Working Principle of Parallel Switching Power Supply

Figure 1-11-a is the simplest working principle diagram of parallel switching power supply, and figure 1-11-b is the waveform of output voltage of parallel switching power supply. In figure 1-11-a, UI is the working voltage of the switching power supply, l is the energy storage inductance, K is the control switch, and R is the load. In figure 1-11-b, UI is the input voltage of the switching power supply, uo is the output voltage of the switching power supply, up is the peak voltage of the switching power supply output, and UA is the average voltage of the switching power supply output.

When the control switch K is turned on, the input power supply UI starts to energize the energy storage inductor L, the current flowing through the energy storage inductor L begins to increase, and the current also generates a magnetic field in the energy storage inductor; When the control switch K is turned from on to off, the energy storage inductor will generate back EMF, and the direction of the current generated by the back EMF is the same as that of the original current. Therefore, a high voltage will be generated on the load.

During ton, the control switch K is turned on, and the voltage El at both ends of the energy storage filter inductance L is exactly equal to the input voltage UI, that is:

El = LDI / dt = UI - during K-ON (1-35)

By integrating the above formula, it can be obtained that the current flowing through the energy storage inductance L is:

Where IL is the instantaneous value of the current flowing through the energy storage inductor L, t is the time variable, and I (0) is the initial current flowing through the energy storage inductor, that is, the current flowing through the energy storage inductor immediately before the switch K is turned on. Generally, when the duty cycle D is less than or equal to 0.5, I (0) = 0, so the maximum current ILM flowing through the energy storage inductance L can be obtained as follows:

ILM = UI * ton / L - during k connection (d = 0.5) (1-37)

Where ton is the time when the control switch K is turned on. When the control switch K in figure 1-11-a suddenly changes from on to off, the energy storage inductor L will release its stored energy (magnetic energy) through the back electromotive force. The back electromotive force generated by the energy storage inductor L is:

The negative sign in the formula indicates that the polarity of the back EMF el is opposite to the symbol in formula (1-35), that is, the polarity of the back EMF of the inductance is exactly opposite when k is on and off. The first-order differential equation of formula (1-38) is solved as follows:

In the formula, C is a constant. It is easy to calculate C by substituting the initial conditions into the above formula. Since the current IL flowing through the energy storage inductor L cannot change suddenly when the control switch K suddenly changes from on to off, I (ton ) is exactly equal to the maximum current ILM flowing through the energy storage inductor L, so formula (1-39) can be written as:

When t is large, the value of the output voltage of the parallel switching power supply will be close to the input voltage UI, but this generally does not happen because the off time of the control switch K can not wait that long.

As can be seen from equation (1-42), when the load r of parallel switching power supply is large or open circuit, the amplitude of output pulse voltage will be very high. Therefore, parallel switching power supply is often used in high voltage pulse generation circuit.

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