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ET227 Transistor: SOA Violation

The ET227 transistor (labeled A from the DC gain tests) I’d been using, ever since the very beginning, failed with a collector-to-emitter short when I started it for a data taking run. In most circuits, that would be a catastrophic failure accompanied by arcs & sparks, but the Kenmore 158 simply started running at full speed and ignored my increasingly desperate attempts to regain control.

OK, those transistors date back to the 1980s (or maybe even earlier), so maybe It Was Time.

I swapped in ET227-B, buttoned everything up, and continued taking data.

Two days later, ET227-B failed with a collector-to-emitter short when it turned on.

Once is happenstance. Twice is coincidence. A third time means I missed the cluetrain.

Although the ET227 can switch 1 kV and 100 A, the Safe Operating Area plot shows that the DC limit passes through 1 A at 200 V:

ET227 - Safe Operating Area

ET227 – Safe Operating Area

Bearing in mind that peak line voltage hits 170 – 180 V, 200 V looks like a convenient upper limit. Also, those limits apply at 25 °C case temperature and drop as the junctions warm up, although the datasheet remains mute as to the difference.

The circuit puts the following elements in series across the AC line:

  • 5 A fast-blow fuse
  • Normally open relay
  • Full-wave rectifier block
  • 120 VAC / 100 W universal motor
  • ET227 NPN transistor
  • 25 T x 2 parallel 24 AWG winding

After screwing around with Spice for a while, I can’t convince myself that the simulation means anything, but the general idea is that closing the relay at maximum line voltage (about 180 V) produces a staggeringly high current pulse through the series capacitances. A small amount of stray capacitance across the motor passes line voltage to the collector, the collector-base capacitance feeds it to the base, the transistor’s gain slams essentially unlimited current against line voltage, and the operating point squirts through the top of the SOA graph.

I made up a snubber from a 220 nF X capacitor and a 5.6 Ω resistor. That won’t have any effect on the spike, because the various stray / parasitic capacitors remain directly in series across the line, so the snubber looks like an open circuit. The snubber does damp the ringing after the spike vanishes, but that’s not the problem.

Some scope shots from ET227-C show the magnitude of the problem; it hasn’t blown yet, but obviously this can’t go on. Note the varying horizontal time scales and vertical current scales (all are at 10 mV/div, with the Tek probe providing the scaling).

At 50 mA/div, the two humps come from the (damped) ringing. This one doesn’t have much of a spike:

Snubbed power on transient - ET227C 50 mA-div

Snubbed power on transient – ET227C 50 mA-div

At 100 mA/div, I must have caught it at a higher point in the voltage waveform:

Snubber 5.6 ohm  220 nF - 650 mA spike - 100 mA-div

Snubber 5.6 ohm 220 nF – 650 mA spike – 100 mA-div

At 200 mA/div, this one looks seriously worse:

Snubber 5.6 ohm  220 nF - 1600 mA spike - 200 mA-div

Snubber 5.6 ohm 220 nF – 1600 mA spike – 200 mA-div

Now, agreed, a 1.6 A spike in a transistor rated for 200 A pulses doesn’t sound like much, but catching the spikes depends on random chance. If the collector voltage starts at 100 V, then that spike comes pretty close to the DC SOA limit; that’s not enough to kill the transistor, but it’s certainly suggestive.

Putting an NTC power thermistor in series would add some resistance to the circuit and reduce the magnitude of the spike, but they’re really intended for power supplies that draw a constant load, not a sewing machine that starts and stops all the time. If the motor runs for a while, then the thermistor will be hot for the next startup and the relay will close with relatively little resistance in the circuit.

More doodling seems in order.

 

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  1. #1 by Jason Doege on 2014-11-18 - 08:20

    What about a soft-start circuit like what is used on the gates of triacs?

  2. #9 by Jason Doege on 2014-11-18 - 08:26

    Also, what do things look like when you turn the ET227 off?

    • #10 by Ed on 2014-11-18 - 09:13

      Absolutely no problem on that side: the base drive is so slow that there aren’t any transients to be found.

      FWIW, that was my first hint: it didn’t turn off. [sigh]

      The little current bump as the ET227 turns on doesn’t pose a problem, either; the relay kills the poor thing.

  3. #11 by Brent Crosby on 2014-11-18 - 10:13

    Maybe wait for a zero crossing before turning it on?

    • #12 by Ed on 2014-11-18 - 11:05

      Remember, it’s a mechanical relay with (at least) half a cycle of chatter built right into the contacts…

  4. #13 by madbodger on 2014-11-18 - 11:22

    This is a deceptive problem, with no obvious (to me) simple solutions. I’ll be curious as to how it plays out. I started to think of a capacitive voltage divider and clamping circuit, but your snubber idea seems more likely.

    • #14 by Ed on 2014-11-18 - 11:51

      The snubber seems to be resonating with the motor inductance, as recent scope shots have a weirdly corrugated flat top. I’m not sure it’s a net win, as it’s not active fast enough during the initial pulse.

  5. #15 by Red County Pete on 2014-11-18 - 11:34

    Blowing up transistors was a side effect when I was developing tests for a power IC. We got pretty good at replacing the fried probe tips, before we changed test platforms… One thought, would an inductor solve more problems than it created?

    • #16 by Ed on 2014-11-18 - 11:49

      Just got off the phone with Eks, who suggested exactly the same thing; you guys know what you’re doing.

      I’m fiddling with some NTC power thermistors that won’t do anything useful, so the inductor is next on the list.

      I must spend more time with the SPICE simulator to see what will happen, though. Eks says it might create a resonant circuit (with all the stray capacitances) that will kill something on the rebound, but an inductor seems less awful than anything else.

  6. #17 by hexley on 2014-11-18 - 12:10

    “the collector-base capacitance feeds it to the base”

    Any chance you could clamp the base to the emitter until the input relay has settled, to kill this parasitic drive? I don’t remember if you already have a current limiter in place, but if so maybe it could be dual-purposed into a “no current at start up” limiter.

    Also, have you verified that there are no spikes when the input is fed with steady DC (perhaps by adding a filter cap at the bridge output) — just to rule out anything untoward happening in the drive circuit when the ET227 is first turned on?

    • #18 by Ed on 2014-11-18 - 17:10

      That internal 13 Ω resistor nails the base to the emitter, which seemed entirely adequate until I started killing transistors, so a dead short might be better.

      However, I really don’t know how the Miller effect works at the device level. The current injection may happen on the “inside” of the base-emitter junction, where an external short-to-emitter couldn’t divert the current. But, if that were true, then the internal base-emitter resistor would be similarly impaired and I thought that squelched the Miller current.

      Thanks for the suggestion; I’ll add it to the list of Things To Try when the obvious tricks don’t work.

      when the ET227 is first turned on

      Judging from the scope traces, the huge spike occurs only when the relay closes, not when the ET227 goes on. There’s a little current overshoot when the collector current reaches the limit level that I (probably mistakenly) attribute to the Miller capacitance, but nothing that could possibly push the operating point outside the SOA.

      But that sounds like handwaving even to me…

      • #19 by hexley on 2014-11-21 - 19:20

        If you are still working your way through this “three-pipe” problem and aren’t tired of kibitzers, there are a few things that would be interesting to know.

        1. Could you grab some scope captures for the collector voltage and emitter current at power-on? Would be interesting to see dV/dt and the associated spike.

        2. Ditto for captures when operating at speed? Would be interesting to see what is going on with collector voltage and current when the commutator “commutes”.

        3. There does not seem to be a flyback diode across the motor in the schematic posted earlier (https://softsolder.com/2014/10/09/optoisolated-et227-transistor-driver/). No doubt the ET227 should laugh off any momentary ringing caused by, say, bouncing brushes in the motor — but adding a diode couldn’t hurt given the current [ouch] state of affairs :-)

        • #20 by Ed on 2014-11-22 - 08:15

          I must dust off the variable isolation transformer and do exactly as you suggest, although I have a few other things in the must-do hopper for the next week. A few scope pictures should be most illuminating.

          A CC reader sent a long note describing an ultra-low-speed motor drive for a radar dish that he built back in the day. It’s not quite applicable as-is, but the principle seems dead on and it’s pretty much a simple matter of software; I think I can junk my firmware doodles and start over on a better path.

          Kibitzers like you folks are a great asset!

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