Electronics > Projects, Designs, and Technical Stuff
Mains switching research break out
BrianHG:
--- Quote from: oschonrock on August 03, 2020, 09:44:52 am ---Let me paraphrase for clarity. I initially understood your requirement for a +/-15V supply like this: Output stages of a driver like the UCC21220 are "functionally isolated from each other". ie they have VDDA/OUTA/VSSA and quite separately VDDB/OUTB/VSSB. in this case the sources are linked and therefore we would therefore join VSSB and VSSA together. I had originally thought: "Ah but we need to trigger one with +15V and one with -15V", but I am now realising (the same as you did?), that because they are both N-Channel devices and the gate we care about (ie the one which needs triggering, and not the one in body diode conduction) will always need a "positive trigger relative to the common sources".
--- End quote ---
No I was mistaken. Since both mosfet 'sources' are wired together, and they are both N-channel mosfets, the only valid gate voltage between both gates and sources is 0v for off, 15v for on. You only need one isolated single 15v supply & a single isolated gate driver hence my additional posts with the new silicon labs IC and single supplies, both DC-DC and 240v AC-DC. (My innitial comment was for an isolated gate drive each going their own way. Use for a completely different application)
See top left diagram on page 5, 'Bidirectional MOSFET Driver Application' of the Vishay VOM1271 Photodiode Output Optocouplers.
oschonrock:
--- Quote from: BrianHG on August 03, 2020, 01:37:50 pm ---See top left diagram on page 5, 'Bidirectional MOSFET Driver Application' of the Vishay VOM1271 Photodiode Output Optocouplers.
--- End quote ---
Yes. That is the exact topology I sketched in my original circuit isn't it?
I found it confusing to:
a) realise that the MOSFET will conduct "in reverse" if you just keep its gate positive with respect to source (a BJT would never do that obviously, and this also answers the MOSFET vs IGBT question for this application. IGBT would require external diode at minimum) - so yes, not body diode, but just "ON" for the one which is "in reverse" (and the other one obviously)
b) the way those circuits are physically drawn on the page screws with my head. ::) Something about the floating supply and having centre between the sources "grounded" with respect to that floating supply, if you understand what I mean...
So all sorted then. They will just both be "on" for the whole time while we give the driver a high (I just didn't realise that was possible and was therefore relying on the body diode in my thinking)
Last problem: I can't find a single channel isolated driver which I can actually buy. TI's page for example has just 6 parts if you select MOSFET, isolated and 1 channel.
https://www.ti.com/power-management/gate-drivers/products.html#o7=Isolated%20gate%20drivers&p1250=MOSFET&p480=1
and none of those are available in the UK, it appears.
I could just use that 2 channel half bridge one for the prototype. ...?
https://uk.farnell.com/texas-instruments/ucc21220d/dual-gate-driver-half-bridge-soic/dp/2843000
BrianHG:
These arent as fast, but they are cheaper and available and you can still do over 25KHz PWM with them. Plus they are optical coupling instead of capacitive. This means 500ns delay from in to out, but they still have a good +/- 2.5amp drive with a 35ns rise and fall output and they are less than half the price of the TI parts.
VOD3120AD
Farnel has stock.
oschonrock:
--- Quote from: BrianHG on August 03, 2020, 04:15:35 pm ---These arent as fast, but they are cheaper and available and you can still do over 25KHz PWM with them. Plus they are optical coupling instead of capacitive. This means 500ns delay from in to out, but they still have a good +/- 2.5amp drive with a 35ns rise and fall output and they are less than half the price of the TI parts.
VOD3120AD
Farnel has stock.
--- End quote ---
Nice one thanks. DIP....sigh. Nice for prototype though.
25kHz PWM is totally excessive. We are not "modulating DC with a sine wave", for which you need maybe 15kHz to do it nicely. But we are just "dimming" an already existing sine wave for which you need, maybe, generously, 100x fundamental ie 5kHz. That's switching every 1.8degrees of phase! More than enough. Only advantage in going higher would be to get above audio and 20kHz would be totally sufficient for that.
How are you working out what PWM switching frequency we can get out of a MOSFET given a specific driver? It's been 30yrs since I have done those calcs. Ultimately down to thermal power dissipation during switching time based on the available charge from the driver? Experience + dark art or something more quantitative?
BrianHG:
--- Quote from: oschonrock on August 03, 2020, 05:43:55 pm ---How are you working out what PWM switching frequency we can get out of a MOSFET given a specific driver? It's been 30yrs since I have done those calcs. Ultimately down to thermal power dissipation during switching time based on the available charge from the driver? Experience + dark art or something more quantitative?
--- End quote ---
There are others here far better suited to help you there. All my designs to date have over 10x headroom on switching the mosfets, so I mostly rely on simple on-resistance and current calculation.
That virshay driver can do 100KHz if you look at the specs, but my head-room practice made me quote you the 25KHz figure. If you ever made anything down at 15KHz, I would never buy it because my ears would go nuts from coil-whine. Many old TVs drove me nuts & I already had a scan-doubled 31Khz large screen TV back in the early 90's when they were in they were over 15K$ (ouch to my pocketbook, but I had peace and silence). Though I can no longer hear 25KHz, I remember as a kid I could hear all the ultrasonic alarms in the shopping malls and it drove me nuts as my parents couldn't hear a thing. I have no interest in affecting children's hearing, so 25Khz is the absolute bottom I would allow in any of my designs.
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