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Double the Carbon, double the fun


simmconn
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I was going to name it 'Double the Carbon, double the voltage, double the fun', but I'm afraid that would be too wordy.

Ever since I measured the output swing capability of the Carbon, I wanted to see if it can drive my ESL speakers. To do that, I'd need a beefed-up version that can output at least 1kVrms before clipping. That's 1414V peak, out of a ±750V to ±800V PSU.

To simply scale up the Carbon amp and the GRHV PSU without major changes to the PCBs, a few hurdles would have to be overcome. First is the 900-V rated 10M90s in the cascode CCS that bottom out with ±450V PSU. When I was browsing the high voltage FETs at Mouser, I found the UnitedSiC UF3N170400B. It's a 1700V 6.8A SiC JFET, naturally in depletion mode, perfect for the upper device in the cascode CCS. The threshold is about 9V, which helps the DN2540 to get further into the constant current region yet keeps the power dissipation still in check (no heat sink needed even with the TO-92 package). The drawback is the package. The TO268-7 isn't too friendly for a DIY thermal solution. I had a few carrier boards made with aluminum-based PCB. According to simulation, the little board would give me 9 degree temperature difference from the package case to bottom of the board when the FET is dissipating 9W. I chose black solder mask, hoping to get a bit better heat radiation capability. 10 boards cost me about $3 at JLC. They also have copper-based PCB with a special process to have the base copper flush with the top surface for a much, much higher K. Too bad the NRE is about $150, so I chose the cheaper route with aluminum. The insulation is good to 3kV so this board can be bolted directly on the main heat sink without additional insulation. Isn't that great?

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The next bottleneck is the 600-V rated STN9360 when the PSU voltage reaches ±600V. I replaced them with the 900-V rated 2SA1968s. Thanks to the cautionary words from spritzer, I started cleaning up traces with high voltage differential that are getting too close to each other. I removed the middle pad 'A' in the 10M90s footprint, and re-wired a couple of traces on the back. With the bias changed from 24V to 13V above B-, the JFET can be used as the output device as well. At almost $10 a pop, they are considerably more expensive than the SiC MOSFET they are replacing. So the full-JFET configuration shown below is more of a proof-of-concept than anything else.

445461540_IMG_2077(1024x768).thumb.jpg.38011052ea2f9d12b2950c6ce4459ea0.jpg

The feedback dividers are changed from 200k:200 ohm to 440K:470 Ohm to maintain the gain and allow the output swing to double, without having the feedback current overtaking the idle current through the input JFET. Two 10pf 1KV caps are in series as a 5pf cap, since I don't have the proper 2kV rated caps. Not a great idea. I was lucky it didn't blow up.

Technically the board is now ready for up to ±850V. I tested the SiC-JFET 'doubled carbon' config with ±400V PSU. The performance is slightly worse than the original Carbon. Is it worth the effort? We'll see.

(To be continued...) 

Edited by simmconn
Minor corrections
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6 minutes ago, kevin gilmore said:

start with a megatron and change the 8 output tubes to 4cx250

then use +/- 3000v power supplies

far more fun. and just a slight bit more dangerous.

maybe it can direct drive Quad electrostatic speakers bypassing the speakers in built audio step up transformer 🙂 Kevin, maybe we need a super high current and super high voltage golden reference psu?

 

Edited by jamesmking
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actually for 4cx250, maximum power supply would be +/-1000 volts and forced air cooling for the tubes

3-500z much better. maximum power supply would be +/-2000 volts no forced air cooling

after that it gets really large really fast.

all tube golden reference power supply is not that hard.

custom transformers will have to use very high voltage wire. will look like flyback transformers from old crt color tv.

 

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1 hour ago, kevin gilmore said:

actually for 4cx250, maximum power supply would be +/-1000 volts and forced air cooling for the tubes

3-500z much better. maximum power supply would be +/-2000 volts no forced air cooling

after that it gets really large really fast.

all tube golden reference power supply is not that hard.

custom transformers will have to use very high voltage wire. will look like flyback transformers from old crt color tv.

 

I wonder if it would then be possible to use the redundant quad esl audio step up transfers in reverse to step down the 4cx250 output voltage so it can drive conventional headphones and/or conventional speakers making the amp totally universal: drives quad esl, drives stax without the step down transformer, drives conventional headphones or speakers with transformer....  drives everything   SPLAT  :tmonk:

 

 

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The second part of the experiment is the PSU. The Circlotron HV power supply (goldenreferencehv900new.pdf) uses an ingenious way to overcome the component voltage rating limitations, by using a floating reference point at 1/2 of the output. Since I'm not pursuing 900V, maybe I can get away with replacing a few parts in the GRHV and settle at 800V? Aside from doubling up on the input and output caps, the STN9360 needs to be replaced with 2SA1968, and KSC5026 replaced with 2SC3675 or 2SC4686A, and the resistor divider updated. Done, I thought.

After a couple of minor volcano eruptions, I had to go back to the drawing board.

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Since they both failed at power up, it looks like either the current limiter didn't work, or it was overwhelmed because the SiC-FET had a D-G breakdown. I put the SOAs of the SiC-FET side by side, comparing their allowed VDS under 0.1A of current at 25C, which is the PSU start up condition:

SOA_Compare.thumb.png.d7f482304497e39f6f5195c231fbe83b.png

Apparently the C2M1000170D is the most robust, allowing 700V (100ms). The G2R1000MT17D scores the second, allowing about 450V. The MSC750SMA170B only allows a little above 300V. Considering it's specified at max Tj, the 25C spec should be better. So what works for the C2M1000170D may not work for the cheaper subs. It could be SOA violations that killed the SiC FETs.

The GRHV soft start uses the cascode CCS to charge the Tantalum cap until the SiC FET VGS is higher than its threshold voltage. The SiC FET turns on slowly so the current limiter has plenty of time to get in action. The output cap is then charged with 0.1A of current over a few seconds until the preset output voltage is reached.

The above assumes that the PNP is ON at the beginning, which is not exactly the case. The output cap needs to be 'pre-charged' to about 1.2V to establish a DC path for the PNP to turn on. The per-charging is done with the SiC FET fully on, and only the current limiter in effect, when the SiC FET is most vulnerable. Although the pre-charge time is short (several ms depending on the output cap), it's better safe than sorry. My solution is to add a diode D5 to establish an initial DC path for the PNP. I chose a low-capacitance small signal schottky diode, with a resistor R2 in series to further reduce the effect of its junction capacitance.

Also during the soft start, the charging current increases quickly to reach the current limit, but the output voltage only ramps up linearly, which means the SiC FET would pass a lot of current when its VDS is still high. It would be great if the VDS can affect the current limit and set it lower during ramp up. So I added R5 and zener D7. D7 has a higher voltage than the normal working dropout voltage, so R5 wouldn't take effect during normal operation. 

After the above mods, the SiC FET has not failed. I also reduced the output voltage to 750V and reduced the input accordingly. The last failure happened a few minutes after turning on, and was caused by thermal runaway of the PNP. It turns out the DN2540 developed some kind of gate leakage, where the voltage drop on its source resistor no longer controls/represents the output current of the CCS. A lot more current gets dumped to the PNP, causing it to burn. I couldn't figure out what caused the damage, perhaps some transient conditions. So I added another zener diode D8 to protect the DN2540. 

750v_PSU_SCH.thumb.png.7c462e4c4bc41dbd624396d87e19a9bb.png

Now both GRHV PSUs are upgraded to 750V output and work stably so far.

The amp boards work okay when connected to the 750V PSU. Now is the time for some measurements. First is the all-JFET configuration.

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And swapping the SiC FET back in as the output device, with visible improvements:

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The SNR is about 6dB worse compared to the original Carbon under 400V PSU, due to the feedback resistor mod in order to get a higher output voltage. Did I get the 1kVrms output I was looking for? Yes. Well, sort of. The problem is, the volcano eruptions ruined two SiC FETs and I don't have any more to populate the other channel. Digi-key has the C2M1000170D back in stock now, but the price has gone up to nearly $10 a piece. I guess I'll keep using the GU-50s on the other channel.

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Nice work @simmconn

The current limiting with Q5 is not fast enough.  Put a depletion mode mosfet CCS in front (left) of M6 (Vin).  Similar to what Kevin did on the T2.  You'll also see this on the Circlotron 900V supply.  The golden reference supply is much more robust this way :)

Good luck!

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Posted (edited)

Thanks. A fixed-setting current limiter before the passing device would also help the SOA issue. I can also do something like the following:

Start_Current_limit.png.286e4efc8f1ed232c57a403f63aa81d1.png

With carefully selected R5 and R8 values, M1 and U4 can split the start up VIN. A resistor can be added to the M1 source so it also acts as a current limiter during normal operation. After all, most of the high voltage Si-MOSFET and SiC FET are designed for switching applications. We need to tread carefully when using them in linear mode.  

Edited by simmconn
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