Design of digital controlled DC regulated power supply

With the continuous emergence of new power electronic devices and circuit topologies suitable for higher switching frequencies, traditional application technologies have minimized the impact of switching power supply performance due to the limitations of power device performance. New power supply circuit topologies and new control technologies can enable power switches to operate in zero voltage or zero current states. In order to improve the working efficiency of switching power supplies, it is necessary to design switching power supplies with excellent performance.

1 Comparison of several digital controlled DC regulated power supply design schemes

1.1 Circuit principles of several design schemes

Solution 1: Use analog discrete components and pure hardware to realize functions. Through power transformer, rectifier filter circuit and voltage regulator circuit, the voltage regulator can output ±5 V, ±12 V, ±15 V stably and can output 0-30 V voltage, as shown in Figure 1. However, due to the large dispersion of analog discrete components and the large influence between resistors and capacitors, the designed index is not high and does not meet the design requirements. In addition, there are many devices used, complex connections, poor flexibility, high power consumption, and many solder joints and lines, which affect the stability and accuracy of the finished product.

Solution 2: This solution adopts the traditional adjustment tube solution. Its main feature is to use a set of dual counters to complete the control function of the system. The output of the binary counter is converted by D/A to control the reference voltage of the error amplification to control the output step. The decimal counter drives the digital tube to display the output voltage value after decoding. In order for the system to work properly, the dual counters must work synchronously.

Solution 3: This solution is different from Solution 1 in that it uses a set of decimal counters. On the one hand, it completes the voltage decoding and display, and on the other hand, its output is used as the address input of the EPROM. The output of the EPROM is converted by D/A to control the error amplification and synchronization problem. However, since the control data is burned into the EPROM, the flexibility of system design is reduced.

Solution 4: This solution uses the 51 series single-chip microcomputer as the control unit of the whole machine. By changing the input digital quantity to change the output voltage value, the output voltage of the switch control power supply changes, and the output voltage is indirectly changed. In order to enable the system to detect the actual output voltage value, the analog-to-digital conversion is performed through ADC0809, and the voltage is indirectly sampled in real time by the single-chip microcomputer, and then the data is processed. The single-chip microcomputer is used to program the output of the digital signal, and the analog quantity is output through the D/A converter (DA0830), and then the output voltage is stabilized through the switch power supply control circuit. The single-chip microcomputer system also takes into account the real-time monitoring of the constant voltage source. After the output voltage is converted from current to voltage, the analog quantity is converted into data quantity in real time through the A/D conversion chip. After the single-chip microcomputer analyzes and processes, the voltage is more stable through the feedback link in the form of data, forming a stable voltage-controlled voltage source. In addition, the PWM-controlled switching power supply has the advantages of high integration, high cost performance, simplest peripheral circuit, best performance index, and can form a high-efficiency isolated switching power supply without industrial frequency transformer. Moreover, its cost is comparable to that of a linear regulated power supply of the same power, while its power efficiency is significantly improved and its size and weight are greatly reduced.

2 Comparison and demonstration of the schemes

(1) Output module

Solution 1 uses a linear voltage regulator to increase/decrease the output by changing its reference voltage. In this way, the influence of the ripple after rectification and filtering on the output ground cannot be ignored. This output can only be measured with a multimeter. In Solution 2 and Solution 3, an operational amplifier is used as the front-end operational amplifier. Since the operational amplifier has a large power supply voltage rejection ratio, the ripple voltage at the output end can be reduced. In Solution 1, the large capacitor connected in parallel at the output end of the linear voltage regulator to suppress the ripple reduces the response speed of the system, so that the output voltage is difficult to track the fast-changing input. The output voltage waveform in Solution 4 is the same as the D/A conversion output waveform. It can not only output DC level, but also generate a variety of waveform outputs as long as the quantized data of the waveform is generated in advance, so that the system has a signal source with a certain driving capability.

(2) CNC module

Solution 1 uses pure hardware to control the output of voltage. The most basic circuit principle analysis requires the calculation of the load size and the selection of the voltage regulator. Solution 2 and Solution 3 use medium and small-scale devices to implement the numerical control part of the system. Many chips are used, resulting in complicated internal circuit interface signals, many interrelationships, and poor anti-interference ability. For example, if the dual counters in Solution 1 count out of sync, the displayed voltage will be inconsistent with the output voltage. Solution 4 uses the AT89C51 microcontroller to complete the functions of the entire numerical control part. At the same time, as an intelligent programmable device, AT89C51 is convenient for the expansion of system functions.

(3) Control module

In this system, a PWM regulation circuit with D/A conversion function, a chopper circuit, a current regulator and an adjustable voltage regulator (LM317) are used to control the output reference voltage. A/D conversion sampling is used to make the output more accurate, with small ripple, expandable current, and easy circuit protection.

(4) Display module

The display output in Scheme 2 and Scheme 3 directly decodes the quantized value of the voltage and displays the output. The displayed value is the input of the D/A conversion. Due to the error introduced by the D/A conversion and the power drive circuit, there may be a large deviation between the displayed value and the actual output value of the power supply. Scheme 4 uses an A/D conversion circuit to sample the output voltage, analyze and process it through the microcontroller, and feedback the data to make the voltage more stable, so that the deviation between the displayed value and the actual output is minimized. Scheme 4 uses a 4-bit digital voltmeter to directly sample the output voltage and display the actual output voltage value. Once the system works abnormally and the deviation between the preset value and the output value is too large, the user can process it according to the information. The query time of the keyboard/display is also used to improve the CPU utilization.

3 Conclusion

As mentioned earlier, although Scheme 3 has many advantages over the previous two, Schemes 1 and 2 are not unfeasible for meeting the design requirements and even have advantages in some aspects. One of the important considerations for adopting Scheme 4 is that the system uses a single-chip microcomputer, which makes further functional expansion more convenient.