[Introduction]Serialized 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 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.
The second lecture, “Loss and Form Factor of Diodes”, introduces the basic knowledge, the basic concepts of diodes and measurement technology, and the third lecture summarizes the analysis methods of rectifier circuits with “Resistive Inductive Load and Back-EMF Load”. This lecture starts with the nonlinear load of the rectifier bridge and the nonlinear load of the Inverter.
Capacitive filtering is a very common nonlinear load in AC-powered power supplies, home appliances, and motor drives.
The peak current of the capacitive filter load is much higher than the average current, which also makes the RMS current relatively large, which causes a large stress on the rectifier bridge current. Due to the large harmonics of the current, it often exceeds the limit of the harmonic current in the electromagnetic compatibility. Generally, the circuit with a power greater than 80W needs to add inductance filtering, which also suppresses the peak current.
Analysis of Capacitive Filter Load of Single-Phase Rectifier Circuit
The capacitor in the capacitive filter load of the rectifier circuit is an energy storage device. Its function is to smooth the ripple Ubrumm of the output voltage of the rectifier bridge. Its stored energy is equivalent to the load energy consumption. The terminal voltage uc is ud, and its function is to reverse Electromotive force, the previous lecture already has the foreshadowing of knowledge.
The diode current in the capacitor filter rectifier circuit is discontinuous and non-linear. Only when the input voltage uL1>ud, that is, during t1~t2, can the current iL1 charge the capacitor iC and supply power to the load iL. Outside the interval, due to the reverse bias of the diode, there is no chance to supply power to the load. To supply energy to the load in a short time, at this time, the peak current is much larger than the average current, and the corresponding effective value, that is, the rms current, will also be much larger (see the simulation data below).
The disadvantage of such a circuit is that the current peak value is large, and the high-order harmonics are large. Due to the limitation of electromagnetic compatibility standards and regulations, generally, the circuit with a power greater than 80W needs to add inductance filtering.
In the “Power Electronics Technology” edited by Liu Jinjun, a single-phase uncontrollable rectifier circuit with capacitor filtering is given. The load size (resistance value) and the filter capacitor have an influence on the θ value and δ value. The current in the figure The waveform is an ideal waveform, and the rising edge is a sudden change.
It can be read from the figure that the delta value is between 40-70 degrees, and the θ value is between 20-40 degrees, unless the design is not in accordance with the rules, and a particularly large or small capacitor is used. Especially large capacitors have limited discharge during the discharge cycle, the busbar ripple voltage is small, and the charging angle θ is small; while for very small capacitors, the busbar ripple voltage is large. Entering the charging state, the charging time is relatively long, and the charging angle θ is large. However, due to cost considerations and the requirement of busbar ripple voltage, the θ angle of charging will not be too outrageous. This table does not apply if there is a filter Inductor.
According to GB/T 7260.3-2003 Uninterruptible Power Supply Equipment (UPS) Part 3: Methods and Test Requirements for Determining Performance, the Informative Appendix Baseline Non-linear Loads, take 1000VA load as an example, RL=121Ω, C=1210μf. For specific regulations, please refer to the continuation “How does the inverter face the nonlinear load of rectification and filtering? “.
Read from the table above: θ=24°, δ=65°
For a half cycle of 10ms, its on-time is t=24/180*10ms=1.33ms, which is very close to the simulation results in the table below.
Note: Rs=0.2Ω, is the line impedance
Knowing that the delta value is between 40-70 degrees and the theta value is between 20-40 degrees, we can calculate the current in the capacitor and resistor when the diode is conducting. This kind of circuit has no input impedance limit, even in steady state, the output current of the rectifier bridge is relatively large.
It can be read in the figure that the diode starts to conduct from t=0 and reaches its peak value at t=θ/2, so the current peak value is calculated as follows:
This is close to the above simulation results.
However, no matter how large the peak current is, since the capacitor does not consume power, the average value of the current output by the rectifier bridge is the average value of the resistor, and the average value of the diode current is half of the average value of the current on the resistor.
The average current of a diode is easy to calculate, but the rms value is needed to calculate the diode losses, and the crest factor needs to be known.
For the design of nonlinear loads, it is recommended to use simulation to obtain the design parameters.
Effect of Line Impedance on Current
The peak value of the line impedance current will have a great influence. The impedance increases from 0.05Ω to 5Ω, and the voltage drop at the peak current on the line is only U=IR=4*5=20V, which accounts for only 6% of the 311V, but it has a negative impact on the suppression of The peak current effect is obvious. According to the provisions of the reference non-linear load in GB/T 7260.3-2003, the active power consumption of the linear resistor in series is 4% of the total apparent power. In the actual system design, the inductance of the AC side or the DC side is used to reduce the harmonic current of the system and also reduce the peak current.
From the theoretical analysis and simulation, it can be seen that the stress of the rectifier diode has a great relationship with the line impedance. The actual system needs to meet the harmonic current limit issued by GB17625.1 low-voltage electrical and Electronic equipment (the input current of each phase of the equipment is less than or equal to 16A) . If it is diode rectification, the harmonic current needs to be satisfied by series inductance, and the peak current will not be too large.
Next lecture preview
“Example of Rectifier Capacitor Filter Load”
Abstract: Taking household air conditioner as a single-phase rectifier capacitor filter circuit and Infineon 22kW general-purpose inverter as a three-phase rectifier capacitor filter circuit as examples, the design parameters are verified by PLECS simulation, and the empirical value of the waveform coefficient is given, which is convenient for engineering calculation.
Source: Infineon Industrial semiconductor, Author: Chen Ziying