Hello,
Thanks, everybody, for your input.
VE1BLL's comments make the most sense to me, and VE1BLL provides some good reasoning behind them. The post about the top supply failing and getting current back-fed into it makes sense (with the original topology that I posted).
So…I’ve attached the modified diagram to this reply/post.
Let’s narrow down the failure case just to make it a more tractable problem: let’s consider only the case where any *one* of the supplies/modules fails.
Perhaps there are three “failure modes” for any one of the 30V modules. When a module fails it could look like: 1) an open-circuit, 2) a short-circuit, or 3) a non-zero and non-infinite impedance.
Considering VE1BLL’s suggestion, and putting the diodes across each supply (anode on neg terminal, cathode on pos terminal), perhaps this would be the behavior in each of the above three cases:
1)
If any of the supplies gets turned off, resulting in an effective OPEN-circuit for that supply, then current would be bypassed through that supply's diode. Thus, we would see 60V on the output. That sounds like a fairly graceful failure to me. If I was to go with my original diagram, when either of the bottom two modules fail, there might be an opportunity for current flow from DCCOM through the diode associated with the failed module (since the cathode of that diode is tied to the negative terminal of the above/adjacent module). I think this is enough of an argument to go with VE1BLL’s suggestion.
2)
If any of the supplies fails in an effective SHORT-circuit for that supply, then all the current would just flow through the effective short-circuit, bypassing even the bypass diode. Eventually, I would anticipate that the failed module in this case would have some thermal problems and would degrade further to an open-circuit condition. But failure modes can be hypothetical a lot of the times.
3)
If any of the supplies gets turned off, resulting in some non-zero and non-infinite impedance, then current would be bypassed through that supply's diode. And again, if any one of the supplies fails, then we would see 60V on the output.
In response to user “itsmoked”, who had two comments:
>>“Why this bizarre setup anyway?”
– Well, that’s why I posted this question. If I knew the answer, I wouldn’t have posted. The devil’s in the details, and it’s very interesting and helpful to see everybody’s input on this!
>>“The practical tip response would be "don't do it"
--Well, that’s not very practical at all since it doesn’t solve the problem. I have used Astec supplies in series before (two 48VDC/25A modules in series) with no problem. Using three 30VDC modules is the only way I can get my power requirement of 4500W for this given product line.
In response to user “waross”:
Shunt regulation is common in motion control systems where a motor is present and the possibility of motor-generated back-EMF exists. For example, we develop “gravity-assist” machines that can be moved up or down. When the operator is moving the machine down, the motor is behaving like a generator and generating a voltage on the DC bus. Also, back-EMF can be present during motor decelerations as a part of normal, coordinated movements (not just in “gravity-assist” situations). The simplest form is a Zener along with a power resistor to regulate the voltage if it rises above a certain value and to shed excess power. More sophisticated versions might use a comparator and some active loads (power FETs), or a tiny microcontroller (e.g. PIC) that monitors DC bus voltage and then switches the line through a power resistor. For example, Maxon Motor makes an off-the-shelf shunt regulator:
In response to user biff44:
The capacitor recommendation is a good one…very interesting. How would you connect three caps: each positive lead of each cap connected to each module’s positive lead, and then the negative leads of all the caps to DCCOM? Or connect both leads of the caps across just the module terminals for each supply (that is, in parallel with the diodes when connected as VE1BLL suggests and as shown in the newest diagram).
Overall, I like VE1BLL’s suggestion, and it also provides me with some idea of what kind of ratings those diodes should have (50A bare minimum…probably 100A to be safe).
I do have a follow-on question:
Do I put a diode on the bottom-most supply (V3)? If that bottom-most supply fails and opens, then it’s diode (D3) would have anode at DCCOM and cathode at the negative terminal of the middle supply…would D3 then be conducting? Would that be OK? I think so….?
Thanks again!
