I am building an arduino controlled RGB LED driver by using WS2803 constant current LED driver, TLP250 MOSFET drivers and IRF540N MOSFETs. This is how it looks like:
The picture got scaled down so it is harder to see, R3, R7 and R11 are 1k resistors.
This circuit is driving 5m RGB LED strip (100 segments) and should consume max 2A/channel. So each MOSFET should need to handle 2A at 13V max. IRF540N is rated at 100V/33A. RDSon should be 44mOhm. Thus I thought there would be no need for a heatsink.
I obviously want to PWM this things (WS2803 PWMs at 2.5kHz) but let's focus on full ON state. The problem I have is that the MOSFETS are seriously overheating in full ON state (no switching going on). You can see the values I measured in full ON state on the picture.
TLP250 seems to drive the MOSFETs correctly (VGS=10.6V) but I do not understand why I get so high VDS (like 0.6V on red LEDs). Those MOSFETs should have RDSon 44mOhm so when 1.4A is flowing through it, it should create a voltage drop of less than 0.1V.
The things I tried:
- removed TLP250 and applied 13V straight on to gate - was thinking that the MOSFET are not fully open but it did not help at all, VDS was still at 0.6V
- removed LED strip and used a car bulb 12V/55W on red channel. There was 3.5A flowing, VDS was at 2V and rising as the MOSFET was heating up
So my questions are:
- why is VDS so high and why is MOSFET overheating?
- even with VDS at 0.6V and ID at 1.4A the power is 0.84W which I assume should be fine without a heatsink?
- would I be better off with a less powerful MOSFET, something like 20V/5A? Or use logic level MOSFETs and drive it directly from WS2803 (though I like the optical isolation of TLP250).
- I have this circuit only on a breadboard at the moment and the wires that connect MOSFET's source to GND get really hot too. I know that this is normal as there is a relatively high current flowing through them but I thought I just mention it
- I bought the MOSFETs in bulk from China, can it be that those are not really IRF540Ns and have quite lower specs?
EDIT: One more thing. I have created this controller based on the MOSFET driver from here. The guy is using separate power sources for TLP250 and for the load (Vsupply, VMOS). I used the same source for both. Not sure if that matters. And my power supply is 12V 10A regulated so I do not think that the power supply is the problem.