8i20 DC power supply companion board.

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16 Jan 2020 00:38 #154986 by blazini36
I've recently decommissioned an 8i20 I was using on my mill due to some issues and the fact that I was never really thrilled about the power supply solution I was using, it took up too much space, too many wires and start up current issues, I always meant to whip up a PCB to handle this a bit neater but never quite got around to it. I am a fan of the 8i20 so I'd like to get a more proper setup for when I recommision it. I'm just thinking it up at the moment, I haven't really dove in yet. Once done I'll slap the project on github.

Power components aren't my strong suit but assuming everything is specified for the max 2200w I'd like to get a couple suggestions on the following if anyone has any:
1) filter cap values and quantity
2) best way to handle initial inrush current of charging the caps on power on
3)on board brake resistors.

The brake resistors I'm not sure about. Trying to figure what a good minimum power resistor setup looks like and fit on a PCB. This would be heatsunk and have a thermostat or some other thermal device connected to an SSR to disconnect drive input power if the brake resistors were overheating.

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17 Jan 2020 16:49 #155066 by andypugh
I control the 8i20 power supply on my mill largely through HAL.

There is one SSRs in the PSU box, one mechanical relay and two power resistors.

One resistor is on the incoming mains supply to limit inrush current. This has an SSR that bypasses it once the bus voltage reaches a threshold.

The mechanical relay has a 240V coil and is wired such that the NC contacts discharge the capacitor when the power goes off through the second power resistor. This relay is _not_ rated for the current / voltage.

As I recall, I use a single 10,000uF smoothing capacitor rated at 450V and with a low ESR. These are very expensive but I found a bunch on eBay and snapped them up.

A HAL component monitors the bus voltage. It turns on the surge resistor bypass relay when the bus voltage hits 90% max. It also will not allow the PSU to turn _on_ unless the bus voltage is < 10V. This is so that the discharge relay only ever has to "make" the circuit to discharge and is never asked to "break".

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19 Jan 2020 00:27 #155122 by blazini36
I'm trying to avoid controlling the power supply with hal, other than an enable signal. Any particular reason for the 10,000uf capacitor? From what I know about drives this size I would assume say 4 x 470uf caps is sufficient, which is the setup I was using previously. I did have a few 4700uf 450v caps laying around that I tried, and besides being huge, the inrush was pretty serious. What value and power rating did you use for the inrush limiting resistor?

I got to playing with a schematic just so I can visualize the idea and came up with this. Missing some passives and probably a diode between the igbt and INA138 but I just wanted to get it on paper at the moment so I can get some advice. I'm not sure that I'll use any discharge circuit, waiting 5 minutes until the HV LED on the 8i20 goes out before playing with stuff hasn't really bothered me.

Thoughts.....or just plain wrong?

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19 Jan 2020 00:45 #155123 by andypugh

blazini36 wrote: I'm trying to avoid controlling the power supply with hal, other than an enable signal.


Why?
I built my first one self-contained, but then regretted it as I had no idea what was going on.
HAL is great for this sort of thing.

Any particular reason for the 10,000uf capacitor? From what I know about drives this size I would assume say 4 x 470uf caps is sufficient, which is the setup I was using previously.


I just checked, and the caps are 4700uF (RIFA PEH169UV447CQ ). I used them because I found a bunch of them cheap on eBay.

I do think that they might be over-sized. This web site can be useful: www.changpuak.ch/electronics/power_supply_design.php
as it graphs inrush and ripple.

> What value and power rating did you use for the inrush limiting resistor?

It's been a while, and I have forgotten. It was sized to take 10 seconds to reach full bus voltage (I have an RCD on the circuit and not tripping that was the main requirement). Then the power was calculated from that. And then reduced by a large factor to account for the veery low duty cycle.

waiting 5 minutes until the HV LED on the 8i20 goes out before playing with stuff hasn't really bothered me.


I have more capacitance, and why would the caps discharge at all with the drives disabled?

Important note: I am just sharing what I did. I have no qualifications to indicate that what I did was what should be done.

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19 Jan 2020 01:22 - 19 Jan 2020 01:33 #155126 by blazini36
"Why?"

Just a philosophy thing. While there is nothing wrong with having hal control something like this, if hardware logic is not controlling it then hal must control it. Which means a configuration issue becomes a safety issue. Besides that it requires more wiring from the controller to the Power supply/drive area. Though I'm not exactly an electronics wizard myself I hope to come up with something small-ish and once the board is designed I slap it together and don't think about it much.

"I have more capacitance, and why would the caps discharge at all with the drives disabled?"
Caps aren't batteries, they cannot hold a charge indefinately. Just the passive circuitry on the 8i20 does in fact fully discharge the motor power caps...albeit slowly. The higher capacitance of your setup will take much longer to discharge but it will in fact discharge just sitting with the motor leads connected to the 8i20 (without the relay). There is a small benefit to the fact that if the caps have not discharged fully there is no inrush to deal with anyway.

Ignore that first drawing not sure what I was thinking putting the transistor before the rectifier.

"Important note: I am just sharing what I did. I have no qualifications to indicate that what I did was what should be done."
well me neither lol. I design boards as a hobby and have been relatively successful. I myself am not "qualified" to design a HV PCB but I will seek advice and play with it until I feel it is safe to test. I also seek advice for stuff like this on electronics forums and if we are lucky enough, maybe the grand wizard PCW himself will grace us with some advice.

Maybe slightly more proper schematic:

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19 Jan 2020 01:30 #155127 by andypugh
You could use a Zener rather than a current shunt? I have no idea whether that is better or worse as an idea.

Both have the problem than if the load current gets very high the surge resistor comes back on. But then that might be a good thing, as it suggests a fault condition.

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19 Jan 2020 02:09 #155128 by blazini36
I used the shunt as that is what the INA138 application notes refer to. It's pretty likely I won't be able to find a suitable shunt anyway so a resister-zener or something similar is more likely. I have to do a bit more research as to what's available for handling that voltage which is why I haven't bothered to try to calculate the passives to set the gain for the INA138.

I'm looking at replacing the INA138 and shunt with a diode and IGBT gate driver which might take a bit of guesswork out of it. I use a Fet driver on another PCB I make but that's for switching speed wheres this is more for isolation.

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20 Jan 2020 00:50 #155215 by blazini36
After educating myself just a bit this is where I'm going at the moment. Still not quite sure how to derive the voltage reference to operate the gate of the IGBT but I'm working on that.

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