Principle and typical application of digital control UPS uninterruptible power supply technology

Traditional UPS uses analog circuit control. For manufacturers and users, analog control has many limitations, whether it is phase control technology or SPWM technology. With the development of information technology, the emergence of high-speed digital signal processing chips (Digital Signal Processor, DSP) has made it possible to apply digital control in a wider range of electrical control fields, and it has also become one of the main development trends.

1. Application advantages of digital control UPS

With the support of high-speed digital signal processing chips, the use of digital control strategies can not only better solve the problems related to UPS power supply analog control, but also add some control functions that are difficult to achieve in UPS power supply analog control. Its main application advantages are:

(1) Digital control can adopt advanced control methods and intelligent control strategies to make the UPS more intelligent and more perfect in performance. Intelligent control represents the latest development stage of automatic control, inheriting the qualitative, variable structure, and adaptive thinking modes of the human brain, and also bringing new vitality to power electronic control. Under high-frequency switching working conditions, the model of the inverter power supply becomes more complicated, which is difficult to achieve good control effects with analog control or classical control theory. The use of advanced and intelligent digital control strategies can fundamentally improve the performance indicators of the system.

(2) Flexible control, easy system upgrade, and even online modification of control algorithms without changing the hardware circuits. The control scheme of a digital control system is reflected in the control program. Once the relevant hardware resources are reasonably configured, the control performance of the original system can be improved by simply modifying the control software, or the control software of different control strategies can be replaced in real time and online according to different control objects.

(3) Improved control system reliability and easy standardization. Due to the high reliability of digital control, the reliability of the entire control system will inevitably be improved. In addition, a unified control panel can be used for different systems (or different types of products), and only some modifications need to be made to the control software. This is very attractive to manufacturers.

(4) The system is easy to maintain. Once a system failure occurs, it can be easily debugged, fault query, history query, software repair, and even online modification and debugging of control parameters through RS-232, RS-485 or USB interface. In this way, self-calibration and remote service can be completed at a low cost, which greatly facilitates the manufacturer's after-sales service.

(5) Good system consistency, low cost, and easy manufacturing. Since the control software does not have differences like analog devices, the control boards with the same control program have good consistency. There are no differences caused by debugging analog devices in analog systems, so the consistency of the same control board will be much higher than that of the analog system. By using software control, hardware software is realized, which greatly reduces the size of the control board and reduces the production cost.

(6) Easy to form a parallel operation system. Since all unit UPS systems are digitally controlled and have corresponding control variables representing the state quantity in the system, it is easy to obtain the information required for current sharing and use the corresponding current sharing algorithm to realize the parallel operation system of UPS.

2. DSP-controlled UPS workflow

The working process of the digital UPS power supply controlled by DSP is as follows: when the mains power is normal and the input voltage and frequency are within the allowable range, the PFC part performs power factor correction on the input, so that the input power factor of the system is about 0.98, while avoiding pollution to the power grid. The input mains power is transformed by the PFC link to obtain a 400V DC output voltage, which provides energy for the subsequent inverter circuit. At the same time, the DC/DC part is still working normally, but because the battery voltage is transformed by the DC/DC circuit to obtain a 360V output voltage, which is slightly lower than the DC bus voltage obtained by the mains power after PFC transformation, it is isolated from the DC bus by the diode, and the DC/DC part runs at no load and is in hot standby state. When the mains power is abnormal, the mains power is off or the input voltage and frequency are not within the allowable range, the DC bus voltage obtained by the mains power through PFC drops rapidly. When it is lower than 360V, the diode is turned on, so that the DC bus voltage is maintained at 360V. At this time, the energy obtained by the inverter is the DC bus voltage obtained by the battery voltage through the DC/DC circuit. Regardless of whether the mains power is normal, the inverter part can work normally. Generally, the battery can provide a backup power supply time of several minutes to tens of minutes. The backup power supply time of a large-capacity battery pack can reach several to tens of hours. For users with diesel generators, the diesel generator can be put into the input end of the UPS power supply within 5 to 10 seconds when the mains power is cut off, and high-quality sine wave power can be provided to users in the event of a long power outage. The processed mains power is also sent to the mains voltage/current phase measurement circuit to generate a mains voltage signal and phase signal for the microprocessor voltage/current measurement and synchronous phase locking. In this way, the uninterrupted power supply function for the load is realized.

3. Composition structure of UPS controlled by DSP

If UPS is to realize digital control, the control functions and algorithms that can only be realized by analog devices can be realized by DSP software programming, so the structure of the whole UPS is much simpler than that of UPS realized by analog devices. As shown in Figure 1, the overall block diagram of digital UPS is shown below. It is mainly composed of input power factor correction, inverter part, DC/DC, etc.

Figure 1 DSP-controlled UPS principle block diagram

4. Key circuit structure of UPS controlled by DSP

(1) UPS power correction circuit

The input power factor correction circuit is shown in Figure 2 and is mainly composed of power tube T5, inductor L1, diode D1, and capacitor C1. It provides power factor correction function for the input part and boosts the voltage to 400V.

Figure 2 Input power factor correction circuit diagram

The working principle of the input power factor correction factor circuit is that the UPS mains power is further reduced from the interference on the power grid through the power factor correction module, and at the same time the power factor and conversion efficiency of the entire UPS system are improved. The power correction module is an AC/DC converter, which completes the input rectification and controls the input current to be a sine wave, thereby achieving a very high input power factor. The power factor correction part must also keep the DC voltage constant and not change with the input. The DC voltage is converted into an AC power supply with appropriate amplitude and frequency in the inverter part. When the UPS works in battery mode, the DC power supply is isolated by DC/DC conversion to obtain the DC voltage required by the inverter part.

(2) Sine Inverter Circuit Structure

As shown in Figure 3, the sinusoidal inverter circuit is mainly composed of capacitor C1, inverter bridge composed of power tubes T1, T2, T3, T4, inductor L2, capacitor C2, etc. The output of the PFC module can generate the pure sinusoidal AC voltage required by the load through the inverter part.

Figure 3 Sine inverter circuit diagram

The sinusoidal inverter of the digital UPS is always in operation. Its working principle is to sample the output voltage and current of the inverter circuit through a sampling circuit, and the obtained sampling signal is input into the DSP, which processes the sampling signal and realizes the sinusoidal inverter circuit control function according to certain algorithms and programs.

(3) DC/DC circuit structure

The structure of the DC/DC circuit is shown in Figure 4, which is mainly composed of a high-frequency transformer, power tubes T6 and T7, rectifier diodes D33, D34, D35, D36, capacitor C31, etc. This part adopts DC voltage loop feedback control, and the converted voltage is connected to the output end of PFC through diode D6.

Figure 4 Structural diagram of DC/DC circuit

Since the battery voltage is relatively low and the inverter does not utilize the DC voltage very well, a DC/DC circuit is needed to convert the battery voltage. There are many DC/DC circuit structures, but each has its own advantages and disadvantages. The most commonly used is the push-pull DC conversion circuit. The advantages of this circuit are simple driving circuit and high output power. It is generally selected as a DC conversion circuit for loads with relatively high power requirements.

(4) Other structural functions of UPS

At the same time, the monitoring program of the entire UPS is realized through SCI and SPI, and communication with the microcomputer is carried out through the SCI port to realize remote monitoring, which is an important structural function of the fully digital UPS.

On the one hand, when a mains failure or power outage occurs during UPS operation, the UPS will use the above communication channel to transmit an alarm signal generated by the mains failure to the computer network powered by it. When the power outage lasts for a long time and the battery pack's supply voltage is lower than the critical discharge voltage, the computer network will complete data preservation and equipment protection under the drive of the UPS power supply to issue an automatic shutdown command.

On the other hand, a friendly human-machine interface is provided to monitor the operating parameters of the UPS in real time, which is convenient for users to modify parameters and query the historical records of UPS operation. The UPS can also be started/automatically shut down on a scheduled basis on the computer network. To achieve the above control functions, RS-232 and RS-485 communication interfaces can also be provided, and users can choose one according to actual conditions. For UPS that require network management functions, a simple network management protocol (SNMP) adapter or adapter card should be configured.

With the development of digital technology, DSP technology has been used by many UPS manufacturers in their products. The use of DSP technology improves the stability and purity of the output voltage of UPS products, and also improves the reliability of UPS products themselves. The application of IGBT technology and high-frequency technology has greatly improved power efficiency, reduced system noise and power loss of the power supply itself, and improved system reliability. The digitalization of UPS does not simply refer to the application of digital devices in the system, such as single-chip microcomputers and FPGAs, but refers to the control of the entire system using the computing power and discrete control methods of digital devices. With the development of digital processing hardware technology and the increase in computing speed, UPS will inevitably develop in the direction of digitalization.