Our Input voltage to the LED driver is 48V. It is a buck LED driver and has a maximum duty cycle so as you know we cannot drive led strings that are 48V or near to 48V.
With some rigs we drop 1V in the long cable from the 48V PSU to the LED driver so the input voltage to the LED driver is only really 47V.
The leds in each channel are 8 parallel strings of 12-in-series ….and they sit on a small bit of MCPCB as you’d expect. –there are three channels, so there’e three bits of this MCPCB with this 8x12 array on them. These three MCPCB pieces are stuck to a large, water cooled heatsink.
Why do you need a simulator, when you have good LEDs to use as a std load?
Yes but we need to test the leds that are 3.2V in Vf, and we also need to test the leds that are 3.9V in Vf (3.9v not 3.8v because we drive them at 440mA). Its going to be virtually impossible to be sure of getting led loads with either of those vf’s. The 3.9V vf led load is needed because it means the led driver is at max duty cycle, and is the worst case for stability…..in other words, if the array containing 3.9v vf leds is stable, then the array containing leds of lesser vf than that will also be stable (I’m speaking about the gain and phase margin of the led driver here)
We also need to test for subharmonic oscillation of the led driver, and we need to do this with minimum vf led array and max vf led array…the driver chip we are using has internal slope compensation but does not say what it is, and the semiconductor company wont tell us what it is.
SunnySkyGuy I believe what you yourself term “ESR” is what I term “Rd” (dynamic resistance)
Your question in regards to the simulator works well at a fixed temp, your design is far from a fixed temp., but a rough estimate could be made to make it work.
OK thanks, we appreciate the simulator is less ideal than the leds themselves, but far far better than a resistive load. We will use leds to test the driver but they will probably end up being 3.5-3.6v leds.
This range is only at a fixed temp 25C I believe, unlike Cree which uses a more useful value of 85'C for White LEDs.
….we think its disappointing that the datasheet doesn’t tell how the leds were mounted for their vf measurement, , it just says , as you say, that it was Ta=25c…..i mean, if they had them mounted on a small heatsink then the vf would be smaller than if they had mounted them on a large water cooled heatsink for the vf test, because the led junction temperature would be higher with water cooled….basically, as you describe, they don’t tell what was the junction temperature which is disappointing.
It means datasheet tests it with a ~1ms pulse so self-heating does not occur
…oops sorry, so you are saying that they did the vf test by just pulsing them at very low duty cycle with 350mA, so that the junction temperature was 25degc when they measured vf?
This, as you allude, sounds like a very non-useful way to do it.
The main reason it wont simulate it due to the Shockley effect of your heat sink and since you haven't measure the junction temp yet using Vf,
we measure junction temperature by putting thermocouple stuck to the MCPCB as close to the leds as possible, then take that as the case temperature, then use the quoted Tjc figure in the datasheet to get the junction temperature…..the method of using vf to measure junction temp we have heard of though it sounds rather time consuming to do
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It means datasheet tests it with a ~1ms pulse so self-heating does not occur
SunnySkyGuy, do you mean that junction temperature was 25degc for their forward voltage measurements?