Electronic – Why aren’t non-isolated DC-DC converters made for high wattage applications

ac-dcbuckdc/dc converterpower supplypower-factor-correction

I am new to electronics. I am presently studying DC to DC converters.

One thing I noticed is that most of the people are using isolated stepdown converters like half bridge, full bridge, or resonant LLC converters for conversion such as 400VDC in and 28VDC out at 1kW. But there are no non-isolated converters like buck converters.

I am just curious to know why this is so?
I am trying to build one AC to DC converter at 1KW, where my boost PFC gives 400VDC output.

I think 7% duty cycle is normal to drive given with the technology we developed (as D = 28/400 => D = 0.07), still i am unable to find any manufacturer who makes such PMIC with that small duty cycle at high power such as 1KW.

Best Answer

Like Aaron said, a topology with a transformer allows for better control of the step down/step up voltage. A duty cycle of 7% seems fine on paper, but in reality you should aim for closer to 30%. In an application with changing load that 7% nominal duty cycle can drop down to 1 or 2% with low or no load.

When using a PFC, especially a boost PFC, the 400V(ish) output is chosen for the AC line. A universal input converter (90VAC-264VAC) will have a rectified voltage of 128VDC to 374VDC. If you are only doing low line (90-130VAC or so) then it doesn't need to boost to 400.

Then there is safety concerns. Any company is going to have to get UL/CE/TUV/KC or whatever other safety agency you can think of. Think about a buck converter, if that top switch fails, how are you going to protect the output from that HVDC? A transformer/coupled inductor provides galvanic isolation.

Then you start getting into efficiency concerns. A buck converter will always have turn-on, turn-off and conduction losses in the switches. There is nothing you can do about it. A resonant topology like a PSFB or LLC can lessen or eliminate the turn-on losses. Look up Zero Voltage Switching. Phase Shifted Full Bridges are really cool and a topology I work with often.

You can use a buck converter for a 1kW source, there are just better options. Of course, better is the enemy of good! For a medium power source like that, you can look into interleaved buck converters. These are essentially 2 or more buck converters connected in parallel.