The PC817C opto-isolators have fairly slow rise and fall times, but they operate in this circuit very much as the data sheet would suggest. Add in the MOSFET and the switching times are a little slower still.
By Julian
Youtuber, shed dweller, solar charge controller aficionado
Yeah one solution would definitely be to employ a logic level mosfet. This circuit usually makes it at least to 5.5 volts when it is on. It's just getting hold ov one. The local electronics shops – Jaycar – are a bit thin on the ground when it comes to mosfets. There's a PSMN1R2-30YL I would like to try and it's fully on at 4.5volts with a Rds ov only 1.3 milliohms. It's a SOT-669 package though, but I reckon I could solder it to a bit ov copper or something. But to get one might be tricky.
That's odd – the Vishay IRF540 datasheet says 77mΩ at Vgs=10v, but the IR IRF540 datasheet has 44mΩ for the same gate source voltage. You could try the IRL540 (I think L stands for Logic level). That has the same 77mΩ spec, but at Vgs=5v.
Interesting you mention the leads not handling the high current. Take a look at the IRF1404 – drain current 202 Amps! But it's a theoretical die current at maximum temperature. Note 6 says that package limitation current is 75A.
It's an IRF540. Supposed to handle 28A (but one wonders how the leads would!!). It has an Rds(on) ov 0.077R but that is at Gs ov 10 volts. I think that's where the trouble is starting, because I am giving it 6V and from a rough calculation from the current flowing in the circuit and the voltage across the drain & source (approx 2.5V) it appears to be running an effective resistance ov 0.25R, which ov course at 10A is 25 odd watts!! No wonder the bugger is getting hot ! 🙂
Yes, it would be a little under 12v.
What's your FET's Rds(on)? I changed the FET in my PWM5 controller from a 17mOhm IRFZ44N to an 8mOhm IRF3205 and it made a big difference to temperature at just 6A. There's no heatsink in my design. 6V on the gate may be enough.
A transformer can also limit the high frequency response too. How many volts are you hitting the gate ov the mosfet with? I assume it's somewhere in the order ov 12v minus the drop through the optocoupler? I am having issues with driving one at present. I am giving it 6 volts and it's on, but I still think it's in linear mode as it's getting kinda hot passing 10 amps.
Size, cost (the optos are 5p each), availability. Also optos can be used for ultra low frequency switching.
Why not use a transformer instead the the opto-isolators?