Testing of HF transformer designed for phase shift full bridge converter(PSFB)

I have designed an HF transformer for a PSFB converter.

Specs:

• Peak Primary Voltage = 800 V
• Peak Secondary Voltage = 176 V
• Turns ratio Np:Ns = 18:4
• Primary Irms current = 12.38 A
• Secondary rms current = 56.29 A
• R_primary = 26 mΩ
• R_secondary = 1.7 mΩ
• L_magnetizing = 9.3 mH
• L_leakage = 26.4 uH (estimated using LCR meter)

I would like to know the recommended testing procedure to test this transformer, before I go for actual testing of the main converter.

My expectation from the test are:

1. Testing whether the core is getting saturated at a rated voltage or not.
2. Verification of the voltage waveform (typical quasi square waveform in PSFB) at secondary during no load. (This is to verify turns ratio)
3. Estimation of core loss (if possible).

Please let me know if you need further information.

• If L_magnetizing = 9.3 mH then it's a very, very, very, very poor design if the L_leakage = 26.4 mH. Nov 18 '21 at 22:01
• Leakage inductance is generally orders of magnitude lower than primary inductance. (Also note the secondary power is higher than the primary power!) Nov 18 '21 at 22:05
• How did you measure the leakage inductance. You should have shorted out the secondary and measured the primary (or vice versa).
– qrk
Nov 18 '21 at 23:07
• My apologies for the typo error. Leakage inductance is in micro Henry not in milli Henry. I have corrected the mistake. Nov 19 '21 at 5:58
• RMS current values are also corrected. Initially, I gave approximate values. Nov 19 '21 at 6:06

It's not clear that you designed the transformer to operate at a maximum flux density and that you chose the wire diameter to optimize DC + AC copper losses. Your DC copper loss looks very low which means you used large diameter wire which will cause very large real AC losses (i.e. heat) unless you are using bunched or Litz wire; or a planer design.
Out of curiosity, what sort of core material are you using and at what frequency?

To test for core saturation, inject a 50% duty cycle (worst case waveform) signal while monitoring the current. A clip-on oscilloscope current probe is very handy for this sort of test. For your design, I would drive the secondary since it's a lower voltage winding. Your current waveform should be square. If you see the current quickly ramping up, depicted by the red spikes in the diagram, then the transformer is starting to saturate. You may want a light load on the output to reduce ringing. If you don't have a high voltage generator, you can use a lower frequency which will cause the transformer to saturate at a lower voltage (refer to the flux density equation you used to see the relationship between frequency, voltage, & flux density). You can calculate core loss by looking up the core loss information in the manufacturer's data sheets (you need to find the power loss graph per unit volume or weight for the core material you are using). You'll need to know the peak flux density which you should have already calculated to determine the number of turns.

An important test you are leaving out is the total copper losses. Copper loss is $$\R_{dc}\$$ (DC loss) plus $$\R{pe}\$$ (proximity effect loss, one of the eddy current losses). Skin effect loss will probably be negligible. A swept impedance analyzer is a very useful tool that can show you the series real losses versus frequency. The loss test is performed when measuring leakage inductance (i.e., when one of the windings is shorted). If you don't have access to a swept impedance analyzer, then set your LRC meter to a frequency close to your operating frequency and be sure to observe the series loss resistance.

You should also test where the resonant frequency is when the opposite winding is shorted. I like to keep the resonant frequency >5x higher than the operating frequency. The resonance test is normally done when measuring leakage inductance with a swept analyzer, although, you can sweep the transformer with a signal generator to find resonance.

You can get a good indication of turns ratio by measuring the unloaded primary and secondary inductances. The ratio will be slightly off since the winding geometries are different. Otherwise, use a sine wave and look at the ratio. You may need to load the transformer slightly to flatten the frequency response.

• Hi, Thank you for your answer. Litz wire is used for winding. Ferrite N87 is the core material and the frequency of operation is 50kHz. Dec 2 '21 at 8:10