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

T3sl4co1l:
LT's Silent Switcher line should be of use.

I don't think I'd suggest a custom design: even with the quirks that a lot of integrated controllers have, the fact is they offer so much more functionality than a modest sized discrete circuit can.  You'll either need hundreds of components to pull it off (the last discrete design I did, used about 100 components per channel), or as many hours programming the MCU/FPGA to provide equivalent functionality in a smaller package.

I don't think you can get away from noise in general: even resonant architectures still have spiky dV or dI/dt in places, stuff like diode recovery and all that, and that will be reflected in the emissions spectrum at some level.  So, expect to need filtering and shielding regardless.  What you're really investigating is, how much filtering is ultimately needed, and at what frequencies?

There may be some interest in a phase interleaved design for example, which acts to reduce or null the fundamental ripple (in effect, multiplying the ripple frequency by N for N phases).  That, with a Silent Switcher style control, could give quite handy improvements.  That could make the difference between, say, board-level shields, versus a full shielded enclosure, or board-level filtering versus feed-thrus.

The aim is to push the ripple up to a higher frequency, so that filtering the fundamental is easier, while also bringing down the worst case harmonics, so that your filter bandwidth does not need to be insane.

Higher Fsw also helps reduce the size of components, but at the expense of higher switching losses.

It doesn't need compressed into so little spectrum as to be a pure tone; like, you could make a class E amplifier at 13.56MHz and keep everything very soft and free from harmonics, and filtering would basically amount to 13.56MHz traps (which could be a bit of space savings actually, over a lowpass of equal attenuation).  Controlling something like that would really be the bigger issue though, and also the control bandwidth would fall outside of the notch filter, which really means that you don't have any filtering from load to source (i.e., the control must respond almost instantaneously to load changes).

I don't know if that kind of filtering is necessarily intended in your application (i.e., in effect, load variations being reflected as source emissions), but it may also be equivalent to the specifications you're given.

If you have to meet dropout/holdup requirements (avionics often have this requirement; yours may too?), you may need a lot of energy storage anyway, and turning that into a highly effective filter may not be much additional cost (and, cost in the most general sense, not just direct BOM cost but cost to the size or weight as well).

If size/weight is priority, it's interesting to note that the attenuation-per-energy-storage ratio (and therefore, the attenuation-per-volume, more or less) is maximized with a modest number of stages (the number depending on how much attenuation is required).  That is, for a given attenuation, a 3rd order (CLC) filter would need a fairly low cutoff frequency and therefore relatively large components, but a 5th or 7th order uses more but smaller components; while a still higher order filter uses simply too many parts with size of diminishing returns (because number of components keeps rising, while cutoff frequency approaches Fstop).  So there's a minima between extremes.

Tim
thm_w:

--- Quote from: max_torque on July 19, 2019, 06:24:04 pm ---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 ---

If space and cost are not an issue, are there no off the shelf modules available that are certified to those specific MIL standards, have you looked?
Maybe certified to a similar standard perhaps. Downside of this is these modules are usually potted so any modifications you do would likely just be additional input or output filtering.


--- Quote from: max_torque on July 19, 2019, 06:24:04 pm ---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.

--- End quote ---

Yes slowing rise time with series gate resistor or similar will cut down EMI.

Look at the standard for emissions graphs, usually they will have some higher allowed dB rating in the lower frequency range. If your switching frequency and second harmonic is below the point where they drop the maximum emissions down, that would probably help. Do you have a copy of the standard?
max_torque:
Thanks for the replies!

Those LT parts look interesting, as they package most of the components into what is persumably a better RFI optimised package than i could manage!

My biggest choice is what switching frequency to use?  There are higher limits at lower frequencies, so that could help. Limits are currently expected to be at the least Land Class C (DEF STD 59-411), but i'd like to get a Class B to give future headroom for the device.




Doing a 10Khz "switcher"  (is it even classed a switcher at that frequency!  :-DD )  looks to have some merits in terms of having the highest limits, but of course, it'll be a 'mare to filter due to that low frequency.


Some of those LT silent switcher parts do have a frequency control pin, so i can at least set and hold a fixed switching frequency. I wonder if a fundamental switching frequency of say 100 to 200Khz has some merit?


IDEngineer:

--- Quote from: max_torque on July 20, 2019, 01:51:12 pm ---Doing a 10Khz "switcher"  (is it even classed a switcher at that frequency!
--- End quote ---
I am embarrassed to admit that back when I was first working on what we then called "DC-DC power supplies", a 10KHz frequency wasn't considered low at all.
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