Digitally controlled switching power supply design

introduction

In high power factor correction AC/DC circuits, dedicated control chips such as UC3842 and UC3855A are widely used to achieve power factor correction, while in phase-shifted full-bridge DC/DC circuits, dedicated power chips such as TL494 and UC3875 are widely used to drive switch tubes. The specific power chip itself is not programmable, has poor controllability, is difficult to expand, and is not easy to upgrade and repair. At the same time, the power chip is an analog control chip, which has errors caused by temperature drift and aging that are difficult to overcome by analog circuits, and cannot guarantee that the system always has high precision and reliability. To overcome the above shortcomings, digital controller DSP can be used to replace traditional analog control chips. At present, digital processing (DSP) technology has gradually matured. The new generation of DSP adopts Harvard structure and pipeline operation, that is, the program and data memory are independent of each other, and multiple operations such as fetching, decoding, reading data and executing instructions are completed in each clock cycle, thereby greatly reducing the instruction execution cycle. In addition, due to its unique register structure, powerful addressing mode, flexible instruction system and powerful floating-point computing capability, DSP not only has a greatly improved computing capability compared to single-chip microcomputers, but also makes it easier to implement high-level languages ​​on this processor. It is precisely because of its special structural design and super computing power that functions that previously required hardware to implement can be transplanted to DSP and implemented with software, allowing some theories and algorithms in digital signal processing to be implemented in real time.

1 Digital Control Switching Power Supply System

The communication switch power supply is mainly composed of a main circuit and a control circuit. The main circuit is mainly composed of a single-phase high power factor correction AC/DC conversion circuit and a phase-shifted full-bridge soft switch DC/DC conversion circuit. It includes a single-phase AC input power supply, a filter network, a rectifier circuit, a Boost high power factor correction circuit and a phase-shifted full-bridge conversion circuit. The control circuit mainly includes a DSP digital controller, which is composed of a DSP, a drive circuit, a detection circuit, a protection circuit and an auxiliary power supply circuit. The principle block diagram of the system main circuit and the control circuit is shown in Figure 1. In Figure 1, E represents the input voltage and inductor current, the output voltage and current, the main switch tube drain voltage, and the sampling circuit; B represents the power switch drive circuit; F represents the output voltage and current, the primary inductor current and the 4 switch tube drain voltage sampling circuit.

1.1 Single-phase power factor correction AC/DC conversion circuit

The single-phase power factor correction AC/DC conversion circuit adopts a Boost type ZVT-PWM converter, and its circuit diagram is shown in Figure 2. This circuit can realize the zero voltage turn-on of the main switch tube S and the zero current turn-off of the diode D.

1.2 Phase-Shifted Full-Bridge Soft-Switching DC/DC Converter Circuit

The phase-shifted full-bridge soft-switching DC/DC conversion circuit adopts a full-bridge DC/DC converter as shown in FIG3 .

1.3 DSP-based hardware circuit design

The digital control circuit with TMS320F2812 as the core is shown in Figure 4. As can be seen from Figure 4, the control system mainly includes the following parts: DSP and its peripheral circuits, signal detection and conditioning circuits, drive circuits and protection circuits.

Among them, the signal detection and conditioning circuit mainly completes the functions of sampling the input current and voltage in Figure 2, A/D, etc. The DSP generates a pulse signal and then drives the power switch tubes in Figures 2 and 3 after D/A conversion.

1.4 System control algorithm software implementation

DSP digital control can realize more advanced and complex strategies than analog control. Compared with analog control circuits, digital control circuits have more advantages: digital PID system has the advantages of short design cycle, flexibility, easy modular management, and can eliminate instability and electromagnetic interference caused by discrete components. The main program of the digital control system is shown in Figure 5. The role of the main program: initialization, including assigning initial values ​​to the control registers. At this time, the system working clock opens CAP1INT and CAP2INT interrupts, collects output signals during the idle time waiting for interrupts, and sets the ADC conversion end flag to 1. To ensure the normal operation of the program, the watchdog must be disabled, the frequency and dead time of the PWM signal must be set, the control registers of general timers 1 and 2 must be set, and the capture control register must be set to detect the falling edge.

2 Experimental results and analysis

Assume that the AC input voltage is 220V, the output voltage is 48V, the output power is 1000W, the efficiency is 95%, and the converter operating frequency is 100kHz.

2.1 Calculation of boost inductor for single-phase power factor corrected AC/DC converter

The calculation of the boost inductor must be obtained under the worst case, that is, when the input voltage is the lowest and the output is fully loaded, the input current is:

The allowable ripple current is generally 20% of the input current, that is:

The minimum duty cycle at minimum line voltage is:

The critical inductance is derived from the basic formula of electromagnetic induction:

Therefore, the boost inductor L can be taken as 470H.

2.2 Calculation of filter output capacitor for phase-shifted full-bridge soft-switching converter

When selecting the output capacitor, the output voltage maintenance time of the capacitor is very important. When the input energy is cut off, the capacitor voltage is required to be maintained within a certain range. The output filter capacitor is calculated by the following formula:

2.3 Simulation results and analysis

In order to verify the feasibility of the design of digital switching power supply based on DSP control and the correctness of parameter selection, the system shown in Figure 1 is simulated using Pspice software, and the simulation waveforms are shown in Figures 6 and 7. Figure 6 is the input AC voltage and current simulation waveforms. It can be clearly seen from Figure 6 that the input current follows the AC input voltage very well, achieving the purpose of power factor correction. Figure 7 shows the output voltage simulation waveform. It can be seen from Figure 7 that the output is a relatively smooth 48V DC voltage. The simulation results are completely consistent with the results of theoretical calculations, achieving the desired purpose.

2.4 Test results and analysis

Finally, a power factor correction experimental circuit based on TMS320F2812 is designed. The experimental results are shown in Figure 8, which shows the input voltage and input current waveforms. The waveforms show that the input current follows the input voltage very well, achieving the purpose of power factor correction. The experimental results show that it is feasible to replace analog controllers with digital controllers in communication switching power supplies.

3 Conclusion

Compared with analog switching power supplies, digital switching power supplies have incomparable advantages, such as reducing the size and weight of the power supply, improving control accuracy, and facilitating maintenance and upgrading.

With the continuous improvement of control theory and implementation methods and the continuous reduction of DSP prices, digitally controlled switching power supplies will become an important research direction in the future.