Isolation and protection for SCR gate circuits

You are basically referring to the behavior of your hardware under single-fault abnormal conditions.These are only considered possible across basic insulation barriers or components without reinforced insulation.

Your test equipment will be expected to provide reinforced insulation between the hazardous potentials and the operator, so that single faults do not result in a hazard to the operator.

Single faults are basically any single short circuit or open circuit condition that may occur. For any specific single fault, a satisfactory safe result is expected - no hazardous conditions presented to user accessible terminals - no fire or sustained source of combustion - no ejected or loose material or connections that might produce a second fault - no opening of wiring that is not specifically designed to open safely (forget about it - no open printed traces or harness wires) - no loss of hipot test integrity.

Some components can reliably perform protective functions without producing an unsafe result - a fuse or other limiter that is correctly sized and placed can prevent many downstream devices from unsafe behavior. Series or parallel components in some situations prevent any abnormal conditions from occurring.

Single faults applied to components on the operator-accessible portions of the circuitry also have to result in the same behavior.

Draw up a table of components and board spacings that don't provide reinforced insulation levels or redundant connections. It is an interesting exercise predict the result of te open or short condition. These should be followed up with practical testing, using the exact components intended in the end use, if you don't want your assumptions overturned at an inconvenient later date.

RL

Thank you for your detailed response. It appears that the safest approach is to use 600 VRMS fuses for the four gate leads. The main disadvantage is increased cost and complexity of installation. If I could find fuses of smaller dimension and lower cost, preferably PCB mounted or in an in-line holder, I would probably go with this option. Any recommendations on alternatives to the KTK and similar fuses?

I also considered fusible resistors, but they seem to be rated at 300 volts and less, and are essentially thermal time delay elements which would not provide the desired fast trip and current limiting of a fast acting fuse.

I have looked at many SCR and IGBT trigger boards, and have not seen any special means of protection from a failure of isolation. It may be that the isolation devices (optoisolators and transformers) have special reinforced insulation and are considered intrinsically safe. It is always possible that any component could fail, and it is impossible to design for total safety under all circumstances.

The DC-DC converters with reinforced insulation, rated at 4000 volts, are probably quite safe under these operating conditions. If I use specially made transformers, also rated and tested at 4000 volts, I think these will also be safe. The 4N35 optoisolators (per Vishay) are specified with a 5300 VRMS rating, with double molding isolation, so I think they will be safe.

As a precaution, I may build a test jig on which I will mount one or more of the optoisolators, DC-DC converters, and transformers. I can measure the insulation resistance at 2500 VDC (or even 5000 VDC), and make sure it is the same at all voltages. Then I can apply 2500 VAC and leave it on for an extended time, like 24 hours, and then remeasure the insulation resistance. I think this will be a fair test for the components themselves. Beyond that, the most likely source of failure will be the PC board itself, environmental contamination, and external wiring problems. We normally hipot the completed SCR boards at 2500 VDC, although we do not specify how long the voltage is applied. It might be good to specify one minute, and also perform a before and after insulation resistance test.

Finally, please elaborate on the requirements for "reinforced insulation". Is it in IEC950? Thanks.

Paul

Under what circumstances do you anticipate large SCR gate fault currents? The paths from the anode and cathode are usually pretty well defined - gate drive signal loops being independant; the gate structure is fairly low-impedance to the drive circuit loop with an internal shorting failure mode.

The impedance that normally limits gate current can also be used to limit fault current, and semiconductor return paths can be integrated into the drive section to shunt this predictably - a shorting condition of the semiconductors or an opening condition of the limiters can produce a satisfactory result.

IGBT and fet gates aren't so predictable due to the failure modes of the gate oxide, but again, limiting impedances and shunting semiconductors here can serve the same function.

Single fault fault behaviour doesn't usually depend on the normal ratings of components - these ratings simply cover normal operation. The aim is no ignition, no flying parts.

Application of test stess voltages for longer periods than are required to perform the test is abnormal, and unless you know what you're trying to prove, its also pointless. Continuous stress outside of the intended application can create corona, surface tracking and non-recoverable component damage.

It will be specified in the standard that you're intending to address, though the insulation thicknesses, layer count, and creepage distances may be device-specific. Lab equipment is EN61010, but transformers could be covered by EN61558... other components may refer back to VDE or BSI publications.

RL

As far as a cheep fuze goes that can have a fair HV standoff for PCB use, consider using a #30 bare wire that starts at a PCB pad, goes "up" away from the PCB at least 1/2 inch, and then back "down" to a second pad a few inches away; the "run" being reasonably parallel to the PCB. If the current capability is not too high, then any arc would nominally extinguish fairly fast, as the amount of material (the wire) to vaporize and provide a continuing arc path is limited. Having the wire a good distance away from the PCB prevents the hot plasma from heating the PCB enough to start carbonizing or vaporizing it. If the power is excessive, the fuze material needs to be in either a hi-voltage liquid (?freons or transformer oil?) or special fire retardant powder like that used in power cartridge fuses. Failing that, use a few feet of space for the interrupter with a high pressure air blast or triggered shotgun to disturb/kill the arc.

There is a tradeoff; the amount of fuzeable conductive material needed for the fuse itself should be as little as possible so that when vaporized, does not lend to a nice highly conductive path about as good as the original conductor. And that *includes* the ends if the current and energy is sufficent to heat up and then vaporize them.

One time, in an Army barricks, someone continually ran thsir radio and someone else objected and got nowhere - so gave them a "grid leak detector" to "improve reception". That was a plug where the wires were twisted together; the idea was to trip the circuit breaker and shut off power. Well, it was a poor connection, and the result was a plume arc shooting about a foot out from the outlet; they tried kicking it to put the fire out to no avail--it grew longer. I discovered it and walked over to the breaker box and snapped all of them off. The estimated current was 10 amps on a 15 amp circuit; would have continued "forever". All of the "grid leak detector" was vaporized and most of that part of the outlet was vaporized and the wiring was in the process of providing ions for the arc.

The only condition which concerns me would involve an actual breakdown of the isolation barrier of the DC-DC converter or the optoisolator. Hopefully that would be no more likely than a failure in the insulation of the gate wires, although they only touch the heat sink of the SCRs. In one case this will be the same potential as the gate, but the opposite heat sink may differ by 480 to 600 VAC when the switch is off. It would be impossible to fuse the gate wires for this, but possibly standoffs could be used.

However, there is one retrofit application in which the SCR assembly is located remotely from the trigger board, and the gate wires are brought through an extensive wiring harness on two twisted pairs inside a shielded cable. It is rated at 600 VAC but it makes me a bit nervous, especially where it goes through a multi-pin connector with many other signal and control wires. It would be nearly impossible to rewire the entire test set, so we take a calculated risk that is (probably) the responsibility of the original designer.

I would only perform such tests as a sort of accelerated lifetime failure test, or essentially a destructive test. I would certainly not subject production units to any more than a one minute hipot at 2500 volts.

We have not generally used any standards for these test sets, other than what I have considered reasonable and safe. It would probably be a good idea to research various standards to see what may apply. I have some NEMA standards that I have referred to, and we try to use components that are adequately rated. These circuit breaker test sets are specialty devices, and we have not sought UL or any other approvals. Some older test sets that we repair and retrofit have had what I consider to be serious safety issues, and we attempt to correct them when we work on them.

I'll check the standards you mentioned. Any others I should investigate? Thanks!

Paul E. Schoen

Choosing a suitable DC-DC converter or optical isolator will involve testing the most likely candidates for suitability.

Obviously the short of the gate lead to anode potential will fire the SCR, protecting the harness wire connections but requiring a fusible link to protect the load and possibly the SCR. If the drive limiting impedance is present in the return path, then any voltage developed on it will also fire the SCR - possibly an undesirable condition if the section is subject to high dV/dT in normal operation.

I suggest that you limit your concerns to the equipment that has developed a need for recognized safety approvals, once the actual requirement is determined.

Drivers are generally located at the drive point for a number of very good reasons, only one of which is the reduction in hazardous real estate.

Safety certification will tend to reduce the practicality of a later retrofit, for better or for worse.

Accelerated life testing does not involve applying factors that are outside the design limits intended. It is more likely to examine operation under stated temperature extremes and rates of change, vibration and design/functional limits, including start-up and shutdown.

Your end-use market will determine and justify those standards that are most sensible.

RL