UPS anti-interference technology based on single chip microcomputer control

O Introduction

With the increasing popularity of computer applications and the development of global information networking, the demand for high-quality power supply equipment is increasing. Uninterruptible power supply (UPS) is a power electronic device developed to meet this situation. While ensuring uninterrupted power supply, UPS can also provide high-quality sine wave power supply with stable voltage, stable frequency and low waveform distortion. It has been widely used in computer network systems, postal and telecommunications, banking and securities, power systems, industrial control, medical and other fields. The microcomputer-controlled uninterruptible power supply has two major advantages: simplified hardware circuits and reduced costs; flexible software design, easy to expand functions, and convenient software modification to achieve different control ideas.

Microcomputer control technology is applied to uninterruptible power supply, mainly in the single-chip microcomputer system. The single-chip microcomputer generates PWM pulse signals and amplifies them to drive the switch of the inverter. However, the anti-interference ability of the single-chip microcomputer is not strong enough. If the anti-interference measures are not appropriate, not only can the above advantages not be reflected, but the reliability of the system may also be reduced, or even fail to work.

This article combines the working principle of UPS, explains the application of anti-interference technology in UPS from the types of interference and the corresponding measures.

1. Block diagram and working principle

1.1 Structure diagram

According to the working mode, UPS is divided into two categories: backup type and online type.

The basic structure diagram of UPS is shown in Figure 1 and Figure 2. It consists of a charging circuit, a battery, a boost circuit, an inverter circuit, a switching switch, an automatic voltage regulator, and other components.

1.2 Basic working principle

Backup UPS: When the mains supply is within the specified voltage range, the input voltage is slightly filtered by the automatic voltage regulation link, and after eliminating some interference, it is directly output to supply the load. When the mains supply is abnormal and exceeds the specified range, the UPS starts the inverter, converts the backup battery voltage into a voltage value equivalent to the normal mains supply, and then outputs it to the load. Its basic topology is shown in Figure 1. The advantages of this type of UPS are high reliability, relatively simple structure, high efficiency, and low price. However, since most of the waveforms after inversion are square waves or quasi-square waves, the power supply quality is slightly poor. In addition, when the mains supply is abnormal, the inverter needs to be started to supply power from the battery, and there is a certain switching time.

Online UPS: Also known as double conversion UPS, it undergoes two conversions, AC/DC and DC/AC, from mains input to UPS output. Its basic topology is shown in Figure 2. Under normal mains conditions, the UPS input voltage is converted into DC voltage through rectification, filtering and other circuits, and then the DC voltage is converted into AC voltage required by the load through the inverter. When the mains power is abnormal, the input is automatically switched to the battery, which is powered by the battery and outputs AC voltage to the load after passing through the inverter. This type of UPS is obviously much more complex in circuit than the backup UPS. Due to the use of secondary conversion, the power supply quality of the load is greatly improved. However, its inverter is always in working state, which puts higher requirements on the reliability and service life of the inverter. As a result, the price of online UPS is much higher than that of backup UPS.

2 Types and sources of interference

Interference in a UPS system controlled by a single-chip microcomputer generally refers to various transient electrical signals generated by various external and internal interference sources, which are transmitted into the system through certain channels, or the internal interference causes a certain degree of impact on the normal operation of the system. Its interference mainly includes the following aspects:

2.1 Interference from the power grid

UPS is a system that provides high-quality power to the load after purification. Its input is the power frequency power with complex harmonics. There are a lot of harmonic sources in the power grid, such as various rectifiers, electronic voltage regulators, nonlinear loads and lighting loads.

2.2 Interference of alternating magnetic field

There are strong alternating magnetic fields around high-power transformers and wires through which large currents pass.

Inside the UPS, the alternating magnetic fields mainly include:

(1) Electromagnetic interference generated by the inverter

When working, the power tube of the inverter is always in the switching state. In order to reduce the power loss, the switching transition time cannot be too long, that is, di/dt cannot be too small, otherwise the switch will overheat and burn easily. However, if it is too large, it will easily cause parasitic oscillation in the circuit, and the noise interference such as harmonic current and high-frequency burrs will be detrimental to the equipment.

(2) Interference generated by high-frequency transformer

There is leakage inductance in high-frequency transformers. When the switch tube just changes from on to off, the energy stored in the leakage inductance and lead inductance of the transformer primary side has nowhere to be released, which will charge the equivalent output capacitance of the switch tube. Since this equivalent capacitance is very small, it will cause a relatively high voltage spike.

This type of interference is the most destructive to control systems.

2.3 Unequal potential interference

If the insulation performance of electrical equipment in industrial production is poor, unstable leakage current will be generated to the ground; using the earth as the electrical grounding wire of the transmission line will also generate large ground current. If the location is not properly selected during the grounding installation of control system equipment, leakage current or ground current will cause potential differences at various points in the system, causing the system to often have uncertain faults.

2.4 Natural disturbances

Natural interference refers to interference caused by natural phenomena in the atmosphere and electromagnetic radiation interference from the universe, such as lightning, changes in the electric field in the lower atmosphere, changes in the ionosphere, and electromagnetic radiation from sunspots, among which lightning interference is the most serious. Lightning will not only cause strong interference in the circuit, but also burn the input module.

2.5 Effects of temperature, humidity and corrosive media on reliability

Components of different grades have their specific operating temperature ranges. When using them, you should refer to the data of each manufacturer and choose higher-grade products according to the situation.

A humid environment or one containing corrosive gases will have a significant impact on components and circuit boards. If the system works in such an environment for a long time, the printed circuit board will be corroded, causing short circuits, and the connectors and IC sockets will be oxidized and rusted, causing poor contact and affecting the reliability of the system. Therefore, the environment should be kept dry as much as possible, and connectors and IC sockets should be used as little as possible, and interconnected by direct welding; various connectors should use gold-plated or other anti-corrosion-treated connectors.

3 Anti-interference measures of the system

In order to improve the anti-interference ability of UPS, we should start from two aspects, namely, consider the hardware and software respectively: we should remove or reduce the interference ability of the interference source, and improve the anti-interference ability of the system itself.

3.1 Hardware anti-interference

(1) AC line filter

In order to meet the relevant electromagnetic interference (EMI) standards, prevent the noise generated by the UPS from entering the power grid, or prevent the noise of the power grid from entering the UPS power supply and interfering with the normal operation of the system, an EMI filter must be applied at the input end of the uninterruptible power supply. This filter can suppress both common mode and differential mode interference signals.

(2) Startup surge current suppression circuit

When the power is turned on, a large surge current will be generated because the UPS's rectifier circuit will charge the filter capacitor. The size of the surge current depends on the phase of the AC voltage at startup and the impedance of the input filter. The simplest way to suppress the startup surge current is to connect a thermistor with a negative temperature coefficient in series between the DC side of the rectifier bridge and the filter capacitor. At startup, the resistor is in a cold state and presents a large resistance, thereby suppressing the startup current. After startup, the resistor temperature rises and the resistance value decreases to ensure that the power supply has a higher efficiency. For high-power circuits, the above-mentioned thermistor is replaced with an ordinary resistor, and a thyristor switch is connected in parallel at both ends of the resistor. When the power is started, the thyristor switch is turned off, and the startup surge current is limited by the resistor. When the filter capacitor is charged, the thyristor is triggered to turn on, achieving the purpose of the short-circuit current limiting resistor.

(3) Active power factor correction technology

The traditional method of suppressing harmonics is to use passive correction technology, but passive correction is currently generally used to suppress high-order harmonics. If you need to further suppress the low-order harmonics of the device and improve the power factor of the device, active power factor correction technology is currently mostly used. Active power factor correction technology is to add active switches to the traditional rectifier circuit, and control the on and off of the active switch to force the input current to follow the changes in the input voltage, thereby obtaining an input current close to a sine wave and a power factor close to 1.

(4) Using soft switching technology

The power electronic switch is turned on when its terminal voltage is not zero and turned off when its current is not zero, which is collectively referred to as hard switching. During hard switching, the switch device will bear high power, generate severe heat, reduce the device life, and generate serious electromagnetic interference. If some measures are taken to change the circuit structure and control strategy, the switch can be turned on with zero voltage and turned off with zero current, that is, the soft switching technology can be adopted to greatly improve the UPS performance.

(5) Correct and good grounding

The thickness of the power line and ground line has a great impact on the reliability of the system. If the ground line is too thin, it will not be able to provide enough current for the system, and the resistance of the power line and ground line will increase. When the current flows through the power line, a large voltage drop will be formed on the power line, affecting normal operation.

The power line and ground line should be as short as possible. If the line is too long, a voltage drop will occur when the current flows through it, and external interference signals may be coupled to the power line. Avoid routing the line close to or parallel to the control signal of large current to avoid causing great interference.

(6) Modular design

The benefits of modular design are:

Reduce the correlation and coupling between the parts, and avoid or alleviate the mutual interference between them;

It is easy to locate faults, facilitate quick repairs, and improve system reliability.

(7) Isolation and buffering

The input and output channels are a way for external interference signals to enter the system. Transformers can be used to isolate AC signals; optocouplers can be used to isolate DC signals. The external input and output channels are isolated from the control components in electrical connection, blocking external interference signals from entering the control system.

(8) Device optimization

Since UPS generally works continuously, it is necessary to screen and compare repeatedly to prevent system failure due to the device itself.

(9) Anti-interference of printed circuit boards

Printed circuit boards should follow the principles of large-area grounding, graded shielding, and short lines for large signals and high-impedance circuits.

3.2 Software anti-interference

According to different sources, software interference types can be divided into two categories: one is that the input and output channels are interfered with, making the system unable to be accurately controlled; the other is that the program is interfered with during execution and runs away. When designing the system, use:

(1) System power-on self-diagnosis

When the system is cold started, it will first perform a self-check on the CPU, RAM, etc. If an abnormality is found, it will report the error and wait for repair to avoid operating with the problem.

Self-examination generally has the following methods:

Power-on self-test; timed self-test; key-controlled self-test.

(2) Using a monitoring timer or watchdog technology

The monitoring timer is a monitoring circuit that resets the system when the system software is disturbed and deviates from the predetermined path. Most single-chip systems are equipped with a monitoring timer circuit.

(3) Setting up software traps

In the fault of the program area, fill the gap with NOP instructions to ensure that the program that was bounced due to interference can enter the normal operation track as soon as possible. Use a strong jump guide instruction to force the captured program to a specified address. Add two NOP instructions in front of it.

(4) All unused interrupt entries are filled with RETI instructions.

4 Conclusion

Uninterruptible power supply is a system where strong and weak electric signals intersect, and there are various interferences. If the system does not have strong enough anti-interference ability, even if the overall design, hardware design, and software design are reasonable, it cannot fully guarantee that the system works well. The actual system interference is complex, and it is necessary to accumulate experience in practice. Only in this way can a better circuit be designed.