Bench power supply output enable mosfet switch design

[AloyseTech]

Hi,

I would like to add a output enable/disable switch to my bench power supply. The output range is 0-15V 0-2A. The load switch will be located after the voltage sense to allow output setting/regulation even if external output is disabled. I need to be able to switch as low as 0V. A -5V 10mA rail is available if needed (for mosfet bias?). I need to control this switch using a 3.3V digital output.

Here is a partial test circuit online simulation : http://tinyurl.com/ydbx5v87

Any help is welcome

Thanks

[xavier60]

This might give you some ideas to try.

[Wolfgang]

Hi,

the idea to do this with a MOSFET is fine. Some things need to be considered, however:

- As your input voltage is possibly lower than a MOSFETs turn-on voltage, you need an external supply voltage for the switch anyway.
- What you want is a low voltage drop, so a MOSFET with a low RdsON.

- The simplest thing here is a fat N-Channel (less RdsON than P-Channel of equal size) MOSFET like an IRFZ44 in the positive supply rail. The gate voltage can be drawn from a 9V block battery with a switch.
  A Zener protection for the gate and a resistor can be used for protection. An IRFZ44 can handle several 10 Amps, so your 2A is no challenge and the voltage drop will be low.
- Battery lifetime is very long because the MOSFET gate draws no current.

[xavier60]

Although it should be common knowledge by now, I must mention that it is very important to place the Gate protection zenner and series resistor directly at the MOSFET's pins to lessen the chance of oscillation.
I have seen rather harmless looking layouts oscillate because of this precaution not being taken.

[xavier60]

Hi,

the idea to do this with a MOSFET is fine. Some things need to be considered, however:

- As your input voltage is possibly lower than a MOSFETs turn-on voltage, you need an external supply voltage for the switch anyway.
- What you want is a low voltage drop, so a MOSFET with a low RdsON.

- The simplest thing here is a fat N-Channel (less RdsON than P-Channel of equal size) MOSFET like an IRFZ44 in the positive supply rail. The gate voltage can be drawn from a 9V block battery with a switch.
  A Zener protection for the gate and a resistor can be used for protection. An IRFZ44 can handle several 10 Amps, so your 2A is no challenge and the voltage drop will be low.
- Battery lifetime is very long because the MOSFET gate draws no current.

A double throw switch could be used to avoid having bleed resistors which would put some drain on the battery.
But the op wants to control it with a 3.3v signal.

[AloyseTech]

I have a tracking pre-regulator at 4V above my output voltage that could be used for driving the gate I guess?

[Wolfgang]

Yes, it could be used. I was a bit sceptic because I wanted to provide really low RdsOn. Even a logic-level FET like the IRLZ44 (22mOhm) would love to have more Volts at the gate.
4Volts should suffice, if you are not too picky.

[001]

See some tricky circuit with BJTs and zeners  here https://www.eevblog.com/forum/projects/how-it-works-current-limiter-and-output-protector-in-pd-power-supplies/

[AloyseTech]

Here is what I have now, what do you guys think about this?
The PMPB20EN has a max RDSon of around 35mOhm at current from 0 to 5A with a VGS of 3V.

[ogden]

Two cents about using just single mosfet: you may think that power supply is disconnected from your circuit, yet connection through MOSFET body diode is still there. Could be trap if circuit have bulk capacitor(s) in the input - current may flow *into* "disconnected" supply if voltages does not match.

[edit] I am afraid that VOUT+3V is insufficient. You want mosfet to be "as open as possible" - so voltage drop on it is insignificant.

[Wolfgang]

Looks great, but does it have the 35mOhm also with only 3V at the gate ?
I would add some gate protection stuff as well.

[AloyseTech]

Two cents about using just single mosfet: you may think that power supply is disconnected from your circuit, yet connection through MOSFET body diode is still there. Could be trap if circuit have bulk capacitor(s) in the input - current may flow *into* "disconnected" supply if voltages does not match.

[edit] I am afraid that VOUT+3V is insufficient. You want mosfet to be "as open as possible" - so voltage drop on it is insignificant.

I'm aware of the body diode reverse connection. In my application it is more a "disable" than a "disconnect" function.

There is a 1R shunt as well before VOUT and VTRACK tracks the voltage before this shunt. So VTRACK = VOUT + 3V + 1R*IOUT.
Figure 8 on the PMP datasheet show that even with a VGS of 2.75V, the RDSon is always below 40mR for current below 2A (with Tj=25 degC).
The VGS range is +/-20V. So I don't need gate protection right?

[ogden]

I'm aware of the body diode reverse connection. In my application it is more a "disable" than a "disconnect" function.

Solution does not cost much: just identical 2nd mosfet.

[AloyseTech]

Adding the second mosfet like attached double the RDson (max would now be 80mR). This is quite higher than what I would like to have... 50mR is OK. I could use two BSC026N04LS, but their are more expensive and bigger.