Design of intelligent power amplifier switching power supply based on DSP

Zhou Jinbo, Zhang Guobao

(School of Automation, Southeast University, Nanjing, Jiangsu 210096)

1 Introduction

Switching power supply is gradually replacing industrial frequency power supply with its small size, light weight, low power consumption, high efficiency, small ripple, low noise, high intelligence, easy expansion, etc., and is widely used in various electronic devices. High reliability, intelligence and digitization are the development direction of switching power supply. Audio power amplifier requires the power supply to automatically adjust the output voltage as the load changes, and then adjust the power to improve the dynamic performance of the power supply and reduce the internal loss of the audio power amplifier, but the current switching power supply cannot achieve this. TMS320F2812 DSP is selected as the main controller of the power amplifier switching power supply to design a low-power. A new type of intelligent power amplifier switching power supply suitable for large power amplifier systems.

2 Design of intelligent power amplifier switching power supply

Figure 1 is the overall principle block diagram of the intelligent audio power amplifier switching power supply. The main circuit adopts an AC-DC-AC-DC structure. The input power frequency 220 V AC circuit passes through the filter circuit and then outputs DC voltage through the single-phase bridge rectifier circuit; the conversion circuit uses a full-bridge phase-shifted inverter circuit to convert the front-end DC power into high-frequency AC power. Then it outputs a stable DC voltage through secondary rectification and filtering; the detection circuit samples the output voltage signal and sends it to the control circuit, and adjusts the output voltage by changing the output pulse width duty cycle of the control circuit; the protection circuit implements overvoltage and overcurrent protection; the power detection circuit samples the current of the conversion circuit, and when the output power exceeds 500 W, it generates an overpower detection signal to drive the control circuit and reduce the output voltage: the auxiliary power supply circuit supplies power to the control circuit and various op amps.



2.1 Power amplifier switching power supply module

Figure 2 is the main circuit of the power amplifier switching power supply, where Vin is 220 V AC input obtained by front-end filtering and full-wave rectification, and the voltage is 300 V. It is the input voltage of the full-bridge inverter circuit. VQ1, VQ2, VQ3, and VQ4 are IRFP460 type high-power MOSFETs, which are used as converter switches. Since the IRFP460 MOSFET is a majority carrier device, the switching speed is extremely fast, and the typical value of the turn-on and turn-off time is generally 20 ns. It has a high breakdown voltage and a large operating current. In addition, the input impedance of the MOSFET is high, and the driving circuit is relatively simple. As long as a voltage of about 10 V is added between the gate and the source, it can be saturated and turned on. L4, C5, and C6 form an auxiliary resonant network. Considering the primary leakage inductance of the transformer, the value of the resonant inductor LT is generally smaller than the actual value. Here, a nonlinear saturation inductor with an inductance value of 34 μH and a 1μF inductance is selected. Considering the magnetic saturation problem of the high-frequency pulse transformer T1, the primary winding is connected in series with an anti-bias magnetic capacitor, VD15 and VD16, VD17 and VD18 are full-wave rectifier diodes, L1, C13, EC1, EC2 and L2, C14, EC3, EC4 are filter circuits for the +35 V and -35 V output circuits, respectively.


2.2 Power amplifier switching power supply module control circuit

The control circuit is based on DSPTFMS320F2812, and mainly includes functions such as generating phase-shift pulse waveform, real-time sampling, power regulation, overvoltage protection, overcurrent protection, overpower protection, filtering algorithm and full-bridge phase-shift algorithm. The built-in 16-channel 12-bit high-resolution A/D conversion circuit of TMS320F2812 is used to realize real-time sampling of voltage and current. The minimum conversion time of each channel is 80 ns, and the input signal level range of the A/D conversion circuit is 0~3 V. After sampling, the PWM waveform phase shift angle that drives the full-bridge inverter switch tube is adjusted through software programming to achieve voltage stabilization. At the same time, when the output voltage and current are too high or undervoltage, the DSP calls the corresponding subroutine to handle sudden abnormal events and play a protective role. At the same time, the output voltage and current signals are sampled by A/D for calculation, the output power can be accurately measured, and the values ​​of the relevant registers of the event manager can be adjusted to adjust the output voltage.

The dynamic characteristics and voltage stabilization accuracy of the controller are closely related to the design of the regulator. In the design of the power amplifier switching power supply, an incremental PID control algorithm is used.

The digital control in the power supply design adopts digital sampling control, that is, the control quantity is calculated according to the deviation value at the sampling time. The discrete form of PID control is:



Where Ts is the sampling period.

Formula (1) is the position PID control formula. In order to increase the reliability of the control system, an incremental PID control formula is used, that is, the DSP only outputs the increment of the control quantity u(k). Formula (1) is the output of the Kth PID controller, then the output of the (K-1)th PID controller is:



Therefore, the incremental PID control algorithm is:





Formula (3) and Formula (4) are the incremental PID control formulas of the control program. Compared with the position PID control, the incremental PID control only has a different algorithm, but it only outputs the increment, which reduces the impact of DSP misoperation on the control system and does not cause integral loss of control. Figure 3 is the implementation block diagram of the PID controller based on TMS320F2812.



2.3 Software Design of Power Amplifier Switching Power Supply

The software design of the power amplifier switching power supply based on DSP mainly realizes the following functions:

(1) The generation of full-bridge phase-shift pulses uses two comparison units in the TMS320F2812 event manager to directly output circuit pulses. From the basic principle of phase shift, the lagging bridge arm has a periodic delay relative to the driving between the leading arm, and its delay angle is the phase shift angle. Set the PWM1/PWM2 output by comparison unit 1 to drive the leading arm switch tubes VQ1 and VQ3 respectively, and the PWM3/PWM4 output by comparison unit 2 to drive the lagging arm switch tubes VQ4 and VQ2. The driving pulses between the upper and lower tubes of each bridge arm are complementary and have dead zones. The driving pulse of the fixed leading bridge arm is issued at time 0 of each cycle. As long as the time corresponding to the phase shift angle φ is delayed, the driving pulse of the lagging bridge arm can be obtained when the comparison event occurs again, thereby realizing free phase shift within the range of 0° to 180°.

(2) Detection and protection of overvoltage, overcurrent and overpower The DSP-based power amplifier switching power supply has protection functions such as overvoltage, overcurrent, overpower and overheating. When an abnormality occurs, the system enters the abnormal interrupt service subroutine for processing and locks the PWM output in time. To prevent malfunction, it is set to read 20 abnormal signals continuously before it is considered as a circuit abnormality, otherwise it will not be processed. The program flow of each module is shown in Figure 4 to Figure 6.







3 Experimental results

Based on the previous analysis, a prototype was designed with a switching frequency of 100 kHz and an output voltage of ±35 V and ±42 V. The DSP-controlled audio power amplifier switching power supply was subjected to a load test. Under light load and heavy load conditions, the output voltage ripple coefficient was less than 0.5%, and the output voltage accuracy was less than 0.5%.

Figure 7 shows the phase shift waveform of the DSP. Among them, channel 1 is the PWM1 output of comparison unit 1, which is the leading bridge arm; channel 2 is the PWM3 output of comparison unit 2. It can be clearly seen from Figure 7 that channel 2 lags behind channel 1 by about 135°. Figure 8 is the critical waveform of zero voltage turn-on of the lagging bridge arm. The input voltage is about 175 V and the output power is 100W. In Figure 8, channel 1 is the gate-source voltage Vcs waveform of the power MOS tube, and channel 2 is the drain-source voltage VDS waveform of the power MOS tube. When VDS is turned off, it is 175 V. It can be seen from Figure 8 that VDS first drops to 0, and then Vcs rises. At this time, the switch tube is turned on at zero voltage. The heavier the load, the more obvious the zero voltage turn-on phenomenon. When the output power is 400 W, the input power is 440 W, and the conversion efficiency of the full-bridge phase-shift converter is 90.9%.




The experimental results show that the power amplifier switching power supply based on DSPTMS320F2812 has a good output waveform, less harmonic content, and excellent adjustability. When the load changes in the full range, the switching power supply can maintain good output performance. Moreover, due to the use of a full-bridge phase-shift soft switching converter, the switch tube works in a zero voltage switching state, so the power consumption of the entire power supply system is small, and it has a good application prospect in high-end high-power amplifier audio.

4 Conclusion

Using DSP as the control core of the audio amplifier switching power supply realizes the digital control of the switching power supply, overcomes the problems of component aging and thermal drift in the analog control system, and solves the problems of low load and calculation accuracy of the single-chip control circuit. The full-bridge phase-shift circuit is used in the audio amplifier switching power supply to effectively reduce the internal loss of the amplifier switching power supply and apply it to high-power audio amplifier systems.

Using the software and hardware resources of TMS320F2812, PWM control, filtering, sampling and various system protection functions are realized, the control circuit is simplified, and the flexibility of power supply design and manufacturing is improved; in addition, the controller has good controllability, is easy to expand, and is easy to upgrade and maintain.