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Part Number: TPS92512
- Direction : MCU(PWM Generate) -> PWM0 -> DIM0 -> TPS92512(PDIM) -> J10(Output to LED)
- Input to device from SMPS : 36V
- LED : 12(Serial) * 12(Parallel)
LED flickering upon PWM duty 60% ~ 0%
- Input to device from SMPS : 45V
- LED : 15(Serial) * 12(Parallel)
No LED flickering
Any component change can help [Case 1] problem?
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In reply to Irwin Nederbragt:
Yes correct. LED package's forward voltage is 3V. so Stack voltage would be 36V.
In reply to Jay Kim:
If Vout is 36V and Vin is 36V you cannot regulate current. Have you looked at the current waveform and switching node. If the output and input are near the same the MOSFET can be on or switching at a very low frequency.
In my previous posting, [Case 2] was
Input to device from SMPS : 45V and LED : 15(Serial)*12(Parallel)
so Vout and Vin are also near the same but is working well with no flickering.
Attached photo is current measure when LEDs are flickering.
Any component value change can help?
1. PWM 250Hz, 100% duty
2. PWM 250Hz, 90% duty
3. PWM 250Hz, 80% duty
Can you try a few things:
Either short out one of the series LEDs (all that are in parallel) to make the stack 11 LEDs instead of 12. Look at and compare the switching waveform of 11 and 12 LEDs. It is unstable with 12 LEDs but that can be for a few reasons such as hitting maximum duty cycle.
Or try raising the 36 volts up to 39 volts or so and see what difference there is in the switching and PWM waveforms.
If the LED stack is 36 volts and you are providing 36 volts this will not work. Also make sure the power source is stable.
If you think this is a stability issue you can try and increase the comp capacitor value but suspect that will not fix this.
The input voltage and output voltage cannot be the same on a buck converter. Either the input voltage needs to be higher or the output strings needs to have a lower voltage. If you run the 12 series LED from 45 volts it probably will work correct.
As for measuring flicker, the buck converter has to be operating correct first. It looks like it's oscillating but that can be from having the input and output voltage near the same. Do some testing with higher input voltage, 45V input for 12 series load and see if it operates correct, then you know the circuit is working.
Does it appear to start working better after it is on for a while (heating up)?
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