The Harm of Harmonic Current of Switching Power Supply
There are high-power switching tubes installed in the power supply, and they will generate harmonics when they act at high frequencies, causing electromagnetic interference to surrounding equipment and affecting the power quality of the grid. Therefore, it is very necessary to suppress the harmonics generated by the switching power supply.
Current suppression methods can be divided into active filtering and passive filtering. Among them, the filtering effect of the former is better, but its technology is relatively complicated, and it is difficult to design in practical applications; the passive filtering method of the latter can also suppress harmonics, and can also achieve the effect of reactive power compensation, but Its control effect is far less than that of active filtering.
With years of experience in the power supply industry, Jinshengyang has developed a highly reliable PFC integrated circuit for the high-frequency harmonics generated by switching power supplies. Through simulation test experiments and field use feedback, the power factor performance of the power supply has been continuously optimized and improved. Active PFC switching power supply products with super harmonic suppression capability, such as: LMF series, LIF series, LOF series, etc., the power factor of the product can reach up to 0.99, which can effectively suppress the electromagnetic interference caused by high-frequency harmonics to surrounding equipment , improve the power utilization rate of the power grid;
02 Analysis of harmonic mechanism of switching power supply
In the switching circuit of the switching power supply, the switching tube has only two working states: on and off. At this time, there will be an AC signal corresponding to the operating frequency in the output voltage, and this harmonic signal will continue to exist in the output voltage. When the current flows through a non-linear load: such as a capacitive or inductive load, the If the applied voltage does not show a linear relationship, a non-sinusoidal current will be formed, thereby generating harmonics.
The suppression of harmonics in the power system is to control the harmonics within the limit value by reducing or eliminating the harmonic current injected into the system. For example, if the pulse frequency of the control signal of the switch is set to 100kHz, it can be seen that: Both the 3rd harmonic and 5th harmonic energy of the odd component of the output fundamental wave exist. In addition, at the rising edge and falling edge, the voltage change rate of the pulse signal is very fast, and the current change rate is also very fast; in this process, a high frequency component different from the control pulse frequency will be generated. It can be seen that in order to control the frequency component of the switching power supply, the switching control pulse should be reasonably selected according to the design needs when designing the switching power supply. In addition, the rate of the control pulse should also be reduced.
03 Hazards of Harmonic Current
In recent years, various faults and accidents caused by harmonics have occurred continuously, and the seriousness of harmonic harm has aroused people's high attention. The harm of harmonics generated by switching power supplies to public power grids and other systems generally has the following aspects:
04 Harmonic current suppression method of switching power supply using EMI filter
EMI filtering technology can effectively suppress spike interference, and can effectively filter out conduction interference and radiation interference. Figure 4 shows an EMI filter, which is composed of capacitors and inductors; it is connected to the input end of the switching power supply, and the high-frequency bypass capacitors are C1 and C5. Differential mode interference is filtered out; L1, C3, C4 and L2, C3, C4 filter out common mode interference in the circuit; actual tests show that when the parameters of components are selected reasonably, the EMI filter can achieve better Harmonic suppression effect of switching power supply.
Using Passive Power Factor Correction Circuits
The EMI filter circuit introduced in the previous section suppresses harmonics. Although it can effectively suppress conduction and radiation interference, it is helpless against input current waveform distortion. Therefore, in order to greatly reduce the harmonic content in the current, it is necessary to analyze the bridge rectifier capacitor filter circuit, find out its input characteristics and make necessary improvements.
One of the passive power factor correction circuits, its components include capacitors and diodes; when the circuit is stable, the input current harmonics will be effectively improved due to the extended conduction time of the rectifier diodes.
Use Active Power Factor Correction Circuitry
Different from the passive power factor correction circuit, the pulse width modulation strategy is used in the active power factor correction circuit, and its control effect is obviously better than that of the passive power factor correction circuit. Its input current can be corrected to a sine wave, the harmonic content is within 10%, and the power factor can also be corrected to be close to 1.
A simplified circuit for active power factor correction adopts double-loop control; wherein, the outer loop controls the output voltage, and the inner loop controls the inductor current; adopting a suitable control strategy can ensure that the peak current of the inductor tracks the change of the upper VDC, An average current with a sinusoidal form is thereby achieved.
Another active power factor correction circuit uses a BOOST boost PFC integrated circuit, and its working principle is analyzed: when the power frequency AC is connected, the input voltage charges C1 through the bridge rectifier circuit, and when the capacitor When the voltage on the circuit rises to a certain value, the main control IC of the PFC circuit will be started, and the corresponding PWM pulse will be given from the GATE pin of the IC, and then the pulse will drive the MOS tube Q1 to make it work in the switch state; through the sampling resistor R3 and R4, the sampling value is sent to the IC voltage loop comparator; at the same time, when the voltage is sent to the IC current detection comparator, an error signal can be obtained through the internal adder, which adjusts the PWM pulse output , to control the current on L1 so that the input current waveform follows the input voltage so that the power factor is close to 1.
