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AC-DC 340v power supply board design help

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blazini36:

--- Quote from: WattsThat on January 19, 2020, 06:16:31 pm ---+1 on the common bus arrangement and the standard “pre-charge” circuit is an appropriately sized fusible power resistor and a mechanical contactor to short the resistor when the bus voltage reaches about 66% of the rated voltage. This is bog standard on three phase ac inverter drives. It’s only when you reach 50kw or so do they resort to replacing three of the six the diodes with phase controlled thyristors for the precharge.

--- End quote ---

Well it's only "common" if there's more than 1 of something using it. If I intended to use a mechanical contractor I really would not have bothered posting because that's rather simple to do. I can read DC bus voltage from my controller and operate a contactor coil to bypass the current limiting device pretty easily but it's not what I'm trying to do. Realistically the solid state components to do this are super cheap, figuring it out is only a bit more difficult which is why I posted on an electronics forum.

For argument's sake we can forget that this has anything to do with servos and just think of it as a self contained AC-DC power supply.

WattsThat:
Sorry, didn’t see your attachment before commenting. I’m not a power electronics guy but I know enough to know that’s not going to work - but it would be an impressive failure when the power was applied.

The INA138 will not withstand 340 volts common mode.

I’d also expect an IGBT to require 10V on the gate to have it fully turned on, gonna need a gate driver there.

https://www.infineon.com/dgdl/Infineon-Simple_High_Side_Drive_Provides_Fast_Sw-Article-v01_00-EN.pdf?fileId=5546d46254e133b401554de58b225d15

Ignoring the above and to answer your question, selecting R3 is a matter of how fast you want to charge the caps and what the selected resistance will withstand with respect to fusing current. To get to 63% of Vin, t = rc and 100% requires 5 t. It’s hard to find fusible resistors in the size that will be required, I’ve seen regular 5-7w ceramic power resistors encased in fiberglass sleeving, sometimes paralleled parts to get to the required ratings.

At the end of the day, hopefully you’ll figure out why mechanical relays get used in this application. Throwing complexity at a simple problem always fails in some way.

blazini36:

--- Quote from: WattsThat on January 20, 2020, 04:28:59 am ---Sorry, didn’t see your attachment before commenting. I’m not a power electronics guy but I know enough to know that’s not going to work - but it would be an impressive failure when the power was applied.

The INA138 will not withstand 340 volts common mode.

I’d also expect an IGBT to require 10V on the gate to have it fully turned on, gonna need a gate driver there.

https://www.infineon.com/dgdl/Infineon-Simple_High_Side_Drive_Provides_Fast_Sw-Article-v01_00-EN.pdf?fileId=5546d46254e133b401554de58b225d15

Ignoring the above and to answer your question, selecting R3 is a matter of how fast you want to charge the caps and what the selected resistance will withstand with respect to fusing current. To get to 63% of Vin, t = rc and 100% requires 5 t. It’s hard to find fusible resistors in the size that will be required, I’ve seen regular 5-7w ceramic power resistors encased in fiberglass sleeving, sometimes paralleled parts to get to the required ratings.

At the end of the day, hopefully you’ll figure out why mechanical relays get used in this application. Throwing complexity at a simple problem always fails in some way.

--- End quote ---

About the INA138, you're right. I was just trying to get an initial drawing down, like I said just spitballing.....not plugging anything in at the moment. I worked on it a bit more today (attached). The INA138 is pointless anyway because a suitable small shunt does not seem to exist. Relay or not it still has to be controlled, and I don't want to externally control the bypass relay. I need a voltage reference from the DC bus side to control the bypass mechanism that I'm still trying to figure out.

I added the NTC as the current limiting device and moved it and the IGBT to the negative DC bus rail. Added gate driven triacs to the input side. The triacs on both input lines are redundant and I'll likely remove 1. haven't added anything to drive the gate of the IGBT yet

station240:
You'd be better off adding a Power Factor Correction (PFC) circuit on the front.
Easy enough to change the feedback resistors to get 340V DC.
TI and others have dedicated PFC controllers, everything else is just a boost converter.

NiHaoMike:

--- Quote from: station240 on January 20, 2020, 05:45:37 pm ---You'd be better off adding a Power Factor Correction (PFC) circuit on the front.
Easy enough to change the feedback resistors to get 340V DC.
TI and others have dedicated PFC controllers, everything else is just a boost converter.

--- End quote ---
Only if he's in a commercial space or is running into the current limits of the supply. PFC doesn't really have much benefit in lower powered residential applications.

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