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Old 2nd February 2011, 02:52 PM   #1
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Default High voltage, low current output stage for class D amplifier

I'm working in a class-D amplifier to drive a pair of (homebrew) electrostatic loudspeakers. My target voltage is something between 1.2kV and 2kV peak to peak. The amplifier will deliver less than 50 mA's in all circumstances and probably less than 1 mA in average. This means that Rds On is going to hurt little, even if it's in the low kilohm range.

There are two problems in this design: Level shifting and output devices. Adequate output devices should be relatively low-current components, as high current capability usually means high gate charge. As for level shifters, the options are to use either a digital level shifter or an optocoupler. However I'm a little confused in the device election so I would like to ask the wise people in the forum for advice in choosing these critical parts.

My idea is to see if it's possible to build a class D amplifier + electrostatic loudspeaker system demonstrating ultra high efficiency and low overall distortion (including the loudspeaker in the distortion computation), rather than building something that fits audiophile tastes, so a conventional half bridge amplifier using raw UcD topology seems fine.

Last edited by ionomolo; 2nd February 2011 at 02:54 PM.
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Old 2nd February 2011, 03:11 PM   #2
jcx is offline jcx  United States
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output device parasitc C makes high V, high frequency, efficiency switching amps impractical, as do the limits of output filtering inductor parasitics with high V*T capability

audio frequency xfmr step up would be an easier option

Last edited by jcx; 2nd February 2011 at 03:15 PM.
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Old 2nd February 2011, 04:26 PM   #3
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The point here is that we are talking about switching a few watts only. Even with 5% efficiency this would fall within the SOA of a single output pair. The output inductor is not much of a concern, as inductors are damaged by excessive current, not voltage, and currents are going to be incredibly small in this design. The capacitor may be more of a concern, but nothing that can't be fixed by spending a few bucks in a HV cap and a larger inductor.

The big problem, as you noticed, is the output capacitance. My second-line defense against it is cascoding. It's an ugly solution for a class-d amp and phase delays will demand very careful attention, but it's a solution after all. However I believe there is some hope left about finding an adequate pair of transistors. The reason is that the voltage is not *that* much higher than in conventional amplifiers, only one order of magnitude, but current is three orders of magnitude smaller. Three orders of magnitude would mean decreasing the number of carriers in the FET by three orders of magnitude to have the same voltage drop, and the parasitic capacitances by a comparable amount. Moreover, since a voltage drop that would be awful for a 50V amplifier is irrelevant in a 1000V amp, we could even relax the on-state voltage requirement by an extra order magnitude. With such a relaxed conditions current-wise, it seems strange not to find a device that suits the design.
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Old 2nd February 2011, 07:40 PM   #4
jcx is offline jcx  United States
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if you calculate circulating power for full V drive into the C load at the highest audio frequency you may end up with more than "a few watts only"

Class A push-pull can be 50% "efficient" - but you have to bias for 1/2 peak current at highest audio frequency into the panel's C

1/2 * 50 mA * 2kV = 50 W

IXTH03N400 Mosfets could do the job in a linear amp - if you can buy them

Class AB is possible, cutting power supply current, mosfet dissipation requirements


the IXYS devices could be used as switches too except for the slow body diode and the need for floating gate drive since complements aren't available at these V levels

I still have doubts about a efficient switching inductor design - mH inductance, kV insulation, spacing requirements, while keeping SRF several 10x the switching frequency min
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Old 2nd February 2011, 08:26 PM   #5
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Quote:
Originally Posted by jcx View Post
if you calculate circulating power for full V drive into the C load at the highest audio frequency you may end up with more than "a few watts only"

Class A push-pull can be 50% "efficient" - but you have to bias for 1/2 peak current at highest audio frequency into the panel's C

1/2 * 50 mA * 2kV = 50 W

IXTH03N400 Mosfets could do the job in a linear amp - if you can buy them

Class AB is possible, cutting power supply current, mosfet dissipation requirements


the IXYS devices could be used as switches too except for the slow body diode and the need for floating gate drive since complements aren't available at these V levels

I still have doubts about a efficient switching inductor design - mH inductance, kV insulation, spacing requirements, while keeping SRF several 10x the switching frequency min
My 5% efficiency and "few watts" here relate to the power dissipated in the loudspeaker, not to the total amount of power flowing back and forth. These are -approximately- independent of frequency. The key idea here is that class D amps have very high efficiency with capacitive loads. Obviously there will be conduction losses, but it would take something like 5 KOhm Rds per device to waste 20% of that power.

With regards to the inductor, the total amount of energy stored should not be any higher than in conventional, low current high voltage designs. Thiner wire, more turns, same flux. Isolation is a problem but I don't believe it's a show stopper.

I will take a look at your devices. I'm also looking at vacuum tubes, but I believe there is some delay mechanism in these that I don't understand. Less than 1 pF of output capacitance is too good to be true, basically rendering the driver design trivial. There is also a part of me who wants to stay away from these faulty things because the entire heather wiring and unreliability of tubes would make debugging much longer in a device that poses an extraordinary danger to work with.

I understand this is an uncommon project and I may have to face many unforeseen complications, but there are things that work in its favor too. Low current means little EMI and less problems with parasitic inductances.

Last edited by ionomolo; 2nd February 2011 at 08:29 PM.
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Old 2nd February 2011, 09:05 PM   #6
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Have a look at KSC5042 bipolar transistors, although they are only 900V, so you'd need to bridge them.
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Old 3rd February 2011, 08:09 AM   #7
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Quote:
Originally Posted by darkfenriz View Post
Have a look at KSC5042 bipolar transistors, although they are only 900V, so you'd need to bridge them.
The electrostatic loudspeaker design is already bridged so the voltage would be a bit on the low side, but the 2.8 pF output capacitance of this transistor is wonderful! I don't know about switching times for bjt's however.

Perhaps somebody could enlighten me about switching times in MOS vs BJT's vs Tubes.
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Old 3rd February 2011, 12:38 PM   #8
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This seems to be the guy:

http://ixdev.ixys.com/DataSheet/DS10...V02N250_S).pdf

2.5 kV, 200 mA, 8 pF output capacitance.

I think I will start designing the level shifter around it.
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Old 3rd February 2011, 01:42 PM   #9
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The major drawback of bipolar transistors (when used for class D) is that there is a variable delay (increases with base current and temperature) from the instant base charge starts to be removed to the instant Vce starts to rise. This delay may range from 200ns in low current devices (500mA) to 2us in high current ones (30A).

When operating bipolar transistors at collector currents not much lower than the maximum rating (>1/4 max. Ic) there are also increased conduction losses during the first 200ns-2us of each switching period while charge is being stored in the B-E junction (which has to be removed later before turn-off can happen). Vce-sat starts high and approaches steady state value as more charge is stored (dynamic saturation process). The higher the base current, the more charge ends up stored.

Go for high voltage MOSFET, they excel at low currents.
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Last edited by Eva; 3rd February 2011 at 01:46 PM.
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Old 3rd February 2011, 03:21 PM   #10
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The IXYS high voltage MOSFET I've just found seems to be the solution. Pity it's not in stock at Digi-Key.

Now the problem left is the level shifting. For this purpose I would like to use an optocoupler or digital level shifter, but I'm worried on the upper side of the totem pole, because the drain is referred to the (unfiltered) output and not to a constant voltage, and high frequency signal may travel along the level-shifter ground and leak trough the mains transformer.
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