to the +5V rail which will power up my entire PCB. Which essentially means that there will be 200mA flowing across the FET.

I proto'd up my system and when I take my voltmeter and measure across the source and drain of the FET I read 0V across the FET. Now I doubt it's really 0V probably in the mV range.

From what I'm seeing it appears that I'm drawing negligible power across the FET and should not provide a heatsink. Even now when I have it powered up it isn't hot at all.

Theta(ja) for this part is = 42 degrees C per watt.

Is this a correct approach to ensuring I do not need a heatsink?

attached is my schematic(simplified)

Thanks
 
Yes, that part has a maximum "ON" resistance of 0.1 ohm so the power dissipated at 200mA is only 4mW, thus you don't need any heatsink.
 
But you can't turn on a P FET unles the gate is more negitive than the source. I think you might do better with a transistor. The FET will still turn on pretty quick once you reach the treshold voltage. See what you think of this one.
 
But you can't turn on a P FET unles the gate is more negitive than the source. I think you might do better with a transistor. The FET will still turn on pretty quick once you reach the treshold voltage. See what you think of this one.
But the gate is more negative than the source in the OPs circuit. Grounding the gate provides a negative gate-to-source (positive source-to-gate) voltage which turns the P FET on.

P.S. A P FET is a transistor as is a BJT.;)
 
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Thanks for the help guys.

But you can't turn on a P FET unles the gate is more negitive than the source. I think you might do better with a transistor. The FET will still turn on pretty quick once you reach the treshold voltage. See what you think of this one.

Could you explain why the BJT would be a better solution? I don't mean to criticize. I just don't understand why it would be a better solution from the PMOS. Aren't BJT's power hogs? So for my application it would dissipate more energy?
 
Could you explain why the BJT would be a better solution? I don't mean to criticize. I just don't understand why it would be a better solution from the PMOS. Aren't BJT's power hogs? So for my application it would dissipate more energy?
BJT's aren't power hogs but they do, in general, dissipate more heat as switches. First, their minimum saturated "ON" voltage is typically a couple tenths of a volt or more, and second, they typically require a base current at least 1/10th of the collector current to be fully saturated, which is usually wasted current.
 
wuchy143,

You would be better to connect the capacitor between the output and the gate. That will give you a slower, more controlled start-up, and the rate of rise of voltage will be less dependent on the value of the load and the exact characteristics of the FET.

What I have suggested works even better where the power supply is established and the FET is being used to start the circuit. That way the capacitor is charged one way when the circuit is off, and is eventually charged the other way when the circuit is on. There is a slow ramp in voltage. If the capacitor is discharged to start with, the output voltage will quickly rise to (supply voltage - FET threshold voltage)
 
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