Circuit idea for high side MOSFET driving - Page 1
Forum section: electronics / projects

author: hkBattousai
author_rank: Regular Contributor
timestamp: August 03, 2014, 11:24:13 am
content: There is a transformer in my circuit which is supposed to be connected to the AC grid when needed. I don't want to leave it connected all the time in order to prevent wasting energy.I am planning to use two MOSFETs to switch the transformer. I came to a conclusion that I need two separate isolated DC supplies to control these MOSFETs. I decided to generate clock pulse by 555 and use capacitive coupling to isolate DC component of the clock pulses, then rectify it to obtain the gate driving DC supply as seen in the schematics below.Would this circuit work? Would the AC grid harm the low voltage side?Do I need C4 and C7?Note 1: The transformer will be switched once in a while at most. Therefore the switching losses can be neglected. (Isn't it?)Note 2: I know I could do this by using MOSFET driver ICs, but they are usually more expensive.

author: nuno
author_rank: Frequent Contributor
timestamp: August 03, 2014, 11:30:37 am
content: Quote from: hkBattousai on August 03, 2014, 11:24:13 amHow about a relay then?

author: SeanB
author_rank: Super Contributor
timestamp: August 03, 2014, 11:32:51 am
content: Simpler and easier would be to use an optically isolated triac to do the switching. Simpler, cheaper and if the load is low you can just use the triac inside the opto itself to drive the load. Otherwise you can use a low current optotriac module which both takes up a lot less board space, has one part to install and will often include zero cross switching on board for free.

author: hkBattousai
author_rank: Regular Contributor
timestamp: August 03, 2014, 11:37:51 am
content: Using relay or optotriac are indeed simpler solutions. I thought this circuit in my mind and I couldn't anticipate it would be this complex. Maybe I should stick with a relay or opto-isolator as it is usually done.Anyway, I just wonder would this circuit work or not. I would like to know if I correctly designed it.

author: mij59
author_rank: Frequent Contributor
timestamp: August 03, 2014, 11:43:12 am
content: Cheap, simple and SAFE.http://i.ebayimg.com/00/s/NzkwWDc5MA==/z/-s4AAMXQyY1TVUDn/$_12.JPG

author: wraper
author_rank: Unknown
timestamp: August 03, 2014, 01:29:28 pm
content: Strange and pretty expensive schematic considering all those high voltage capacitors and very dangerous. What I can see briefly trying to understand how this would work are instantly destroyed gates of the both mosfets. As well as 555 destroyed and all circuit under the mains voltage. It is almost the same as applying mains voltage between gate and source through diode and 1 uf capacitor in series.

author: wraper
author_rank: Unknown
timestamp: August 03, 2014, 01:35:00 pm
content: Also there are schottky diode simbols on your schematic. They rarely exist rated above 100V.

author: mij59
author_rank: Frequent Contributor
timestamp: August 03, 2014, 01:41:14 pm
content: Quote from: wraper on August 03, 2014, 01:29:28 pmMaybe its should look something like this.http://www.discovercircuits.com/PDF-FILES/FETSWITCH.pdf

author: hkBattousai
author_rank: Regular Contributor
timestamp: August 03, 2014, 01:49:45 pm
content: Quote from: mij59 on August 03, 2014, 01:41:14 pmThis was exactly what I was trying to do. It should had been like this.Anyway, I will use optotriac in my circuit.

author: wraper
author_rank: Unknown
timestamp: August 03, 2014, 01:52:02 pm
content: Quote from: mij59 on August 03, 2014, 01:41:14 pmHard to compare them as they are completely different.OP's schematic will work like this picture for half wave and the same for another mosfet on second half wave = BOOM.

author: mij59
author_rank: Frequent Contributor
timestamp: August 03, 2014, 02:04:32 pm
content: Quote from: wraper on August 03, 2014, 01:52:02 pmSome designs are not hindered by any knowledge.

author: wraper
author_rank: Unknown
timestamp: August 03, 2014, 02:24:59 pm
content: Quote from: hkBattousai on August 03, 2014, 01:49:45 pmneed to notice that there are pitfalls when driving inductive load with such circuit.

author: rob77
author_rank: Super Contributor
timestamp: August 03, 2014, 02:36:58 pm
content: Quote from: mij59 on August 03, 2014, 01:41:14 pmi think so.i don't get why in the OP's schematic the 2 mosfets are "230V apart" from each other...wiring the neutral to the transformer permanently and having the mosfet switch (in fact solid state relay) at the hot wire only would be a better and simpler solution.btw... what's wrong with the old-school relays ?it's simple, isolated, safe, cheap... tons of reasons to use a simple relay

author: mij59
author_rank: Frequent Contributor
timestamp: August 03, 2014, 02:43:09 pm
content: Quote from: rob77 on August 03, 2014, 02:36:58 pmIn the schematic only the "switch" is drawn.