Driving MOSFET on main voltage with an isolated converter - Page 1
Forum section: electronics / general

author: ManniW
author_rank: Contributor
timestamp: January 05, 2024, 08:53:28 pm
content: I have this circuit which is connected to 230V AC main voltage.I need some way to drive those 2 FETs. It will be switching at 100 Hz, so using a simple gate drive transformer does not seem to be an option at such a low frequency. My original idea was, to use a 1:1:1 coupled inductor and attach it to a PWM pin of my microcontroller and simply operate an unregulated flyback to provide those 2 isolated output voltages. I can then use those for a gate driver IC. However, such an unregulated flyback seems to be not a great idea, so I am wondering what the actual, proper way of achieving an isolated gate driver power supply is. It needs to be optimized for cost and size, so it should ideally be a small and cheap converter.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 05, 2024, 09:39:30 pm
content: Quote from: ManniW on January 05, 2024, 08:53:28 pmUse a gate driver chip with built in isolation.

author: ManniW
author_rank: Contributor
timestamp: January 05, 2024, 11:28:26 pm
content: Those chips require isolated voltage aswell, at least I haven't found one with an integrated dc-dc-converter. That basically brings me back to the original question, how do I create the isolated voltage for such a driver chip?

author: langwadt
author_rank: Super Contributor
timestamp: January 06, 2024, 02:31:50 am
content: what is the diodes for? why would you switch 230V AC at 100Hz?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 06, 2024, 05:19:36 am
content: There are drivers with built in power. They use the switching on the device. But I'm not sure if they would work at 100 Hz. But I would look into it. Or use a isolated power supply. There are chips for that. Fly back power supply. Very low power.

author: BrianHG
author_rank: Super Contributor
timestamp: January 06, 2024, 05:29:16 am
content: search for 'photovoltaic optoisolator'. They arent the fastest things, but no secondary supply needed as these mosfet gate driver optoisolatior use their internal IR Led to feed a photo solar cell creating 7-8v to drive the gate. No other parts needed.Note that this is how solid state AC/DC mosfet output relays are made, the manual way.

author: Faringdon
author_rank: Super Contributor
timestamp: January 06, 2024, 10:45:48 pm
content: You are switching at the mains zero cross aswell?....i am not sure if those photovoltaic drivers could be gauranteed to give you a switching at the zero cross.But yes, you seem to have it, an isolated 1W ACDC module with its secondary ground to the source of the fets...and just use that to switch the fets....though i see you have diodes in there...so you could just have a diode bridge on the mains , then have each fetacrossinstead of the two lower diodes of the bridge, and use a simple linear regulator to provide power for the gate driver.The attached LTspice and jpeg gives some ideas...you can use eg a digital isolator to switch the gate driver thats in the isolated supply...infineon also do some good isolated drive stuff...so to do silicon labs, and analog.com etc etcThe attached coudl be made simpler if using gan fets since they dont have the diode in them....as you can see , in the attached i had to turn the blessed fets upside down and then feed them with an isolated supply...but if they had been GaN's...then i coudl just have driven them straight off the post rectifier linear reg that you can see there....But yes, havign thought some more...what about just a cheap 1W isolated ac/dc module, and then a digital isolator to pass the signal to whatever you have in it, eg gate driver ic.Or do your own 1w flyback isolated with eg a Tinyswitch from power integrations.