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SMPS for dormant tube audio amp project

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GK:
http://www.glensstuff.com/200vbuck/200vbuck.htm

The above is a link to a description of a mains-powered buck regulator I built nearly 10 years ago as part of a power supply unit for an audio amplifier project that has been sitting uncompleted on shelf since (which I'm revisiting and finishing off). This was to serve as a preregulator for a simple unregulated converter providing the 800V plate supply as well as +/-140V rails for the solid-state driver circuitry (the latter cleaned up and dropped down to +/-120V by a linear reg.

Performance wise the old design is still plenty adequate for the job at hand, but I'm contemplating perhaps redesigning the whole shebang and shrinking it all down to a single PCB if possible.

The IGBTs I used are obsolete now and there are probably better off-line half-bridge driver ICs available now too. At the moment I'm interested in how much higher I could practically raise the switching frequency to so as to shrink down my magnetics. For off-line voltage levels at this kind of power level, using more modern MOSFETs/IGBT/driver ICs, what is considered decent or run of the mill switching frequency wise in 2019?

GK:
Incidentally the buck regulators for the heater supplies with valves (just mocked up on a spare chassis for testing/evaluation) - low ripple, 60s linear voltage-ramping soft-start, crow-bar OVP, etc.



T3sl4co1l:
Nice!

Hm, IGBTs were an odd choice; the high side fine, but the low side wasn't doing anything.

These days, anything up to 400kHz would be pretty normal I guess, and you can always go higher with GaN if you feel like it!

Schottky diodes are available in all voltages now, from 20V to 100V (conventional, Pt-Si I think, guard ring), to 200-300V (field-effect "super barrier" and superjunction), to 600V+ (SiC).  The only advantages held by fast PN diodes are cost and leakage (and maybe surge current).

MOSFETs, especially higher voltage types, are far better performance than they used to be.  Performance of all types has continued to improve incrementally; high-Vds parts have improved sharply (formerly, a quadratic performance blow at high Vds; superjunction removed this limitation).  There's also SiC and GaN, with different Vgs requirements; SiC has better Rds, and somewhat better speed, at high voltage ratings; GaN has crazy speed and is recently available in higher voltages (500V) as well as low voltages (30-200V).

If you want to keep the synchronous converter, shop for MOSFETs with Rds(ON) < 0.5V / Iout(max) or thereabouts.  That way you avoid body diode conduction (except at switching edges).  This is harder to do at higher and higher voltages, as you need quadratically beefier transistors, incurring significant cost and Qg.

Regarding switching edges: consider using as marginal a dead time as possible.  It only takes 10s, 100s of ns for the body diode to get forward-biased, and then you incur full hard-switching recovery and loss.  Some interleave isn't even a bad thing, as long as the switching loop has a controlled impedance (i.e., you may want to add stray inductance and a snubber).  The key insight is that, with controlled impedances, you aren't working against a steep cliff; rather, the switching losses increase gradually to either side, and you want to ride the valley of soft switching inbetween.

Dead time could even be controlled based on load current or switching voltage, which, I don't know of any controllers that attempt to do that, but an adventurous analog type like yourself might..?

Wait, I have seen some controllers that do that, but only sort of; the one example was a resonant controller, which extends dead time under conditions expected to have low load current (i.e., high switching frequency).  But still, nothing full-on watching load current, or commutation voltage, and operating accordingly.

Tim

GK:
The low-side IGBT conducts in the forward direction when the load current drops below the peak ripple current of the inductor - it's there to ensure that the regulator always operates in continuous current mode and is how the control loop is made to operate entirely unperturbed right down to a load current of zero.

A MOSFET with low rds-on would give less conduction losses when conducting in the reverse direction than a IGBT here, but given the low average load current relative to the peak and the fact that the low-side switch is only on for ~40% of the time, the efficiency gains I think wouldn't go beyond a single watt. But either way I'm not fussed - MOSFET or IGBT - whatever gets the job done.

I have stock of all sizes of ETD formers and cores in F48 material, which is said to be good to 500kHz, but is in reality is getting a bit lossy by then as far as I can tell. ~200kHz or so might be a good target/all-round compromise.

GK:
Does anyone here have experience in efficiently grinding ferrite? This stuff is frigging hard and you can forget about a hand file! So far the best thing I have found for the job is these little abrasive flap wheels, but it's still a slow and tedious process.

I'm going to potentially have a mini hobby production run of direct off-line flyback transformers here soon which will need to be gapped, and I'm not looking forward to it.



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