# IRF640N and Attiny13 as a PWM module for 70W LED.

Hello!

I'm thinking about making a high frequency PWM (about 25kHz) dimmer for my LED strip 12V 70W.

Will it work? Do I need a heatsink?

From the IRF640N data sheet.

RDS(on)Static Drain-to-Source On-Resistance –––––– 0.15Ω VGS = 10V, ID = 11A2

The RDS(on) is only specified at 10V which means that a 5V (or3.3V) output of your mystery Arduino will not fully turn the MOSFET on.

Look for a logic level MOSFET (RDS(on) specified at 5V or less. IRL540 for instance.

Why 25khz?
Default PWM frequencies (~500hz or 1khz) are high enough for a LED strip, unless you want to do something weird.
A higher frequency results in higher switching losses (hotter fet), and maybe poor low level dimming.
The IRF640 is not suitable for low voltage (high Rds(on)) or 5volt logic (high Vgs(th)).
Use a logic level fet with low Rds(on).
Leo..

You’ll probably need a heatsink. 70W at 12V is almost 6 Amps.

The datasheet for the MOSFET might give you the power dissipation allowed without a heatsink. And you can calculate the power dissipation of the MOSFET as Rds(on) as Power (Watts) = Currnet2/R. Note that the MOSFET can end-up dissipating more power with PWM because it’s partially-on during the switching-time.

Or, you can just try it. If the MOSFET gets too hot to touch, it’s probably too hot and you need a heatsink. Or if the MOSFET dies, that’s a good clue too.

I need 25khz, because it’s a video light. The shutter is opened for 1/50 of a second and the matrix is not that fast inside my camera. It reads the information line by line. So, the lower frequncy I have the thicker dark lines I get in the frame. I’ve found out that about 12kHz is fine for the camera, but the dound it produces is unbearable. I played with a LED dimmer bought in a led shop. It is based on a 555 timer. I’ve changed a cap and got 35kHz, but the MOSFET turned off after 2 minutes getting so hot that I couldn’t touch it. It has an AOD518 as a MOSFET. I’ve tried to change it to IRF640N. But it could work only at 1kHz also getting very hot. I think it may be caused by the low voltage and current on the gate. The signal comes to the gate through an operational amplifier. And it has the same vcc with the 555 timer. When I rise the voltage it turns off. So, I’ve decided to make a PWM with my Attiny13 since I’ve already done this for a fan with a small 2n2222 transistor.

Tofer:
Hello!

I'm thinking about making a high frequency PWM (about 25kHz) dimmer for my LED strip 12V 70W.

Will it work? Do I need a heatsink?

No it won't work if you use a non logic-level MOSFET rated for 200V (and thus a very high on-resistance)

Choose a logic-level device with an on-resistance of below 0.03 ohms and you'll probably not need
a heatsink.

However for 25kHz rate a MOSFET gate driver chip is required, and they normally work at 12V, so
a non-logic-level MOSFET is OK in that scenario.

For a video light, a COB LED setup with constant current drivers makes more sense (to me) than a LED strip.
A PT1115 chip has no problems driving 20watt worth of COB with almost pure DC (no flicker at all).
And can be dimmed with DC (from and Arduino PWM pin with RC filter).
Leo..

For a video light, a COB LED setup with constant current drivers makes more sense (to me) than a LED strip.

I'd agree with that!

Switching-constant-current (technically controlled current) dimmers use PWM or something like PWM for control, but the output voltage & current is "smoothed" through an inductor.

Or, I assume it's not one-long-strip? You can simply turn-off segments of the strip to adjust the brightness and you wouldn't need a microcontroller.

Plus a 12V (or 5V) LED strip has power-wasting current-limiting resistors so it's not efficient for lighting (if that matters).

Or, a halogen lamp won't flicker but an AC dimmer isn't as easy to build and of course halogen lamps are not as efficient as LEDs so you'd need more wattage.