Switchmode power supply design - transformer vs buck converter design

[haxby]

Hi all,

Are there any power electronics experts here? I'm a noob in terms of SMPSU design, but not a noob in general electronics design.

The problem:  I want to run 5KW 240VAC appliance(s) from a Prius hybrid battery pack.

I have the car, and I have a 5kw UPS inverter. Unfortunately the inverter runs on 120V DC. It was designed for 10x 12V lead acid batteries in series.  The Prius battery pack is 240V DC.

So I am looking at designing a SMPSU to step down 240VDC to 110VDC at 5KW.

I have looked at buck converter designs, but would you agree that an isolated primary/secondary switchmode transformer topology would be better for this purpose?

I can't find any high power buck designs on the web, so there must be something that limits the power of this topology? Maybe there is a power electronics designer here that can help explain why?

[Weston]

An isolated converter is generally going to require more active devices and be less efficient than an unisolated converter such as a buck. Unless you really need isolation (which may already be provided by the UPS?) you would be better off with an unisolated converter.

There is nothing that inherently limits the buck converter output power. I think why you are finding few designs comes down to demand, 240V to 120V at a few kW is a relatively uncommon requirement. If you want to see what the power stage would ~ look like at that power level a lot of reference designs exist for boost converters as they are widely use for a PFC front end. A buck is just a boost converter in reverse, so all you would need to do is swap the location of the diode if it is a non-synchronous design.

Possibly more importantly, a 5kW buck converter is going to be a sizable undertaking and cost. Is there no way you can modify the UPS to run off 240V or the battery pack to output 120V?

[digsys]

This path is fraught with problems - driving one high power converter from another has many more added issue - switching resonance / interference patterns etc etc
Why not sell the one you have and purchase a proper one. There are many out there these days, and likely better efficiency / power / Vin-Vout etc
Plenty of EV car forums to check out

[David Hess]

I have looked at buck converter designs, but would you agree that an isolated primary/secondary switchmode transformer topology would be better for this purpose?

Isolation is not only safer but may be required to prevent ground loops.

The other reason that a transformer may be used with a buck topology is that the duty cycle is the ratio of output to input voltage.  At high or low conversion ratios, the transformer adjusts the duty cycle to a more convenient value.  This is not a consideration in your case but isolation would still be advantageous.

I can't find any high power buck designs on the web, so there must be something that limits the power of this topology? Maybe there is a power electronics designer here that can help explain why?

High power buck converters are feasible and even common in the transformer isolated form but not for converting between two high voltages simply because it is not a common application.

Switching converters require a large inductor as an energy storage element but provide regulation of the output voltage.  The alternative if the input voltage is relatively constant is an inverter which provides no regulation, but uses a transformer and requires no large inductor for energy storage.  In addition, the input and output ripple currents of an inverter are low because most of the output power is provided through the transformer which considerably relaxes the ripple current requirements of the input and output capacitors.

[NiHaoMike]

It would be easier to find a UPS that uses a 200V battery voltage.
http://priups.com/riddle/answer-1.htm

It might be worth opening the UPS and checking how it's designed - maybe it just boosts the battery to 340V or so and would only need some modifications to use a 200V battery.

Or get another Prius inverter assembly (they're cheap - I just got one for $90 to use in a solar inverter project) and use it for the boost converter and output stages.

[haxby]

Thanks all for your help....

I'm going to have a crack at building one. I found a great youtube channel with some great practical advice:

https://www.youtube.com/user/subcooledheatpump/videos

Thanks again!

[NiHaoMike]

You'll also want to check out Damien Maguire and the Openinverter project.