mosfets are hot when 7.2V is used instead of 5V

dknguyen said:

The pull-up resistor was never required to begin with if your comparator has a push-pull output. If your comparator has an open-drain output then the resistor should be required and it would be a little strange that it works without it.

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The comparator is an LM393 with an open collector output:

Thats why I used the 680 ohm resistor in the first place but apparently it made things worse when 7.2V was applied

mik3ca said:

Thats why I used the 680 ohm resistor in the first place but apparently it made things worse when 7.2V was applied

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That is pretty weird. Huh.

The LM393 has an output current that is guaranteed to be only 4mA. If the supply is 7.2V and you want the output to go as low as it can then 4mA in the 680 ohms is a voltage drop of 2.72V so the output low voltage will be 7.2V - 2.72V= 4.48V which will not turn off many IRF9530 Mosfets and will not turn on many IRF520 Mosfets. Use a resistor that is 7.2V/4mA= 1.8k then it will work if you replace the Mosfets with logic level Mosfets.

Just out of curiosity ... the schematic in post #1 has +V and 5V.... and yet you say you are changing the +V. Are you also changing the 5V or is it a regulated voltage. If the 5V is fixed at 5V then that is your problem. The PMOS transistor will never be able to turn off completely and I'm willing to bet that is the one that gets HOT.

The +5 to the rest of the circuitry is regulated. by the LM2940 low-dropout regulator. (I have to double check the part number but its something like that).

Ok, so based on things, I am currently on the right track with removing the 680 ohm resistor entirely because the 680 ohm in place with V+ being 7.2V did make at least one of the mosfets hot but removing that resistor caused the mosfets to not be so hot (more like maybe a notch warmer than room temperature).

So maybe that's it. maybe it was the 680 ohm resistor that forced a mosfet to always be on making the continuous output resistance at 2 ohms? (0.56 for rds on resistance? plus parallel impedance of both capacitors and inductors in series) and maybe 7/2=3.5A * 7= 21W
but with 5V... 5/2=2.5A * 5=12.5W ... hmm that can't quite be right?

I'm just trying to understand math because I made 680 ohm resistor as open circuit and earlier someone suggested using 1.8K resistor yet I determined through experimentation that 680 ohms is a resistor too low to use. Then again, I should probably increase the inductance so that output resistance increases at the same frequency. hmm...

The 680 Ohm resistor tied to 5V is causing your main problem..... and you are definitely getting confused in the math. I will try to elaborate later.

Did you see ronsimpson's comment in post #11? There is no point in worrying about the power dissipation of that resistor if the IRF9530 is not getting turned off. Or did I miss something?

With the 680 ohm resistor connecting the gate of the P-channel IRF9530 to 5V and the 7.2V on its source then it might never turn off causing both Mosfets to get hot. But the value of the 680 ohm pullup resistor is too low for the LM393 to make the N-Channel IRF520 to turn off which causes it and maybe the other Mosfet to get hot. The supply must be 10V or more for those ordinary Mosfets to work properly, the IRF520 should be a logic-level IRL520 (see the "L"?) and the P-channel Mosfet must also be a logic level one since your supply is too low for many ordinary Mosfets.

Instead, buy a few hundred ordinary Mosfets from different distributors and test them. You might find a couple that work with a supply that is only 5V to 7.2V.

Exactly what audioguru says. The datasheet for the PMOS says the gate threshold is 2V-4V ... the difference between 7.2V and 5V places it right there in that 2V to 4V range. This is the voltage in which the PMOS transistor begins to conduct, for saturation voltage you need at least 10V as mentioned above. I did not think about the LM393 not having enough oomph to overcome the 680 Ohm pul-up, but audioguru is exactly right, the NMOS and PMOS are more likely both ON at the same time causing a high shoot through current.