Electronic – how to build a high-power (4kW) switched-mode DC-DC converter

conversiondcdc/dc converterpowerpower supply

I would like to build a switched-mode DC-to-DC converter which should support up to 4000 W of power. It should be able to act as a CCCV power-supply, so supply a constant current Imax up to the point where voltage Vmax is required to have that current, and then supply a constant voltage Vmax with a diminishing current.

I would like it to be modular, to be able to swap-out a few components to change the Imax and Vmax. I do not want to have knobs to change the Imax and Vmax like on a general-purpose lab power supply, as these parameters will be changed very rarely, when I change the battery configurations, maybe every few months, and I do not want the risk of me or someone else accidentally turning the knobs, and then connecting it to the batteries with wrong parameters which could kill very costly batteries – as I want this to support up to 4kW, you can imagine the batteries will be large. No, I don't have batteries supporting such current yet, but within years I could have, so I want this device to be easily expandable as my battery park grows.

EDIT – I modified my expectations, which I hope will make it easier to build:

The configuration I would like to have:

  • Input: rectified 230VAC mains (that will be something around 325VDC?)
  • Output: CCCV with exactly Imax=15.0A, Vmax=230.4VDC

Parameters:

  • input and output electrically isolated
  • low ripple (1% is OK, 2.5% is too much)
  • efficiency at least 90%, I am willing to spend up to 10% more for every extra 1% efficiency that I gain over 90%.

In the future I would like to be able to install modules to be able to accept a different voltage input, for example from solar panels, so a design that will accept a wide voltage input range (say 100-400VDC) would be a plus.

I would be grateful for some basic pointers into how to build such a beast, possibly using some ready-made blocks, but I don't want to buy the whole device, as I want it to be modular and expandable (unless some DIY kit is available).

I could take four Chinese 48V (adjustable to 58.8V) 15A CCCV supplies for less then 200$ each and connect them in series to do the job, but then I have "black boxes" which if a single component inside brakes, not having the schematic will be difficult to fix. So I would prefer to spend even twice as much for a modular, self-made design, so if something breaks I can easily exchange the faulty element, and if my desired voltages change I can just rebuild part of the device without buying a whole new one.

Best Answer

Designing a power supply is not a trivial task. I've spent more than 10 years in the employ of professional companies which do this sort of work.

Some thoughts:

  1. Input-side - that's huge power you'll be drawing from the mains. I'd estimate that you'll need at least 20A service. You'll also need some hefty EMI filtering to avoid polluting the mains with the switching noise from the converter. Don't forget about inrush limiting, surge suppression, fuse sizing and X-capacitor discharge circuitry.

  2. PFC - Most designs need some sort of power-factor correction to ensure that sinusoidal current is being drawn from the mains. You'll most likely need a multiphase PFC to handle this sort of current effectively.

  3. DC/DC converter: You'll definitely need a soft-switching topology to achieve any reasonable sort of efficiency on the primary side. Look into the zero-voltage-transition full-bridge converter, aka phase-shift full-bridge (Intersil, Texas Instruments and others make controllers for this sort of topology) and look into really rugged MOSFETS (I've used IRFPS40N50Ls for 3kW designs). You'll need really good transformer core material (consider Ferroxcube or Nicera) to keep the losses low. You may want to even consider running two 2kW converters, and sum the currents on the secondary-side with a current sharing circuit.

  4. Rectification: There isn't an efficient way to rectify such high voltage. You won't be able to take advantage of synchronous rectifiation (in my estimation) so you'll be burning power in Schottky diodes.

  5. Thermal management: You need to ensure that your magnetics and switching devices are all in their safe operating areas, and design heatsinks / install fans to ensure that they all stay out of any potential thermal runaway conditions.

  6. Protections: Over voltage, over current, over temperature, short-circuit, line surge, ESD, EFT ... all things that you need to design protection against.

  7. Regulatory / Safety: You'll need to ensure that you meet proper creepages and clearences, that your touch current is safe, that all the critical magnetics and power train devices do not result in the power supply becoming unsafe during any single abnormal event, that your thermal management is keeping the parts within their safe operating areas, that you're using regulatory-approved parts for safety-critical functions (X- and Y-caps, MOVs, optoisolators, etc.)

Are you sure you really want to try and design such a thing yourself? There are books out there that explain how to design, but becoming proficient is a life's work in itself.