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Improved Partial Cancellation Method for High-frequency Core Loss Measurement

Equivalent circuit
Fig.1. The equivalent circuit of inductive partial cancellation method
Accurate high-frequency core loss measurement is critical for power converter design, especially for integrated voltage regulator application. Partial cancellation method is a promising candidate, which is designed to cancel out loss error caused by phase discrepancy without finely tuning cancellation component values. Fig.1 shows the equivalent circuit of HouÂ’s inductive partial cancellation method. The voltage of the core sample, the voltage of air core transformer, and the current flowing through the core sample are measured. The basic idea of this method is utilizing the loss error in meas-ured air core transformer loss to cancel out loss error in measured core loss. However, it as-sumes a small value of phase discrepancy caused by probe delay between voltage probe and current probe, which is not valid for high frequency. A detailed error analysis under this case shows the probe delay between voltage probe and current probe must be limited within a range to obtain the accuracy. An initial compensation method by using a simple circuit with a high Q capacitor is proposed to solve this problem. Fig.2 shows the circuit for initial compensation. By measuring capacitor voltage and its current under sinusoidal excitation, the high Q capacitor loss is obtained by integrating the product of its voltage and current in a cycle. By extracting the equivalent series resistance (ESR) loss, the probe delay between voltage probe and current probe is calculated out from the rest of the measured loss. It shows the probe delay changes with frequencies and a little difference will induce measurement error for higher frequencies (> 20 MHz). Therefore, it is necessary to initially compensate the probe delay before using partial cancellation method. Finally, experimental results verify proposed method. The core loss of metal flake composite manufactured by TOKIN Corporation is measured three times at 60MHz with sinusoidal frequencies. The results with initial compensation are constant and stable compared with results using delay values from the probe datasheet. As a promising method for high frequency core loss measurement, partial cancellation method assumes phase discrepancy as a small value, which is not valid for high frequency. This paper first performs the error analysis under high frequency test conditions. The result shows large phase discrepancy will introduce non-negligible error and phase discrepancy needs to be controlled in a small range to limit measured loss error. An initial compensation method is proposed to control phase discrepancy in a small range effectively by using a high Q capacitor. Finally, experiments verify the measurement accuracy of the improved partial cancellation method.
Compensation circuit
Fig.2. Initial compensation circuit with high Q capacitor
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