Ingineer said:
If anyone is considering using a bunch of 48V power supplies in series to make a QC, why would you then add another buck (or boost) stage? Why not just control the existing PWM circuit in the bank of power supplies? Probably as simple as adding an optoisolator (you might not even need that depending on the power supply design).
These enterprise-class power supplies are usually very well engineered and have good efficiencies, so it seems silly to add something external, thus adding loss, when you could just control the existing circuit. In addition, you'd be dealing with high voltages and currents in your buck stage, so any failures could be catastrophic. (For you, your LEAF, or your home!)
As Cor mentioned, In series operation a failure of one unit could result in high voltage reverse polarity applied to the outputs. There absolutely must be some investigation done and possibly modification to harden the system. Applying high-current reverse DC to the output would most likely result in the large electrolytic filters exploding and/or venting. Even a momentary reversal at low current could result in destruction of the output sensing and feedback circuits. When you are dealing with this much power you must not cut corners and "rig" it.
-Phil
Yes, controlling directly would be best solution. However, a couple of problems:
1. no-one seems to have schematics for those so we'd have to reverse-engineer the units to understand how they are controlled
2. even if we are successful at that, we might not be successful at getting the timing specs that we need for voltage rise / fall speeds
3. With 8 supplies, you would actually need 8 optos and associated circuitry. Of course, still less than building a whole new power stage but does entail a bit more engineering.
4. One will still need a control stage for sensors, running comms, executing commands etc. So all we're saving would be the actual buck power stage which is not that difficult to build once the layouts / driver circuits are all designed.
Also, the added benefit of adding a universal buck stage after the set is ability to go all the way down to zero volts output, making it a universal power supply. Not as interesting for this particular application, I would presume, but is quite interesting for others.
On a series connection - yes, every supply will have to have a reverse diode connected to the outputs. Then a control board will have to have an ability to sense a drop in input voltage and either (1) adjust its power draw accordingly, or (2) shut down entirely. (1) and (2) can be combined - say, a reduction in draw on a single-supply failure and complete shutdown on multiple. This way there is some built in redundancy and ability to 'limp-charge' at, say, 70% of the rate until a replacement supply can be swapped.
Valery.