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Class D Switching Power Amplifiers and Power D/A conversion

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Old 4th December 2012, 09:20 PM   #101
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In my SOCD design (stopped due to no time) I used MAX961, so I would automatically get a inverted drive signal.

See New self oscillating post filter feedback topology...

Maybe there is something in the design you can use, like analogue input filter, supply circuits, blocking diodes,....
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Old 4th December 2012, 10:45 PM   #102
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Hm... and you skipped the level shifter between comparator and IRS and simply accepted that the speaker terminals are at +78V DC vs GND.
Also possible, but not my preference.

The blocking diodes is definitely a key question and one of the main risks in my approach. From the view of a proper high power design they would be indicated, no doubt - yes. But then I have the same amount of power devices like paralleling 3x IRFB4227. ...circuit is already now to complex. On the other hand - just 4 devices more, not to bad.
Hard switching with the body diodes of the IRFP4668 is the reason why I reworked my design from 400kHz to 320kHz.
I remember earlier real life measurements with the IRFP4668 where I was not comfortable with the body diodes of that beast for a clocked 400kHz design, unfortunately I messed the documentation....
For a self oscillating type which reduces the switching frequency at high levels I think (or pray?) it should be possible to get it going. I guess most critical will be the situation of a short, which results in high currents and high switching frequency until the protection trips.

Do you have real life comparisons on hand at 300kHz hard switching 50A at 170V of the IRFP4668 with and w/o blocking diodes?
P.S. To bad that you stopped your SOCD.
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Old 5th December 2012, 06:09 AM   #103
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Default My 2 cents.

Every thing is fine when the ambient temperature is close to 25-35C, you have RDS to shunt the body diode and it works great, but in reality in high power amplifier, the temperature goes up and then the RDS shunting no longer helps you anymore[RDS wents up], more the body diode conducts more the Qrr charge/discharge takes place, more the Trr it takes, thus the reverse recovery current increases much more than expected, more EMI, less reliability. PRONE to failure in field.

The only proven and bruteforce solution which gives you peace of mind is series schottky with anti parallel high speed but soft recovery diode which must be rated for 125C for its specs, not to get fooled by some datasheets showing specs with 25C, 20V, 30A etc like deceptions. Look for 125C, 400V, 30A values and associated di/dt and Trr results along with it.

Yes the dissipation rises because of series schottky but not much, but in reality what you get is a reliable product. Free from ill-effects of temperature rise.

Case history:
Crest audio launched LT series class-D which had cross-conduction/low reverse recovery current design using FREDs and series 300nH inductors, but the reliability went down considerably.Then they switched back to series schottky in CD series of amps. PL380 from QSC is another example of this.

I have seen people making claims that parallel low RDS mosfets and you can get away with body diode conduction[RDS shunting Body diode] but what about high power-high temperature operation where RDS starts increasing and it no longer shunts the body diode effectively[You don't have air condition fitted inside the amp, though the dissipation in class-D is much less but it happens when output power is at kilowatt levels.]??. Also by paralleling mosfets you again need a hefty gate drive for them, what about that?


Think about that.............rest, the choice is yours.

Last edited by Workhorse; 5th December 2012 at 06:31 AM.
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Old 5th December 2012, 08:05 AM   #104
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I have found that you don't need the series Schottky + Inverse parallel fast diode. Simply using a suitably rated Schottky connected in inverse parallel between drain and source of each MOSFET does just as well, with much lower losses. A high-speed hall-effect current probe feeding a 'scope shows that this works effectively to bypass the current away from the internal MOSFET diode.
The reliability of my VA/PA amplifier designs, when operating under short-circuit conditions, improved enormously when I added the inverse Schottky. (It's a fact of life that 70/100V output commercial amplifiers used for Public Address and Voice Alarm installations all too often find themselves working into short circuits!)
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Old 5th December 2012, 09:10 AM   #105
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This is true only when rails are lower than 150V, above that you can't use schottky trick.
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Old 5th December 2012, 09:23 AM   #106
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I'm using 200V Schottkies in a fully balanced/bridged output stage using +/-85V rails with no problems.
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Old 5th December 2012, 10:05 AM   #107
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Only possible if the Schottky Vf is less than Vf of body diode at a given freewheeling current and temperature. No matter Schottky's are available upto 250V as single diodes, one such is available from OnSemi but the VF at rated current is already close to 0.6V which is similar to VF of body diode in similar case mosfet.

Last edited by Workhorse; 5th December 2012 at 10:09 AM.
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Old 5th December 2012, 10:21 AM   #108
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Very true, but on my boards both the MOSFET and the Schottky are surface-mount parts only a few mm from each other. The Schottky heats up as the MOSFET heats up so the forward voltage of the Schottky drops as the temperature increases.

It's true that as my amps are designed to feed 100V rms directly from the amplifier at only 3 or 4 amps maximum, the freewheeling current is not very high.
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Old 5th December 2012, 10:25 AM   #109
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Quote:
Originally Posted by Workhorse View Post
.....rest, the choice is yours.
How unpleasant. In case of failure, I cannot blame anybody than me!

I fully agree that shunting the diodes by the N-Chanel will not work (especially at high temperatures), except you use unsaint low Rdson in relation to the concerned currents.
Furtheron I decided to double check simulation results of switching losses with manual calculation of switching losses (instead of praying...).
Result: Simulation contains Qrr, but it seems like the increase of Qrr with di/dt is modeled in a way that shows systematical lower values than the data sheet. Also sim temperature seems to have almost no influence below 100C, but extremely catastrophic impact already 125C - not realistic.

Manual calculation for 170V/50A/320kHz/125C in hardswitching situation tells me turn ON losses of 462W and turn OFF losses around 82W. Total 544W.
Sim would indicate total losses of 390W at 100C.
SeeingThat's the typical situation of 'it can work' and 'completely unexplainable defects'
Not my cup of tea. Especially when considering a realistic case temp of the MosFet of 75C (instead 25C) the allowed losses of the IRFP4668 are not 520W anymore, but 345W.

@Ouroboros:
Yes, I also think that a shottky can help in case of lower voltages, on the other hand for lower voltages in the mean time there are MosFets with pretty good body diodes up to 100V and acceptable body diodes up to 150V.
As you and workhorse already indicated, at 200V and current levels around 50A todays shottkys won't save my world

To bad. Time schedule killed. Originally I intended to set up the layout in a rush and have a PCB in hands during winter holidays. Forget that.
Synergies between the power stages of SystemD_MD and SystemD_2kW will be low, some essential parts of the 2kW power stage have to be designed from scratch.
Not sure if I should be sad or happy.
Most likely happy. Designing from scratch is more hobby fun than doing a layout in a rush.
fun ahead

Many thanks to all of you for your comments and special thanks to Tom for critically asking if I really want to rely on the body diodes!
I agree the simplistic approach would be to brave with respect to todays components.
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Old 5th December 2012, 11:23 AM   #110
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Have you considered cascode switching using a common-base (gate) bjt or igbt in place of series schottky?
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