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Mosfet Quiet Bootstrap
Mosfet Quiet Bootstrap
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Old 15th October 2010, 06:27 PM   #1
acoustixman is offline acoustixman  United States
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Join Date: Feb 2006
Location: Boulder City, Nevada
Default Mosfet Quiet Bootstrap

A question for the solid-state experts:

My first power amp utilizes a +/-68V supply rail to provide gate-drive headroom for 150W -> 8 ohms. I used cascode stages in a symmetrical VAS, follwed by class-A EF gate drivers to [excellent] Exicon lateral MOSFETs in SF. The sonic results are simply wonderful and it truly paints a 3D picture for each nuance of every song. Only problem was in using such a high supply voltage for my 'household' audio loads. I wanted to keep the PS simple while accounting for the VAS cascode, forward EF bias, and gate drive voltage penalties, probably totaling around 12-15V of lost voltage across the fets at full-swing. Ultimately the thing dissipates too much heat all the time for obvious reasons, despite that I bias it in class-AB (or class-B if you are from the school of Mr. G. Randy Slone).

My next scratch-build concept is to use a +/-55V supply for the FET drains and a small dedicated center-coupled supply (say around +/-70V) for the "smart end" of the amp. I may still build an amp like this for elegance and PSRR; HOWEVER my current project is a retrofit in order to salvage a nice chassis with a good +/-55V power supply (it is a 'blown' SAE amplifier - blown like an abused circuit, not like a dragster intake)...

Enter bootstrapping...Now comes the question:
I want to provide a bootstrap voltage margin to the "smart end" while directly connecting the drains to the +/-55V rails. I am PSRR-paranoid though, so I cooked up this circuit. I have not found any references to lead me on this so naturally I am curious about my prospects. I should actually say that every reference I *have* seen about bootstrapping simply uses a capacitor and a diode or resistor from the incoming supply.

My goal is to "mildly" admit some power to expand the margins, sort of assuming that low/mid-band energy will be available to energize this circuit. Because I prefer elegant biasing techniques for my numerous cascode elements, I don't want to simply use big caps that will force the rails to simply keep their distance from the output via brute force; I want *a little* energy to be rectified and subdued, with a limited voltage value, while not allowing entry of bootstrap diode switching transients. Perhaps this reasoning is questionable; I covet any and all insights.

The attached circuit is the present state of my supply design. It is now built, and optimistically I am going to power it up this weekend (obviously at DC it will degenerate to having the bootstrap rails at a diode drop inside the power rails). Can anyone offer pros, cons, expectations, or suspicions about this design? Perhaps someone has some simulation capability and would care to explore it? I am very confident in my PSRR (also using cascodes on diff-amp) but I don't want to be injecting mid-HF junk (without the inductors) if I can compromise and get just a little boost. Basically I'm trying to combine the strengths of something like a flyback converter with low-pass filtering to give some extra emf when needed to avoid drive-clipping (because it is so dissapointing to clip the gate drive when the output devices are right in their wheelhouse).

I understand that this circuit will lag and attenuate compared with a brute-force big-cap bootstrap, but the intention is to be "subtle" and store *some* useful energy during dynamic passages of MUSIC (as compared with the capability to follow LF sine waves on the supply rail), and I am not giving myself any requirement to be able to put the FETs nearly into Vds clip at any time; i.e. I am never going to count on having all 12-15 extra volts available. I would simply like to have 150 nice clean watts into 8 ohms.

The gates are protected against overvoltage. Output rail to bootstrap connection is at audio-band feedback pickup node (I use multi-band feedback as outlined by W. Marshall Leach). Circuit is entirely polarity-symmetrical but I have shown only the positive side. Amp will be used for "pink" music, not DC or multi-amp bass. Speaker loads will be approximately resistive because I want them that way. EF stage moves about 10mA DC, while the diff-amp, VAS, and biasing nets combine for about another 12mA. The power supply is adequate for the output goals.

Diagram notes:
Original design is degenerate via removal of D1, D2, L1 & C8, with connection between output supply and driver supply. Second approach above would be the same but without this connection and using separate supplies. Jogged line shows wire-to-board from chassis power to amplifier.

Thanks all so much for *any* tips and thoughts!!!
Attached Images
File Type: jpg mfts_amp_bootsubcirc_101015.JPG (40.6 KB, 143 views)
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Old 16th October 2010, 12:15 PM   #2
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
the concept is simple and can easily be added to any amplifier+PSU.

Try it.

I doubt you will measure any significant increase in front end supply voltage due to the very short time, if any, that the output peaks are able lift the voltage.
regards Andrew T.
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