what can go wrong with this circuit

Zak28 · Jun 27, 2018

Its an AC mains powered circuit running in the US and its object is to discharge capacitor into coil with a gang of IGBTs driven by a 555 oscillator. I certainly will not exceed any components limits and will choose them appropriately and of course it will be fused on the mains side. My concern is wiring it up correctly to mains power.

Im certain the circuit checks out valid but I never made mains powered stuff before. Will anything in this circuit be destroyed or will the circuit not work because of an improper mains hookup?

Greatly appreciated.

KlausST · Jun 27, 2018

Hi,

no diode values. Maybe the used ones are not suitable.

Current limit is about 8.85A RMS. We don´t know if this is OK or not.

Wire is marked with "20m". I assume this means 20 mOhms. This maybe is just the DC resistance. Don´t know if impedance matters.
Maybe the wiring impedance causes voltage peaks that may harm the IGBTs.

From a safety point of view the circuit is not isolated. Via the diodes there is connection to mains. Thus it is be dangerous to touch the circuit .. even the NE555 part.

You need to keep on safety regulations of your country. Especially creepage and clearance distances.

Klaus

Zak28 · Jun 27, 2018

KlausST said:
...From a safety point of view the circuit is not isolated. Via the diodes there is connection to mains. Thus it is be dangerous to touch the circuit .. even the NE555 part...

The only way I can think of that would fix this is using earth ground

KlausST said:
...no diode values. Maybe the used ones are not suitable...

Measured diode power in ltspice is less than 584.55mW with

Code:

RFN30TS6D FastRecovery 30A 600v

which are similar but not the same, I could not get spice to work with my diodes model, but its close enough to my diode which is:

Code:

IDW30E65D1FKSA1 DIODE GEN PURP 650V 60A TO247-3

KlausST said:
...Wire is marked with "20m". I assume this means 20 mOhms. This maybe is just the DC resistance. Don´t know if impedance matters.
Maybe the wiring impedance causes voltage peaks that may harm the IGBTs...

You assumed correct they are 20mΩ but how can it harm the 650v rated IGBTs?

Code:

IGW75N65H5XKSA1 IGBT TRENCH 650V 120A TO247-3

Greatly appreciated

CataM · Jun 27, 2018

You assumed correct they are 20mΩ but how can it harm the 650v rated IGBTs?

With the inductance the wire has. Also depends on how fast the IGBTs switch and the current it switches. I doubt there will be a problem to the IGBT's big output capacitance.

Zak28 · Jun 28, 2018

CataM said:
With the inductance the wire has. Also depends on how fast the IGBTs switch and the current it switches. I doubt there will be a problem to the IGBT's big output capacitance.

Its a 16awg @ 1.8meter extension cord cabling, I suppose some sort of other cabling is ideal to mitigate parasitic inductance, perhaps speaker cable?

FvM · Jun 28, 2018

The only way I can think of that would fix this is using earth ground

Mains has already earth connection, usually neutral is earthed. Shorting a node of the rectifier circuit to ground is not possible, otherwise you get destroyed rectifier and tripped mains fuse.

Your internal circuit ground carries mains voltage like all other circuit nodes, it must be isolated against earth ground and never being touched.

Cable inductance can cause overvoltage during current switch-off, may destroy the IGBT. Can be avoided by placing D5 next IGBT and 4.7mF capacitor.

c_mitra · Jun 28, 2018

The only way I can think of that would fix this is using earth ground

Not correct. Even if you connect the ground to an earth ground, there is a current path (via the reverse leakage of the diodes) to the mains that can be a safety hazard.

CataM · Jun 28, 2018

Its a 16awg @ 1.8meter extension cord cabling,

~3uH inductance. Characteristic impedance's order of magnitude is somehow large. Simulate it please with the IGBT's model.

Otherwise, follow FvM's advice.

Zak28 · Jun 29, 2018

FvM said:
...Your internal circuit ground carries mains voltage like all other circuit nodes, it must be isolated against earth ground and never being touched...

I still dont understand the issue since its inconsequential if 1. the chassis is double insulated (non metalic chassis) and 2. I do not touch the circuit whilst its energised.

As of now the claims people have made have caused me to believe the circuit in its current iteration is improperly grounded/hooked up to mains.It seems floating circuits from mains is more dangerous than any other energized circuit since the return path is literally everywhere, it being the ground. I believe my circuit is properly wired and nothing would fail or harm anything granted it isnt contacting anybody, aside from the high voltage spike which was mentioned and yet the gang of IGBT capacitance would likely have made it a non issue.

FvM said:
...placing D5 next IGBT and 4.7mF capacitor...

I thought D5 would prevent high voltage kickback from the coil. Is D6 is in the proper position that D5s intended purpose is and also mitigate high voltage from cabling inducance?

Greatly appreciated.

- - - Updated - - -

CataM said:
...Simulate it...

Unfortunately the simulator hasn't built in IGBT support, work rounds - rather gimmicks, are the only solution for this particular package. I simed the circuit with both wires replaced by 3uF inductors @ same resistance and indeed an enourmous voltage crept up, D6 resolved the issue. Fortunately the diodes non repetitive forward current is 240.0A @ 10ms, far above the 107A ~2mS measured. I presume the IGBTs capacitance would have not saved it from catastrophic failure at the voltages the simulator showed.

CataM · Jun 29, 2018

Unfortunately the simulator hasn't built in IGBT support

Yes it does, once you download the model.
Go to the "Simulation models" part: https://www.infineon.com/cms/en/pro...-igbt-without-anti-parallel-diode/igw75n65h5/

You do not need both D6 and D5. Use D5 where D6 is.

I presume the IGBTs capacitance would have not saved it from catastrophic failure at the voltages the simulator showed.

Probably, but expect voltage spikes to be much less with IGBT model. Simulate it please and see by yourself. Your idealistic switch model has no capacitance modeled.

KlausST · Jun 29, 2018

Hi,

I presume the IGBTs capacitance would have not saved it from catastrophic failure at the voltages the simulator showed.

Why guessing?
Why not simply add a capacitor to your simulation to see what happens?
I assume you get faster (than writing a post and wait for response) and more detailed results than a forum can give (without a member does the simulation job)

Klaus

FvM · Jun 29, 2018

I still dont understand the issue since its inconsequential if 1. the chassis is double insulated (non metalic chassis) and 2. I do not touch the circuit whilst its energised.

It's unclear what you are actually doing and where you are expecting problems.

In post #3, you started to discuss possible earth connection of your internal circuit ground (rectifier-, IGBT emitter) and people told you must not do this because mains neutral has already earth connection and earth connection of another circuit node causes a short. I hope this is clear now.

I don't understand what the "floating" problem should be. I see you understand the necessity of safe insulation of all circuit parts against inadvertent touching. That's good.

As for D5/D6: D6 is the right place for the diode to protect the IGBT against flyback overvoltage.

Zak28 · Jun 29, 2018

FvM said:
...As for D5/D6: D6 is the right place for the diode to protect the IGBT against flyback overvoltage...

I believe its best for D5 remain were it is as a redundant safety measure.

FvM said:
It's unclear what you are actually doing and where you are expecting problems

In post #3, you started to discuss possible earth connection...

AFter KlausST mentioned

KlausST said:
From a safety point of view the circuit is not isolated. Via the diodes there is connection to mains....

I had the notion it was improperly attached to mains thus I made the claim to use earth ground, I was so much believing the notion that I committed the fallacy of improperly using earth ground.

CataM said:
...Simulate it...

I found that model before but could not find any guides on using IGBT's in the simulator since there are is no IGBT symbol. The diode does not sim also for some reason.

Greatly appreciated.

Zak28 · Jun 30, 2018

The IGBT incurs a ~4kv dampened ring per turn on with the 4nF capacitance it has ofcourse this was only obtained by attaching a parallel 4nF capacitor to the idealized switch. I hope somebody points to a good guide to IGBT simulations with LTSPICE.

KlausST · Jun 30, 2018

Hi,

The ringing is expectable....use an RC combination to suppress ringing.

4kV is obviously too high. An even bigger capacitor won't be suitable, thus you still will need a voltage limiting device ... like D6.

Klaus

Zak28 · Jun 30, 2018

KlausST said:
...like D6...

Its ideal for this circuit.

FvM · Jun 30, 2018

The simulation visualizes the effect of wiring inductance, but the switch model hasn't much to do with real IGBT behavior, in so far the waveforms are mostly irrelevant.

If you want to see more than what is already known before, you'll use real IGBT and diode models.

Zak28 · Jul 1, 2018

FvM said:
...If you want to see more than what is already known before, you'll use real IGBT and diode models.

Would it sim if I place a BJT symbol and use the IGBT model?

andre_luis · Jul 1, 2018

Zak28 said:
The IGBT incurs a ~4kv dampened ring per turn on with the 4nF capacitance it has ofcourse this was only obtained by attaching a parallel 4nF capacitor to the idealized switch.

Simulators often do not work well when probing open nodes. The above -4Kv is just a numerical flaw on the computational process, it is unrealistic once you are using an idealized switch. If you add a very high resistor in parallel to the switch (eg. 10M) this would not happen.

CataM · Jul 1, 2018

Zak28 said:
Would it sim if I place a BJT symbol and use the IGBT model?

I have tried IGBT models of Infineon but they do not work in LTSpice. Gives the error "QTOT1() function" not implemented but the function is there...

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