Making Switching Loss Power Measurements

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.

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Explore posts in the same categories: Ask Scope Guru Q/A, Oscilloscope Fundamentals, Power

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