Testing Transformers used in Switch Mode Power Supplies

Most electrical equipment is powered by a switch mode power supply. Switching the power at high frequency, 25kHz to 250kHz typically, reduces the size and cost of the power supply. One of the key components in a switching power supply is the transformer. The transformer consists of a ferrite core set, two or more coils wound on a coil former or bobbin, lead wires or printed circuit pins, and a clip or some other means of holding everything together. Butler Winding manufactures custom high frequency transformers and tests 100% of them to make sure they meet all design criteria.

Almost all our transformers are tested on a Voltech AT3600 transformer tester which is calibrated yearly. A four wire Kelvin connections is made to make certain that the voltage and current measurements are made as close as possible to the device under test commonly referred to as the DUT, see figure 1. The first test performed on the DUT is either continuity or resistance. Continuity makes sure the DUT is connected correctly and test fixturing or test leads are correct. Resistance testing additionally measures winding resistance which ensures the correct wire gauge has been used.

Figure 1: A Voltech AT3600 automated transformer tester with a 200W, 50kHz ferrite
EE core transformer under test connected with Kelvin flying test leads.

The second test on the DUT is series inductance. This makes sure the correct type of ferrite material has been used and the correct number of turns has been wound on the bobbin. The test conditions this measurement is made at must be carefully selected. The voltage must be chosen so that the resulting magnetic field density within the ferrite core is between 5 – 10 Gauss. The test frequency should be relatively low, like 1kHz – 10kHz, so you are well below the self resonance frequency of the DUT.
Based on their application, some transformers have an intentional air-gap in the ferrite core. It is common to have an air-gap on the center leg of a ferrite E core set to ensure the core does not saturate if it is operated in a unidirectional application like a switching flyback transformer. If the DUT has an air-gap then leakage inductance is commonly measured. This makes sure the gap is the correct size, 0.001” to 0.040” typically, and that the windings are positioned correctly.

Turns ratio testing is performed on the DUT by applying a signal to one winding and measuring the transformed signals on all the other windings. Polarity is also tested to make sure the windings were wound is the correct direction; this is commonly referred to as the transformer dot convention.

Since isolation is often a transformer requirement, HIPOT testing either AC or DC is performed on the DUT. Voltages range from 100V to 5kV. The high voltage is applied across winding to winding or winding to core and the resultant leakage current is measured. A maximum current limit is set commonly 250uA.

Figure 2: EFD15 size test fixture to which facilitates fast transformer testing. The operator inserts the transformer, clamps it down, and pushes one button to test it. A green/red light switch indicates the pass/fail results.

Test fixturing is used for rapid 100% testing of all transformers. Both thru-hole and surface mount transformers are tested. See figure 2 for our recently tooled EFD15 surface mount test fixture. All test data is saved under its part number and manufacturing date. Special tests beyond what was discussed here can be made. Contact Butler winding to discuss further.

Written by Mike Horgan
Engineering and Quality Manager at Butler Winding