Design of program-controlled switching power supply based on MSP430F169

　　This system is based on TI MSP430F169. The voltage can be preset, the step voltage is 0.1V, the output voltage range is 20V~36V, and the output current is 0~2A. The preset voltage, measured voltage, measured current, and measured efficiency can be displayed. The system is mainly composed of the smallest single-chip system, PWM signal control chip TL494, switching power supply boost main circuit, on-chip A/D and on-chip D/A. The system presets the voltage value through the keyboard and sends it to TL494 to form a closed-loop feedback loop. The voltage on the constantan wire is sampled to indirectly calculate the current and display it. This system has the advantages of fast adjustment speed, high precision, low voltage adjustment rate, low load adjustment rate, high efficiency, no need for additional auxiliary power supply board, and small output ripple.

　　1. Scheme demonstration and comparison

　　1.1 Selection of the main control CPU

　　Solution 1: Use AT89S51 single-chip microcomputer for control. It is relatively simple to connect A/D and D/A to 51 single-chip microcomputer, but due to the simple function of 51 single-chip microcomputer, it is relatively complicated to implement for such a complex system.

　　Solution 2: Use the ultra-low power single-chip MSP430F169, which is a fully integrated mixed-signal system-level MCU chip. It integrates 12 A/D and D/A chips, and this single-chip computer has very rich resources. Using the JTAG method, it can be downloaded and debugged online through the USB port, which is very convenient to use, and the low power consumption facilitates the improvement of overall efficiency.

　　1.2 Selection of DC-DC main circuit topology

　　There are two types of DC-DC conversion: isolation and non-isolation. Although the input-output isolation method is safe, the efficiency will be reduced due to the leakage and loss of the isolation transformer, and the winding of the isolation transformer is complicated, so the non-isolation method is selected. There are several specific solutions:

　　Solution 1: BUCK topology. See Figure 1. The switch tube V1 is controlled by a PWM wave with a duty cycle of D, and is alternately turned on or off. After passing through L and C filters, a stable DC output voltage Uo=D*Vd (D≤1) is obtained on the load R. Since the input voltage is 18V and the output voltage is 20~36V, it cannot meet the requirements.

　　Solution 2: BOOST topology. See Figure 2. When switch V1 is turned on, the inductor stores energy, and when it is turned off, the inductor outputs energy. As long as the inductor is wound properly, the required output voltage of 30 to 36V can be achieved, and the output voltage Uo presents a continuous and smooth characteristic.

　　Solution 3: BUCK-BOOST topology. See Figure 3. Since the circuit belongs to the buck-boost topology, the control is relatively complex. Since this question only requires boosting, solution 2 is selected.

　　1.3 Selection of control methods

　　Solution 1: Use a single-chip microcomputer to generate PWM waves to control the on and off of the switch. According to the feedback voltage sampled by the on-chip A/D, the duty cycle is changed programmatically to stabilize the output voltage at the set value. The load current is sampled on the constantan wire and input into the single-chip microcomputer after the on-chip A/D. When the voltage reaches a certain value, the switch tube is turned off to form overcurrent protection. This solution is mainly implemented by software, the control algorithm is relatively complex, the speed is slow, and the output voltage stability is not good. If you want to achieve automatic recovery, it is relatively complicated to implement.

　　Solution 2: Use constant frequency pulse width modulation controller TL494. This chip can be push-pull or single-ended output, the operating frequency is 1~300KHz, the output voltage can reach 40V, there is a 5V voltage reference, the dead time can be adjusted, the output stage source current can reach 200mA, and the driving ability is strong. There are two error comparators inside the chip, a voltage comparator and a current comparator. The current comparator can be used for overcurrent protection, and the voltage comparator can be set to closed-loop control with fast adjustment speed. In view of the above analysis, solution 2 is selected.

　　1.4 Current working mode selection

　　Option 1: Continuous current mode.

　　The current is in continuous working state. When the next cycle comes, the current in the inductor has not decreased to zero, and the current in the capacitor can be replenished in time. The peak value of the output current is small, and the output ripple voltage is small.

　　Solution 2: Current discontinuous mode. In discontinuous mode, when the inductor energy is released, the next cycle has not yet arrived, and the capacitor energy cannot be replenished in time. The peak current of the diode is very large, and the requirements for the switch tube and the diode are very high. The loss of the diode is very large, and because the current is intermittent, the output current AC component is relatively large, which will increase the loss on the output capacitor. For the same power output, the peak current of the discontinuous working mode is much higher, and the ripple of the output DC voltage will also increase, resulting in large losses.

　　In view of the above analysis, this design adopts plan 1.

　　1.5 Solution selection to improve efficiency

　　The factors that affect efficiency mainly include the power consumption of the microcontroller and peripheral circuits, the efficiency of the power supply circuit of the microcontroller and peripheral circuits, and the efficiency of the DC-DC converter. Therefore, we use the ultra-low power consumption MSP430 microcontroller, a high conversion efficiency chip to power the peripheral circuit, and low-loss components and excellent control strategies.

　　2. Detailed software and hardware analysis

　　2.1 Overall hardware block diagram design (see Figure 4):

　　The microcontroller controls the voltage step through the keyboard, and the microcontroller controls the D/A to provide a reference voltage, which is compared with the feedback voltage of the output voltage. The voltage error amplifier inside the TL494 generates a high or low level to control the pulse width change to adjust the output voltage change. After repeated adjustments, the output reaches the set value. The feedback adjustment of the voltage after the reference voltage is output is automatically adjusted by the TL494, and the adjustment speed is fast.

　　2.2 Theoretical analysis and parameter calculation

　　2.2.1 Selection of main circuit components and parameter design:

　　2.2.1.1 Selection of magnetic core and wire diameter. When alternating current passes through a conductor, the current will concentrate on the surface of the conductor. This phenomenon is called skin effect. When current or voltage is conducted in a conductor with higher frequency electrons, it will gather on the surface of the conductor instead of being evenly distributed in the cross-sectional area of ​​the entire conductor. The selection of wire diameter is mainly determined by the switching frequency of this system. The larger the switching frequency, the smaller the wire diameter, but the smaller the current allowed to pass, and the switching loss increases, and the efficiency decreases. The frequency used in this system is 44K. From the table, it is known that the penetration depth at this frequency is 0.3304mm, and the diameter should be twice this depth, that is, 0.6608mm. The AWG wire specification selected is 21#, and the diameter is 0.0785cm (including paint). The magnetic core selects iron-nickel-molybdenum magnetic core, which has a high saturation flux density, is not easy to saturate under a large magnetizing field, has a high magnetic permeability, and has good magnetic performance stability (low temperature rise, high current resistance, low noise), and is suitable for switching power supplies.

　　2.2.2 Control circuit design and parameter design:

　　The control circuit uses TI's TL494 to generate PWM waveforms and control the conduction of the switch tube. Rt and Ct are selected as 102 and 24K, and the frequency is 44KHz. The soft start circuit is realized by connecting resistors and capacitors at pins 14 and 4, and is realized by charging and discharging. The start time is 10mS, Ct=10uF, Rt=1K. Pin 13 is grounded, and a single tube output is used to further reduce the internal power consumption of the chip. TL494 is shown in the figure below.

　　2.2.3 Efficiency Analysis:

　　Output power calculation formula: η=Po/Pi, input power calculation formula: Pi=Ui*Ii.

　　Since the topic requires that the DC/DC converter (controller) can only be powered by the Uin port and no auxiliary power supply can be added, the power consumption of the microcontroller and some peripheral circuits should be as low as possible. For this reason, when designing this system, the microcontroller uses the ultra-low power microcontroller MSP430F169. The system integrates 8 12-bit A/Ds and two 12-bit D/As. The power consumption of the external A/D and D/A is reduced. Improving efficiency mainly requires reducing the loss of the converter. The losses of the converter mainly include MOSFET conduction loss, MOSFET switching loss, MOSFET driver loss, diode loss, output capacitor loss, and control part loss. These losses can be reduced by reducing the switching frequency and other methods. The losses at each level are mainly: 1. Conduction loss: 2. Switching loss: 3. Gate-level driver loss: 4. Diode loss: 5. Output capacitor loss:

　　The specific losses are as follows:

　　Conduction loss and switching loss are mainly for the switch tube. Based on the selection of IRP540, the power consumption is 0.4W.

　　2. Another major loss is the diode loss. The normal conduction voltage drop of the diode is 0.7V, and the loss is Pd=0.7*Ii. The gate-level drive and output capacitor losses mainly involve selecting low-power devices and low-ESR capacitors.

　　2.2.4 Protection circuit design and parameter design:

　　Selection of the size of constantan resistor: constantan wire mainly plays two roles, overcurrent protection and testing load current. Constantan wire is connected between the rectifier input ground and the load ground. The smaller the better, so that the voltage between the two grounds is very small. However, if it is too small, it will cause misjudgment of overcurrent protection due to interference problems, and the requirements for the post-stage operational amplifier are relatively high. After experiments, it is better to choose a 0.1 ohm resistor. Because the resistance is too small and difficult to measure, first measure the 1 ohm resistance, and then cut off one tenth of its length.

　　The TL494 chip has a current error amplifier. It can be used for overcurrent protection. The voltage drop on the constantan resistor is compared with the pre-adjusted value. If the current is too large, the output is high, which prevents the PWM signal from being generated. The switch tube is in the off state, which reduces the output voltage and forms a protection function. Once the output voltage is reduced, the output current is reduced, the detection voltage is reduced, and the current error amplifier will output a low level to regenerate the PWM waveform, so the circuit has a self-recovery function.

　　2.2.5 Design of digital setting and display circuit:

　　Since the feedback voltage measured when sampling at the output end is one-twenty-fourth of the output voltage, that is, the output is 36V when the voltage is 1.5V, and the output is 30V when the voltage is 0.834V, the design uses a 12-bit D/A conversion accuracy of 0.61mV (reference voltage is 2.5V), which is directly output to TL494 to provide a reference voltage. In addition, three A/D chips are set up to collect output voltage, output current, and input current respectively. In order to reduce power consumption, the design uses 128*64, a large screen, and more display content. When the backlight is not in use, it automatically turns off to reduce power consumption.

　　2.3 Hardware Circuit Design

　　2.3.1 The main circuit diagram is as follows:

　　2.3.2 The main CPU PCB diagram is as follows:

　　2.4 Software Design

　　The software design of this design is relatively simple. It is designed to minimize the peripheral circuits as much as possible for the sake of efficiency. Therefore, the microcontroller drives the peripheral chips directly through the I/O port, without using the bus method. The overall software design flow chart is shown in Figure 6.

　　3. Conclusion

　　By searching for a series of information and designing and debugging the circuit, we finally achieved very good results, and all technical indicators reached a very high level. However, there are still many problems with the circuit, such as the use of ultra-low power microcontrollers in power supply design, and the microcontroller's anti-interference ability is not good, so more attention should be paid to it in the future.