Measuring Power Electornic Signals (600 V +) using 50 Ohm terminations?  - Page 1
Forum section: electronics / projects

author: Glenn0010
author_rank: Regular Contributor
timestamp: November 02, 2020, 02:47:31 pm
content: Hi All,I'm just wondering if anyoune has tired measuring IGBT switching waveforms using 50 Ohm terminations ex 950 ohm res > 50 ohm cable > 50 ohm termination, giving 20: 1 ratio.Are there any pitfalls using this method? And do the reults represent what is actually happening? I've never tried this but wondering wether it would workCheers

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 02, 2020, 03:08:16 pm
content: Yes, I have done similar using non-inductive power resistors, physically in a linear string, on a refractory board.That was used as a temporary work- around ( not recommended as permanent test gear) when the voltages exceeded the range of the high voltage probes available.( eg 3000V)The method is not really suitable for accurately measuring fast transients due to the stray reactances.A safer way is to try to just measure circuit currents, using a Rogowski coil.These are available in various high voltage ratings, with a lead to converter module giving 0 ~ 10 V etc for the oscilloscope

author: MarkL
author_rank: Unknown
timestamp: November 02, 2020, 05:32:20 pm
content: If you're trying to directly measure a 600V waveform, a probe with 1k total resistance is not the right thing. At 600V, the probe will draw 600mA, and at the worst case duty of 100%, would dissipate a total of 360W (with 18W of that in the 50R terminator).

author: David Hess
author_rank: Super Contributor
timestamp: November 03, 2020, 01:02:28 am
content: It would work fine with a higher attenuation ratio. High impedance probes use a transmission line into a 1 megohm input but there is nothing to prevent using a higher attenuation low impedance probe into a 50 ohm input to support higher bandwidth. The reactance of the attenuator at the probe tip needs to be compensated which will raise in the input capacitance however power electronics will have no trouble driving it.But why do you need such a fast 600 volt probe? IGBT circuits are not fast enough to require better than even 100 MHz performance and there are plenty of high voltage high impedance probes faster than that.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 03, 2020, 03:43:22 am
content: Quote from: Glenn0010 on November 02, 2020, 02:47:31 pmP=V^2/R = 600^2/1000 = 360W. 950ohm resistor will take most of the hit and can instantly vaporized. the 50ohm termination will take 18W of power, you'll need this as termination..https://www.ebay.com/itm/JFW-30-Watt-50-Ohm-Type-N-Male-High-Power-Termination/174075321277?hash=item2887b29bbd:g:e0wAAOSwoMFdsbaubut then you'll need something fancy to get 360W 950 ohm resistor, the question is for what? is it worthy?

author: DaJMasta
author_rank: Super Contributor
timestamp: November 03, 2020, 06:52:16 am
content: Make sure your cable is rated for that much power, but also, why not just an inline high power attenuator? A beefy RF attenuator should mean your termination doesn't have to be so substantial and then you have a resistor with integrated heatsink of appropriate sized characterized for high frequency use.Also could look at designs for a Cantenna RF load - basically a power resistor in a shielded can full of oil for thermal capacity. Would likely give you good results with pulsed measurements, though I think you need a bigger can for CW operation at that load.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 03, 2020, 01:09:42 pm
content: But be careful about recommending RF attenuators and Rf loads.They usually present 50 Ohm etc at both the input and output ends.What is needed here is a high input resistance ( I used 10's of kOhm in a string of power resistors) and a 50 Ohm output resistance configured as a divider.The other problem is the ground side of 'scope going out to a circuit which probablywon't be at ground potential, giving common mode errors to 'scope.I think using a Rogowski CT and a small current probe on the gate, where possible, gives a lot of circuit info (Except for on-state voltage) and overcomes a lot of problems.