P channel Mosfet weird output

Thread Starter

MikeScott

Joined Oct 29, 2021
6
Hi readers. Using an N channel and a P channel Mosfet to control an underload from a TTL circuit, and getting a weird unexplainable result. Here is the circuit:
AUIRF4905SL P Chanel Mosfet circuit.jpg
Looks simple enough, but look at the gate vs the drain signals:
signal in signal out 180deg.JPG
As you can see the gate has a perfect square wave going in, but the drain ? Anything but a clean output. With or without the load. The supply feeding the source is clean, and there are no other components on the board to interfere, and I replaced the Mosfet to eliminate possibility of faulty component.
Any ideas ?
 

Papabravo

Joined Feb 24, 2006
17,580
It is the 20K resistor trying to discharge the gate capacitance of the P-channel FET and pull the gate back up to the source voltage. What do you suppose is the RC time constant might be.
 

Thread Starter

MikeScott

Joined Oct 29, 2021
6
It is the 20K resistor trying to discharge the gate capacitance of the P-channel FET and pull the gate back up to the source voltage. What do you suppose is the RC time constant might be.
Hi. Thanks for the response. If the problem caused by the 20K resistor why is it not visible on the signal at the gate? If you are correct what would you suggest?
 

Ian0

Joined Aug 7, 2020
4,119
Does the "high current load" include any decoupling capacitors? If so, you are watching them discharge.
 

Thread Starter

MikeScott

Joined Oct 29, 2021
6
Does the "high current load" include any decoupling capacitors? If so, you are watching them discharge.
I thought of that. Load is a coil (with a reverse diode). The problem exists if the load is connected or disconnected, which rules out the load as the culprit. There are no capacitors in the circuit, though a thought comes to mind as the circuit board is a prototyping pcb (parallel tracks) and I wonder if that is acting as a capacitor
 

Ian0

Joined Aug 7, 2020
4,119
That waveform would look about right for the situation with no load (there will be MOSFET drain-source capacitance which might be higher than the prototyping board)
So - asking the dim question - are you sure that the load is connected?
What happens if you put a resistor (1k or so) between drain and V- on the prototyping board next to the MOSFET?
 

Thread Starter

MikeScott

Joined Oct 29, 2021
6
That waveform would look about right for the situation with no load (there will be MOSFET drain-source capacitance which might be higher than the prototyping board)
So - asking the dim question - are you sure that the load is connected?
What happens if you put a resistor (1k or so) between drain and V- on the prototyping board next to the MOSFET?
I cut the surplus pcb track, no change with no load, but add the load and a major change, I get -50v spike ( I had to switch the scope probe to 10:1. here is the scope image of the FET drain (at 10:1) so the - spike is -50v and the + side is good
IMG_1442.JPG
I then disconnect the coil load and used a 1K resistor as you suggest and I get a perfect square wave out in the drain
 
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