Archive for the ‘Power’ category

Measuring AC Power with Advanced Math

March 9, 2010

Question: How do I measure the AC power using the advanced math capabilities – I wanted to use the equation p = |V| x |I| x cos(Phase( V-I)) but don’t seem to get the correct answer.

Answer: Just to recap, it looks like you are making a power quality measurement on the input to your power supply.  To do that, you are measuring voltage and current on the input, then performing math to determine the Real Power from the measured Apparent Power. 

Whenever you are making power measurements, it’s critical to deskew your probes.  Your voltage and current probes have different propagation delays, and even the pathways behind each channel of your oscilloscope have different delays and gain.  The differences in pathway for the voltage measurement and current measurement introduce timing errors and amplitude errors in to your power measurement, since power is the product of voltage and current. This, of course, will distort your phase measurement and affect your results.

To deskew your probes, you’ll need to adjust the delay and offset of each oscilloscope channel to compensate for the differences in pathways.  To do this, you can use a deskew kit.  This deskew kit provides a fixture and pulse generator.  The deskew pulse generator provides a stimulus signal to the deskew fixture which is then routed to the voltage and current probes.  The propagation delay and gain of each path can then be adjusted using the channel adjustments (deskew and offset) in the scope to align the two waveforms.

Or, if you are using automated power analysis software like DPOPWR or DPO4PWR/DPO3PWR, you can use automated deskew in the software.  The static de-skew function automatically adjusts the delay between selected voltage and current channels based on an embedded table of propagation times for the probes.  Or, each probe may have its propagation delay embedded in its internal memory which the oscilloscope reads.  This technique offers a quick and easy method to minimize de-skew.  DPOPWR even provides an automatic deskew function in which the scope adjusts the waveforms for you.

Automated power analysis software will also automatically measure power quality parameters like apparent power, reactive power and real power (also known as true power) for you.

For more resources on power measurements, I’d suggest looking at

Best of luck!


Line Power Quality

January 29, 2010

Question:  Can you please discuss the differences between Instantaneous Power, Apparent Power, Reactive Power and True Power? How do these all relate?

Answer: True Power, Apparent Power and Reactive Power are typically measured on the power line input to determine the impact of the power supply on the power source that feeds it. Instantaneous Power is the instantaneous voltage multiplied by the instantaneous current. It represents the power on the power line at any one point in time.  Apparent Power is actually calculated by determining the RMS voltage of the voltage waveform for an entire waveform acquisition and multiplying it by the RMS current for the same waveform acquisition.

To determine True Power, the oscilloscope determines the phase difference between the voltage and current waveforms. True Power is then simply the Apparent Power multiplied by the cosine of the phase angle difference. Similarly, Reactive Power is the Apparent Power multiplied by the sine of the phase angle difference.

Making Floating Measurements

January 26, 2010

Question: How do I know where to hook up my probes if I don’t know where ground is?

Answer: Differential probes allow you to make floating measurements. For switching loss measurements, you’ll want to look at the voltage across the switching device.  For example, with a MOSFET, you would connect your differential probe to the drain and the source to measure the voltage drop across the MOSFET.

Making Switching Loss Power Measurements

January 22, 2010

Question: When characterizing the switching transistor in a power supply, you would have to make very high voltage measurements and very low voltage measurements.  How do you do that with an oscilloscope?

Answer:  This is a common problem when making switching loss measurements.  When the switch is in the ON state, the voltage across the switch is usually millivolts to a few volts.  In the OFF state, the voltage across the switch can be quite high, upwards of 750 V for some applications.

With an 8-bit oscilloscope, measuring a 750 V signal and a 100 mV signal in one acquisition can be a problem.  To see the high voltage signal, the scope needs to be set to, say, 100 volts per division.  However, at this setting, an 8-bit oscilloscope will only be able to resolve about 4 V at the low end.  An 8-bit oscilloscope doesn’t provide enough dynamic range.

Typically, power measurement software software packages will allow you to enter the Dynamic On Resistance or VCE(sat) values from your switching device’s datasheet.  The software will then calculate the voltage when the switch is ON. 

  • For MOSFETs, you would enter the expected on-resistance between the drain and source of the device when it is conducting.
  • For BJTs and IGBTs, you would enter the expected saturation voltage from the collector to the emitter of the device when it is saturated or ON.
  • The software then does the rest.

Or, one other option is to enhance the vertical resolution of the “on” Voltage measurement by engaging the HiRes or boxcar averaging feature of the scope to increase the vertical resolution of the measurement.