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I’ve made a simple PWM dimmer for LED lights (50W) using IRLZ44N MOSFET and Arduino Uno. PWM frequency is 490 Hz. The Arduino is programmed in a way that dimming tries to mimic daylight using analogWrite(). The circuit looks like this:

enter image description here

It works ok and dimming is plausible to the eye. However, I've compared gate (blue) and drain (orange) pins on the oscilloscope and measured something like this:

enter image description here

It seems like it takes significant time for the voltage on the drain to rise.

My question is: How can I improve this circuit in order to get the better looking square wave voltage on the drain of the MOSFET?

Should I amplify the current from the PWM Arduino pin to charge the input MOSFET capacitor quicker or is there something else?

Thanks for consideration!

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    \$\begingroup\$ Have you tried anything? Like reduce the value of the 220 ohm resistor, maybe down to 100 or zero? \$\endgroup\$
    – Justme
    Jul 7, 2021 at 19:44
  • \$\begingroup\$ Is the rise time causing a problem? MOSFET running too hot? Unacceptable nonlinearity in brightness at low duty cycle? Your gate resistor and the gate capacitance form a RC network so you will always see some sort of exponential. \$\endgroup\$
    – vir
    Jul 7, 2021 at 20:11
  • \$\begingroup\$ Is the blue signal measured at the gate itself or at the Arduino output? \$\endgroup\$
    – John Birckhead
    Jul 7, 2021 at 20:41
  • \$\begingroup\$ the drain voltage should lower when the gate goes high, something is off with the graph. If you want to lower the voltage rise time on drain, you would want to pull the gate down harder, like a 2.2k or so, so the FET shuts off quicker. Do you have any capacitors? Some LED strip tape can act like a capacitor itself, which would round the edges as well. \$\endgroup\$
    – dandavis
    Jul 7, 2021 at 21:25
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    \$\begingroup\$ A very warm welcome to the site. Please can you edit your question and specify the PWM frequency (within the text, don't add in comments or under an 'Edit' banner at the end). Thanks and, again, welcome. \$\endgroup\$
    – TonyM
    Jul 7, 2021 at 22:00

1 Answer 1

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I see two possible solutions:

  1. Depending on your PWM frequency you might be seeing the capacitance from the LEDs. This might be normal. Try to removed the LEDs and replace with a resistor and measure the signal again. If it's a nice square wave then you have your answer.

  2. The MCU isn't charging/discharging the gate fast. Think of the gate to source junction as a cap. You have to pump charge into it to turn it on and remove charge to turn it off. It might be a good idea to look into a gate pump circuit or at least build a simple circuit with a small mosfet or bjt that controls the gate of your other big MOSFET. There is some capacitance between the gate and the source.

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