“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.

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Author: Chen Ziying

Summary

In the sixth issue, the AC/DC conversion of rectifier circuits is widely used. Among them, diode rectification is 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 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.

The second lecture “Loss and Form Factor of Diodes” introduces the basic concepts of diodes and measurement technology. From the third lecture, various loads of the rectifier bridge, including resistive loads, resistive inductive loads, loads with back EMF, and capacitive filter loads are introduced. . Finally, the inverter design strategy for nonlinear loads is explained with standard and test waveforms.

Analysis of Resistive-Inductive Loads in Single-Phase Rectifier Circuits

Purely resistive loads are relatively simple and will not be given any further examples. Resistive and inductive loads are still commonly used, such as DC motor loads. In order to conform to the habit of fast reading on WeChat, a single-phase circuit is used as an example to analyze its discontinuous current mode and continuous current mode, and some current waveforms are given. If necessary, you can read the second chapter of Mr. Ruan Xinbo’s “Power Electronics Technology”. Mr. Ruan spent 45 pages explaining the rectifier circuit very systematically.

Resistive Inductive Loads—Continuous Current Mode

The ideal continuous mode load is a square wave with a little ripple in the current. Due to the inductance of the distribution (leakage inductance of the transformer), the current rate of change of the square wave current is not as high and the edges are not as steep.

When VT1 and VT4 are turned on at an angle, the ud voltage is equal to u2 minus the forward voltage drop of the two rectifiers. The waveform follows the input voltage u2 during the conduction period of the rectifiers. It is assumed in the analysis: ud=u2.

Starting from the corner, the current id keeps rising until the voltage uR on the resistor equals u2. In the figure, since the inductance is assumed to be large and the load resistance R is small, it appears that the current peak (maximum id) is close to the zero-crossing of the ud voltage. When uR>u2, the Inductor releases energy at this time, and the current gradually decreases. Once u2 is negative in the second half cycle, but the current is still positive, the energy is fed back to the grid at this time. This phenomenon is more clearly seen in discontinuous current mode.

In the case of diode rectification, the current is commutated when the input voltage crosses zero, and in the case of thyristor commutation occurs at the corner. The current id(0) during commutation, that is, the id(θ) of the first half cycle. The larger the inductance L is, the more energy is stored, and the smaller the load resistance R is. At this time, the id(θ) is also relatively large, and the current ripple is not large.

So the current on the rectifier tube is also a square wave.

The rectifier losses are:

The average value id of the diode current is half of the average value of the current output by the rectifier bridge, and the effective value can be obtained by looking up the table to obtain the square wave form factor of the rectifier circuit. This makes it easy to calculate the losses on the diode.

In fact, here, the rms current is also easy to understand. Since the average value of the inductance is zero at steady state, Id is only determined by the load resistance.

Since the average value on the inductance is zero in the steady state, Id is only determined by the load resistance, and the current is a square wave. The RMS value is equal to the average value and the peak value, so the problem becomes simple.

Resistive Inductive Load Discontinuous Current Mode

Analyzing the overcurrent continuous mode and looking at the current discontinuous mode, its characteristics are obvious, and the commutation process is listed below in the form of a journal.

t1=ωt1: T1, T4 are turned on, ud=u2, the load current id rises from zero, the grid supplies power to the inductor and the resistor, and the inductor stores energy;
uR=u2: The voltage on the resistor is equal to the input voltage, the inductor voltage is zero, and the current id reaches its peak value;
uR>u2: The voltage on the resistor is greater than the input voltage, the current id begins to drop, and the inductor releases energy. During this period, the energy of the resistor comes from the grid and the inductor;
π: u2 crosses zero and goes to the negative half cycle. The inductor current lags behind the voltage, and the current is still positive. At this time, the inductor feeds back energy to the grid and supplies power to the resistor.
t2=ωt2: The inductor energy is discharged, the current id is zero, and the T1 and T4 thyristors are turned off
t3=ωt3: start of negative half cycle

The discontinuous current mode current is not square wave, and the form factor needs to be obtained by simulation.

Load with back EMF

This is also a common form of rectifier load. Looking at the schematic diagram, I think that this is the simplest charging circuit. The load can be regarded as a DC voltage source. For the rectifier circuit, they are the back EMF load.

The rectifier circuit with back electromotive force only has current when the input voltage is higher than the back electromotive force E, and the load current is discontinuous.

Due to the short current conduction time, when the resistance in the loop is relatively small (the resistance in this type of circuit is loss, which is often very small), when the same average current is output, the peak value and effective value of the current are relatively large, and the power factor low, so a reactor is generally required in the design. This characteristic load will be explained in detail in the next two units with its application and its wide range of capacitive loads.