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Ultra quiet step down switching converter topology? 28v ->10V, 5A
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Wolfgang:

--- Quote from: max_torque on July 19, 2019, 06:24:04 pm ---I have an existing power supply device that drops around 28Vdc to 10Vdc for a Defense application that currently only requires an average current of 500mA, enabling that psu to be a simple linear dropper, that obviously dissipates an average of 9W, but that is very quiet (no switching or high frequency content).

A new application i'm looking at looks to require around 3A continuous, which is getting difficult to do linearly as the power required to be dissipated is now around 54W.

As such i'm looking at options for moving to a switching buck converter, but i need to be very careful of the EMC aspect, as the MIL RFI/RFC standards are draconian for this application .

So, some questions:

1) What topology should i start looking at?   Overall efficiency is not really important, the current device necessarily handles 9W of loss, which would be a hugely in-efficient switcher for just a 3A transfer.   Only voltage reduction is required as there is always at least 8 volts of voltage headroom.

2) is there any merit in following the switching stage with a low voltage drop linear one?  My suspicion is that at the frequencies that are important (harmonics of the base frequency) that high frequency content will probably pass right though any linear stage anyway?  One advantage of a switcher with a linear follower is that the basic switcher can be very slow and not that responsive, relying on the linear bit to provide the low impedance output across a decent bandwidth.

3) If i significantly oversized the switching elements (mosfets), then i could accept higher switching loses, and hence have slow rise times, is this practical? There is obviously a trade off between switching speeds and switcher size and cost, with lower speeds meaning larger inductors and larger capacitors to get the same current ripple.

4) Any merit in a parallel dual path architecture to interleave the current pulses from the switching elements

5) Off the shelf switching controller, or DIY approach with custom logic?



Space/size is not really an issue, nor is cost particularly, and the existing linear PSU does not include any current limiting beyond some output resistance and some PTC fusing

--- End quote ---

Hi,

whats the PARD level allowed in mV ? I think well below a mV over 0-10MHz is possible, if ...

My approach would be:

- a good input filter, cutoff well below switching frequency
- Low switching frequency
- risetime limited switching signals to cut harmonics down (inefficient, but thats not the main issue).
- low fT bipolar transistors
- LC output filter for switcher
- linear postregulator with some preload
- very good shielding between switcher and postregulator

a resonant Royer could do, maybe look up some Jim Williams App notes.

regards
  Wolfgang


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