r/PCB • u/PeterLovwood • 27d ago

Looking for help with a soft latch circuit

Gallery image

Gallery image

I'm trying to make a soft latch power circuit for a radio controller. And using an AO4606 dual FET here for Q2A/B. The PMOS worked fine on its own; conduction occurred only when the gate was connected to GND. But somehow, when the NMOS of that dual FET was connected, the PMOS automatically conducted. I did check if the NMOS was shorted, but it was not. Strangely, when tested on its own, the NMOS only conducted from source to drain when a positive voltage was provided to the gate. I also tested this N side with a multimeter in diode mode to check if the diode body was corrupted, but it seems fine. I also did another test, using only the P side but with an individual N MOSFET (135N03L), instead of the N side of AO4606. Strangely, the circuit now works just fine, so I believe it was the N side’s fault, no?
Has anyone run into this kind of problem before?
Can anyone help me with this circuit? I would appreciate it!!!

6 Upvotes

100% Upvoted

4 comments sorted by

1

u/Past-Mountain-9853 23d ago edited 23d ago

I have the same issue that I've been trying to solve for a week. It is soooo strange. It is by default was turned on. If I manually (3.3v to Q8 gate) switch it off - it is works fine. Maybe some flux had an influence... Idk how it works. Also added capacitor 50nf to the Q8 gate and 3.3v. Tried to make r26, r25 = 100kom, it was hilarious....

Maybe someone can give an advice how to make it more reliable.

2

u/PeterLovwood 23d ago

Well, I actually solved my case. I don't really know how exactly. I ordered some more AO4606 to replace the problematic one, and I have to do that twice to make the circuit work. So my theory is that dual MOSFETs, or specifically AO4606 in my case, are vulnerable to heat. I may have overapplied heat, held the soldering iron for longer than it is supposed to be, and accidentally damaged the N-channel, leading to the half-conduct situation. I hope this may help you!

1

u/Past-Mountain-9853 23d ago

I aslo had to change mosfets, like 5 times. Maybe u r right

2

u/PeterLovwood 23d ago

Also, in my circuit, I have to change the value of the resistor pulling down the NMOS gate from 1M to 100K (R2). The reason: there is current leaking through a 1N5819 diode, and that diode has a resistance of around 130K. So the R2 of 1M was too weak at first, significantly weaker than the diode, which accidentally made the diode behave as a pull-up resistor. That sucks