Need Advice for High Current, High Voltage, High Speed, Switching

[townsend]

Hello everyone!

I'm new here - just freshly registered. I have a PhD in Neuroscience, with other degrees prior in Mathematics and Computer Science, and prior to that I earned diplomas from college in the areas of Electronics and Electrical. I don't profess to be a formally recognized "Engineer" and do not have the PEng designation, but my formal study of the area at the college level was carried through as I worked on my Undergrad, Master's and Phd, and I do have two patents registered to my name in the area of EEG acquisition electronics.

Put simply, I might not be familiar with all the jargon that PEng's use, but I have a good grasp of the principles and am able to design my own stuff and debug/troubleshoot other peoples' designs. So, please excuse me in advance if I don't seem to follow what you are saying - its just a matter of us speaking different working languages.

So, here's my question:
I am in the middle of trying to implement an idea, but I'll spare you all the details and only include what is necessary to help describe my problem. I have a system operating at about 680V and about 40A, and I need to be able to switch a component in and out about 120 times per second. I am under the gun to get a prototype completed fast, and so instead of working on my own switching mechanism, I chose to use a solid state relay (SSR). I chose one with random on/off (i.e. it doesn't wait for a zero crossing, but switches when you tell it to). I also need optical isolation to simplify the connection to the microcontroller that will be handling the timing.

To guarantee perfect operation, I really need sub-millisecond timing accuracy. I also don't want to break the bank with the project, so I wanted to keep the cost of the switch under $50. Unfortunately, the only solution that I found indicated in the datasheet that the device would switch on (and off) within 1 millisecond of being told to do so.

I was disappointed but checked my calculations and found that although not ideal, this would produce "acceptable" results. Without providing all the details, the potential for the delay will result in the output voltage of my project being generated to be a little higher or lower than desired, but still within acceptable limits.

1/ Can anyone point me to a SSR that can respond within 100usec, is within budget, and meets the voltage and current requirements?
2/ Can anyone point me to a resource to help me design my own switching system so I don't need to use a ready-made SSR?

Regarding question 2 above, I specialize in low current low voltage analog and digital applications, so "power electronics" is not a familiar area for me. That's why I'm here seeking some advice. Thanks in advance!
-gt-

 

[townsend]

Thanks a lot for taking the time to think that through and make those suggestions. I purchased just the PSU unit of the equipment ($7000 for the entire machine, but just under $400 for the PSU by itself). I'd rather spend $400 and fail than $7000 and fail!

It arrived yesterday but is 1/4 mile away in the USA. I'm in Canada, but I'm going on a Caribean cruise next week, and will drive into the states and to an airport there. I'll bring it back with me upon my return ... duty free and tax free, because of the time spent in the states.

So I won't have it until I'm back from my (fairly short) vacation. When I get it home I can cut it open and see what is there and report back to you. As far as charging the caps up, I've carefully considered all that the members have enlightened me on concerning caps, and put together a REALLY old-school solution that will cost about $25 in total (Excluding the costs of the additional filter caps and the transformer). I think you'll laugh pretty hard when you see it. So far you've been spot on, always thinking ahead, so I suspect that my description is enough to get the wheels in your mind turning, and you'll probably be able to deduce where I'm going with this based on my very sketchy description above. I can probably breadboard and pre-test it without the transformer, since charging that much capacitance remains a challenge even at 220V. So once I get it constructed, I'll fire it up and monitor the currents and if it works, I'll send a schematic and some pictures and wave forms.

I suppose by then you'll have figured it out and posted what I have in my head.

Thanks again for the help!
-gt-

Update: Still on vacation. I logged in remotely to check the shop and nothing was running. I found out there was a thunderstorm and a power failure. (whew!) I've made arrangements for someone to go over to reset the equipment.

It turns out that the same transformer that I require, when custom built in Canada is $600 ... I have offers from various factories in China who will build it for as little as $98 ... but the problem is shipping is - shipping from China is expensive - three or four times the cost of the product. So I'm still working on this. If I can get a solution, I can buy a dozen more for $1200 and ship in a crate by sea for less than $1000 shipping for the lot. However, at this point I only require a single unit. I'll ultimately use underground cooling, but to reduce the number of unknowns during development, I'm thinking I might buy a proper radiator with fans just for testing purposes. If I combine shipping of the transformer with the radiator, it might be a cost effective option.

Anyway, I'm back from vacation in a few days and I can get started on Frankenstein-ing the PSU. The first hurdle will be to determine it has any power monitoring electronics on the three-phase side that requires defeating.
-gt-

 

[townsend]

Update: Still on vacation. I logged in remotely to check the shop and nothing was running. I found out there was a thunderstorm and a power failure. (whew!) I've made arrangements for someone to go over to reset the equipment.

It turns out that the same transformer that I require, when custom built in Canada is $600 ... I have offers from various factories in China who will build it for as little as $98 ... but the problem is shipping is - shipping from China is expensive - three or four times the cost of the product. So I'm still working on this. If I can get a solution, I can buy a dozen more for $1200 and ship in a crate by sea for less than $1000 shipping for the lot. However, at this point I only require a single unit. I'll ultimately use underground cooling, but to reduce the number of unknowns during development, I'm thinking I might buy a proper radiator with fans just for testing purposes. If I combine shipping of the transformer with the radiator, it might be a cost effective option.

Anyway, I'm back from vacation in a few days and I can get started on Frankenstein-ing the PSU. The first hurdle will be to determine it has any power monitoring electronics on the three-phase side that requires defeating.
-gt-

Well, I cracked open the PSU, and where I had hopped to see six diodes forming a three-phase bridge rectifier circuit, I instead saw six devices with three terminals marked SDG, so they were FETs. I do not have a proper schematic, but from what I could gather, it seemed that there was a handful of components including an optoisolator on each FET. My guess is that it probably has some sort of "soft-start" power up circuit to deal with the uncharged filter capacitors and eliminate the inrush current. Once the caps are mostly charged and ready to go, the FETs are probably left on full and operated as if they were regular diodes.

Anyway, if the input stage had been simpler, I would have use a pair of mechanical $11 contractors rated at 50 amps from Amazon.com and a bank of light bulbs (load resistors were $1000 at Digikey.com, hence the bulbs) to slowly charge the caps through a resistance for a minute or two and then short the resistor out and switch in the load. If I had of continued, I later found I could get the same resistor from Alibaba for only $100, so I would have used that instead of the light bulbs.

Anyway, it seems that the circuitry is sophisticated and probably monitored by a microcontroller and it is unlikely (even if I obtained a proper schematic) that I could be successful at modifying it.

In the meantime, I had other irons in the fire, and discovered that the same quality of unit that was $4000 in Canada to produce 380-3P from 220-1P power is available on Alibaba for about $600 including shipping to Canada. I talked to the engineer at the supplier at length and grilled him on various aspects concerning quality of the sine wave output and power efficiency. The bottom line is that he guaranteed me that my 5.3 kW load would draw 8 amps on each phase wire, and that if I checked the current on the 220 V side, it would be less than an amp in excess of 24 Amps. I don't think I can beat this solution. It is cheap and efficient. So I guess that pretty much concludes things.

Thanks to everyone who participated - this was a fantastic learning experience for me, and you are a great bunch of guys! I appreciate everyone's help. Thanks so much!
-gt-

 

[D.A.(Tony)Stewart]

Will this question take another 50 comments before we get a real design spec for the load? V, I, Z(f) , time, power , energy? ,source impedance, incl. X, diode, cap ESR , load regulation error , MTBF , power cycles, target BOM cost, efficiency ... or whatever it takes to define all the requirements in a table or list.

 

[Easy peasy]

The input fets may well be active diodes or a fully active front end - is there an isolating transformer in the psu ?

are there HV caps ( electro's ) near the input ? about 800V worth ? if so it inplies an active front end for power factor correction ? please send psu data if you have it

 

[D.A.(Tony)Stewart]

2/ Can anyone point me to a resource to help me design my own switching system so I don't need to use a ready-made SSR?

Regarding question 2 above, I specialize in low current low voltage analog and digital applications, so "power electronics" is not a familiar area for me. That's why I'm here seeking some advice. Thanks in advance!
-gt-

You sound like a smart guy, but I don't know how much theory you know about RLC impedances, back EMF and popular solutions for SSR's. I suspect the reason you want a ready-made SSR is because you believe your effort will result in a cheaper or better solution, yet you have not given the real reasons or confidence you can deal with eddy-current losses, stray capacitive loading, stray inductance , transient triggered gates from dV/dt or dI/dt. But I think you are over-anxious and need to answer some of the questions I have suggested.

https://www.electronics-tutorials.ws/power/solid-state-relay.html