How to probe high-side Mosfet gate - Page 1
Forum section: electronics / projects

author: Romain
author_rank: Regular Contributor
timestamp: June 04, 2020, 08:38:23 am
content: What solutions are there to probe high-side N Mosfet gates in a 40-100V motor controller application?I'm trying to build a list that could be useful for beginners and more advanced engineers.1. Normal scope + differential probe (USD150-300):https://www.eevblog.com/product/hvp70/2. Scope with isolated inputs (USD 10k):https://cleverscope.com/news/cs448/Video on the CS448:

author: filssavi
author_rank: Frequent Contributor
timestamp: June 04, 2020, 09:07:40 am
content: Differential probes will work in a pinch, only you Usually get massive ringing due to inductance/capacitance of the lead’s probes and the high dv/dt.The gold standard right now are optically isolated probes, in the lab we have the luck of having a tek isoVu with the new mso58 and comparing the same signal between it and the normal differential probe is night and dayThat scope seems nice, and 10k$ is much less than what you would pay a single isovue and scope (I think it is in the 60k region msrp however we had accademic pricing for the 1gig bandwidth). However you are trusting a niche manufacturer,and also that scope is basically a 1 trick pony, with a normal scope you can do much more...Also it is really rare to need more than 1 high side gate channel, as usually once the driver is nailed you only work on timing and for that a regular probe is enoughSecond caveat is that if you are not using SiC/GaN and pushing really hard when switching all this is not needed

author: Dave
author_rank: Super Contributor
timestamp: June 04, 2020, 10:05:13 am
content: At work we're using a R&S RTH1054 for this exact purpose. Does the job quite well.Contrary to popular belief, the dv/dt in motor controller power stages are often not even that high, because they're limited to reduce the EM emissions. It's always a balance between reducing the switching losses and passing the EMC tests.

author: filssavi
author_rank: Frequent Contributor
timestamp: June 04, 2020, 11:46:00 am
content: Quote from: Dave on June 04, 2020, 10:05:13 amIn case of general industrial motor drives, which are predominantly Si IGBT based you are correct, howvever that is far from being guaranteed, With wide bandgap devices, in decent surface mount packages/modules, and a good layout with loops as tight as possible, and good bypassing on the DC link, you can push the switching times way down if you need. Usually it is done where power density is desired such as on vehicular applications(planes and electric cars).The voltage range the original poster is interested in however is still low enough where he might even get away with a 100:1 passive probe (if grounding the negative dc link is not an issue)

author: free_electron
author_rank: Super Contributor
timestamp: June 04, 2020, 12:54:07 pm
content: isolation probe.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2020, 07:03:53 pm
content: If an expensive, direct measurement is infeasible, measurements can be made more indirectly in test fixtures.Direct operation: kill DC bus, and measure the high side directly. The output is at GND, so it can be probe-grounded without issue. Additional tests can also be made (e.g., active load on the local supply to test for load capability and (differential) transient immunity, etc.) This does not subject the driver to switching noise.Ambient noise: set up a transient generator, and couple it into the driver in various ways (without compromising the probed signal quality). Example: high voltage step or impulse into a wire or plate, placed near the driver. This tests induced electric field noise.Signal side noise: if the driver is isolated, set up some basic stimulus (e.g., a 555 to generate waveforms, a battery to supply power, etc.) on the isolated signal-input side. Pack it all into a metal box, and drive the box (ground) with the transient generator. Tie the high side driver to ground plane. This tests the isolation barrier for transient immunity, dV/dt, that sort of thing. Don't forget to check with respect to phase of the driving signal, in case it's prone to hiccups coincident with its own switching.If the driver is bootstrap, probably ignore this step. (Some signal-to-power-ground noise can still be tested on these, but also you have a fully qualified IC on your hands, so these tests probably aren't a big deal anyway.)Power side noise: this gets harder to test; an isolated probe is hard to avoid. An option is to tie the high side driver into ground plane, and subject the inverter pins to transients. Probe signal quality can be okay thanks to the ground plane. Grounding and shielding must be very good. This tests magnetic immunity.This combination of tests, still misses edge cases with combinations of electric and magnetic noise, and has to make some assumptions about the layout and grounding.You might also consider filling in for a probe, with some cute logic circuits to monitor levels or state instead. Example: if you are concerned about generating runt pulses of either polarity, or interruptions to steady levels, you can use a pulse detector and latch that into an LED. As you increase stimulus (load voltage or current, or one of the above transient tests say), if at some point the light turns on, you can set up a more specific test to recreate the symptom in a more controlled environment.That said, several of these tests may be of interest in general, as:1. Low side drivers aren't generally subjected to such fields, so if their layouts or circuits differ, potential problems may go unnoticed.2. You can test to much higher voltages, or dV/dt, than the application requires. With a suitable transient generator, you might test the circuit to malfunction, and see its true limits; now you have a measure of how much guard band you have above normal operation.3. You can test with more waveforms (steps, impulses, CW..), and at more frequencies (PRF, modulation, burst patterns, etc.), than the inverter itself is likely to generate.One last thing -- note that I haven't even mentioned the gate drive waveforms themselves. This is a design quantity; you should have very little reason to measure it specifically! It depends on the driver (voltage, current and speed), transistor (gate charge, resistance), load (drain/collector voltage Miller effect, and source/emitter degeneration), and how they are connected (likely the gate connection will have a much higher TL impedance than the driver, so manifests as simple stray inductance). The only parameter which varies with operation, is the load, and you can measure that on the low side, and assume it is representative for all channels -- again, assuming the layouts are consistent and well behaved.Tim

author: daqq
author_rank: Super Contributor
timestamp: June 04, 2020, 07:16:10 pm
content: I used two normal probes, subtracted the data and got the resulting waveform. No special hardware needed. Not ideal obviously, but works well enough for some purposes. Also, if you have a high res scope, you won't even know the difference.

author: filssavi
author_rank: Frequent Contributor
timestamp: June 04, 2020, 07:33:05 pm
content: Quote from: daqq on June 04, 2020, 07:16:10 pmThe old two probe subtraction trick can work for general electronics, however it is not applicable to high side gate drivers since it has basically no CMRR, this means you will have huge ringing during the commutation transients. that basically render looking at the gate useless (you can see if the transistor works by looking at the output voltage or current.Also you might not want to connect the negative of the dc link to ground if you use an isolated power supply, as it tends to cause huge common mode noise issues through it's class y filter capacitors (high frequencies pass straight through the isolation transformer as it was not even there because of parassitic winding capacitance.Last but not least if you go too much higher than 100V you risk blowing up your scope input channel on the negative probe due to the almost unavoidable overvoltage on turn on of the high side due to parassitic inductances

author: daqq
author_rank: Super Contributor
timestamp: June 04, 2020, 07:39:43 pm
content: QuoteAre you sure about this? What if you have a standard 10:1 probe rated to a reasonable voltage.I did this in a 300V circuit with standard probes (one probe on an IGBT G, one probe on an IGBT E), it worked. The display might not have been absolutely perfect, but it was reasonably accurate, no exploded scopes.

author: filssavi
author_rank: Frequent Contributor
timestamp: June 04, 2020, 08:08:08 pm
content: First of all I might have been a little hyperbole happy, you are not going to literally explode the scope front-end, however there is a good chance you will either throw off the calibration or slightly damage the frontend (it will not stop working, only it will have worse performance).As for the voltage rating usually the probe rating takes into account the voltage divider effect (obviously if probe and scope are mismatched that might not be true). For example the TPP1000 that comes as standard with the MSO58 from tektronix is rated at 300V rms that is 420ish volts peak. if you take a look at the scope specs it lists an absolute maximum input voltage of plus or minus 42 volts, which is exactly what the probe is rated at.So you were close to that limit, now how much overvoltage you will get really depends on how your board layout is done and what load are you driving (how inductive it is) but on a standard IGBT machine drive at 300V having 100 to 200V overvoltage is quite typical when not in open circuit,

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2020, 09:06:31 pm
content: Quote from: filssavi on June 04, 2020, 07:33:05 pmThe CMRR of the probe pair needs to be calibrated before use. The variable function on the oscilloscope corrects the DC CMRR and the probe compensation trims the AC CMRR. In the past special probe pairs were available for this application which provided very good performance.The probes in other respects operate like any high impedance probe and present a high DC resistance the source shunted by the probe tip capacitance and when properly used, will work better than the differential input of a high voltage differential probe. Ringing problems indicate misuse of the probe and apply to any probe include a high voltage differential probe. The big advantage of using common probes is the availability of a coaxial connection.But a good high voltage differential probe is much easier to use.

author: uer166
author_rank: Super Contributor
timestamp: June 04, 2020, 10:13:45 pm
content: Subscribing to the thread, I would love to see a <$1k solution to measure high-side Vgs directly over reasonable edge rates (say a 200V common mode swing over 40ns), normal Si, not GaN or SiC.

author: filssavi
author_rank: Frequent Contributor
timestamp: June 05, 2020, 05:13:42 am
content: Tektronix high voltage differential probes start at 1100€ for the P5200A, it does not have too much bandwidth(50MHz) but it will do if you just want to see what the gate is doing (signal integrity leaves a bit To be desired with the long antennas they call probe leads but that is unavoidable at this price point)Keysight N2791A starts even lower at 700ish dollars, however it has an even lower bandwidth (25 MHz).I am agire you can find something cheaper from China, however I would not touch those with a 10 foot pole

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2020, 08:19:02 pm
content: I think high voltage differential probes from companies like Tektronix and Keysight are lower bandwidth because they are more realistic. At higher bandwidth, the common mode rejection is so difficult to maintain that the extra bandwidth is of questionable use.