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Old 20th July 2011, 07:09 PM   #51
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Gate leakage increases with Vdg and also with temperature. This not much
of an issue unless it increases as the chip ages. Typically that is not the
case from the units I have examined after many hours of operation, but
I have seen very leaky devices get worse.

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Old 20th July 2011, 10:02 PM   #52
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Quote:
Originally Posted by wrenchone View Post
I would worry about the solder bond to the package and the epoxy at temperatures much above 150C... I wonder how they do the bond-down anyway?...
175C is a typical "shutdown" temp built into devices of "normal" Silicon. It also can be expected to be 15-20 X accelerated life. (1 day would be the equivalent of 15 or 20 days!). I would expect the package to be the stumbling block.
I've been downstairs a few times There are several processes and I do not know any SiC process details but: Silicon die are ussually soldered to the package substrate with a silver paste in an oven the reaches 350-450C over the coarse of an hour maybe, depending on the process (450C is very common). Talk about accelerating life! The die bond process must be considered in the big scheme of things. Every process the die goes through may expose it to more heat. Growing further oxide thickness or defusing junctions deeper. Therfore higher temp processes are first and every sucsessive one lower temp. There are lower temp, coductive epoxy die bond processes.
The die likely makes at least one direct electrical contact with the pkg substrate. Ussually the Drain of a MOSFET. The Gate and Source can be wire bonded. Bonds are ussually acoustically welded(what a coincidence). Many wires are used on the Source. There are other pakaging configurations but we're thinking TO-247 here. If it is encapsulated the epoxy used becomes soft at some temp ussually about 175. It can move around and it can exert pressure on the wire and wire bonds. There is also bumps, but we'll save that for another time.
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Old 20th July 2011, 10:20 PM   #53
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I'm quite familiar with the bonding processes for typical Si parts. A lot of the time conductive epoxy is used. Some use solder - I used to see that a lot on TO-3 parts. I don't know what they use for SiC. I gave one of the process/packaging guys downstairs from me an 085 to play with. I never heard anything back from him - time to corner the bloke and make him cough up.
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Old 22nd July 2011, 09:01 PM   #54
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I have one channel working more or less satisfactorily, though somewhat shy of the desired value for output stage bias current (120ma vs 150). I went ahead and loaded the 8 ohm tap of the output transformer with a 10 ohm resistor, and did open loop and closed loop excitation with sine and square waves. The first picture shows result from sine excitation. The left-hand set of waveforms is with partial feedback (R7 is connected, C2/R6 are not). The gain under these conditions is ~62X (36 dB). With the loop closed around the transformer 8 ohm secondary (using R6, with partial feedback resistor R7 still in place - C2 is not stuffed), the gain is 14.6 (23.3 dB).
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File Type: gif OL-CL_SIN.gif (19.1 KB, 786 views)

Last edited by wrenchone; 22nd July 2011 at 09:14 PM.
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Old 22nd July 2011, 09:08 PM   #55
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This next set of pictures shows open loop/closed loop square wave excitation. Closing the loop around the transformer tightens up the square wave response. There is output overshoot, but it is just the overshoot of the input waveform amplified. The amplifier looks stable without compensation capacitor C2. The other breadboard I have exhibits low gain and a distorted triangle output with 1kHz square wave excitation. I suspect an accidentally zorched output jfet (ouch!) or a defective output transformer. We'll see tomorrow. When I get both channels satisfactorily operating, I'll push some tunes through them. I'll also do a gain-phase plot of the working channel when time permits.
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File Type: gif OL-CL_SQ.gif (17.6 KB, 774 views)
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Old 25th July 2011, 02:40 AM   #56
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OK, both channels are up, and I ventured powering up a pair of so-so Infinity 2-ways hanging around in the lab, using a fairly venerable Panasonic portable CD player as a source. All the roaring and screaming and fet-fiddling is worth it. Crisp, detailed, lifelike.

I still need to fiddle with the bias current on one of the amps, but the simple tube-derived SE circuit works a treat. Some effort changing around the input fets may be worthwhile, but the Supertex input fets didn't appear to harm the sound any. Obvious one to try would be some of the smaller Ixys depletion mode devices. I may also get better results by pushing more through the input fet and reducing the drain loading resistor, but that experiment will wait until later. This guy will definitely make the trip to Burning Amp.
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Old 1st August 2011, 07:20 AM   #57
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Last week, I powered up both channels of the amp in the lab at work and got a "thumbs up" from the co-workers still hanging around. The amp went home with me to be fitted with a case, fan (throttled down DC fan) and a decent power supply. It was running at work off a nifty Xantrex HV bench supply, a nice piece of kit for a tube fancier with deep pockets, but essential also for folks doing SMPS development.
For the power supply, I plan to try an isolated 115/230V step-up transformer with a full wave bridge, which should give me about 320VDC before my capacitance multiplier. I'm curious to see if one transformer (150VA) will hold up powering both channels - if not, I'll try a dual power supply approach, as I have two of the transformers and paid almost nothing for them. First priority is scrounging around in the basement to see if I can scare up a suitable case.

The attached schematic is the one I'm currently using. The values of the parts denoted with asterisks should be changed to get the proper bias current, depending on the parts you have on hand. Leaky 085s (that is, ones with leaky gates) won't work at high drain voltages with a 10k gate resistor, and should be used in low voltage projects - more on that later, as I have far more projects in process than time to work on them.
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Old 2nd August 2011, 10:52 AM   #58
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wrenchone,Would you please share you used output transformer primary Impedance?thanks.
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Old 2nd August 2011, 05:18 PM   #59
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Look it up, it is the One Electron UBT-1 - I made no secret of it. Also, the impedance is mentioned in the very first post of this thread.

Last edited by wrenchone; 2nd August 2011 at 05:32 PM.
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Old 3rd August 2011, 12:44 AM   #60
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Got it,thanks.
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