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I have the following circuit (see attached) where I am using a SI2302 MOSFET to switch an LED using a micro-controller (internal pull down.)

enter image description here

The problem I am seeing:

Upon switching between 3.3V (HIGH) and 0V (LOW) on the gate, the voltage measured on the drain pin switches between 0.6 and 0.7 volts, which makes the LED just slightly brighter/dimmer when switching, but it won't turn it on or off fully.

I would expect to see approximately 3.3 volts at the drain when the MCU output is low (MOSFET not conducting) and approximately 0 volts at the drain when the MCU output is high (MOSFET fully conducting).

I have read through some similar posts, but the issue there was that the MOSFET was probably broken or soldered the wrong way around. I have 6 of these circuits in parallel and all show the same issue.

UPDATE: Gate is driven by an STM32L051. Tried it with an external 10k pulldown resistor between gate and source. Same outcome. Tried it by disabling the STM32 and manually switching the Gate. Same outcome.

FINAL UPDATE: It was my bad, I soldered SI2303 instead of SI2302. Thank you all very much!

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  • \$\begingroup\$ Add an external pull down between gate and source and get back to us. \$\endgroup\$ Jun 22, 2021 at 14:52
  • \$\begingroup\$ Which device drives the gate and how? Is the software controlling the pin checked? \$\endgroup\$
    – Justme
    Jun 22, 2021 at 15:13
  • \$\begingroup\$ @Justme an STM32L051 microcontroller. The voltage on the gate seems reasonable, switching between 0V and ~3.4V. \$\endgroup\$
    – lazerlini
    Jun 22, 2021 at 15:16
  • \$\begingroup\$ @StainlessSteelRat done with a 10k resistor, no difference. \$\endgroup\$
    – lazerlini
    Jun 22, 2021 at 15:16
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    \$\begingroup\$ @StainlessSteelRat you are correct. I replaced one MOSFET with one from another batch (other manufacturer, older mouser shipment), now it works. Thank you very much! \$\endgroup\$
    – lazerlini
    Jun 22, 2021 at 15:50

2 Answers 2

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If you are seeing 0.6V across the MOSFET then it's highly likely that you are measuring the forward drop of the body diode. So, either

a) The MOSFET is misplaced (i.e. mirrored order -- D to S and vice-versa), or

b) That's a PMOS instead.

For (b), the body diode is reversed compared to an NMOS. So, even if you connect it as supposed, the body diode will always be forward-biased.

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  • \$\begingroup\$ thanks for the reply! I am certain that it is connected the right way around, the datasheet mouser.ch/datasheet/2/258/SI2302A(SOT-23)-V1-1626736.pdf also specifies N-Channel Mosfet. \$\endgroup\$
    – lazerlini
    Jun 22, 2021 at 15:07
  • \$\begingroup\$ @lazerlini Then you've swapped drain and source. \$\endgroup\$
    – Hearth
    Jun 22, 2021 at 15:08
  • \$\begingroup\$ @Hearth drain is connected to the LED, source is connected to ground \$\endgroup\$
    – lazerlini
    Jun 22, 2021 at 15:15
  • \$\begingroup\$ @lazerlini How do you know that? The datasheet doesn't appear to say what the pinout is. \$\endgroup\$
    – Hearth
    Jun 22, 2021 at 15:15
  • \$\begingroup\$ @Hearth on page 1 under "internal structure" it says 1. GATE 2. SOURCE 3. DRAIN, and in the top view to the right you can see the pins labeled with 1, 2 and 3 \$\endgroup\$
    – lazerlini
    Jun 22, 2021 at 15:17
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If you are seeing 0.6 volts at the drain with respect to 0 volts when the gate is high then you have a problem with either the MOSFET or how you have connected it to your circuit or your gate drive voltage is inadequate.

With a decent drive voltage on the gate, the MOSFET should be able to deliver an "on" resistance of easily below 0.1 ohms. Given that you have a 20 ohm resistor in series with the LED (that will limit the current to between 50 mA and 100 mA), you should expect to see a drain voltage of no-more than 10 mV.

Added to this is the problem of it only showing 0.7 volts when the gate is undriven and therefore, despite you saying this: -

I have read through some similar posts, but the issue there was that the MOSFET was probably broken or soldered the wrong way around.

I have to conclude that the MOSFET is broken, soldered the wrong way round or not adequately driven on the gate.

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