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3 Posts authored by: jchang Employee

If you are dealing with more than a couple tens of amperes of AC current and want to make flexible current measurements, consider a Rogowski current probe.

 

A Rogowski coil is an electrical transducer used for measuring AC currents such as high-speed transients, pulsed currents of a power device, or power line sinusoidal currents at 50 or 60 Hz. The Rogowski coil has a flexible clip-around sensor coil that can easily be wrapped around the current-carrying conductor for measurement and can measure up to a couple thousand amperes of very large currents without an increase in transducer size.

 

How does a Rogowski coil work?

The theory of operation behind the Rogowski coil is based on Faraday’s Law which states that the total electromotive force induced in a closed circuit is proportional to the time rate of change of the total magnetic flux linking the circuit.

Download the "6 Essentials for Getting the Most Out of Your Oscilloscope" eBook.

 

The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. The key difference is that the Rogowski coil has an air core as opposed to the current transformer, which relies on a high-permeability steel core to magnetically couple with a secondary winding. The air core design has a lower insertion impedance, which enables a faster signal response and a very linear signal voltage.

 

An air-cored coil is placed around the current-carrying conductor in a toroidal fashion and the magnetic field produced by the AC current induces a voltage in the coil. The Rogowski coil produces a voltage that is proportional to the rate of change (derivative) of the current enclosed by the coil-loop. The coil voltage is then integrated in order for the probe to provide an output voltage that is proportional to the input current signal.

 

 

Advantages

Rogowski coil current probes offer many advantages over different types of current transducers or sensing techniques.

  • Large current measurement without core saturation -- Rogowski coils have the capability to measure large currents (a very wide range from a few mA to more than a few hundred kA) without saturating the core because the probe employs non-magnetic “air” core. The upper range of the measurable current is limited by either the maximum input voltage of a measuring instrument or by the voltage breakdown limits of the coil or the integrator circuit elements. Unlike other current transducers, which get bulkier and heavier as the measurable current range grows, the Rogowski coil remains the same small size coil independent of the amplitude of current being measured. This makes the Rogowski coil the most effective measurement tool for making several hundreds or even thousands of amperes of large AC current measurements.

 

 

  • Very flexible to use -- The lightweight clip-around sensor coil is flexible and easy to wrap around a current-carrying conductor. It can easily be inserted into hard-to-reach components in the circuit. Most Rogowski coils are thin enough to fit between the legs of a T0-220 or TO-247 power semiconductor package without needing an additional loop of wire to connect the current probe. This also gives an advantage in achieving high signal integrity measurement.
  • Wide bandwidth up to >30 MHz -- This enables the Rogowski coil to measure the very rapidly changing current signal – e.g., several thousand A/usec. High bandwidth characteristic allows for analyzing high-order harmonics in systems operating at high switching frequencies, or accurately monitoring switching waveforms with rapid rise- or fall-times.
  • Non-intrusive or lossless measurement -- The Rogowski coil draws extremely little current from the DUT because of low insertion impedance. The impedance injected into the DUT due to the probe is only a few pico-Henries, which enables a faster signal response and very linear signal voltage.
  • Low cost Compared to a hall effect sensor/transformer current probe, the Rogowski coil typically comes in at lower price point.

 

Limitations

  • AC only -- Rogowski cannot handle DC current. It is AC only.
  • Sensitivity - Rogowski coil has a lower sensitivity compared to a current transformer due to the absence of a high permeability magnetic core.

 

Applications

Rogowski coil current probes have a large number of applications in broad power industries and power measurement applications. The following are some examples of Rogowski coil applications:

  • Flexible current measurement of power devices such as MOSFET or IGBT device as small as TO-220 or TO-247 package or around the terminals of large power modules
  • To measure power losses in power semiconductors
  • To monitor currents in small inductors, capacitors, snubber circuits, etc.
  • To measure small AC current on a conductor with high DC current or in the presence of a high DC magnetic field.
  • To measure high frequency sinusoidal, pulsed, or transient currents from power line frequency to RF applications
  • To measure current in motor drives and, in particular, power quality measurements in VSD, UPS or SMPS circuits
  • To evaluate switching performance of power semiconductor switches (double pulse tester).
  • Power distribution line monitoring or utilities pole probe monitoring
  • Smart grid applications
  • Plasma current measurement

 

Conclusion

There are a number of different ways of measuring electric current where each method has advantages and limitations.

 

The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. However, Rogowski coils have the capability to measure large currents (very wide range from a few mA to more than a few kA) without saturation because of its non-magnetic “air” core. The air core design also has a lower insertion impedance to enable a faster signal response and a very linear signal voltage and is very cost effective compared to its hall effect sensor/current transformer counterpart. This makes the Rogowski coil the most effective measurement tool to make several hundreds or thousands of amperes of large AC current measurement.

In oscilloscopes and oscilloscope probes, bandwidth is the width of a range of frequencies measured in Hertz. Specifically, bandwidth is specified as the frequency at which a sinusoidal input signal is attenuated to 70.7% of its original amplitude, also known as the -3 dB point. Most scope companies design the scope/probe response to be as flat as possible throughout its specified frequency range, and most customers simply rely on the specified bandwidth of the oscilloscope or oscilloscope probes. This often leaves them wondering if they are indeed getting the bandwidth performance at the probe tip. This article provides some step-by-step instructions on how to simply measure and verify the bandwidth of your probe with an oscilloscope you may already have.

Oscilloscope Gaussian frequency response

Figure 1 An example of an Oscilloscope Gaussian frequency response

 

To measure the bandwidth of an oscilloscope probe, a VNA (vector network analyzer) is often used, which can be very expensive and difficult to learn how to use. Also, because typical passive probes are high impedance probes that should be terminated into 1 Mohm of an oscilloscope, it makes the traditional VNA s21 method hard to implement because it is 50 ohm based system.

 

The other way to get bandwidth is to use a sine wave source, a splitter, and a power meter and sweep the response directly. If you do this, you must set this up to run using a remote interface such as GPIB or USB. Doing it manually is very laborious, subject to mistakes, and requires extensive effort every time you want to evaluate a tweak, etc.

 

An easier way of measuring probe bandwidth, especially for the lower bandwidth probes (say, <1 GHz passive probe) is the time domain approach utilizing only an oscilloscope with the built-in step signal source, the ‘differentiate’ function, and the ‘FFT’. To be able to use this method, your oscilloscope should support the function of another function output. If you don't, an alternative is to pull the time domain waveform data out of the oscilloscope, import it into the PC based analysis tool such as Mathlab or Excel, and apply the math functions on the step data there.

 

When you apply a step function to your system, then you will get the step response. If you then apply the differentiate (or derivative) to this step response, you obtain the impulse response, and then take the FFT of the impulse response to obtain the frequency response of the system.

 

Keysight’s Infiniium real-time oscilloscope is an excellent tool for this quick bandwidth testing. Here is the step by step procedure of the testing. For this bandwidth measurement example, a N2873A 500MHz 10:1 passive probe with an Infiniium MSOS804A 8 GHz oscilloscope is used.

  • Use a performance verification fixture such as Keysight’s E2655C with a 50 ohm BNC cable to connect the Aux output of the oscilloscope to the input of the oscilloscope. The Infiniium oscilloscope has an Aux output port with fast edge speed (~140 psec, 10-90% for Infinium S Series) for probe calibration. It is very important to note that the rise time of the signal source should be faster than the probe’s rise time, and the frequency response of the source is reasonably flat over frequency.

Probing 25 ohm signal source with Keysight E2655C

Figure 2 Probing 25 ohm signal source with the Keysight E2655C performance verification fixture

 

  • Connect the probe to the PV fixture to measure one edge of the source. Use as short a probe ground as possible to reduce probe loading associated with ground leads.

Ch 1 (yellow) = signal source (Aux output) as loaded by the probe

Ch 2 (green) = the measured output of the probe

 

Probing fast edge

Figure 3 Probing fast edge

 

  • Place the rising edges at center of the screen. Trigger on the measured output of the probe (ch2) and use the averaging or high resolution acquisition to reduce the noise on the waveform.
  • Use the oscilloscope’s built-in math function to differentiate the step response. Now you get the impulse response of the channel 2 where the probe is connected to. Assign the differentiated output of the step response into the F1 of the oscilloscope.

 

Built-in math function to differentiate the step response

Figure 4  -- Use the oscilloscope’s built-in math function to differentiate the step response.

 

  • Apply the built-in FFT Magnitude function on the impulse response (F1) of the measured step signal. Rescale the FFT to 100MHz/div (the center frequency at 500 MHz with the 1 GHz of frequency span across the screen) and 3dB/div vertically.

 

FFT magnitude function

Figure 5 -- Apply the built-in FFT Magnitude function on the impulse response

 

  • Now you have a plot of bandwidth. Since the vertical scale of the FFT plot is set to 3 dB/div with the horizontal scale set to 100 MHz/div, you can see the probe has ~660 MHz, as you pick the point in the FFT trace falling by 3 dB.

 

Plot of bandwidth

Figure 6 Now you have a plot of bandwidth

 

There is one catch to this. The way we do differentiate in some of the oscilloscopes is taking the best fit slope to three adjacent points and then assign this slope to the center point. This can really hose the bandwidth measurement up if you don't have enough sample density on the edge, so experiment with sample density and make sure it doesn't affect the bandwidth.

 

Conclusion

Utilizing the built-in mathematical capabilities available in modern digital oscilloscopes, it is possible to derive the frequency response or the bandwidth characteristics of a probe based on the measured step response of a fast step signal. Among those several test methods, the time domain approach is the easiest for an oscilloscope user to duplicate without having a need to use expensive test instruments.

Keysight N2820A current probe

Many oscilloscope users measure current flowing through a circuit by measuring the voltage drop across the current sense resistor. But if you’re lucky enough to have a Keysight N2820A high-sensitivity current probe, it is even more than that. While the N2820A (and N2821A) is designed to measure low level current signals with high sensitivity, accuracy and dynamic range, the underlying architecture of the probe is actually a voltage probe. It essentially is an ultra-sensitive voltage probe with extra-low probe loading measuring the voltage drop across the sense resistor in the current flowing path and dividing the voltage by the sense resistor value to derive the current flow. Or simply put, I = V/R.

 

 

You can use the N2820A current probe as a voltage probe in just a few steps:

 

1. Don’t use the current sense resistor. Use the user-defined head (Keysight N2825A) with the probe and connect the +, - input of the probe head directly to the DUT like you would to measure voltage with a voltage probe.

Keysight N2825A with N2820A oscilloscope probe

 

connecting your current probe to the oscilloscope

2. Keep the common-mode voltages of the signal input to less than ±12V, and differential input should be less than ±1.2V. If you have a signal with some DC component that exceeds the differential input range of the probe, you will rather AC couple the signal on the oscilloscope.  

 

3. Choose 1Ω Rsense in the oscilloscope menu. That way, you can get the correct values on your measurement. E.g., 1 mA = 1mV.

Choose 1-ohm Rsense in the oscilloscope menu

 

Measuring voltage on a Keysight InfiniiVision oscilloscope

4. If you have a Keysight Infiniium oscilloscope, you can go a step further and add external scaling to turn the units back into volts (with 1:1 attenuation ratio). Check the External Scaling under the channel’s probe menu and select ‘volt’ as the units. If you are using a Keysight InfiniiVision oscilloscope, you would interpret the voltage reading in ‘Amphere’ as ‘Voltage’.

 

Measuring voltage on a Keysight Infiniium oscilloscope

 

When using the N2820A as a voltage probe, the key characteristics are:

  • Bandwidth (-3dB) : 500 kHz (zoom-in), 3 MHz (zoom-out)
  • Measurable input range: 3uV - 1.2V (differential), 12V (common mode)
  • Input impedance : 3 Gohm (giga ohm) differential, 1.5 Gohm single-ended
  • DC amplitude accuracy : 3% or 10 uV whichever is greater
  • Probe gain factor: 300X (zoom-in), 2X (zoom-out)
  • Offset : scope vertical positioning only (no probe offset)