mosfetopto-isolatorphototransistor
I have a high power 12V rail that I would like to keep separate from the logic supply, and so would like to optocouple the N-MOSFET drive for an external high power LED light. Does this seem like a reasonable schematic?
The optocouple used is a Vishay VO618A (datasheet) (CTR 100% to 200%). The N-MOSFET's datasheet is here.
I have a few questions:
After receiving IRF540N from a reputable seller I can definitely confirm that the ones I was originally using are counterfeits.
After replacing fake one with a genuine one I got Vds=85mV on the red channel. What I was not expecting though is that the genuine FET got hot after a minute or so. And then I realized that those FETs are not generating much heat themselves but rather get heated up (and quite a lot) from the breadboard and the wires (Connor Wolf mentioned it). Short wires connecting FET's source to GND are screaming hot when this is in full ON state. Moving FETs off the breadboard confirmed that the source of the heat was the breadboard/wires. Fake one was getting hot but I could actually cool it down just by touching it. Genuine one was somewhere between the room temperature and luke warm. Btw. measuring Vds directly on FET pins vs measuring it 1cm away on the breadboard made around 200mV difference (85mV on pins, 300mV on breadboard).
Here are some pictures, fake on the left, genuine on the right and manufacturer's part marking on the bottom:
Although there are more IRF package markings possible as shown in this document I could not find any similar to the fake one (which only supports that this is a counterfeit). Also the cutouts on the top of the back plate are rectangular vs round on the genuine and in the spec.
Thank you guys for all your comments! The circuit now works as expected (PWM included).
Further down I'm suggesting something that should work in a lot of applications but maybe not yours (due the the high gate capacitance of the MOSFET). Because of that I'd consider using a DC-DC isolation module to provide isolated power to gate circuits at the MOSFET. Here's a circuit that is 90% indicative: -
B2 would be supplied by the DC-DC converter and U1 can be isolated as well although in this particular diagram they show both sides of the coupler grounded (directly below U1). DC converter like this would be OK: -
It can continuously supply 250mA and this can be used in conjunction with a reasonably sized electrolytic capacitor to provide the high current surge needed to charge the gate.
I'd also consider the simplicity of using a photovoltaic opto isolator such as the Vishay VOM1271.
It can switch on in 53 us into a 200pF load and produce a drive voltage of about 8V making it suitable for a lot of MOSFETs. Of course if the MOSFET gate capacitance is 2nF then it will take about 0.5 milli seconds to turn on.
Best Answer
It's reasonable. The maximum gate voltage on that particular MOSFET is 16V, so you have a bit of margin from the 12V supply but not necessarily a comfortable one. If transients are possible you should add a gate protection zener and series resistor. If the '12V' is automotive, this is absolutely essential. It will also allow you to substitute a different MOSFET in the future that may have a lower Vgs(max) rating. At least leave spots on the PCB.
Your turn on will be reasonably fast, but turn-off will be very slow (maybe more than 0.1ms), because the gate charge has to drain out through the 10K resistor and the opto is lethargic with high resistances (they spec it with 75 ohm loads which is most unrealistic).
My rule of thumb is to allow 2-3:1 margin (depending on temperature range and pedigree of the opto supplier) from the minimum guaranteed CTR at room temperature to account for temperature effects and CTR aging.
If you use the VO618A-3 version, the minimum CTR is 100%, so with 2.5:1, 3mA is enough, so the series resistor can be 1.2K. This will also reduce the aging- you have 17mA+ which is overkill. Or use 1K and drive it right from the micro.
You can read app notes and sharpen your pencil if you want to evaluate further at your particular temperature range etc.