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High voltage buck converter

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Ptzf:
Hello!

Here is what I want to do: convert a steady 325VDC (rectified mains) to a variable DC voltage in the range of 250-300V, depending on voltage feedback. The current at the load will be around 1-1.5A. I would like to use a buck converter to do this, as it is simple and possibly cheap. Switching frequency would be say 300 kHz. The named voltages imply a duty cycle of roughly 70%-90%.

Are there any obvious flaws in this plan of using a simple buck converter? Should I use some other topology to achieve the design goal (simple, cheap and with voltage-mode feedback)? Why I'm asking is because search results on the web seem to give buck converter duty cycles at 50% max (perhaps because of PWM controller current mode operation difficulties) and max 100V input IC buck controllers. Will the high duty cycle be a nonlinear problem for the inductor-diode circuit? In short: I lack domain knowledge :)

T3sl4co1l:
Current mode. Always current mode.

Other than that, yes, that's perfectly doable.  Consider an N-ch MOSFET, and a bootstrap gate driver.  Make sure the controller has some way to limit maximum duty cycle, so it doesn't stay latched on (100% duty), which starves the bootstrap supply.

Current mode is easiest if you don't need common grounds, between input and output.  You can place the shunt resistor there, in series with the output ground (upstream of the filter caps, so it reads inductor current ripple).  If you do require common ground, you can still use a "flying" sense method, but it gets more difficult (either a current sense amp, with quite high voltage rating and CMRR, or a current transformer to read switch current).

You could also use another topology, like flyback or SEPIC, to get a wider range of input and output voltages, and consequently, different positions for current sense (which may turn out to be more convenient).  These take more reactive power (bigger inductor, dual windings) so they are less efficient than the buck converter would be.  Sometimes the advantage is worth the cost, depends.  Just putting that out there.

Tim

langwadt:

--- Quote from: T3sl4co1l on July 28, 2018, 09:11:05 pm ---Current mode. Always current mode.

Other than that, yes, that's perfectly doable.  Consider an N-ch MOSFET, and a bootstrap gate driver.  Make sure the controller has some way to limit maximum duty cycle, so it doesn't stay latched on (100% duty), which starves the bootstrap supply.

Current mode is easiest if you don't need common grounds, between input and output.  You can place the shunt resistor there, in series with the output ground (upstream of the filter caps, so it reads inductor current ripple).  If you do require common ground, you can still use a "flying" sense method, but it gets more difficult (either a current sense amp, with quite high voltage rating and CMRR, or a current transformer to read switch current).

You could also use another topology, like flyback or SEPIC, to get a wider range of input and output voltages, and consequently, different positions for current sense (which may turn out to be more convenient).  These take more reactive power (bigger inductor, dual windings) so they are less efficient than the buck converter would be.  Sometimes the advantage is worth the cost, depends.  Just putting that out there.

Tim

--- End quote ---


if common ground isn't need doesn't it make more sense to turn everything up-side-down like many of the off-line LED supplies?

T3sl4co1l:

--- Quote from: langwadt on July 28, 2018, 10:44:07 pm ---if common ground isn't need doesn't it make more sense to turn everything up-side-down like many of the off-line LED supplies?

--- End quote ---

I prefer average current mode to peak current mode (which is easier in that case), but that's definitely an option too.

Tim

David Hess:
I am inclined to think that there are better ways now but back in the 80s, Tektronix used a bootstrapped sort of SEPIC converter to convert a 340 volt line into to 42 volts DC.  This allows driving an n-channel buck transistor without a level shift and easy current mode control although Tektronix just used current limiting.

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