See how the resonance phenomenon solves the power supply voltage output spike problem!

It is well known that switching power supplies have many advantages over linear power supplies, the most prominent of which is their high efficiency. High efficiency brings or creates many other advantages. However, the switching power supply has an outstanding disadvantage, that is, the spike forming component is large in the output voltage, and the spike is very large.

The spikes cause a number of problems that create high frequency disturbances to the input grid and output loads, affecting the operation of the load and other equipment.

This limits its application in many areas. This article discusses how to use the resonance phenomenon to achieve the purpose of reducing spikes with a simple processing method.

First, the cause of sharp points is how the spikes are produced.

Originally, the inductors, capacitors, and mounting circuits that can actually be made have a certain distributed capacitance and distributed inductance.

They form a relatively complex circuit with one or several resonant frequency points. When the voltage harmonics of the same frequency are detected, attenuating oscillations are generated. The first positive half wave is a spike. In practice, the distributed capacitance is below a few PF and the distributed inductance is below a few MH. This resonant frequency is approximately in the range above OMc. Since the filter circuit accepts the voltage of the square waveform, the voltage waveform contains a lot of high-order harmonics, that is, the amplitude of the higher harmonics is relatively large. In the currently used design, the fundamental frequency of the switching power supply is generally set in the range of 30 to 300 Kc, so the higher harmonics in the range above 10 Mcc will have a larger amplitude, so the formation of spikes is almost inevitable. The switching power supply generates a square wave voltage with a variable duty cycle from the switching tube, and then becomes a flat DC output voltage through the LC inductor-capacitor filter circuit, with spikes superimposed thereon. The problem is that such spikes are difficult to filter out. This is related to the magnitude of the voltage, the shape of the mounting circuit, etc., and is generally in the range of several tens of mV to more than one hundred mV.

Second, the method of reducing sharp points

How can we reduce the spike and make it in a few millivolts or even less than one millivolt? From the above analysis of industrial technology development, it can be seen that it is difficult to have great effect in the filtering process, because it is necessary to eliminate the distributed capacitance inductance. Not enough.

The use of multi-stage filtering can reduce the sharpness, but makes the system unstable and easily causes self-excitation.

You can think of ways to weaken the higher harmonic amplitude of the switching waveform. After the expansion of the Fourier series, it can be seen that the amplitude of the square wave higher harmonics is inversely proportional to the number of times.

An=K/n

n is the harmonic order and K is a constant.

The amplitude of the triangular wave higher harmonic is inversely proportional to the square of the number of times.

An=K/n2.

That is to say, the amplitude of the higher harmonics of the triangular wave is much smaller than that of the square wave.

To be more realistic, let's take a look at the trapezoidal wave.

The relationship between the higher harmonic amplitude of the trapezoidal wave and the number of times is: An = K / (n2 - c).

c is another constant, related to the slope of the trapezoid.

That is to say, the higher harmonics of the trapezoidal wave are basically inversely proportional to the square of the number of times when the number of times of the harmonic wave is large (more than ten times).

In this way, if the front and rear edges of the square wave are slowed down, the amplitude of the higher harmonics will be much smaller, which is advantageous for reducing spikes. We can reduce the front and rear edges by reducing the switching speed of the switching tube. This method is simple, but the tube consumption rises sharply, and the loss of switching power supply loss is lost. It is difficult to achieve the desired effect, and it is practically impossible. The figure below shows the relationship between the loss of the switching tube and the switching time in a typical switching power supply. The switching frequency is 25Kc. It can be seen that when the switching time is increased to 1HS (100oS) or more, the tube consumption will rise sharply. And the switching time of 1MS or more can effectively reduce the spike. The general switching time is below 200nS. The switching frequency has a tendency to increase, reaching more than lKc, this method will not work.

The development of technology requires us to solve the problem of not increasing the loss and effectively reducing the spike.

After repeated research and experimentation, it is found that the resonance phenomenon itself can be used to solve the spike problem brought about by it. A resonant circuit is inserted between the switching tube and the filter. Its resonant frequency is designed to be about 10 times the switching frequency, about 2Mc, so that the voltage waveform reaching the filter is a square wave superimposed with an attenuated sine wave.

The frequency of the sine wave is the frequency of this resonant circuit, and because of its low frequency, it is easily filtered by the latter filter. The spikes can be as small as desired. It can be observed on the oscilloscope that the front and rear edges of the switching waveform are attenuating the front and rear half waves of the sine wave, so it is not steep. This resonant circuit also uses an LC circuit with minimal losses. The switch tube still maintains high-speed switching, and the tube consumption is still very low. This method works well. In the experimental power supply developed in the laboratory, the spike is as small as 5 mV.

In a finely tuned power supply, the spikes can be as small as an oscilloscope with a 5mV/div.

Third, the conclusion

Resonance is a very interesting electrical phenomenon. It is well applied and can solve many problems in scientific research and production and achieve excellent results. This article discusses its successful application in switching power supplies.

Under the premise of not reducing efficiency and not significantly increasing the cost volume, the spikes are effectively reduced, and the quality of the switching power supply is improved.

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