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I'm building a capacitor discharger with a thyristor for high-voltage impulse generation into an inductive load. To make the circuit easier to use, I wired the thyristor as a high-side switch. Then I built a floating gate driver circuit around it. The isolated ground is connected to thyristor's Cathode, and the driver output is connected to the Gate. The idea is that, when the thyristor turns on, the potentials of both its cathode and gate rise up to a high value, but the isolated driver always applies a bias to its gate, in spite of the rising potentials.

Isolated Driver

The isolated high-side thyristor gate driver is built using an isolated DC-DC converter with 3000 Vpk functional isolation, and an isolated MOSFET driver.

Isolated thyristor gate driver

The 12 V isolated voltage is generated by an isolated DC/DC converter module with 3000 Vpk functional isolation, Murata NMV0512SAC (26 pF isolation capacitance). The gate driver is generated by an isolated MOSFET gate driver (kind of an abuse, I know, but as long as it can generate a driving pulse within its ratings, it should work), TI UCC5304 (0.5 pF isolation capacitance).

According to an ABB application note, for high-current, high di/dt applications, it's important to generate a momentary surge of high peak current to the gate (as high as 100% of rated absolute maximum gate current) to trigger the thyristor to its fullest degree. Thus, R26 and C41 are used to provide a surge of 2.3 A of peak current. After a few microseconds, R27 takes over, reducing the steady-current gate drive to 0.4 A.

Clamping TVS diode D7 has been removed during the test.

Thyristor

The core of the circuit is shown in the follow schematic.

schematic

simulate this circuit – Schematic created using CircuitLab

The actual device prototype is fairly complex and includes multiple subcircuits and an expensive 1000 V thyristor. But for the purpose of this question, I've simplified the circuit to its core and downscaled both voltage and components, and I could still replicate this problem. The capacitor is charged by a high-voltage supply to 130 V, then the thyristor is triggered by the control logic to discharge the capacitor.

The component values of the load are required by design of the pulse-shaping circuit and cannot be changed.

Two tests have been done, I performed the first test with the switch SW1 open, disconnecting the rightmost part of the load (actually by uninstalling R2, there's no physical switch). The next test was performed with SW1 closed, discharging high impulse current into a low-impedance load. Calculations have been done to ensure that both the maximum di/dt and the maximum non-repetitive surge current ratings of the thyristor were not exceeded.

Results

Unfortunately, tests showed that the circuit does not work as expected.

Oscilloscope screenshot of the first test, with SW1 open

During the first test, the switch SW1 is open. Channel 1 is a 10x oscilloscope probe connected between the left side of R26/R27 (driver side) and DRIVE_GNDC, Channel 2 is a 10x oscilloscope probe connected between the right side of C41 (DRIVE_G) and DRIVE_GNDC, and the measured waveforms behaved as expected - a microsecond voltage spike at the leading edge by the RC differentiator, followed by a steady-state low drive voltage, lasting for 1 second.

Oscilloscope screenshot of the first test, with SW1 closed

During the second test, the switch SW1 is closed and the same measurement setup was repeated. Now the waveform no longer behaves as expected. An undershoot as high as -33 V is generated in this test (look at the change of scale!). What also immediately followed by the test was a noise similar to a "zap" or "bang". After the test, the thyristor is destroyed - it became very leaky and the capacitor can no longer be charged.

This is actually not the worst result. During the process of testing this circuit, an undershoot as high as -100 V was experienced at higher voltages. Here's another example.

Another oscilloscope screenshot with higher undershoot

Remarks

If a 15 V bidirectional TVS diode is installed across the gate driver, the circuit can survive much longer. It was tested up to 800 V before the TVS diode was destroyed. But it was entirely accidental, during the design the TVS diode was only meant to handle small transients, not a huge undershoot I just described. Naturally, the TVS diode was eventually destroyed after a couple more tests and failed as a short-circuit. But it seemed to save the thyristor in this case, as the circuit worked again after replacing the diode, thus thyristor was not (completely?) destroyed.

It's also worth noticing that this is already the second prototype on a PCB. The first prototype was constructed on a perfboard with an improvised gate driver but similar in principle. Mysterious driving transistor destructions were also observed, but the problem went away after a high-voltage diode was installed in series with the gate. And that prototype was successfully tested up to 1000 V and 500 A per design without any problems. On second thought (unconfirmed), that prototype may (or may not) have the same problem, and the diode may have blocked this destructive undershoot, and only the initial peak voltage arrived at the thyristor's gate. It may work fine for a while, but probably at the risk of incomplete thyristor turn-on - this may reduce its reliability.

Update: Layout

This layout is clearly sub-optimal from the perspective of high-speed electronics. Unfortunately, high-voltage electrical spacing requirements created a lot of layout constraints. Although there's no regulatory requirements for functional isolation, good design practices call for around ~5 mm of electrical spacing for uncoated through-holes at 1 kV for reliability. I found it was difficult to use ground planes at the output side around the thyristor and its load, Thus, all nets are routed as traces.

PCB layout around the thyristor

The output drive and ground of the gate driver was routed to the thyristor as two individual traces. Power and gate driving nets were routed as different traces as Kelvin connections, with no common path. The pulse-shaping network at the output was routed by traces as well.

The oscilloscope probe placement was also sub-optimal. Due to the lack of test points, ground springs are not an option. The probe ground was clipped at the labelled through-hole in the picture. The probe input was clipped at the "Gate" pin of the thyristor.

And this is how the load current is returned to its source.

PCB layout of the return traces

Update: Cathode, Anode and Gate with Respect to GNDB

Could you show us the voltage waveforms at the thyristor's anode and cathode with respect to GNDB? I have a feeling that the cathode voltage might be rising above the anode voltage. - Jonathan S.

Here are the traces.

Channel 1 (yellow) is Cathode, Channel 2 (blue) is Anode. I don't think there's anything unusual. Sure, there's a small inductive spike at turn-off, but I don't think it has anything to do with the initial gate undervoltage.

waveforms at the thyristor's cathode and anode with respect to GNDB

Channel 1 (yellow) is Cathode, Channel 2 (blue) is Gate. This still clearly shows the gate undervoltage.

waveforms at the thyristor's cathode and gate with respect to GNDB

Question

What's going on? What is responsible for the massive undershoot when the isolated driver is supposed to maintain a constant bias voltage on the thyristor's gate?

My current hypothesis is that high-voltage thyristor has an internal resistor across the gate and cathode (10 Ω, as I already measured), thus, the isolation of the driver is essentially breached by this low-value resistor, making the assumption of isolation no longer valid. Is it the correct culprit? If so, how can an isolated high-side thyristor driver be constructed? If not, what is the real problem that I've missed?

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    \$\begingroup\$ Think about the isolation capacitance of the "so-called" isolators and think about the dv/dt and, work out what spurious currents can arise with a very high dv/dt during capacitor discharge. \$\endgroup\$
    – Andy aka
    Nov 12, 2022 at 22:07
  • \$\begingroup\$ Which Thyristor are you actually using? And what's the dV/dt on the cathode? How's the PCB laid out? Are you using a Kelvin connection for the thyristor's gate and cathode pins? \$\endgroup\$ Nov 12, 2022 at 22:41
  • \$\begingroup\$ @JonathanS. To demonstrate this problem, TYN640 was temporally installed and tested at 130 V to produce the oscilloscope screenshots #1 and #2, exactly as indicated on the schematics. During earlier tests, the thyristor was IXYS CS60-16IO1 as designed. \$\endgroup\$ Nov 12, 2022 at 22:46
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    \$\begingroup\$ Drive the thyristor with a pulse transformer. Then you won’t need an isolated 12V supply and eliminate a lot of stray inductive and capacitive paths. \$\endgroup\$
    – Kartman
    Nov 12, 2022 at 23:14
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    \$\begingroup\$ A "snapping" sound is rather characteristic for high-current thyristor pulse circuits. The extreme current rise times cause intense magnetic fields that exert physical forces on the circuit, causing the PCB to bend slightly, which makes that noise. \$\endgroup\$ Nov 12, 2022 at 23:51

1 Answer 1

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Upon further investigation, the problem has been solved with a rather anticlimactic conclusion. The isolated power supply design itself is pretty solid, the true culprit is a basic and unforgivable mistake: an unintentional violation of the Kelvin connection design rule.

I thought I'm already familiar with the basic techniques in high-speed electronics, such as common-impedance coupling, Kelvin connections, star ground, ground plane, or minimizing loop area, I thought I simply wouldn't make this mistake. The mistake was nevertheless made due to an oversight - after I have been starring at the same schematic and layout for too long, I became too desensitized to notice that. After a night of sleep, a reexamination of the layout immediately showed the mistake.

Let that be a lesson for everyone. If you're troubleshooting a problem that doesn't make any sense, double check your assumptions.


Post-mortem of the PCB layout mistake

Due to an oversight, the load current, up to 500 amps, was returned directly via the tiny gate driver traces!

Incorrect PCB layout

Hindsight is Always 20/20

schematic

During the first test, the switch SW1 is open. [...] the measured waveforms behaved as expected - a microsecond voltage spike at the leading edge by the RC differentiator, followed by a steady-state low drive voltage, lasting for 1 second. [...] During the second test, the switch SW1 is closed and the same measurement setup was repeated. Now the waveform no longer behaves as expected. An undershoot as high as -33 V is generated in this test (look at the change of scale!).

This immediately suggests a Kelvin connection violation caused by R2 & L2 followed by SW1.

What's more revealing is that the undershoot was almost a mirror image of the output, something I've already noticed when I posted the question, and I even deduced it was a ground bounce. But because I assumed Kelvin connection was already enforced during the entire design process, I did not recheck this assumption.

Conclusion

The solution was obvious: lifting the component leg of R1 and connecting it directly to the thyristor's Cathode with a jumper wire. After applying this fix, the correct gate drive waveform was obtained, and the problem has been solved.

Gate driver waveform after applying the fix

Loose ends

  1. During the test, I spent much time looking for the source of the sparking/arcing noise. Initially I thought the isolated DC/DC module was arcing and I replaced it with a high-quality one but the problem persisted. I also suspected the hand-winded inductors were arcing (due to bad insulation) or the capacitors themselves were arcing (due to rapid charge/discharge), but couldn't find its source. Eventually I suspected the problem was a mechanical microphonic effect of the inductor windings but couldn't be sure. The correct solution was given by Jonathan S.

    A "snapping" sound is rather characteristic for high-current thyristor pulse circuits. The extreme current rise times cause intense magnetic fields that exert physical forces on the circuit, causing the PCB to bend slightly, which makes that noise.

    This further distract me from the actual problem.

  2. A code review of the control firmware showed that a flag was set incorrectly, and under certain situations, may charge the capacitor bank to the maximum voltage possible in spite of the input setting after the first firing of the impulse. This likely also caused further confusions during troubleshooting.

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    \$\begingroup\$ Oops! Well, stuff like that happens sometimes! I'm rather surprised that the tiny Kelvin ground trace didn't instantly get vaporized, though... \$\endgroup\$ Nov 13, 2022 at 13:54

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