Dimming an LED with a capacitor across it

Question

I want to dim my ceiling LED light. I disassembled it, and it turns out that this is a led strip with many leds in serial. Its power supply is a 40V 450mA one, and I have built a circuit that could dim it with PWM from a microcontroller. Unfortunately a very slow flickering can be seen when I decrease the duty cycle under a threshold, and above this threshold it is just lighting with the same brightness.

For me it seems that the LED has a capacitor that stops the PWM from being effective if the duty cycle is high enough, and if it is lower, it starts to discharge, and when it discharges under the forward voltage of this LED strip, it slowly charges up again until the LEDs light up again.

The circuit I am driving the LED with is:

I have tried this with other lamps and it worked well.

I try to drive it with an ESP using the WLED firmware. It runs with an 1 kHz frequency in theory.

I probed the voltage between the source and the drain because I have the lamp built in the ceiling, and I have the negative side of the lamp and the negative side of the power supply wired down to my desk, but I think it shows the symptoms well, just when the drain-source voltage is low, the lamp will be high.

And also my scope is a cheapish USB one but the better ones are expensive for me and my use cases, sorry if the pictures are bad.

When duty cicle is high:

When it is off (It oscilates a bit because it picks up the mains voltage, but it is not that much):

And here is when the weird flickering happens:

My question is: I have 11 of this lamp, and I want to dim them individually. How could I dim these in a way so that the components would not cost too much?

I don't want to modify the lamps, as I rent this place and I don't own them.

Thank you very much.

Edit:

The problem might be that my power supply is actually a current source. It is ambiguous for me if it is or not, but I uploaded it according to the comments.

Answer 1

The driver looks like a 450mA current source with a 40V max output voltage. The label isn't clear, but normally voltage sources would be specified with "40V", here the extra "max" hints that it's a constant current driver.

That's not compatible with the PWM you're using: when the MOSFET runs at duty cycle lower than 100%, average current drawn by the LEDs will be lower. But the driver will try to enforce its constant current setting and increase output voltage to compensate, which increases current in the LEDs while the MOSFET is on: the LEDs will run at a lower duty cycle, but with larger peak current, and the average current will be the same.

So the driver will try to do the opposite of what your FET is doing, and it won't work.

Unfortunately a very slow flickering can be seen when I decrease the duty cycle under a threshold, and above this threshold it is just lighting with the same brightness.

This sounds like the driver believes the LEDs are disconnected when your duty cycle is low, so it just goes to sleep for a while, which will result in the kind of blinking you're seeing. When your duty cycle is high enough the driver will be able to regulate current in spite of the MOSFET so indeed you will get constant brightness.

In other words, this sort of PWM dimming works fine if the power supply is a voltage source, and the LEDs have some form of current control, for example resistors as in the usual LED strips. But it won't work with a constant current supply.

Since your LED strips consist of a series string of LEDs without resistors, the simplest solution would be to replace the drivers with dimmable constant current ones. These drivers can usually be controlled by 0-10V analog or PWM on a separate input, and they will adjust output current accordingly. But you'd have to buy one driver per light.

Another solution would be to use a constant voltage supply and add a resistor to set the LED current, then PWM with a MOSFET. This allows wiring many lights to the same supply. This would even work with constant current supplies that just clip to their maximum output voltage when the load is too low (they behave as current-limited voltage sources in this mode), but it looks like yours go to sleep instead, so it won't work.

None of these solutions seem compatible with a rental apartment though...

If you want to build interesting LED lights, I'd recommend getting some high quality high CRI LED strips, some aluminium profiles, a 24V constant voltage supply (for example Meanwell), this can be successfully controlled with an ESP32 without trouble.

Answer 2

I simulated your circuit using best guesses for unknown quantities. I was unable to reproduce the behavior, and here is the result at 20% duty cycle:

Note the high capacitor charging current spikes. They are actually about 15A peak.

I also did a simulation with a 1A current source, limited to 68V with a zener. It takes a while to stabilize, but by 100 ms it looks like this:

I tried to duplicate the waveform seen when flickering occurs, and I was able to get something similar by using a triangular wave at 100 Hz, simulating a poorly filtered rectified signal. Adding a capacitor across the power supply roughly duplicates the rising voltage during OFF times.

You can also see that the LED current varies widely. Adding a large capacitor across the LEDs just causes excess current to be drawn. But a large capacitor (1000 uF) across the power supply really seems to help, although it results in much higher current pulses when the MOSFET is ON. So varying PWM does not much affect brightness.