# Diode loss and form factor – a detailed explanation of a seemingly simple rectifier circuit (2)

The rectifier circuit is the basic content of the power electronics course, and it occupies a large amount of space. However, in the book, the important basic issues in engineering are not discussed or covered in detail. Especially the important rectifier capacitor filter load is often ignored, so that the rectifier Circuit design has not received enough attention in the design of medium and small power systems, which directly affects the system cost and reliability. For high-power rectification, such as new energy electrolysis of hydrogen, rectification knowledge is essential.

Author: Chen Ziying

Diode Loss and Form Factor

The AC/DC conversion of the rectifier circuit is widely used, with a wider power range and more quantity than the DC/AC inverter. In order to reduce harmonic current, active PFC is more and more widely used, but diode rectification is still the mainstream solution in motor drive, and the power range is very wide, so it is very important to understand the engineering design of diode rectification.

The rectifier circuit is the basic content of the power electronics course, and it occupies a large amount of space. However, in the book, the important basic issues in engineering are not discussed or covered in detail. Especially the important rectifier capacitor filter load is often ignored, so that the rectifier Circuit design has not received enough attention in the design of medium and small power systems, which directly affects the system cost and reliability. For high-power rectification, such as new energy electrolysis of hydrogen, rectification knowledge is essential.

Designing a good rectifier circuit begins with answering two main questions, current and losses across the diode.

Diode loss

The characteristics of the diode are:

Taking a 100A 1200V IGBTEasy3B module with a three-phase rectifier bridge as an example, the model is FP100R12W3T7_B11, the IF=f(VF) characteristic of the rectifier diode is shown in the figure, and its linearization expression is:

The input of the diode rectifier bridge is AC current, but the DC current flows through a single diode. Then, the loss on the diode, that is, the average power on the diode is:

In the formula, the quantification of the current involves the current average value IAV and the current rms value IRMS across the diode. What flows on the diode is the DC current, we calculate the average power consumption (power) under DC, so the first part of the power consumption (power) is the product of the average DC voltage and the average DC current, here we need to know the flow through The average current of the diode; while the resistive loss (power) is the product of the square of the rms value of the current and the resistance value, where the rms value of the current flowing through the diode needs to be known.

In order to simplify the engineering calculation, we give the waveform coefficient, that is, the ratio of the rms value of the current to the average value of the current, for different load waveforms of various rectifier circuits. In the actual calculation, we only need to know the average current.

If the rectifier circuit is dealing with sinusoidal current, or square wave current under inductive load, the current waveform coefficient can be obtained by looking up the table. If it is a nonlinear load such as rectifier capacitor filtering, it needs to be obtained through simulation and testing, and can also be summarized. Some empirical form factors for engineering design:

Table: Crest factor, average factor and form factor of rectifier circuits

Table: Coefficients of a sine wave

test

A convenient way to test the current is to measure it at the AC input end of the rectifier bridge. If there is an AC input inductance, it needs to be measured between the inductance and the rectifier bridge, i.e. iLn in the figure.

If it is not a sinusoidal current, the rms measurement must be done with a true rms table, and then use the form factor to convert the average current across the diode.

Why the emphasis on true RMS? This is because the general meter head measurement principle is: the pointer of the watch is deflected according to the average value, and the dial is scaled according to the effective value of the sine wave. The relationship between the peak value, RMS value and average value of sine wave voltage/current is:

Taking the sinusoidal voltage as an example, everyone is more familiar with it. If it is not a true RMS meter, when the meter reading is 220V RMS, the actual meter measures an average value of 198V.

Therefore, if you really want to measure the average value of a non-sinusoidal waveform, you need to use an average value table, which is relatively small. It is recommended to use a true RMS table to measure the current and then convert the peak factor.

calculate

Taking the sinusoidal current as an example, calculate the average power on the diode: Assuming that the rms current of the diode is 25A, then the average current is 22.5A.

Simplify the calculation with the average value, and the error is not large under the sine wave:

However, if it is a nonlinear load, the error will be large. The series of articles will discuss the impact of nonlinear loads on rectifier diodes and inverters from an application point of view. Please pay attention to the serialization.