Magna-Power Electronics shares its method ©istockphoto.com/VLADGRiN by Ira J. Pitel S witching power supplies utilize power semiconductors to switch between conducting and nonconducting states. Together with passive filters, waveforms of different magnitudes can be produced by defining the on/off periods of the switching states. Efficient power conversion requires switching periods in the order of 25-500 ns, which produce unwanted voltage transients at the output terminals of the power supply. Further transients can be produced by silicon diodes during the reverse recovery period of current. Reducing the magnitude of switching voltage transients is difficult and depends on careful placement of low-impedance capacitors physically located across the output connections of the power supply. The measurement of these transients requires a special setup to obtain repeatable results. In addition to the challenges of differential measurements, common-mode electrical noise can further complicate the measurement process. For power supplies with isolated outputs, internal stray capacitance causes the output terminals to vary in voltage with respect to the ground, mainly at the switching frequency. Capacitors applied between the outDifferential Mode put terminals and the ground help Output Capacitor EMI Source to mitigate these voltages, but like with Parasitic Impedance switching transients, they are diffidc Power Supply V + 0 cult to manage. The measurement of switching Jig Fixture transients with high common-mode BNC Connector Load voltages requires an oscilloscope ~ and probe with a sufficient bandV0width to measure fast transient signals with high common-mode ~ rejection. Second, and more imporCoaxially Wound tantly, is the technique for feeding Common-Mode Common-Mode Transformer signals to the oscilloscope. OscilEMI Source loscope probes with conventional ground leads cannot be used. The BNC Connector Oscilloscope Digital Object Identifier 10.1109/MPEL.2014.2312276 Date of publication: 20 June 2014 18 IEEE PowEr ElEctronIcs MagazInE z June 2014 fig 1 The electrical diagram of the test fixture with an illustration of EMI sources. 2329-9207/14©2014IEEEhttp://www.istockphoto.com/VLADGRiN