Electronics > Projects, Designs, and Technical Stuff
Mains switching research break out
BrianHG:
--- Quote from: oschonrock on August 03, 2020, 01:08:41 am ---Unfortunately I don't understand your comment on gate transformers, it sounded like it could be interesting. Any sketch/link/part number?
--- End quote ---
You know, any low voltage, IE 15 vin 15v out transformer. Put AC 15v in and get AC 15v out, galvanically isolated.
Some people find or have custom transformers made. Some use telecom transformers or network transformers. It might make you like easier just to use an isolated DC-DC converter, 5v in, 15v out.
You can have one designed with 1 primary and 2 secondary, to be used in conjunction to power a true high speed optocoupled mosfet gate driver which drives the gate high and low, like a logic gate output. Such a device is designed for high speed switching, but you would also need 2 of them for each gate.
This is what a 2 channel HV isolation driver looks like. Though, you would be wiring the output a little differently and the input side would need inverted inputs.
https://www.mouser.com/ProductDetail/Texas-Instruments/UCC21220ADR?qs=qSfuJ%252Bfl%2Fd5KgCu6IE%2FAWg%3D%3D
With 40ns delay and 5ns switching, you may now officially PWM cleanly into the 100KHz region, maybe 1MHz.
But, the HV side needs a continuous clean +/-15v isolated DC-DC power supply to create the +/- gate voltages centered from the mosfet's central 'source'. Something like this:
https://www.mouser.com/ProductDetail/Cincon/EC1SA05N?qs=%2Fha2pyFaduhjhDCN%2Fb9F8eSzvtBCYVIMWKpaSLbxmGc%3D
Otherwise you would need 2 of your power supplies, one for each gate.
Now, I need not say that you need to know what you are doing at every junction and how things function at high AC mains voltages, otherwise, such a project could end in electrical shock or fire.
oschonrock:
--- Quote from: Ian.M on August 03, 2020, 01:43:57 am ---Errrr Guys . . . . . . .
Meanwell IRM-02-12S is a 12V power supply, not a bridge rectifier.
Now we've cleared up that confusion, there are three things I don't like about it.
--- End quote ---
Oh, is that where the rectifier came from. Thanks for clearing that up, and sorry to cause confusing. Note to self: Give more verbose description of circuit.
--- Quote from: Ian.M on August 03, 2020, 01:43:57 am ---
1. No suppression of back-EMF spikes from inductive loads. Unless you use avalanche rated MOSFETs you *NEED* a beefy bidirectional TVS diode across the pair of MOSFETs. It may also benefit from a RC snubber to control dV/dt below the TVS breakover voltage.
2. Heavy load on the opto + limited CTRR at elevated temperature makes it too easy to get insufficient gate drive if your drive to the opto LED is weak. Unless you can guarantee sharp 10mA drive pulses, it would be a good idea to run the opto with a higher load resistor and add a Schmidt trigger input gate driver between it and the MOSFETs.
3. That simple ZC opto circuit is going to give a rather mushy ZC pulse. You may wish to consider using one of the many snap-action active ZC detector circuits, possibly separate ones for rising and falling zero crossing Their optoisolator phototransistors can always be paralleled if you don't want to discriminate between the crossing directions.
Adding load current monitoring could also be rather useful. It also opens up the possibility of protecting the MOSFETs by detecting over-current and rapidly driving the gates off, then locking out the drive till the next ZC pulse. Some additional inductance in series with the load may be required to give the over-current trip time to react.
--- End quote ---
These are all super useful. Thanks and I agree. I don't quite understand point number 3. Will look into that. Or do you have a useful link?
Do you have experience with those photovoltaics? like that VOM1271 in above post? Looks almost too good to be true? How can that have enough energy to switch properly? Even with the 12V SMPS in there you make some valid point about snappy efficient switching...so these photowhatsits...??
BrianHG:
--- Quote from: oschonrock on August 03, 2020, 01:44:37 am ---
--- Quote from: BrianHG on August 03, 2020, 12:34:05 am ---photovoltaic optocouplers, each one
--- End quote ---
Actually these devices are new to me. You mean this yes?
https://www.vishay.com/docs/83469/vom1271.pdf
Not cheap, and not sooo available (yet?), but they look fab! I can ditch the whole annoying isolated 12V SMPS! Not sure of the limitations yet, ie I suspect that they top out at a certain size MOSFET? Too much energy required to switch efficiently - gate capacitance?
Thank you. That might really move it forward.
--- End quote ---
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&autoRedirect=true&minQty=2000
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.
oschonrock:
Yeah, the gate transformers what I thought, and had seen that before, don't fancy it.
But this:
--- Quote from: BrianHG on August 03, 2020, 01:45:31 am ---This is what a 2 channel HV isolation driver looks like. Though, you would be wiring the output a little differently and the input side would need inverted inputs.
https://www.mouser.com/ProductDetail/Texas-Instruments/UCC21220ADR?qs=qSfuJ%252Bfl%2Fd5KgCu6IE%2FAWg%3D%3D
With 40ns delay and 5ns switching, you may now officially PWM cleanly into the 100KHz region, maybe 1MHz.
But, the HV side needs a continuous clean +/-15v isolated DC-DC power supply to create the +/- gate voltages centered from the mosfet's central 'source'. Something like this:
https://www.mouser.com/ProductDetail/Cincon/EC1SA05N?qs=%2Fha2pyFaduhjhDCN%2Fb9F8eSzvtBCYVIMWKpaSLbxmGc%3D
--- End quote ---
That looks like a "proper" solution. These are the sorts of drivers that will give this much more flexibility in terms of switching strategies.
That little power supply is quite fine. It would be annoying to gave to use it in addition to the 230VAC SMPS I have there already, but if I "require" 5V on the "safe right-hand side" of the schematics and use that - by relying on the 1000V insolation which that little block provides...then that's a gain. ..?
BrianHG:
--- Quote from: oschonrock on August 03, 2020, 02:04:40 am ---Yeah, the gate transformers what I thought, and had seen that before, don't fancy it.
But this:
--- Quote from: BrianHG on August 03, 2020, 01:45:31 am ---This is what a 2 channel HV isolation driver looks like. Though, you would be wiring the output a little differently and the input side would need inverted inputs.
https://www.mouser.com/ProductDetail/Texas-Instruments/UCC21220ADR?qs=qSfuJ%252Bfl%2Fd5KgCu6IE%2FAWg%3D%3D
With 40ns delay and 5ns switching, you may now officially PWM cleanly into the 100KHz region, maybe 1MHz.
But, the HV side needs a continuous clean +/-15v isolated DC-DC power supply to create the +/- gate voltages centered from the mosfet's central 'source'. Something like this:
https://www.mouser.com/ProductDetail/Cincon/EC1SA05N?qs=%2Fha2pyFaduhjhDCN%2Fb9F8eSzvtBCYVIMWKpaSLbxmGc%3D
--- End quote ---
That looks like a "proper" solution. These are the sorts of drivers that will give this much more flexibility in terms of switching strategies.
That little power supply is quite fine. It would be annoying to gave to use it in addition to the 230VAC SMPS I have there already, but if I "require" 5V on the "safe right-hand side" of the schematics and use that - by relying on the 1000V insolation which that little block provides...then that's a gain. ..?
--- End quote ---
You can find 240v side supplies which have a center-tap +/-15v output.
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. Since you arent 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.
The TI chip is only rated 3KVrms for 1 minute only between logic side and the isolated sides.
That little supply is rated 1kv minimum continuous. I think that the TI chip is also only 1kv continuous.
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