Generation and Suppression of Electromagnetic Interference in Switching Power Supplies
Before making any power loss measurements within a switching power supply, it is important to synchronise the voltage and current signals to eliminate conduction delays. This process is known as "offset correction". The traditional method is to calculate the time lag between the voltage and current signals and then manually adjust the time lag through the offset correction range of the oscilloscope. This is a very long and tedious process.
A simpler method is to use an offset correction jig and a TDS5000 series oscilloscope. To perform an offset correction, differential voltage and current probes are connected to the test points of the offset correction fixture. The offset fixture is excited by the oscilloscope's Auxiliary output or Cal-out signal. If desired, an external source can also be used to excite the offset fixture.
The offset correction capability of the TDSPWR2 software automatically sets up the oscilloscope and calculates the conduction delay due to probing. The offset correction function then uses the oscilloscope's offset correction range and automatically compensates for time lag. The test setup is now ready for measurement**.
3. Calculating power loss on non-periodic switching signals
Measuring dynamic switching parameters is simpler if the emitter or drain is grounded. However, you need to measure the differential voltage on the floating voltage. If you need to characterise and measure the differential switching signal to ground, it is best to use a differential probe. You can use a Hall effect current probe to see the current through the switching device without disturbing the circuit itself. The TDSPWR2's automatic offset correction function can then be used to remove the conduction delay explained earlier.
The TDSPWR2 software's "Switching Loss" function automatically calculates the power waveform and measures the minimum, maximum and average power loss of the switching device from the captured data. These data are very useful when analysing the power dissipation of a switching device. As shown in Figure 4, these data are displayed as Turnon Loss, Turn of Loss and PowerLoss. These data are very useful when analysing the power dissipation of a switching device. If you know the power loss at turn on and turn off, you can begin to address voltage and current jumps to reduce power loss.
