Power dissipation in PWM-dimmed LED? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: August 18, 2014, 09:49:01 am
content: An LED driver, ZXLD1360, have a dim pin, to be driven by PWM.To get full 1:100 dimming range, it should be driven at under 500Hz.As a result, the LED is essentially driven with a PWM waveform, with peak at it's full current's voltage, and zero in-between.Question is - how does that affect the power dissipation?Obvious part is that the heat dissipation will be based on LED's full power efficacy point, which is less than what it would be at the lower one.Less obvious is - how does the fact of PWM affect it?Let's say the LED have Vf of 15V at 1A current. Normally, it would dissipate 15W (ignoring the *0.75 factor for light output).If it was to be driven at 0.25A, it would have Vf of about 13V, and power dissipation of 3.25W.However, what values to use for driving it with a 0.25 duty cycle PWM?Is it RMS current, I=Ip*sqrt(D) = 0.5A?What about Vf?Assuming Vf of 1A, it would be 7.5W.Assuming Vf of 0.5A, it would be 6.75WAssuming RMS Vf, it would be 3.75WWhich one is it?Basically, how to estimate the power dissipation of an LED driven by PWM?

author: Cside
author_rank: Regular Contributor
timestamp: August 18, 2014, 09:57:41 am
content: why is the Vf dropping from 15V ro 13V ?also "Assuming Vf of 1A..." Vf (foreward voltage) is not a current.As a first pass the power should be the foreward voltage (supply voltage in your case?) x the LED current at given voltage x duty cycle.

author: Marco
author_rank: Super Contributor
timestamp: August 18, 2014, 10:20:11 am
content: You are mixing up a lot of stuff. As a first approximation power simply goes down linearly with duty ratio.In principle due to efficiency droop a greater percentage of the power will go to heat, but cooling design generally isn't precise enough to care about such small differences. Exactly how much is hard to say, efficiency droop is not given parameters in datasheets.

author: Artlav
author_rank: Frequent Contributor
timestamp: August 18, 2014, 10:37:34 am
content: Quote from: Cside on August 18, 2014, 09:57:41 amBecause Vf of an LED is current dependent?Quote from: Cside on August 18, 2014, 09:57:41 amMeaning "assuming Vf at current of 1A".Quote from: Marco on August 18, 2014, 10:20:11 amThe origin of the question is that some components produce waste heat by RMS current, so simply V*A*D would give quite a bit less than expected.The difference can be more than two times, which matters a lot when selecting a heatsink.

author: owiecc
author_rank: Frequent Contributor
timestamp: August 18, 2014, 10:42:44 am
content: Maybe the attached graph will help. Start with DC on the right. When you lower the duty cycle, the average power you dissipate in the LED lowers, the Tj also lowers. This causes the Vf to increase a little. You can even see heating and cooling of the die during PWM dimming. For your calculations of power start with DC and assume linear change with duty cycle. This will be good enough. Either way you need to size the heatsink for D=100%.For more you can check out mydissertation.

author: Artlav
author_rank: Frequent Contributor
timestamp: August 18, 2014, 04:58:15 pm
content: Some testing later, constant current seems to be better.I had two LEDs with the same build and driver running:-One with a 256Hz PWM, duty cycle of 20%, from 500mA current source.-Other with a constant current of 100mA.The PWM one appears noticeably dimmer, but it's heat sink is 2-3 degrees hotter (thermocouple at the base of the star).I tried to measure the brightness with a luxmeter, however it appears to have failed - the PWM one shows 15000 lux (at 1 cm away), the CC one - 80000 lux, which i find suspicious.Bottom line is - i'm getting more light for less heat with CC dimming than with PWM.And i'm curious how that works and how to estimate it.Quote from: owiecc on August 18, 2014, 10:42:44 amThat's quite interesting, thank you.

author: mikerj
author_rank: Super Contributor
timestamp: August 18, 2014, 07:52:55 pm
content: Quote from: owiecc on August 18, 2014, 10:42:44 amThe problem is that even though the average current is lower, the peak current remains the same, as does the peak Vf. This means you get higher total Pdiss in the LED compared with driving it from a constant DC current.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 19, 2014, 01:21:44 am
content: Quote from: Artlav on August 18, 2014, 04:58:15 pmThe things that break peak-current PWM are lower LED efficiency at high power, higher Vf, higher I2R losses, higher losses in the power supply (if using switchers) and likely a bunch of other parameters.You can have the best of both worlds by using a switching constant-current source - dimmable by changing the switching regulator's reference voltage to change the output current.

author: owiecc
author_rank: Frequent Contributor
timestamp: August 19, 2014, 09:41:25 am
content: Quote from: Artlav on August 18, 2014, 04:58:15 pmCC will always be better due to higher efficacy at lower current levels. You can compare the difference on the attached figure. Red: CC, grey: PWM. E.g. for that particular green diode you get twice the lumens at 0.2*Ipeak.Quote from: Artlav on August 18, 2014, 04:58:15 pmIt is a bit hard as you will need some radiometric data. It requires either some expensive HW or a tricky calorimetric setup. Maybe this would be nice for a student project: find some cheap, easy and practical way in which these things can be estimated