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 make the power switch work in zero voltage or zero current state. In order to improve the working efficiency of the switching power supply, it is necessary to design a switching power supply with excellent performance.

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

1.1 Circuit principles of several design schemes

Scheme 1: Use analog discrete components and use pure hardware to realize functions. Through power transformers, rectifier filter circuits and voltage stabilization circuits, the regulated power supply can stably output ±5 V, ±12 V, ±15 V and can adjust the output voltage from 0 to 30 V, 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 indicators are not high and do not meet the design requirements. In addition, there are many devices used, complex connections, poor flexibility, and high power consumption. At the same time, there are 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
Scheme 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 schemes 2 and 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 scheme 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 scheme 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
Scheme 1 uses pure hardware to control the output of voltage. The most basic circuit principle analysis requires the calculation of the load size, which is related to the selection of voltage regulator tube. Schemes 2 and 3 use medium and small-scale devices to implement the CNC part of the system. Many chips are used, resulting in complicated internal circuit interface signals, many interrelationships in the middle, and poor anti-interference ability. For example, if the dual counters in Scheme 1 count out of sync, the displayed voltage will be inconsistent with the output voltage. In Scheme 4, the AT89C51 microcontroller is used to complete the functions of the entire CNC 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 widener and an adjustable voltage regulator (LM317) are used to control the output reference voltage. The A/D conversion sampling makes the output more accurate, with small ripple, expandable current, and easy circuit protection.
(4) Display module
The display output in schemes 2 and 3 directly decodes the quantized value of the voltage for display output. The displayed value is the input value 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. In scheme 4, an A/D conversion circuit is used. By sampling the output voltage, analyzing and processing it through the single-chip microcomputer, and through the data feedback link, the voltage is made 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 deal with it according to the information. The query time of the keyboard/display is also used to improve the CPU utilization.

3 Conclusion
As mentioned above, although Scheme 3 has many advantages over the previous two, Schemes 1 and 2 are not unfeasible for completing the design requirements, and they also 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.