Technical Information Site of Power Supply Design

2019.06.06 AC/DC

Evaluation Results: Efficiency and Switching Waveform

Design Example of Isolated Quasi-Resonant Converters Using SiC MOSFET

As an evaluation of a circuit explained as a past example, the results for efficiency and switching waveform are presented. The overall circuit can be seen in the previous article.

Evaluation of Efficiency

As efficiency evaluation results, graphs show the efficiency and output power for three types of input voltage, and the efficiency and output current for each input terminal.

This graph shows the efficiency for input of 300 V DC, 600 V DC, and 900 V DC to the DC input terminal. The basic specifications assume a 24 V/1 A output, and so good efficiency is observed in the vicinity of 24 W; characteristics that maintain high efficiency to as low an output power as possible are ideal. For the example of an input of 300 V, up to 15 W (Iout of approx. 0.63 A), efficiency of approx. 90% is attained, and even at 5 W (Iout approx. 0.21 A), the efficiency is maintained at 80% or above. High efficiency is maintained over similarly broad output power ranges for other input voltage conditions as well.

This graph shows efficiency when 300 V DC is applied to DCIN (red), 300 V DC is applied to ACIN (green), and 300 V AC is applied to ACIN. As a result, the highest efficiency is obtained for DC input to DCIN without passing through a rectifying circuit.

Evaluation of Switching Waveform

By measuring the power or the voltage and the current, the efficiency can be calculated without any particular need to observe the switching waveform. However, it is extremely important that the waveform at important places in the switching power supply be observed in order to confirm that there are no anomalies such as spikes or oscillation. Below are the switching waveforms of the drain voltage and drain current of a power switching SiC MOSFET. The drain voltage waveform is of the quasi-resonant type. Relative to the upper waveforms, Iout for the lower waveforms is twice as great. The turn-on and turn-off times and the differences in drain currents should be compared.

These waveforms are close to ideal. They can also be used as references to determine whether a prototype circuit is normal or not.

There are other parameters to check as well. This is a set of articles relating to AC/DC converters, and should be referenced. They deal with flyback converters, but the basic approach is similar.

Key Points:

・The circuit example shown is used to measure and study efficiency.

・Circuit components are examples, and others can be selected.

Design Example for PWM Flyback Converter

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