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Mains switching research break out

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oschonrock:

--- Quote from: BrianHG on August 03, 2020, 01:52:02 am ---
Those are great for ON-OFF switching and chopping, but useless for multi-KHz high speed PWM.  For high speed PWM throughout the sine wave, you need the item I listed in 1 post above.

They are only $1.67 each (for 10) here:  (~1$ or less in significant quantity)
https://www.ttiinc.com/content/ttiinc/en/apps/part-detail.html?mfrShortname=VOP&partsNumber=VOM1271T.&customerPartNumber=&minQty=10&customerId=
At that price, plus you don't need that 12v power supply, you save money by using 2 of these, 1 for each gate.  Remember, they guarantee a minimum of 7.8v for your gate voltage, so, choose your mosfet wisely, or run 2 in series.

Remember, with these optos, you want to drive the LED with 30-50ma each, that's 60-100ma total for both.  This means coming out of your UC, I would add a transistor or strong small mosfet (one of those cheap 1.2amp sot23 logic level mosfets) with 2 separate series resistors on the drains to drive each opto.

--- End quote ---

Yeah that makes total sense. 2 good options there.

1. photovoltaics  for chopping (lose the SMPS), or
2. +/-15V isolated supply powered from low side with a HF MOSFET driver (not much more expensive either) to add PWM capability as well

Both are better than what I have. Thank you.

oschonrock:

--- Quote from: BrianHG on August 03, 2020, 02:10:03 am ---
You can find 240v side supplies which have a center-tap +/-15v output.

--- End quote ---

Phew...Too many parts.... Link?   I agree that might be better. More robust and fewer bridges across the isolation.


--- Quote from: BrianHG on August 03, 2020, 02:10:03 am ---That TI chip drives the gate both high and low with a good 4amp surge making a true high frequency switching solution possible and 15v means the cheap bottom end mosfets will be completely turned on and off. 


--- End quote ---
Yeah this is what we want. Also means I can easily produce multiple versions of this thing with different MOSFET ratings. 100W -> 2.5kW @ 240VAC. That driver will laugh at all those.


--- Quote from: BrianHG on August 03, 2020, 02:10:03 am ---Since you aren't driving the output 2-n channels shorting a DC output high and low, you only need a series resistor of something like 20ohm between each TI chip output and each gate of each mosfet.

--- End quote ---

You mean I don't need shoot though prevention...? Or you mean high-side bootstrap?

Can ditch the optocoupler too...obviously

The uC now needs to provide 2 coordinated signals, but has total control for all switching / chopping / PWM  strategies?

Thank you. This has been very helpful.

oschonrock:

--- Quote from: oschonrock on August 03, 2020, 02:28:55 am ---
--- Quote from: BrianHG on August 03, 2020, 02:10:03 am ---
You can find 240v side supplies which have a center-tap +/-15v output.

--- End quote ---

Phew...Too many parts.... Link?   I agree that might be better. More robust and fewer bridges across the isolation.


--- End quote ---

This works. Bit overkill at 340mA and expensive @ £8-9 each.

https://uk.farnell.com/recom-power/rac10-15dk-277/power-supply-ac-dc-2-o-p-10w/dp/2822844

If anyone has better options let me know.


BrianHG:
According to your circuit, your making an AC PWM on/off switch, not a variable output DC PWM supply.

A variable DC PWM output supply has a topology where when you turn on the low side and high side mosfet simultaneously, you short the V+ rail to your GND.  You are not making this, so in the TI datasheet, the example circuit on page 1 with the added diodes and resistors to tame the turn on and turn off speed do not apply to your design as both mosfets are either on, or off at the same time.  All you need is a series resistor from each output to each gate.

I made a little mistake, you only need 1 cheaper single DC-DC 15v isolated converter and a single mosfet driver as your mosfet sources are connected together.  You only need to go from 0v to 15v from the 'source' on both.

In this case, here you go, faster & cheaper & 3.75kv isolation: (remember, best to have a series resistor on the output pin driving the gates as you shouldn't drive them with an absolute 4 amp 8ns fall time pulse.)
https://www.digikey.com/product-detail/en/silicon-labs/SI8261AAC-C-ISR/336-4490-1-ND/8540659

Plus here is the cheapest isolated 3kv DC-DC converter: (Digikey Datasheet typo, this guy is only 1.5kv rated.)
https://www.digikey.com/product-detail/en/cui-inc/PDSE1-S5-S15-S/102-6294-ND/10229829

I have better faith in this one rated at 3kv isolation: (This one is authentic 3kv isolation for only a few cents more)
https://www.digikey.com/product-detail/en/cui-inc/PEME1-S5-S15-S/102-6340-ND/10229875
(You may need a parallel 15.2-16v zener diode on the output of these DC-DC converter as they have a minimum load of 7ma, otherwise the voltage may creep above the spec 15v.)

BrianHG:
4 amp output version of the silicon labs gate driver IC:
https://www.digikey.com/product-detail/en/silicon-labs/SI8261BBC-C-ISR/336-5208-1-ND/9175803
Only ~13 cents more.

BTW, my above change means you only need 1 photovoltaic optoisolator as well, or 2 in series to get 15v output gate drive.

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