Probing SMPS with oscilloscope

[tggzzz]

Wrong on both accounts.

1) Never ever use an isolation transformer. Only use differential probes. Isolation transformers are a death trap in untrained hands c.q. in an  unsuitable measurement setup. 

2) Without an isolation transformer the primary side of the power supply is ground referenced just like the oscilloscope. You can still make useful measurements but there will be some offset. I have done this myself many times.

Everyone has their opinions and this subject goes round and round endlessly.  There's nothing wrong with the way I mentioned and it is often a lot more convenient and less noisy.  You can even do it with isolated or differential probes if you like.  I fail to see how the isolation transformer itself is unsafe--the unsafe situation always involves some mistake or another and you could just as easily make the same--or different--mistake without one.  At least a proper isolation transformer will limit current to a few amps and then stop (fuse or breaker) if you do short something.

Isolation transformers are bad in the hands of novices and people who think they understand the risks (and I hate to say it but you seem to fall in this category). Think about the situation where you clip the ground of your grounded oscilloscope to a random point in the primary side of a PSU. You'll ground the PSU AND bypass the GFI (you should have) making the isolation the isolation transformer provides go down the toilet. On top of that: If things go wrong you can fry your probe (and scope if you are unlucky). It is very easy to clip the ground of the second probe to a different point and creating a short between those points. Working safely should not depend on not making mistakes; people do make mistakes.

Besides all that a differential probe costs the same as a good (=with safety rated isolation between primary and secondary) isolation transformer. The whole point of trying to prove an isolation transformer is a good solution is totally moot. There are better ways to limit the current through a circuit as well. For example by putting a lamp in series or use a quick acting low amp breaker. But even then there are much better ways to test SMPS safely. A method I use when the control circuitry is acting weird is to use a low voltage PSU to power the controller chip and provide the DC bus voltage (say 30V to 40V); that way you can usually test everything without needing to apply mains at all.

People need to consider how things fail, as well as how they work. Hence you make sound points.

But I don't like the series (incandescent) bulb technique.

An SMPS keeps the output power constant. If the input voltage is reduced then it will try to increase the input current, which can damage components.

[CDaniel]

If you work in the primary side of a SMPS you must use an isolation transformer ... without it you can't measure anything right with the ground clip not used  And you can't touch the SMPS primary ground , as the scope ground is tied to earth.
If the farnell probe is not made by a crappy chinese company like the one on e-bay , for sure I would chose it .

Wrong on both accounts.

1) Never ever use an isolation transformer. Only use differential probes. Isolation transformers are a death trap in untrained hands c.q. in an  unsuitable measurement setup. 

2) Without an isolation transformer the primary side of the power supply is ground referenced just like the oscilloscope. You can still make useful measurements but there will be some offset. I have done this myself many times.

How you can state such nonsense ...  Then how do you measure and not die in the secondary side ( isolated from mains ) of any device ? Isolation transformer  make the primary of the SMPS just like a secondary side , isolated from mains . There isn't any difference ( beside the output voltage of course ) between a 230V(120V)-12V transformer and a 230V(120V) - 230V(120V) .  The concept is not complicated at all , if you think a moment . If you touch one of the secondary 230V(120V) wires nothing happens ...

[bdunham7]

Isolation transformers are bad in the hands of novices and people who think they understand the risks (and I hate to say it but you seem to fall in this category).

Novices and idiots should stay out of SMPS guts no matter what equipment they have.  I don't know why you think I fall into this category, I have routinely worked with systems and devices much more dangerous than a garden variety SMPS and I still have 10 fingers and 2 eyeballs after all these years.  CRT Color TV's anyone?  I'm overhauling an old HP 6515A 1600V power supply right now and I assure you, I'm not getting buzzed.

Think about the situation where you clip the ground of your grounded oscilloscope to a random point in the primary side of a PSU. You'll ground the PSU AND bypass the GFI (you should have) making the isolation the isolation transformer provides go down the toilet. On top of that: If things go wrong you can fry your probe (and scope if you are unlucky). It is very easy to clip the ground of the second probe to a different point and creating a short between those points.  Working safely should not depend on not making mistakes; people do make mistakes.

DUDE!  Can you read?  Did you bother to read?  Did you miss the 'no ground clips' part?  Who in the f*ck would do any of those things?   And do you think the result would be any different without the isolation transformer?  If they did, the big smoking mess and fried scope would be a good lesson--that they should adopt a different hobby or line of work. And why do you keep harping about the isolation transformer not being effective against touch voltage when I've expressly said that I'm not using it that way on an SMPS?  Working safely means understanding what you are doing and following the rules you have laid out using that understanding.  If I'm working on something like this using a ground-referenced scope, it is all hands-off and no ground clips, or if necessary one ground clip attached to my reference ground, not some 'random' point.

Besides all that a differential probe costs the same as a good (=with safety rated isolation between primary and secondary) isolation transformer. The whole point of trying to prove an isolation transformer is a good solution is totally moot. There are better ways to limit the current through a circuit as well. For example by putting a lamp in series or use a quick acting low amp breaker. But even then there are much better ways to test SMPS safely. A method I use when the control circuitry is acting weird is to use a low voltage PSU to power the controller chip and provide the DC bus voltage (say 30V to 40V); that way you can usually test everything without needing to apply mains at all.

If you like diff probes better, by all means do it that way.  Inventing silly scenarios that might happen due to sheer ignorance by the user isn't much of an argument--and don't forget, your diff probes won't protect you if you lick the filter capacitor terminals.

[2N3055]

Isolation transformer solves only isolating from distribution network. First moment you connect ANY part of circuit on isolated side of transformer to a ground clip of scope YOU and your circuit is NOT isolated anymore. You just referenced it to scope ground that is earthed... Also it doesn't solve anything if you want to measure on BOTH primary and secondary side of SMPS at the same time with same scope..

Or if you want to measure voltage on inductor AND output voltage at the same time.

Diff probe has no grounded side, so you can scope around anything. like you would with multimeter. That alone is worth buying diff probe..

Micsig diff probe is very inexpensive. Buying good 100x or 1000x probe and iso transformer is not cheaper. And is more limited in use and dangerous in many ways. Fact that it CAN be done is not proof it SHOULD be done..

[CDaniel]

And how and why would you want to measure both primary and secondary side in the same time if your scope's  channels are not isolated? This is not about safety , even at 12V separate windings transformer you can destroy your circuit or measure garbage doing that . And of course in the same circuit if you don't know that your channels ground clips are connected ...

[2N3055]

And how and why would you want to measure both primary and secondary side in the same time if your scope's  channels are not isolated? This is not about safety , even at 12V separate windings transformer you can destroy your circuit or measure garbage doing that .

With two differential probes.... 

And there are many reasons. For instance, on PFC corrected PSU with isolated secondary you might want to look at output while looking at regulating or PFC circuits that are on the primary side...

[CDaniel]

If you want to do that for the living better buy a scope with isolated channels ... 
Many differential probes are for low voltage and don't have good bandwidth.

[2N3055]

If you want to do that for the living better buy a scope with isolated channels ... 
Many differential probes are for low voltage anyway

Isolated channels scope are more of industrial/automation stuff.. Most of them are very basic scopes, with some specialised stuff..

SMPS R&D scope would be 12 bit Lecroy with hi voltage diff probes and current probes...

And probes are for voltages you buy them for. And most of them are NOT low voltage. Low voltage is 50 -70 V..
1300V is not low voltage and neither is 5300V.

[tggzzz]

And probes are for voltages you buy them for.

And frequencies.

Never forget to understand the specification of max voltage as a function of frequency!

[2N3055]

And probes are for voltages you buy them for.

And frequencies.

Never forget to understand the specification of max voltage as a function of frequency!

Absolutely, frequency derating curves are very important... Thank you ...

[CDaniel]

And don't forget that the max voltage stated is peak to peak , so for rms is not that much 
1300Vpp even without derating is not enough for probing safely across the  transformer primary , especially a flyback where could be some high voltage spikes

[tautech]

Very unusual to find a SMPS without back EMF control on the primary.
Lots of MOSFETS would be toast without it.

[CDaniel]

OK , but not allways you deal with a lets say "modern" flyback where a 700V Mosfet can be used

[tautech]

OK , but not allways you deal with a lets say "modern" flyback where a 700V Mosfet can be used

Na, shit RDS _ON

[2N3055]

And don't forget that the max voltage stated is peak to peak , so for rms is not that much 
1300Vpp even without derating is not enough for probing safely across the  transformer primary , especially a flyback where could be some high voltage spikes

When working on SMPSs RMS rating is not important, only P-P... As I said, if you work on something that is 2000 V you buy probe for that...

[CDaniel]

Yes you measure peak to peak , but I mentioned clearly that the probe rating is for peak to peak not rms ...

[bdunham7]

Isolation transformer solves only isolating from distribution network. First moment you connect ANY part of circuit on isolated side of transformer to a ground clip of scope YOU and your circuit is NOT isolated anymore. You just referenced it to scope ground that is earthed... Also it doesn't solve anything if you want to measure on BOTH primary and secondary side of SMPS at the same time with same scope..

Or if you want to measure voltage on inductor AND output voltage at the same time.

I don't know why the dangers and difficulties of SMPS testing have take on such mythical proportions compared to other similar devices which have been around for many years.  There's no special sauce, extreme energy levels beyond compare or complications beyond understanding of anyone with basic knowledge of the subject. 

First, while isolation may protect you from touch voltage in a basic setup, I'm not using it that way here, so there is still a touch voltage hazard if you don't use the hands-off method.  This is no different than using diff probes--if you are probing circuits by hand, they're live to touch.  I'm intentionally referencing a point in the circuit to ground, the most common point being the negative terminal of the input bridge.  And yes, in many cases you CAN measure both primary and secondary sides simultaneously with a single non-isolate scope with no voltage hazard or damage.  I'll provide an example here, measuring "the voltage on an inductor AND output voltage at the same time".

I have an old Delta ATX SMPS, unfortunately it is non-PFC, which makes it a lot less interesting, but it will still serve.  It's the only handy 'laying around' PSU I have right now.  I take the case cover off and add two test points at the + and - of the bridge, where there just happen to be handy empty holes.  I then install a jumper to tie the negative of the bridge to ground.  Then the PSU gets plugged into the isolation transformer. 




The ground clips are initially removed from the probes, and here I've connected one probe (1X) to the 3.3V output and the other (100X) to the +BRIDGE test point I installed.  Unfortunately the 100X probe hook is obstructed in the picture and I've already put everything away.  I turn the power supply on and observe the result on the scope, then set the trigger to capture the noise pulse on the 3.3V output.  Note that CH1 is +350V, CH2 is 3.3V or so, one primary, one secondary and nobody died.





Now I've said 'hands off' for safety, but in reality I often do some careful poking around.  I'll bet the differential probe advocates do so as well and probably with both hands at times.  As long as I use one non-grounded probe and keep my hand behind the guard, I'm pretty safe.  The main danger is damage due to shorting two close points with the probe, a danger that has nothing to do with isolation or probe type--it can happen with a DMM as well.  So I find an interesting point on the input side of the main transformer, attach a test point and hook it up.





Well, there's some noise, which is what happens when you don't have ground clips.  Clearly we can see what part of the CH1 signal is correlated to the noise (not that it was a big mystery), but if we want a more accurate and proportional signal, we can connect a ground clip to the ground reference point.  It's tempting to use a more convenient ground point, but that's where the errors can start to happen.  Add the ground clip and the signal looks good.




And there you have it.  This is, IMO, a completely safe--as safe as anything else with exposed high voltages--method of probing an SMPS.  Yes, is requires some knowledge, planning and care--but differential probes are no more an excuse for carelessness than isolation transformers are.  Stupid hurts, no matter how much gear you have.  Obviously in this case, the ATX-type PSU has a grounded-common output, which may not be the case in other types of devices, so as my favorite math professor used to say, "mutatis mutandis".

As for all of the other issues that have been raised, I don't want to bother quoting everyone, but every system has it's advantages and disadvantages.  I can probe 4 points simultaneously this way with one scope--that would require 4 differential probes.  A cheapo diff probe might be less expensive than my iso-variac, but then you start talking about 12-bit LeCroys and Rogowski coils.  I'm not an SMPS researcher and if I was I wouldn't be asking someone like me for advice on  EEVBlog, would I?  I'm just fixing stuff and this is one way to do that.   

EDIT: My pictures got out of order somehow, I'll maybe sort them later.

[nctnico]

If you work in the primary side of a SMPS you must use an isolation transformer ... without it you can't measure anything right with the ground clip not used  And you can't touch the SMPS primary ground , as the scope ground is tied to earth.
If the farnell probe is not made by a crappy chinese company like the one on e-bay , for sure I would chose it .

Wrong on both accounts.

1) Never ever use an isolation transformer. Only use differential probes. Isolation transformers are a death trap in untrained hands c.q. in an  unsuitable measurement setup. 

2) Without an isolation transformer the primary side of the power supply is ground referenced just like the oscilloscope. You can still make useful measurements but there will be some offset. I have done this myself many times.

How you can state such nonsense ...  Then how do you measure and not die in the secondary side ( isolated from mains ) of any device ? Isolation transformer  make the primary of the SMPS just like a secondary side , isolated from mains . There isn't any difference ( beside the output voltage of course ) between a 230V(120V)-12V transformer and a 230V(120V) - 230V(120V) .  The concept is not complicated at all , if you think a moment . If you touch one of the secondary 230V(120V) wires nothing happens ...

But if you clip it to ground using an oscilloscope probe (otherwise you can't measure anything) the SMPS will be ground referenced without any protection from the GFI. But this has been written and explained so many times before.... 

[bdunham7]

But if you clip it to ground using an oscilloscope probe (otherwise you can't measure anything) the SMPS will be ground referenced without any protection from the GFI. But this has been written and explained so many times before.... 

True, of course.  But relying on GFI to mitigate touch voltage hazards is just as foolish as anything else, especially in an SMPS where intermediate stages may either be isolated enough not to trip the GFI or contain sufficient energy to finish frying you even if the GFI trips.  Of course, if you really want to, you can add a GFI after isolation easily enough. 

[2N3055]

Isolation transformer solves only isolating from distribution network. First moment you connect ANY part of circuit on isolated side of transformer to a ground clip of scope YOU and your circuit is NOT isolated anymore. You just referenced it to scope ground that is earthed... Also it doesn't solve anything if you want to measure on BOTH primary and secondary side of SMPS at the same time with same scope..

Or if you want to measure voltage on inductor AND output voltage at the same time.

I don't know why the dangers and difficulties of SMPS testing have take on such mythical proportions compared to other similar devices which have been around for many years.  There's no special sauce, extreme energy levels beyond compare or complications beyond understanding of anyone with basic knowledge of the subject. 

First, while isolation may protect you from touch voltage in a basic setup, I'm not using it that way here, so there is still a touch voltage hazard if you don't use the hands-off method.  This is no different than using diff probes--if you are probing circuits by hand, they're live to touch.  I'm intentionally referencing a point in the circuit to ground, the most common point being the negative terminal of the input bridge.  And yes, in many cases you CAN measure both primary and secondary sides simultaneously with a single non-isolate scope with no voltage hazard or damage.  I'll provide an example here, measuring "the voltage on an inductor AND output voltage at the same time".

I have an old Delta ATX SMPS, unfortunately it is non-PFC, which makes it a lot less interesting, but it will still serve.  It's the only handy 'laying around' PSU I have right now.  I take the case cover off and add two test points at the + and - of the bridge, where there just happen to be handy empty holes.  I then install a jumper to tie the negative of the bridge to ground.  Then the PSU gets plugged into the isolation transformer. 

(Attachment Link)
(Attachment Link)

The ground clips are initially removed from the probes, and here I've connected one probe (1X) to the 3.3V output and the other (100X) to the +BRIDGE test point I installed.  Unfortunately the 100X probe hook is obstructed in the picture and I've already put everything away.  I turn the power supply on and observe the result on the scope, then set the trigger to capture the noise pulse on the 3.3V output.  Note that CH1 is +350V, CH2 is 3.3V or so, one primary, one secondary and nobody died.

(Attachment Link)
(Attachment Link)
(Attachment Link)

Now I've said 'hands off' for safety, but in reality I often do some careful poking around.  I'll bet the differential probe advocates do so as well and probably with both hands at times.  As long as I use one non-grounded probe and keep my hand behind the guard, I'm pretty safe.  The main danger is damage due to shorting two close points with the probe, a danger that has nothing to do with isolation or probe type--it can happen with a DMM as well.  So I find an interesting point on the input side of the main transformer, attach a test point and hook it up.

(Attachment Link)
(Attachment Link)
(Attachment Link)

Well, there's some noise, which is what happens when you don't have ground clips.  Clearly we can see what part of the CH1 signal is correlated to the noise (not that it was a big mystery), but if we want a more accurate and proportional signal, we can connect a ground clip to the ground reference point.  It's tempting to use a more convenient ground point, but that's where the errors can start to happen.  Add the ground clip and the signal looks good.

(Attachment Link)
(Attachment Link)

And there you have it.  This is, IMO, a completely safe--as safe as anything else with exposed high voltages--method of probing an SMPS.  Yes, is requires some knowledge, planning and care--but differential probes are no more an excuse for carelessness than isolation transformers are.  Stupid hurts, no matter how much gear you have.  Obviously in this case, the ATX-type PSU has a grounded-common output, which may not be the case in other types of devices, so as my favorite math professor used to say, "mutatis mutandis".

As for all of the other issues that have been raised, I don't want to bother quoting everyone, but every system has it's advantages and disadvantages.  I can probe 4 points simultaneously this way with one scope--that would require 4 differential probes.  A cheapo diff probe might be less expensive than my iso-variac, but then you start talking about 12-bit LeCroys and Rogowski coils.  I'm not an SMPS researcher and if I was I wouldn't be asking someone like me for advice on  EEVBlog, would I?  I'm just fixing stuff and this is one way to do that.   

EDIT: My pictures got out of order somehow, I'll maybe sort them later.

You can write whole book of rationalizations to justify your opinion... It's irrelevant.
You are wrong.

I said it before, fact that so far YOU survived and YOU didn't destroy valuable equipment is just something that didn't happen to you.
It will if you keep pushing luck.  I was thinking same as you, and then one day ground clip that was on right place snapped off and while it was falling touched stuff.. BOOM....
I knew everything right and it still happened. Shit happens.

Now imagine someone that is beginner, with limited funds.. Odds are they will destroy little equipment they have and maybe hurt themselves.
Because you preach that it is safe, if you know what you're doing.
Problem with beginners is that most of them are not aware how little they know...
They tend to doing things hastily, and they overestimate their abilities and understanding.

Most of them are used to how with multimeter they can poke around ANYWHERE with any of the probes...
It is how they think and it is not natural for them to think where is ground reference... That is something you have to learn and have to get used to...
For you it was so long time ago you forgot how it is ...

It is against the law in most world countries to work on ungrounded equipment. Because it is dangerous. When things are grounded properly, you KNOW where your ground reference is...

And with diff probe you don't get crap injected in signal. That is what they do, reject common noise and common mode signals...

[nctnico]

But if you clip it to ground using an oscilloscope probe (otherwise you can't measure anything) the SMPS will be ground referenced without any protection from the GFI. But this has been written and explained so many times before.... 

True, of course.  But relying on GFI to mitigate touch voltage hazards is just as foolish as anything else, especially in an SMPS where intermediate stages may either be isolated enough not to trip the GFI or contain sufficient energy to finish frying you even if the GFI trips.

No, wrong. If you touch anything in the primary side of a PSU which causes a current to flow to ground through you, the GFI trips because there is an unbalance between hot and neutral.

Of course, if you really want to, you can add a GFI after isolation easily enough.

The latter is the stupidest thing to do because you'll need to ground reference the secundary side of the isolation transformer and thus voiding the purpose of the isolation transformer. I sincerely hope this is a brain fart and you wrote this in error...

And ofcourse a GFI isn't 100% fail safe but you can have 2 in series if you like (a regular one as part of the installation of your home and an extra sensitive one for your bench).

[bdunham7]

I said it before, fact that so far YOU survived and YOU didn't destroy valuable equipment is just something that didn't happen to you.
It will if you keep pushing luck.  I was thinking same as you, and then one day ground clip that was on right place snapped off and while it was falling touched stuff.. BOOM....
I knew everything right and it still happened. Shit happens.

What does all that have to do with isolation or not?  I've burnt up my fair share of stuff before I learned, and got buzzed once or twice.  The reason I wrote all that is to show a safe way to do things, not a way to 'get away' with something.  This is the result of learning, not luck.

Most of them are used to how with multimeter they can poke around ANYWHERE with any of the probes...
It is how they think and it is not natural for them to think where is ground reference... That is something you have to learn and have to get used to...

Now that you mention it, I think the most common probing mistakes are actually made with DMMs.  Poking around and short two points with the probes and ZAP!--dead chip.  Forget to move the cables from the AMP jack and BRZZT, burnt probes.  And you want to hand these 'beginners' another toy so they can 'safely' poke around inside a high-energy device before acquiring knowledge about....grounds?  Probably using both hands at once? Seriously? 

It is against the law in most world countries to work on ungrounded equipment. Because it is dangerous. When things are grounded properly, you KNOW where your ground reference is...

Now you and ntnico are smart guys, I'm sure, but did you READ?  The whole point of how I do it is that I select the ground point and ground it.  Obviously I know where my ground is, that's the point.  And I've no idea what laws you are talking about--does that include battery powered camera flashes?  Are you saying isolation transformers are illegal? 

And with diff probe you don't get crap injected in signal. That is what they do, reject common noise and common mode signals...

Yours must be magic because the CMRR of bargain-basement diff probes is the main reason not to have them. 

[bdunham7]

No, wrong. If you touch anything in the primary side of a PSU which causes a current to flow to ground through you, the GFI trips because there is an unbalance between hot and neutral.

I appear to have put too many concepts in one sentence.  Let's simplify and you can agree or disagree with each as you like. 

First, relying on GFI for protection against touch voltage hazards is improper.  Period.  It's nice to have, but should not be your primary plan because 1) you still get a shock and 2) sometimes GFI doesn't trip or trips slowly.

Next, depending on the architecture of the SMPS, there may be portions of it that are at high potential relative to ground but do not cause a current imbalance that the GFI can detect, or detect easily; this would be in high-frequency switched sections.  The GFI may have trouble sensing currents at high frequencies, or some intermediate stage may have a fault in its isolation.  Or, in some cases, the output side of the SMPS may be high voltage and ground referenced.  I know that isn't 'primary' but zapping is zapping.

Last, the filter capacitors may retain a significant charge and the GFI can't do much about that.

The latter is the stupidest thing to do because you'll need to ground reference the secundary side of the isolation transformer and thus voiding the purpose of the isolation transformer. I sincerely hope this is a brain fart and you wrote this in error...

You're a smart guy, but here's an opportunity to learn something from a knuckle-dragger like me.  Please read at least twice:

GFI devices, despite having the term 'ground' in their name, typically do not have or need any internal reference or connection to ground.

Yes, seriously.  Two power lines in, two power lines out.  If there is a ground connection on the device, it is there for the electrician's convenience and is not connected internally.  Many GFI devices are actually permitted to be installed in older homes that do not have ground wires at all by simply omitting the ground wire and putting on a sticker that says "No Equipment Ground".  And I have actually tried GFIs with the isolation transformer to measure their trip current so this is not a theory. 

And ofcourse a GFI isn't 100% fail safe but you can have 2 in series if you like (a regular one as part of the installation of your home and an extra sensitive one for your bench).

At least with my home's electrical equipment, that is only possible if using  something called a combination Arc-Fault detector in the service panel, which has a much higher GFI trip level (it trips after you are dead, not before).  Otherwise, the manufacturer specifically prohibits using the devices in series.  I can't remember why.

[nfmax]

A method I use when the control circuitry is acting weird is to use a low voltage PSU to power the controller chip and provide the DC bus voltage (say 30V to 40V); that way you can usually test everything without needing to apply mains at all.

I've used this approach as well. Many 'universal' type power supplies will actually start up on 50V to 60V, at least unloaded. This lets you check them out functionally, for example after you have found and fixed 'the' fault (there's always at least two). If you use a low current limit on the PSU, you also have some protection against dead shorts, MOSFETs being turned hard on, etc.

[David Hess]

Now you and ntnico are smart guys, I'm sure, but did you READ?  The whole point of how I do it is that I select the ground point and ground it.  Obviously I know where my ground is, that's the point.  And I've no idea what laws you are talking about--does that include battery powered camera flashes?  Are you saying isolation transformers are illegal?

That is what I use my isolation transformer for; it allows me to move the safety ground to exactly the same point as my probe ground preventing the ground loop which would otherwise contaminate the measurement.  This also applies to the ground for differential probes for the reason given below; differential probes have a third connection whether it is explicit or not.

And with diff probe you don't get crap injected in signal. That is what they do, reject common noise and common mode signals...

Yours must be magic because the CMRR of bargain-basement diff probes is the main reason not to have them.

Exactly, common mode rejection falls with frequency and commonly available differential probes have poor common mode rejection to start with.  Good common mode rejection is the exception and not the rule.