P-Channel MOSFET - soldering temperature

Using Arduino General Electronics
1

Hi,
I am using circuit in the picture as high-side switch for my 5V bus. I have been successfully using it with BS250 MOSFET. Now I need capability to carry more current so I went with IRLML6401. Unfortunely I am unable to make it work. Drain to GND voltage drops from 5V to about 4.2V when gate voltage is going high.

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I tried to solder it 6 times (every time new MOSFET) but it consistently doesnt work. Now SOT-23 is fairly tiny and I am tring to solder it with iron, as I do not have reflow gear. Could it be that I damage the MOSFET in the process? I was lowering the temprature of the iron and went as low as 250*C.

Am I damageing MOSFETs or is there something else to take into considaration?

2

Probably the long term heat is killing it. Purchase some solder paste, it can be gotten for a buck or so. Clean the pads then with a hot iron, I use 700F, place a new part on the pads and touch with the iron, the solder should flow in a second or two. While heading with the iron use your tweezers to hold it in place with a gentle downward pressure pinning it to the board. I am assuming you have at least a 40 watt temperature controlled iron with a small tip. This should work for both FETs.

3

In your schematic you have got the source and drain of Q2 transposed.
That body diode is going to conduct.

If the schematic is what you actually have, then that is the problem.

This is what the schematic should be like:
Revised Schematic

3 Likes
4

  • What is your load ?

  • Under load, what is the Vds values for each MOSFET .

5

As Gil wrote, it's the time you dwell on the pin is the problem. Any pin soldering should take less than a second.

1 Like
6

Hi,
Can you please post some images of your project?
So we can see your components, we need to identify the package of the P-CH.

Thanks.. Tom.. :smiley: :+1: :coffee: :australia:

7
8

Thank you for all your replies. I am yet to go thru soldering tutorials.

You are right. It is backward on the drawing, got it right in my application.

Currently OLED display for testing, eventually when I get it to work I would like to be able to charge mobile devices from 5V bus.

When the switch is ON:
p-cha Vds = -0.03V
n-ch Vds = 0.00V

When the switch is supoused to be OFF:
p-cha Vds = -0.70V
n-ch Vds = 5.00V

Will upload if the necessary but well its a mess atm. Here is circuit in wider context, I am using ready made modules. Idea is to cut 5VDC bus off when MCU goes to deep sleep.

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9
  • Says you have some serious problem, wiring or component failure.
  • Move the top of R1 to the left of R3.
10

Could this be due to me overheating during soldering FET?

11
  • Show us your actual component connections.
12

Like so?

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Do you mean board layout?

13
  • Also,

IMG_3046
IMG_30461713×1295 373 KB

14

You have the gate of the 2N7000 connected to the LV connection on the Logic converter, surely it's a mistake

1 Like
15

IMG_3047
IMG_30471408×721 166 KB

16

Photo of the board will do?
The button was wired incorrectly, had to cut legs to make it work

Well... the idea was to supply logic level converter with the same pin that turns the 5VDC bus on and off.

Not sure if I can make that change, but will see.

17
  • Yes, make sure we can follow the wiring and PCB traces.
  • Then in version 2
18

PCB is black, so not sure traces will be visible on the photo. I do not have PCB with me atm so will upload later. Thank you.

19

Never a good idea to use an I/O pin as a power source. There is probably something in the logic converter that is preventing the gate from going to zero

20
  • Not following your decision as this is a typical logic level converter.

IMG_3048
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