PWM chopper type AC purification regulated power supply controlled by microcontroller

    Abstract: This article introduces the basic principles of traditional sinusoidal energy distribution AC purification regulated power supply and how to transform it using high-frequency chopper and single-chip microcomputer technology.

    Keywords: High frequency chopper AC voltage regulator AVR

At present, among various AC regulated power supplies, the AC purified regulated power supply using sinusoidal energy distribution technology is a technologically advanced regulated power supply. This kind of power supply mainly controls the equivalent inductance of the inductance regulating branch by changing the firing angle θ of the thyristor, thereby stabilizing the output voltage. It has the characteristics of high cost performance and good reliability.

    However, this method generates more harmonics, larger inductance losses, and significant noise, especially causing great interference to the power grid. To this end, the author uses high-frequency PWM chopper technology to transform it, replace TBIAC with MOSFET or IGBT, and adjust the equivalent inductance by adjusting the pulse width of the high-frequency AC chopper. The above problems are better solved.

The principle of traditional sine wave AC purification power supply is shown in Figure 1.

    In Figure 1, T is an autotransformer with an air gap. The input AC power is connected to point B of T, and a stable AC voltage is output from point C. L, L1 and L2 are linear inductors, and L and the bidirectional thyristor V form the inductance regulating branch. L1 and C1 form a 3rd harmonic filter, and L2 and C2 form a 5th harmonic filter to reduce the distortion of the output voltage. Pulse phase control technology is used to change the V and conduction angle of the triac, thereby adjusting the equivalent inductance value of L, and obtaining the compensation voltage from the N2 winding of T to achieve voltage stabilization.

The key to transforming the traditional sine wave AC purification power supply with high-frequency chopping technology is to replace the bidirectional thyristor with a high-frequency AC switch. There are two forms of high-frequency AC switches: rectifier bridge + IGBT type and MOSFET anti-series type, as shown in Figure 2.

    The rectifier bridge + IGBT type is suitable for high-power power supplies, and the MOSFET anti-series type is suitable for small and medium-power power supplies. The following is a detailed introduction to the high-frequency chopper-adjusted AC regulated power supply controlled by a single-chip microcomputer with a rectifier bridge + IGBT as the AC power switch and the AVR series single-chip computer 90S8535 as the control core. Its schematic block diagram is shown in Figure 3.

Since it is an inductive load and a freewheeling circuit cannot be added like DC chopper, it is necessary to add a turn-on and turn-off buffer circuit to the IGBT. High-frequency AC switch control adopts EPWM DC equipotential modulation technology. In order to make the waveform half-wave odd symmetry and quarter-wave even symmetry, to eliminate the cosine term and even harmonics in the Fourier series, the carrier ratio N=fc/fs=4k, K=1, 2, 3 …, fc is the frequency of the triangular wave, fs is the mains frequency; modulation M=Δt/TΔ=ΔU/ΔUc, Δt is the pulse width, TA=1/fc is the period of the triangular wave, Uc is the amplitude of the triangular wave, and ΔU is the deviation of the output voltage , the formula of triangle wave voltage is:

The output voltage deviation ΔU is the sampling voltage, and the equations for the starting and ending points of the trigger pulse are:

In the formula, TΔ=2π/N, the values ​​of the starting point angle and the ending point angle of each trigger pulse are:

α1=(TΔ/2)-(TΔ-2)(ΔU/Uc)=π/N(1-M)

α2=[π/N]（1+M）

α3=[π/N](3-M)

α4=[π/N](3+M)

Since the PWM chopper waveform is mirror symmetric and origin symmetric, its Fourier series will only contain odd harmonics in the sine term, that is:

Learn calculations, when n=KN±1 (K=1,2,3,4…)

When n≠KN±1, b n≠KN±1 =0

For the fundamental wave, n=1

It can be seen from the above ratio that the larger N is, the higher the harmonic frequency is. All high-order harmonics in uLe can be filtered out using a small LC filter.

For example, to find the equivalent inductance Le, uL=uLe, uL1=ULmsinωt, for uLe, when the higher harmonics are ignored (higher harmonics are filtered out by L and C) uLe=MUmsinωt, when uL, uLe are The effective value expression is: UL=MUle. If both sides are divided by the effective value IL of the current, we can get:

ωL1=MωLe,Le=L1/M

The EPWM schematic waveform is shown in Figure 4.

    AVR90S8535 is an 8-bit RISC structure microcontroller. When the operating voltage of the 8MHz crystal oscillator is 5V, the single instruction cycle is 125ns. When it contains an 8MHz crystal oscillator and a 5V operating voltage, the single instruction cycle is 125ns. Contains 8 channels of 10-bit ADC, the fastest conversion time is 65μs, 16-bit timer/counter with analog comparator and two 8/9/10-bit PWM functions, and programmable watchdog timer, very Suitable for high-speed PWM controller. As can be seen from Figure 3, AVR90S8535 completes the sampling of AC input, output voltage and output current. At the same time, the zero-crossing synchronization signal generated by the sensing branch is also input to the microcontroller. The frequency of the zero-crossing synchronization signal is 100Hz. An interrupt is triggered every time the zero-crossing synchronization signal is input to the microcontroller. In the interrupt subroutine, the microcontroller calculates the EPWM duty cycle based on the output sampling voltage, and the duty cycle is given to the PWM timer of the microcontroller. The PWM timer of the AVR90S8535 is set to a 16kHz PWM output. PWM chopping frequency is not good if it is too low or too high. When the chopping frequency is too low, the freewheeling current of the controlled inductor charges the parallel capacitor of the AC switch when the AC switch is turned off, which increases the withstand voltage of the AC switch. When the chopping frequency is too high, the turn-on loss of the AC switch will be too large. The microcontroller also completes functions such as fault protection, digital setting and display of input and output. The program flow is shown in Figure 5.

    Using high-frequency chopper and single-chip microcomputer technology to transform the traditional sine wave AC purification power supply has resulted in a technological breakthrough although the cost has increased. It makes the product display intuitive and easy to set up. The harmonic stress endured by the inductor and capacitor is greatly reduced, and the power consumption is reduced. Especially the harmonic current component at the input end is greatly reduced, which can meet the current increasingly stringent harmonic requirements. Standard requirements determine the future development direction of power supplies.