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Old 10th Jul 2018, 6:24 pm   #1
astral highway
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Default Gate drive/ (r) 'scope measurements

Calling professional power engineers,

I still have a load of gate driver waveform checking ahead of me, high and low side.

I know better than to connect a 'scope probe and ground probe to a gate driver chip's output, particularly when the chip's power supply is also referenced to my DC power supply's ground.

What's best practice for taking measurements here, please?
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Old 10th Jul 2018, 7:14 pm   #2
kalee20
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Default Re: Gate drive/ (r) 'scope measurements

Why not? It's how I've always done it!

Only when the gate drive is NOT ground-referenced do you have a problem. In that case, you might be able to get away with floating the scope (isolation transformer), or floating the circuit-under-test (power it from a battery). Or use a differential input, as long as you don't exceed the common-mode range.

When you are confident that the gate drive circuit is good, with clean fast edges, you can monitor it for pulse-width etc with a little temporary ferrite-cored transformer (capacitively coupled if necessary)
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Old 10th Jul 2018, 10:26 pm   #3
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Default Re: Gate drive/ (r) 'scope measurements

Hi Peter,

I’m fine with a single device - pic shows some lovely waveforms from one such chip as well as the load of the power device. (So to emphasise, pic does not illustrate my query)

But when I’ve been looking at a pair of low and high side drivers together, I’ve blown up a few instantaneously, suggesting a shorted output that was not apparent. They’re pricey little things so I don’t want to repeat this finding.

There is I note a DC component and I realise I can - and do in the circiuit design that’s active outside the scope of the test - put a DC blocking capacitor either high or low side, but I’d ideally like to see the waveform before this.

I have in the past used an isolated power supply but I need something juicier to get the big pulses into a load that emulates the gate capacitance of a big IGBT half bridge.

My ‘scope has a plastic case and is double-insulated so I could take your advice and lift the earth connection for this purpose.
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Old 11th Jul 2018, 1:24 am   #4
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Default Re: Gate drive/ (r) 'scope measurements

Al,

I once struggled with these sorts of issues for years, especially in analyzing SMPS waveforms with live above ground issues and high vs low side driver issues.

And I tried all the tricks of course, isolating the test object's earth, isolating the scope earth, using coupling capacitors and coupling transformers, using the scope as a differential probe (by adding the two channels and inverting one and not using the probe earths) and finally using actual differential probes.

But in the end I abandoned all of those methods in favor of the only one that I found works all the time, without measurement errors and without mishaps or accidents:

This is the Tektronix 222PS (Power Scout) scope which has fantastic isolation on its probe inputs, you can connect the probe earth anywhere to any circuit under test with no problems.

These scopes are still readily available, they are yellow & easy to spot (not to be confused with the plain 222) .

The only thing is that the original probes had an oddball division ratio and have become very hard to get, but I worked out a small modification to use standard probes, which then means a X100 probe can be added further extending the high voltage capability.

If you do get a 222ps (which would forever solve your measurement issues in the high voltage department) here is the article on how to use standard probes with them;(the high voltage warnings in my article are to cover the fact that some people have a limited understanding of high voltage risks, but others will know where to set the limits for measured voltages)

http://worldphaco.com/uploads/USING_...CILLOSCOPE.pdf

Hugo.

Last edited by Argus25; 11th Jul 2018 at 1:36 am. Reason: add remark.
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Old 11th Jul 2018, 8:46 am   #5
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Default Re: Gate drive/ (r) 'scope measurements

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Originally Posted by astral highway View Post
Hi Peter,


My ‘scope has a plastic case and is double-insulated so I could take your advice and lift the earth connection for this purpose.
You may find it has EMC capacitors bridging the scope's (now disconnected from mains ground) common and the line and neutral of mains. At the high frequencies you're trying to float the thing at you may have enough current in them to create some drama.

Just because there is no longer a copper connection doesn't mean that something is floating at higher frequencies.

It can be quite difficult designing an isolating transformer to isolate at high frequencies. Leakage inductance says you want to interleave windings, interwinding capacitance says you don't.

David
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Old 11th Jul 2018, 10:09 am   #6
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Default Re: Gate drive/ (r) 'scope measurements

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But when I’ve been looking at a pair of low and high side drivers together, I’ve blown up a few instantaneously, suggesting a shorted output that was not apparent. They’re pricey little things so I don’t want to repeat this finding.
Looking at the low side is generally easy, because typically your power supply's ground reference is sufficiently close to the scope's ground reference that no noticeable current flows when you clip on the scope ground and everything carries on working. The tricky part is the high side, as you've observed.

The first way I'd try to do this is to make sure the power supply I'm testing is not connected to ground. Run it from a battery, or from a power supply whose output isn't grounded. Most lab power supplies are like this. Make sure nothing on your power supply is grounded, anywhere. Then you can clip your scope ground wherever you like and make measurements relative to that reference point. This works well if your reference point is a pretty low-impedance one that doesn't move much, such as the positive supply rail. However, if your reference point is something like the opposite gate drive or a bootstrap rail which has a lot of fast edges on it or a relatively high impedance, you may find that grounding it causes trouble. This method also means you can't make more than one measurement simultaneously with the same scope with different reference points.

Sometimes it's worth trying to use the two scope inputs in 'A-B' mode as a differential amplifier. How well this works depends on the scope. Some have a good common mode rejection ratio like this, others don't. Seeing the scope in the photos, I remembered an experience I had with one of my clients, doing some power supply design. They had a very similar GWInstek scope, and using the 'A-B' mode was a total disaster. It turned out that the common-mode range was limited to the screen height, so it was impossible to get accurate traces on which we could actually see anything. This is a pitfall of modern digital scopes which don't have 'proper' analogue input stages and try to do everything with digital maths. I went home and fetched an analogue scope to do the job with.

The best way of tackling such measurements is traditionally a differential amplifier. I have a couple of Tektronix 7A13 modules which are outstanding: 100+MHz bandwidth, good CMRR, and for most practical purposes a common-mode range of several hundred volts. I don't know what their modern equivalent is, sadly, though the Powerscout-type isolated scopes work very well. They exist as both CRT and LCD instruments.

Incidentally, a good way of getting really accurate views of fast pulse waveforms such as gate drives is to use your scope's 50 ohm input termination, if it has one, then use a piece of thin 50 ohm coax (top tip: there are lots of cheap cables on eBay made with RG174 wire or similar. They're great for hacking apart for this sort of thing) from there to the point you want to measure. Strip the end quite short, so less than a centimetre of braid and centre core are showing. Solder the braid on to your ground point and solder a surface-mount resistor of a few hundred ohms or so on to the point you want to measure, then to the centre core of your coax. Hey presto, an instant and cheap probe which will give you accurate results up to several hundred megahertz, without the vagaries of dangly probe ground wires and compensation capacitors. This doesn't solve the ground problem, though.

Chris
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Old 11th Jul 2018, 10:50 am   #7
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Default Re: Gate drive/ (r) 'scope measurements

You can buy a probe from Tek which is a bnc (with housing and logo) with a length of 50 Ohm coax and a resistor at the probe tip already made!

Pushing a couple of grand, though, now. But it does have a nice stabby tip and a handy grounding sleeve.

I had some old Sealectro probe ends with SMC connectors right behind them, just a near resistor end to go on a cable. Someone nicked two of the set of three, and the other one got dropped on its tip, breaking the glass bead. Moral, don't let other people use your personal gear...

David
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Old 11th Jul 2018, 11:22 am   #8
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Default Re: Gate drive/ (r) 'scope measurements

Hey folks, i’m out and about, so please excuse the truncated replies.

Hugo: thank you ! that’s a radical solution and also a really comprehensive one. I’ll investigate with a view to buying such a scope. Your research paper is detailed as ever and I’ll take the time to read it more deeply. But your ingenuity and attention to details shines through, once again.

David: good call, thank you ! hadn’t thought of that specific pathway for current

Chris: thank you -

I’m not sure though how I can check the grounding of my bench power supply without opening it up.

Is the output socket marked ‘ground’ next to ’-ve’ likely to be connected to mains safety earth? Otherwise what is the possible configuration here?

Really interestingl comments on the limitations of some ‘scopes and the modules you mention look spot on.

I also like - and will emulate - the coax segment used to get clean readings

I’ll report back on due course , may be a while . It’s facinating stuff, if also sometimes a cause of a little frustration.

Thanks again folks
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Last edited by astral highway; 11th Jul 2018 at 11:36 am.
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Old 11th Jul 2018, 12:13 pm   #9
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Default Re: Gate drive/ (r) 'scope measurements

Al,

I also could have mentioned that the same isolation that the 222ps scope affords I think is provided by the Tektronix A6902B Probe Isolator, which you will see for sale in the usual places. The cost of them though is about the same as the entire 222ps scope, so I have never gone for one, but they would make any scope act like a 222ps from the isolation perspective, they also had the non standard high voltage probes which are often missing (or damaged) from the isolator units you will see for sale. If you wait you might get a good one with probes that you can then use with your existing scope.

Hugo.
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Old 11th Jul 2018, 1:13 pm   #10
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Default Re: Gate drive/ (r) 'scope measurements

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Chris: thank you -

I’m not sure though how I can check the grounding of my bench power supply without opening it up.

Is the output socket marked ‘ground’ next to ’-ve’ likely to be connected to mains safety earth? Otherwise what is the possible configuration here
A quick ohmmeter test between the negative output terminal and the mains plug's earth pin will tell you. There is frequently a 'ground' or 'earth' terminal on the front panel which is connected to the case metalwork and thus mains earth, but is unconnected to the output.

It's conventional for the output of bench power supplies to be floating with respect to earth. For a start, it means it's safe to connect power supplies in series, which is often handy (even if you just want +/-15V for an op-amp). For example, I have an ancient (germanium power transistors! yay!) Solartron power supply in front of me which has two outputs, both floating, and features an extra earth terminal on the front panel if you want it. The ubiquitous Farnell LT30 family work the same way - the L30-5 behind me has red and black output terminals and a separate green earth terminal. In the office there's a modern Chinese switch-mode bench supply whose output is definitely floating and I don't think it has an earth terminal anywhere, though the metal case is connected to mains earth.

Chris
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Old 11th Jul 2018, 3:30 pm   #11
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Default Re: Gate drive/ (r) 'scope measurements

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You may find it has EMC capacitors bridging the scope's (now disconnected from mains ground) common and the line and neutral of mains. At the high frequencies you're trying to float the thing at you may have enough current in them to create some drama.
Yes - unfortunately, which is why I qualified my suggestion with the words "You might be able to get away with..."

High-side drivers are a pain! For all high-side drivers I've needed, I have driven with a transformer. And on the same transformer, I have had an identical winding to drive the low side, even if not strictly necessary. At least by doing this, I can be certain that the difficult-to-assess high side is the same as the easy-to-monitor low side.

If you cannot float the scope, then float the unit-under-test. I suggested operate from batteries, for maximum isolation from earth (including capacitively). Much better than using a bench power supply.

Hugo's solution, that of buying a custom 'scope, is going to be the best (when Tektronix decide to make a piece of test equipment, they do it well). But I have no experience of this.
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Old 11th Jul 2018, 10:46 pm   #12
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Default Re: Gate drive/ (r) 'scope measurements

Could you use a battery powered 'scope?

There are several old analogue 'scopes still around, eg, the Telequipment D32 which has built in batteries, Sony/Tektronix 314 or 316 which run on an external car battery (these are 10MHz dual channel 'scopes), and several miniature 'scopes from Philips, Sinclair, etc. (some of which are single channel).

These dual channel scopes do NOT have the independently isolated inputs of the Tektronix 222, and when running on batteries with the scope ground (and exposed metalwork) well above true ground, you might be well advised to wear rubber gloves! But they are more readily available, and cheaper than a 222.

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Old 11th Jul 2018, 10:52 pm   #13
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Default Re: Gate drive/ (r) 'scope measurements

I always used differential probes for doing just this on gate drivers, driving high power IGBTs.
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Old 12th Jul 2018, 12:29 am   #14
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Default Re: Gate drive/ (r) 'scope measurements

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Could you use a battery powered 'scope?

Stuart
Battery scopes are a good option.

The one I would recommend is the Hitachi V509, which is a compact scope, can be powered from 12V, has TV H & V sync standard, a helpful delay timebase & has a super intense sharp focus CRT beam as the small crt runs from 10kv EHT, can be seen in daylight, and is 50MHz bandwidth, they come up on ebay from time to time.

(The 222ps of course runs from batteries too, as well as its AC adapter, but has the advantage of isolation to the extent that the scopes input stages are isolated and over-voltage protected, so high voltage transients do not damage it easily either and the dual isolated inputs allow two channel display of waveforms with completely disparate relationships to ground or any other potential, so really its pretty hard to beat).

Another possibility, the Tek 464/466 scopes had a inverter option (an add in pcb) for battery use, its quite clever. There are extra windings on the power transformer which are driven by transistors in a self oscillating circuit with a small toroidal core with sharp saturation properties (its basically a Royer Oscillator) I think off hand it causes the mains power transformer to then run at about 400Hz from the DC supply.

Last edited by Argus25; 12th Jul 2018 at 12:37 am.
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