I am using a P-channel MOSFET (IRF9640N) rated at 200V and 11A. I am using it for high side switching of a 100 ohm load. The Gate-Source has been shorted to check the current flow through the load resistor during off state. At lower voltages (<40V) the MOSFET works fine and the voltage drop across the load resistor is 0 V. However, increasing the supply voltage further leads to some offset, and this voltage across the load is equal to (Applied Voltage-40) volts. I have tested other MOSFETS to check if there is any faulty component but to no avail. I even tried with an NMOS at low side (IRF540N) to see the effects and have similar trends (dc offset at off state) although the effect is observable on above 50V.

 

The voltage is exceeding the maximum VGS specification.

 

The voltage is exceeding the maximum VGS specification.

No, the gate and source are shorted so Vgs=0V. The 40V is actually supply voltage at the source of PMOS.

 

Hi pg,
Do you have a circuit to post, it would help.?
E

 

No, the gate and source are shorted so Vgs=0V. The 40V is actually supply voltage at the source of PMOS.

Post the test circuit

 

Hi pg,
Do you have a circuit to post, it would help.?
E

It i

 

Hi pg,
With that circuit, the Vgs is 0V.
The PMOSFET will not conduct.
E

 

Hi pg,
With that circuit, the Vgs is 0V.
The PMOSFET will not conduct.
E

The problem is, for Supply voltage more than 40V, there is some leakage current, and hence some residual voltage across the load resistor, which should not be the case provided the mosfet is assumed to be perfectly off for this condition.

 

Hi pg,
The datasheet states the possible leakage current.
Could you explain what the PMOS circuit will be used on?
E

Update:
What is the value of the leakage current that you measure, and at what temperature?

 

Hi pg,
The datasheet states the possible leakage current.
Could you explain what the PMOS circuit will be used on?
E

Update:
What is the value of the leakage current that you measure, and at what temperature?


View attachment 324858

So the aim is to use the PMOS circuit for switching of a very low resistance solenoid load (order of 1 ohm, 350uH). The PMOS should provide voltage across this load for a very short period of time (100us-150us). During switching, I have observed that the MOSFET works absolutley fine during ON state, providing desired output voltage across the load. However, as mentioned, during off state of mosfet at high supply voltage is giving me the trouble with a leakage current. The leakage however is much more in the order of few mA.

 

hi pg,
As you may know, the d/s gives the typical parameters, so the leakage current could be higher for some devices.
Do you have other PMOS of the same type, you could try and compare leakage?
E

 

hi pg,
As you may know, the d/s gives the typical parameters, so the leakage current could be higher for some devices.
Do you have other PMOS of the same type, you could try and compare leakage?
E

As per my requirement, I need some high side switching device that could give tolerate 100V, so to be on a safer side, I want this mosfet that could tolerate 200V. I couldn't find any other device that fits the requirement.
Another question, what if a I apply a positive bias between gate and source for this PMOS?

 

Hi pg,
There is limited info on that +Vgs, for a PMOSFET, but this link explains some effects.
E
https://electronics.stackexchange.c...ens-if-you-apply-a-positive-vgs-to-a-p-mosfet

 

hi pg,
As you may know, the d/s gives the typical parameters, so the leakage current could be higher for some devices.
Do you have other PMOS of the same type, you could try and compare leakage?
E

I have tried MOSFET of same type, they have same behaviour.

 

So the aim is to use the PMOS circuit for switching of a very low resistance solenoid load (order of 1 ohm, 350uH). The PMOS should provide voltage across this load for a very short period of time (100us-150us). During switching, I have observed that the MOSFET works absolutley fine during ON state, providing desired output voltage across the load. However, as mentioned, during off state of mosfet at high supply voltage is giving me the trouble with a leakage current. The leakage however is much more in the order of few mA.

Do you have a scope picture of the waveform?
Do you have a diode across the solenoid?
How are you measuring "leakage current"?

We have a schematic of a 100 ohm load, but you are driving 350uH. (which acts much different than a resistor) Please post a schematic of the gate driver and mosfet and load.
Clearly there is something we cannot see.

 

Do you have a scope picture of the waveform?
Do you have a diode across the solenoid?
How are you measuring "leakage current"?

We have a schematic of a 100 ohm load, but you are driving 350uH. (which acts much different than a resistor) Please post a schematic of the gate driver and mosfet and load.
Clearly there is something we cannot see.

1. The solenoid is very sensitive, so don't want to risk it with under prepared circuit. That's why all the current results and discussion are based on a purely resistive dummy load of 100 ohms.
2. In the simulated circuit with same components is working absolutely fine and I have an anti-parallel diode attached.
3. Using the oscilloscope, I am measuring the off state voltage across the load (i.e. drain to ground) which is then used to calculate leakage current via ohms law.
4. Here is the attached waveforms a. Supply Voltage below 40V with perfectly fine on state (=V supply) and off state(=0 V) and b. Supply Voltage (above 40V [46V here]) with perfect on state characteristics yet dc off set of ≅ 6V during off state.
[Note:1 that the gate driver circuit for this has not been published here]
[Note:2 Green waveform is drain to ground (across load resistor of 100ohm) and yellow waveform is across gate to ground.]
5. For the sake of assurance, I have simply removed the PMOS gate driver and shorted the source and gate. This way the MOSFET should be "OFF" all the time and there should be no voltage across the 100 ohm load. However I am still getting the same residual offset voltage across the load.

 

Where did you purchase those MOSFET's from, if you don't mind me asking?

 

Where did you purchase those MOSFET's from, if you don't mind me asking?

ElectronicsComp.com

 

Is the transistor running hot. Like burn your fingers, make you yell hot?
If I take a 35V P-MOSFET and do that test I get the same results as you. (curve tracer test)

Nice scope. Where I work, most of the time, they only use that brand for a reason.

 

Is the transistor running hot. Like burn your fingers, make you yell hot?
If I take a 35V P-MOSFET and do that test I get the same results as you. (curve tracer test)

Nice scope. Where I work, most of the time, they only use that brand for a reason.

No, its normal. I then replaced it with an NMOS (IRF740 and IRF540) with relevant drain, source and gate connections, both of which successfully blocked the voltages (I went as far as 80V). I think there is problem with the batch of PMOS I am using.