PWM Dimmer for LED with DC-DC Boost Converter - Page 1
Forum section: electronics / projects

author: lja246
author_rank: Contributor
timestamp: October 22, 2022, 05:41:30 am
content: Hi all,I'm planning to make a handheld LED torch using a 50W COB LED. The LED will need 30v so I am using a DC-DC boost converter to boost the battery voltage up. I want the LED to be dimmable so I was planning to set the boost converter to 30v and 1 amp limit and then use a mosfet switched with PWM to dim the LED. I was wondering if the constant on and off of the load from the PWM would cause the boost converter to have stability issues. If so what would you suggest I use to dim the LED using a digital interface, e.g not a simple Potentiometer or trim pot.BTW I am only running the 50W LED at 30W to increase its life and the brightness difference between 30W and 50W isn't a massive difference anyway.

author: ajb
author_rank: Super Contributor
timestamp: October 24, 2022, 03:16:35 am
content: Since your LED driver should be operating in constant current mode, shunt dimming is more effective than series dimming. That keeps the current flowing continuously and a switching supply is efficient enough that a dead short in CC mode is no big deal. Look for this feature in the datasheet when choosing a driver IC, some can sense the external pwm control signal and will adapt the control mode during off times. More importantly, the driver will be designed to tolerate shunt dimming. Or, of course, you could buy a ready-made driver module that just has a pwm input pin. There are some decent options for those that are great for just getting something that works quickly

author: thm_w
author_rank: Super Contributor
timestamp: October 25, 2022, 01:54:16 am
content: Never heard of shunt dimming, personally I probably would not use that in this casehttps://training.ti.com/dimming-methods-pwm-dimmingOP is using some off the shelf boost module, normal dimming as they describe should be OK if voltage adjusted appropriately for LED Vf.The bigger concern is, are you really going to dissipate ~25W of power in handheld device.Do you plan to build a flashlight with a massive heatsink and a fan on it? Or is it only going to tolerate max power for a few seconds?I would add in some temperature monitoring, at the least a temperature cutoff switch to shut off the LED at 60-70C case temp or so.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 25, 2022, 05:17:25 am
content: Just adjust the current limit to adjust the brightness.

author: lja246
author_rank: Contributor
timestamp: October 25, 2022, 08:26:48 pm
content: Quote from: thm_w on October 25, 2022, 01:54:16 amIt will have a rather large heatsink on it that i've already tested and at 50W power for the LED for over an hour the heatsink only got to 40 degrees with the fan on 75% but yes I am planning to include temp sensors which can either increase the fan PWM or decrease the brightness of the LED. It is going to be handheld but mainly it will be mountable so the extra size for the heatsink isn't a large concern.Quote from: NiHaoMike on October 25, 2022, 05:17:25 amWhats the easiest way to adjust the current on essentially an off the shelf boost converter module using for example a stm32 where I can have a display and buttons to increase and decrease the brightness?

author: thm_w
author_rank: Super Contributor
timestamp: October 25, 2022, 09:20:37 pm
content: OK wasn't expecting you to already have a fan, thats good then.Adjusting the boost current is a bit more complicated IMO, depending on how the boost module is designed.If there is just a current sense resistor going to the boost IC, you'd have to add some series resistance then a tap point which you could maybe pull down to divide the current signal.

author: szan
author_rank: Regular Contributor
timestamp: October 26, 2022, 01:18:56 am
content: Quote from: lja246 on October 22, 2022, 05:41:30 amWhy not to limit current in boost converter since it is SMPS to drive LEDs at a few predefined currents levels simply by changing DC-DC converter output current limits?

author: thm_w
author_rank: Super Contributor
timestamp: October 26, 2022, 01:28:16 am
content: Quote from: szan on October 26, 2022, 01:18:56 amRead what I said above.Sometimes its easier to not mess with the supply and just PWM. Then you get hundreds of brightness steps.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 26, 2022, 05:09:18 am
content: Can you find out what controller chip is used? Most can be operated with variable current limit by injecting a signal into the current sense circuit.

author: lja246
author_rank: Contributor
timestamp: October 26, 2022, 05:23:05 am
content: Quote from: NiHaoMike on October 26, 2022, 05:09:18 amI've been doing tests using an off the shelf boost converter that uses the UC3843AN IC however i'm not completely set on which IC I want to use. I only need one that can supply the power needed to drive the LED from a minimum voltage of 19.6V. If someone had any suggestions on a better boost converter IC for my application or how to easily control the current/voltage output on the UC3843AN without a potentiometer it would be greatly appreciated.Or it might just be easier to use this boost converter and then a separate MOSFET with PWM.idk I'm open for suggestions.

author: Psi
author_rank: Super Contributor
timestamp: October 26, 2022, 07:02:52 am
content: If you run the brightness PWM under 1khz it shouldn't affect the DCDC at all.I wouldn't go under 120Hz or you will start to see visual artifacts when you move it quickly across something.

author: szan
author_rank: Regular Contributor
timestamp: October 26, 2022, 09:16:07 pm
content: Quote from: thm_w on October 26, 2022, 01:28:16 amMaybe it could be easier to boost voltage above expected Vf of those LEDs at highest needed current levels to be able get maximum power when needed, but also use step down in LEDs final stage to fine tune its current limit...To get rid of 100Hz flickering of my home 100W IR light bulbs used as additional warm red lights for winter days, probably I will use Siemens 230VAC to 3 phase 1kW inverter capable to easy control output frequency and power induction motorsFor the moment I've added 1N4007 diodes in series to limit power to 50W since only one half of 230VAC mains gives nice warm red light in a room and probably extends its lifespan, however now I have 50Hz red flickering

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 27, 2022, 04:07:50 am
content: Quote from: lja246 on October 26, 2022, 05:23:05 amTry connecting the COMP pin (pin 1) of the 3843 to a variable resistor going to ground and monitor the voltage while adjusting. If you're in luck, you'll be able to get a pretty good control over the LED current that way and then you can replace the variable resistor with a voltage clamp circuit.