Attempt at a quick and dirty high voltage high frequency amplifier for testing insulation at frequency.
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Take a look at the project logs for more details and continued work. YouTube Overview:
JFETs, Poor performing transformer and a Fluke 415B supply.
You can see significant performance improvements, and under ideal situations this amplifier will hit ~3kV/uS slew rate, with a 1kVpp output.
Changes made to improve performance:
The new power supply was just an isolation transformer (120/240 windings), with a bridge rectifier, and a switch to flip the output between series and parallel for a 120 or 240 AC output (180 or 340 DC). The image below has the new setup.
After all the changes the amplifier showed significantly better performance. With the new power supply I am able to push at-least 100mA of current as opposed to my limited Fluke supply of 30mA. The next challenge is a thermal one, so I used a thermal camera to examine the actual operating temperatures. I was concerned about the 2 transistors mainly, and at ~65mA of Bias the lower cascade TO-92 was slightly beyond its 1W rated operating point, with a temperature of 133C.
Here is a newer shot of the 3dB performance.
Unfortunately I found after taking this picture the circuit is highly sensitive to capacitance loading on the output. This was using a 100:1 scope probe with a 7pF capacitance. Putting a 10:1 probe with 17pF greatly reduced the 3dB bandwidth to ~650kHz from the ~1Mhz shown in this image.
So it continues to appear that a transformer coupled high voltage amplifier is probably not a great design. If I used huge amounts of bias current, the feedback could compensate but the power dissipation would be an issue long before achieving significantly better performance.
At a more reasonable frequency like 200kHz, this amplifier does perform well with minimal output capacitance. For example, we achieve 1kVpp below at 200kHz, this is 600V/uS slew rate which is a decent accomplishment, and on-par with real lab solutions although the drive capability of this amplifier needs fairly ideal conditions.
Also note you can see the malformed bias current waveform, this is due to bottoming out the current to 0mA every cycle, the only reason the output wave is not distorted is due to the inductor supplying that current for a short period. Basically this is the maximum gain without more bias current.
As alluded to in the last post, making a wide band transformer coupled amplifier is challenging. More challenging then I initially thought. Below are some notes as I worked towards an optimized transformer.
Starting out, I knew I needed to minimize leakage inductance and inter-winding + cross winding capacitance. This is the key to any high performance transformer, especially at high frequencies.
When talking about parasitics, there are two main ideas. Both are at odds with each other:
Transformer design typically comes down to finding a compromise and that is hard to do without complex simulations or just building up some designs and taking tests. So I used my experience in building SMPS transformers to guesstimate a starting point.
Starting off, since this was going to be a high voltage and high frequency design, I have never used u-core ferrite, but it is what is used in old high voltage TV flyback transformers. I figured leakage inductance would be an issue, but the separation of windings would make parasitic capacitance nearly non-existent.
In the image above, you can see two windings on the transformer and two more I made to take measurements with. just different winding counts and 28-40AWG to fit as many turns on a single layer as I could.
I shot a video of winding these transformers, basically just a Kapton film that has wire wound on top of it. I tried optimizing winding counts to see if there was any room for improvement, but the U-Core just had to low of permeability, requiring a lot of windings to get the needed inductance and thus incurring leakage inductance and capacitance. In the end what I was after was not that. So both the shape and actual material were less then ideal.
After some googling, I found a German company (Vacuumschmelze) making torrids with a nano-crystalline high performance material, This would give me 10x more permeability, and basically allow me to reduce my windings by 5-10x for the same inductance values, resulting in lower parasitics. This material also has a higher saturation point vs typical ferrite (1.2T vs 0.4T)
Looking at the picture below, the green is the new material / transformer. The blue is a typical ferrite. My u-cores actually had a permeability of ~2000, so even worse then this plot shows. If you are not a transformer expert higher permeability means:
The cores are more expensive then typical, but if you work with magnetics ever, I strongly recommend you check this stuff out.
Below is a chart of real measurements of the transformers shown above. Note the Torrid has 10uH primary inductance at lower frequencies hence the '*'. You should note the massively better leakage inductance results.
I recently procured some United SiC JFETs, which looked to be a solid answer for any active region high voltage or high current project. I figured they may be a good start for this amplifier, and I could drive them in a trivial cascode configuration with a high voltage supply and signal generator. Basically in a few hours I could see how they performed.
First I found a simulation model, and discovered the high frequency performance was not amazing. I assume this is due to drain to gate capacitance, and even in cascode it provided a challenge. The other issue is a resistive only coupled output would limit my "high voltage" to a few hundred volts.
This was enough for me to construct a circuit on a bare PCB for testing.
I then attempted to add a transformer. I initially used a simple model that looked promising, but upon constructing a U-Core ferrite transformer with a 1:3 step-up, and measuring the parasitics, I found the simulation and real life to come shockingly close, both with poor performance...
The low frequency was limited by the primary inductance of the transformer (Lack there of), while the high frequency was limited by the parasitics of the transformer and JFET. Unfortunately fixing one of those issues would make the opposite worse. This resulted in a very narrow band of operation.
Basically I needed to really go back to the drawing board and fix both the silicon and the transformer. I ended up finding some 400V rated depletion mode NFETs (DN2540). People commonly think it is easy to find high voltage FETs, but 99% of the FETs you see online are designed for switching and can not be operated in the linear region for use in amplifiers like this.
These new FETs had less parasitic capacitance, although a lower voltage and power rating as well. Then in an effort to improve the transformer, I found a unique ferrite product from a German company (VACUUMSCHMELZE). It has significantly higher permeability then typical cores. This allowed more primary inductance with 1/3 the turns of the U-Core I was using. This then also resulted in less leakage inductance and lower inter-winding capacitance.
See the next log for more info.
Collin Matthews
michal777
Caleb W.
Szoftveres