Quick Q #2: earthing circuit ground to mains earth??

[Al (astral highway)]

V interested to hear what the professionals have to say about this...

The IGBT-driven tesla coil I'm building contains logic/ control circuits and also power components, on the same board.

The control circuits derive their power from an 18V transformer which feeds two regulated power supplies, 15V, and 5V for logic. When driven on, the two IGBTs derive their power directly from the mains: 240V- and 240V+ but these are isolated from the logic and control circuit by gate drive transformers.

So the Q: the schematic shows circuit ground to mains earth. I'm not questioning this, but just trying to establish why it is not left floating? I'm *guessing* because strong currents could be induced into the board by the proximity of the main resonant circuit, which will sit directly above the circuit board. Instantaneous currents in the primary are minimum 100A but can be set much higher, according to the design.

Skipping a long way ahead, when I finally mount the circuit board on its aluminium corner stanchions and connect it to a metal case, should I still separately earth this case? or is it okay to rely on a single earth point if there's enough mechanical contact between the stanchions and the case? If so, what's the best configuration for the two earths?

hope that makes sense?

Thank you

[G8HQP Dave]

All external metalwork should be grounded. The incoming mains ground should go straight to the case. Then other things can be grounded to the case too. You have two safety issues here: the medium voltage but very low impedance mains supply, and the high voltage but medium impedance Tesla coil output - you need to follow best practise for both of these.

[Al (astral highway)]

G8HQP Dave, thank you. Very clear, will do exactly this with the earths.

Am I right, do you think, about the reason for ground being earthed? I got the impression that by by contrast, most consumer equipment contains floating grounds, or is this wrong?

[Al (astral highway)]

Originally Posted by G8HQP Dave

You have two safety issues here: the medium voltage but very low impedance mains supply, and the high voltage but medium impedance Tesla coil output - you need to follow best practise for both of these.

Good call. Will give this a lot of thought in the build.

Oh, actually three safety issues: the two you mention, plus the pulsed low impedance, medium voltage output from the switching IGBTs?

[Sean Williams]

Earth bonding in the application you mention would seem fairly critical to me.

If the transformers that drive the IGBT fail, and the devices then fail, the usual failure mode for semiconductors is a short circuit - ergo, mains will appear where it shouldn't.

This could then result in a situation where exposed metalwork will then be rendered live.

Frankly quite a dangerous situation.

Personally I would not leave it to chance.

[trobbins]

Originally Posted by astral highway

So the Q: the schematic shows circuit ground to mains earth. I'm not questioning this, but just trying to establish why it is not left floating?

Can you point to a schematic?

Given the risk of poorly bonded metalwork, and need to control parasitic currents in nearby metalwork, I'd suggest fixing an earth wire strap or wire from each piece of metalwork to the next (using mains protective earth type bolting of the wire/strap), in addition to relying on the constructional screws/bolts holding the metalwork together.

[Al (astral highway)]

Originally Posted by trobbins

Can you point to a schematic?

I'd suggest fixing an earth wire strap or wire from each piece of metalwork to the next (using mains protective earth type bolting of the wire/strap), in addition to relying on the constructional screws/bolts holding the metalwork together.

Hi there, the schematic is huge - four A3 landscape pages, so not practical!

Thanks to you and also to Sean for clarifying best practice. This is very clear and I'll make a note to do this.

Cheers,

Al

[trobbins]

Al, Is the mains input section of the schematic small - scannable? The logic etc sections aren't of interest to the topic, but the IGBT power path and mains connection to that is.

[McMurdo]

Does the control logic have any connections to the outside, eg pc interface etc?

[Al (astral highway)]

Originally Posted by trobbins

Al, Is the mains input section of the schematic small - scannable? ... the IGBT power path and mains connection to that is.

Hiya! Thank you - sorry I don't know your name, what is it?... here's a scan of the two relevant sections. The IGBT circuit is on the same board as the logic. I take Sean's point about the consequences of a worst-case-scenario failure of the gate xformer, on the other hand, the risk seems very low.

I can't quantify the risk, but it would involve a catastrophic failure of the insulation on both secondary and primary windings of the gate transformer. In the (unquantifiably possible --see following para about logic protection) event that the IGBT's fail short circuit, the fuse protection would protect against this a few hundredths of a second.

Having said this, there is, anyway, real-time over-current sense and protection circuitry in the logic. This design protects the expensive IGBTs if even a spurious over-current event takes place in either/both of the two resonator primary/ IGBT current pathways.

As is self-evident, the normal transient protection and gate protection is in place around the two power devices and the cct board tracks around these devices are the minimal possible length to design-out unwanted inductance.

[Al (astral highway)]

Originally Posted by McMurdo

Does the control logic have any connections to the outside, eg pc interface etc?

Hi Kevin. There is potential for external modulation via the on-board logic with a low-voltage (TTL level) signal.

Cheers,

Al

[trobbins]

Al, I had interpreted your info as saying the mains neutral was somehow connected to protective earth or chassis inside. Those schematics show mains input is all floating, as required, but I can't see any EMI management components between mains active/neutral and protective earth input ??

So I think your concern is related to grounding the hazardous secondary side, rather than floating the hazardous secondary HV winding? If so, then yes there are good reasons to ground that secondary HV. Are there any connections to that HV winding, other than to earth one end? How is the voltage/current/power to that winding managed by the controls?

Ciao, Tim

[Al (astral highway)]

Originally Posted by trobbins

..I can't see any EMI management components between mains active/neutral and protective earth input ??

Good call, Tim. There's no EMI management in the design. However, I've pulled a suitable component from some scrap equipment and will make sure I build this in. Thanks.

Originally Posted by trobbins

Are there any connections to that HV winding, other than to earth one end?

There are no other connections to the secondary, only earth. The plasma discharges into air from a spike in the toroid. This design seems unusual as it is customary for the earthy end of the big secondary inductor to be connected to the base of the primary.

Originally Posted by trobbins

How is the voltage/current/power to that winding managed by the controls?

Tim, the pulsed primary current flows through a current sense and feedback transformers. Thsese are monitored by the gate driver and shutodwn logic ccts. Peak current in the primary can be accurately present on the primary current sense circuit, theoretically to a level that would only be tolerated by far larger IGBTs.

Gate drive is synchronously inhibited if an overcurrent event is sensed.

Cheers, Al

[trobbins]

Al, cloning a commercial product is fraught with dangers if you are just relying on a schematic.

I can only recommend you use an isolating transformer before your clone, and have robust protective grounding to all metalwork (anywhere in cooee of the clone) and a common ground point for the clones circuits (primary side, secondary side, control) and the protective grounding and the protective earth.

The power pumped in to the transformer primary windings has to go somewhere. At the point of IGBT turn-off, if that inductive power can't adequately couple to a load, then all available winding voltages transfer the energy out to terminal voltages (or winding distributed capacitances) until a protective part connected to a winding terminal starts to conduct, or leakage loading back to ground on the HT rises (ie. through air and across insulation), or ringing dissipates energy via parasitic loop resistance. The peak current limit setting on the IGBTs, and the operating frequency, end up controlling the energy put in to the transformer (and hence transferred out via the various pathways).

[Al (astral highway)]

Hi Tim, thanks for gettting back to me

Originally Posted by trobbins

...I can only recommend you use an isolating transformer before your clone,

Good call, yes, I have a constant voltage transformer lined up.

Originally Posted by trobbins

The power pumped in to the transformer primary windings has to go somewhere. At the point of IGBT turn-off, if that inductive power can't adequately couple to a load, then all available winding voltages transfer the energy out to terminal voltages (or winding distributed capacitances) until a protective part connected to a winding terminal starts to conduct...

...which is why this should be tuned at very low power initially. The best coupling between the primary and secondary is found, and turns or even fractional turns are added to the primary or taken off it. Once the thing is carefully tuned, there is no operating condition under which the primary is not optimally coupled to the secondary.

If the control side of the board senses overcurrent, the shutdown is synchronised with the tiny window when no current is actually flowing through the half bridge... (Arggh, what's the technical word I need here?)

Tim, on your point about the dispersal of the energy stored in the primary coil and series capacitors, here's a practical anxiety... From a smaller (test) circuit i built before, a big headache is having an infallible shut down sequence.

Because at shut down, If we switch off the resonator circuit while the control circuit is still operating, does all the energy stored in the primary/ series capacitors go where it should? Or do the IGBTs end up dealing with a huge inductive spike?

In fact I'm not sure that this problem goes away if we first switch off the control circuitry and then the resonator circuit?

Be interested to hear your thoughts on any of that, Tim.

Cheers,

Al

[trobbins]

Oops, my last paragraph missed the resonant caps aspect and the tesla coil application - sorry for waffling on.

I would have called it the deadtime - from discontinuous flyback application.

The topic of protection is probably quite complex due to resonant circuitry, and possibly a variety of loosely coupled paths for primary coil energy if current is stopped from flowing through the IGBTs and main filter caps.

I would have thought that a substantial MOV from OUT+ to OUT- with a 1mA DCV that exceeds the max DC_SUPPLY voltage by some margin, so normally there would be no current through the MOV except for capacitive current. But the IGBTs have TVS's across them of about 420VDC rating, and I think one side or the other will conduct when the OUT+ exceeds about +/- 210VDC from OUT-. TVS's probably have a narrower tolerance than typical MOVs, so perhaps using some extra TVS's would not be a detriment, and would both spread the love, and lower the peak voltage experienced by the IGBTs.

I guess some form of crow-bar load could also be switched across OUT+ to OUT- as part of turning off control circuitry - eg. using say 600V FET and series diode and dump resistor (n-channel for one direction, and p-channel the other).