Abstract: This paper briefly describes the harm of harmonic currents（”harmonic” for short） in AC/DC switched-mode power supplies, briefly analyzes the mechanism of the harmonic current generation in AC/DC switched-mode power supplies, and also gives the corresponding suppression methods.
Keywords: switched-mode power supply, harmonic current, harmonic suppression, MORNSUN active PFC AC/DC enclosed switched-mode power supply
AC/DC switched-mode power supplies are equipped with high-power switching transistors, which generate harmonic current at high frequency, causing electromagnetic interference to surrounding equipment and affecting the power quality of the grid. Therefore, it is necessary to suppress the harmonic current generated by the AC/DC switched-mode power supply.
The harmonic current suppression methods can be divided into two types: active filtering and passive filtering. Among them, active filtering has a better filtering effect, but its technology is more complex and has certain design difficulties in practical application. Passive filtering is capable of harmonic suppression and can also play a reactive power compensation effect, but its control effect is far less than that of active filtering.
With 24+ years of experience in the power supply industry, Mornsun has developed high-reliability PFC integrated circuits for the high-frequency harmonics generated by AC/DC switched mode power supplies, optimized and improved the power factor performance of power supplies, and designed and developed AC/DC switched-mode power supply products with active PFC circuits with strong harmonic suppression capability, such as LMF series, LIF series, LOF series, etc. The power factor is up to 0.99, which can effectively suppress electromagnetic interference caused by high-frequency harmonics to surrounding equipment and improve the power utilization of the power grid.
How the harmonic generated by AC/DC switched mode power supplies?
In the switching circuit of an AC/DC switched-mode power supply, the switching transistor has only two operating states: on and off. At this time, there will be an AC signal in the output voltage corresponding to the operating frequency, and this signal is continuously present in the output voltage. When the current flows through a non-linear load, such as a capacitive or inductive load, the relationship with the applied voltage is not linear, becoming a non-sinusoidal current, thus generating harmonic current.
Hazards of Harmonic current
In recent years, various failures and accidents that are caused by harmonic current have been occurring, and the seriousness of harmonic current hazards has attracted high concerns. Harmonics generated by AC/DC switched-mode power supplies are harmful to the utility grid and other systems as follows:
1. Fire hazards：
1) Make the components in the utility grid produce additional harmonic loss, reducing the efficiency of power generation, transmission, and use of electricity, and a large number of odd harmonics flowing through the neutral line will make the line overheat or even fire.
2) Affect the normal operation of various electrical equipment. In addition to the additional loss caused by harmonics on the motor, it also generates mechanical vibration, noise and overvoltage, makes the transformer partially overheat, and results in capacitor overvoltage failure.
3) Will cause local parallel resonance and series resonance in the utility grid, thus amplifying harmonics and significantly increasing the hazards of (1) and (2) as mentioned above, and even causing severe accidents.
2. Other hazards：
1) Can lead to false operation of relay protection and automatic devices, and can lead to inaccurate measurement of electrical measuring instruments.
2) Interference with the neighboring communication system, produce noise, reduces the quality of communication, and even will lead to data loss and make the communication system unable to work properly.
Suppression method for AC/DC switched-mode power supply harmonic current
1. Using EMI filter
EMI filter can be effectively suppressed on Electrical Fast Transient（EFT）, which can effectively filter out conducted interference as well as radiation interference. As in Fig 4, shows a circuit of an EMI filter, which is composed of capacitors and inductors. To connect it to the input of the AC/DC switched-mode power supply, C1 and C5 are high-frequency bypass capacitors, whose role is to filter out the differential mode interference between the input power lines. L1, C3, C4 and L2, C3, C4 to filter out the common mode interference in the circuit. The result of practical tests shows that when the parameters of the components are reasonably selected, EMI filter can achieve good harmonic suppression effect of the AC/DC switched-mode power supply.
2. Using Passive Power Factor Correction (PPFC) Circuits
In the previous section, the EMI filter circuit described to suppress harmonics, although it can effectively suppress conducted and radiated interference, it cannot suppress input current waveform distortion.
Therefore, to reduce the harmonic content of the current significantly, the bridge rectifier capacitor filter circuit needs to be analyzed to identify its input characteristics and make the essential improvements.
The circuit diagram on the right in Figure 5 shows one of the passive power factor correction circuits, whose components include capacitors and diodes. When the circuit is stabilized, the input current harmonics are also effectively improved because the conduction time of the rectifier diode is extended.
3. Use active power factor correction circuit
Different from the passive power factor correction circuit, the active power factor correction circuit uses a solution of pulse width modulation (PWM), and its control effect is significantly better than that of the passive power factor correction circuit. The input current can be corrected to a sine wave with a harmonic content of 10% or less, and the power factor can be corrected to close to 1.
In Figure 6, a simplified active power factor correction circuit is shown, which uses dual-loop control. In this case, the outer loop controls the output voltage and the inner loop controls the inductor current. Adopting a suitable control solution ensures that the inductor’s peak current follows the change in VDC, resulting in an average current with a sinusoidal waveform.
Figure 7 shows another active power factor correction circuit, which uses a BOOST PFC integrated circuit, and its working principle is as follows: when the industrial-frequency AC power is turned on, the input voltage through the bridge rectifier circuit to charge the capacitor（C1）, and when the voltage in the capacitor rises to a certain value, it will start the main control IC of the PFC circuit, and the GATE pin of the IC will provide the corresponding PWM pulse, and then the pulse will drive the MOS tube (Q1 ) to make it work in the switching state. Through sampling resistors （R3 and R4）, the sampled value is sent to the IC voltage loop comparator. Meanwhile, when the voltage is sent to the IC current detection comparator, an error signal can be obtained after an internal adder, which regulates the PWM pulse output and controls the current on L1 so that the input current waveform follows the input voltage to achieve a power factor close to 1.
Certification standards of AC/DC switched-mode power supply harmonic current
In order to ensure that the harmonic currents of SMPS products comply with the standard requirements and meet the relevant design requirements, we have the following standards for harmonic currents in AC/DC switched-mode power supplies.
Tested in accordance with IEC61000-3-2:2019.
a. The tested sample shall be subjected to initial functional testing.
b. AC/DC switched-mode power supplies rated at 75W and above must undergo this test (in the future, it may be 50W and above).
c. Different power supply products can be divided into A, B, C, and D level ITE for testing.
d. Input voltage and frequency, generally take the rated input voltage and frequency for testing (such as 100V-60HZ; 240V-50HZ). The output is connected to a resistor equivalent to the rated maximum load. Exclude harmonic current and power within the first 10S after the power-on operation. The device under test should not exceed 10% of any observed cycle in standby mode.
e. Test requirements:
–The test voltage variation range should be maintained within ±2.0% of the rated voltage, and the frequency variation range should be maintained within ±0.5%; the phase angle between each two phases of the three-phase power supply is 120°±1.5.
–The harmonic current of the test voltage shall not exceed the following values:
This paper analyzes the principle of harmonic generation in AC/DC switched-mode power supplies, and because of the negative impact of harmonics on the power grid, measures need to be taken to suppress harmonics, finally discusses several methods of harmonic suppression. In the process of harmonic suppression, MORNSUN Power with a professional R & D team, and 24+ years of experience in the area of power supply design and manufacture, has provided the corresponding solutions for different field application.
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