PMOS tube as a switch cannot be turned off

S3S4S5S6

PMOS tube as a switch cannot be turned off [Copy link]

 

The schematic diagram is as shown above. The base of transistor Q12, Zero_Ctrl, gives high and low levels to control the on and off of the PMOS tube. High level turns on, low level turns off. The actual circuit is that the LED behind the D pole of the PMOS tube is always on. I measured the output voltage of 4.38V, the base voltage of the transistor is zero, the collector voltage of the transistor is 4.98V, the G pole voltage of the MOS tube is 4.97V, and the S pole voltage is 4.97V. When VGS=0, shouldn't the tube be turned off? When I give the base of the transistor a high level, the PMOS tube is turned on and the output becomes 4.98V

Is this due to incorrect selection of PMOS tube materials or a problem with the circuit design?

LTspice simulates this circuit structure and finds that it can realize the switching function normally

maychang

[Is this due to incorrect selection of PMOS tube materials or a problem with the circuit design? ]

I suspect that the leakage current of your P-channel MOS tube is too large.

wenyangzeng

This circuit has a BUG. The lower end of R108 should be directly connected to the G pole of the PMOS tube to provide sufficient "downward" bias.

tagetage

Just remove R110.

S3S4S5S6

tagetage posted on 2024-9-11 20:45 Just remove R110. .

R110/C88 are removed, no use

tagetage

How can such a simple circuit go wrong? ? Without IO control, directly connect R110 to power or ground for manual testing.

dcexpert

Disconnect the circuit on the right side of R109 to see if it functions normally and whether other parts of the circuit are affected.

S3S4S5S6

dcexpert posted on 2024-9-11 21:29 Disconnect the circuit on the right side of R109 and see if the function is normal, and see if other parts of the circuit are affected.

The right circuit has not been connected yet.

S3S4S5S6

tagetage posted on 2024-9-11 21:24 How can such a simple circuit go wrong? ? Without IO control, directly connect R110 to power or ground for manual testing.

You can try this

S3S4S5S6

wenyangzeng posted on 2024-9-11 20:09 This circuit has a BUG. The lower end of R108 should be directly connected to the G pole of the PMOS tube to provide sufficient "downward" bias.

You can try this

S3S4S5S6

maychang posted on 2024-9-11 19:53 [Is this because the PMOS tube material is not selected correctly or there is a problem with the circuit design? ] I suspect that the leakage current of your P-channel MOS tube is too large.

Is this leakage current described in the general chip manual?

littleshrimp

Check if the MOS tube package is drawn wrong. D S is drawn upside down. The body diode continues to supply power to the next stage when the MOS is shorted.

S3S4S5S6

littleshrimp posted on 2024-9-11 21:54 Check if the MOS tube package is drawn wrong. D S is drawn upside down. The body diode continues to supply power to the next stage when the MOS is shorted.

There is no problem with the packaging of the MOS tube. I have checked it.

Nubility

Which of your two 5V is the source?

maychang

S3S4S5S6 posted on 2024-9-11 21:34 Is this leakage current explained in the general chip manual?

[Is this leakage current described in general chip manuals? ]

This is it:

maychang

S3S4S5S6 posted on 2024-9-11 21:31 The circuit on the right has not been connected yet

[The circuit on the right has not been connected yet]

Just connect a heavier load (smaller resistance, such as several thousand ohms) to the right side.

S3S4S5S6

Nubility posted on 2024-9-11 23:21 Which of your two 5V is the source

you guess

1350774640

R110 is too big

Pathan

Check if the package is done wrong, it seems that DS is reversed

Nubility

S3S4S5S6 posted on 2024-9-12 08:18 Guess

No need to guess, 5-0.6=?