Design of Micro UPS for 5V Power Supply of PC

1 Introduction

With the development of electronic technology and the progress of society, electronic equipment is becoming more and more closely related to people's work and life. These electronic devices, such as PCs and various embedded systems, have higher and higher requirements for power supply quality. Sudden power outages or unstable power supply during work will cause data loss, equipment damage, machine malfunctions, and paralysis of the entire system, which are very serious hazards. Therefore, safe and reliable power supply is an important issue that we have to face seriously. There are many ways to solve the problem of stable power supply, such as filtering, linear voltage regulation, UPS, etc. UPS is the only device that can completely solve the problem of stable power supply.

UPS (Uninterruptible Power Supply) is a power supply that provides uninterrupted, high-quality, efficient and reliable stable voltage to the load. UPS can be divided into AC UPS and DC UPS according to its output voltage. The working principle of AC UPS is that when the mains power supply is normal, one mains power supplies the battery through the rectifier until the battery is in a floating charge state, and the other power supply is directly supplied to the load through the conversion switch after being stabilized by the voltage regulator. When the mains power is interrupted, the conversion switch disconnects the input of AC power, the AC/DC charging stops working, and the battery supplies power to the load through the inverter. The working principle of DC UPS is that when the mains power supply is normal, the conversion switch connects to the output of the DC stabilized power supply, the load is directly powered by the DC stabilized power supply, and the DC stabilized power supply is used to charge the battery until it is fully charged; when the mains power is interrupted, the output of the DC stabilized power supply is interrupted, the conversion switch disconnects the connection with the DC stabilized power source, and the battery supplies power to the load through the DC/DC.

For DC power-consuming equipment such as PCs and embedded systems, the use of DC UPS has more outstanding advantages than AC UPS, such as simple circuit, fewer components, low cost, low power consumption, small size and weight, high reliability, easy manufacturing and convenient maintenance.

The DC UPS designed in this paper is a Boost circuit based on the MAXIM668 control chip of MAXIM. The circuit structure of this scheme is simple, independent switching, and has high output accuracy. Finally, the correctness of the theoretical analysis and the feasibility of this scheme are verified through experiments.

2 UPS system working principle

Figure 1 shows the block diagram of the UPS system in this article. The UPS system consists of a mains detection unit, a UPS, and a switching unit. The working principle is that the detection unit detects in real time whether the mains power is interrupted. When the mains power is normal, the drive signal of the Boost circuit is blocked, the UPS does not work, the switching unit switches to the switching power supply, the PC is directly powered by the switching power supply, and the battery is charged to a floating charge state by the switching power supply output; when the mains power is interrupted, the drive signal of the Boost circuit is turned on, and the switching unit switches to the UPS, and the PC is powered by the UPS. In this way, a stable DC voltage can be provided to the PC when the voltage is normal or interrupted.

The UPS system detection unit is shown in Figure 2. The detection unit consists of a rectifier bridge, an RC circuit, and an optocoupler isolation circuit. Its output out generates a logic level according to whether the detected mains power is interrupted. When the mains power is normal, the mains power is rectified by the rectifier bridge, and the RC is used for rectification and output filtering to obtain direct current. After passing through R2, the optocoupler is turned on, so that the output out of the detection unit is low, thereby blocking the UPS control signal and the UPS does not work; when the mains power is interrupted, the optocoupler is not turned on, and the detection unit outputs out high, turning on the UPS control signal and starting the UPS. It is worth noting that the value of C1 here is small to ensure the real-time detection of the mains power.

The dotted box in Figure 1 shows the UPS designed by the system, which consists of a control drive circuit, a boost circuit, and a battery. Figure 3 shows its circuit diagram. The control drive circuit uses the MAXIM668 chip of MAXIM. Here, the battery is used to power the chip. MAXIM668 is a fixed-frequency PWM controller that works in current mode. During normal operation, the PWM signal output by it is controlled by voltage closed-loop feedback regulation and current peak regulation to achieve the purpose of voltage regulation output. SYNC/SHDN is a chip select signal, which is effective at high level and is controlled by the output out of the detection unit. A 1.25V reference voltage is generated internally at the REF terminal. The FREQ terminal is connected to the resistor Rosc to control the operating frequency of the chip. The FB terminal is the feedback terminal, and the operation of MAXIM668 is controlled by comparing the voltage division result of the feedback resistors R2 and R3 with the 1.25V voltage at the chip REF terminal, forming a voltage closed-loop feedback regulation. The CS+ terminal is connected to an external current sensing resistor, and the pulse width of the PWM signal at the chip EXT output terminal is controlled by comparing the current value on the current sensing resistor with the current peak value determined by the voltage regulator inside the chip, forming a current peak regulation. The main circuit of the UPS is a Boost circuit. By controlling the pulse width of the PWM signal at the EXT output end, Vout is guaranteed to be a stable 5V. The battery not only supplies power to the Boost circuit but also to the control chip MAXIM668.

The principle of the switching unit is shown in the dotted box in Figure 4. Its basic principle is to enable the switching power supply or UPS to supply power to the load when the mains power is in a normal or interrupted state, thereby ensuring stable power supply to the load. The relays in the figure are all in the normally open state.

The detection unit output out and UPS output Vout work together to control whether the switching unit is in operation. In the control logic, Vout is first made into a two-stage Schmitt inverter, so that it outputs 0 when it is not working and outputs 1 when it is working normally. Then the transformed Vout and out are made into an AND logic. When out and Vout outputs become high level at the same time, relay M1 is turned off and relay M2 is closed; when either out or Vout output becomes low level, M1 is closed and M2 is turned off.

Due to the mechanical characteristics of the relay, the switch will be turned off faster than it is closed when the switch is in action, resulting in a gap in the load power supply when the switch switches the switching power supply and the UPS. This paper designs a control delay shutdown and immediate closing link to solve this problem.

When the mains power goes from interrupted to normal, the control logic output is 0. At this time, for M1, the control signal is 1, so it is immediately closed; for M2, the control signal is 0, so it is delayed closed. Vice versa.

3 Calculation of key parameters

3.1 MAXIM668 chip frequency calculation

The frequency range allowed by MAXIM668 is 100KHz-500KHz, and the frequency setting formula of the chip is shown in formula (1). From formula (1), we can know that Rosc can be obtained by selecting the working frequency fosc of different chips. In this paper, fosc is taken as 100KHz, and Rosc can be obtained

3.2 MAXIM668 feedback resistor calculation

The UPS system indicator requires an output voltage of 5V, according to the formula set by the feedback resistor of the MAXIM668 chip

Here, Vout = 5V, VREF = 1.25V. Select R3 = 10KΩ, then R2 = 30KΩ.

3.3 Calculation of Inductance Value in Boost Circuit

The battery in this article uses three 1.2V batteries connected in series to power the Boost circuit at 3.6V. In order to ensure reliable power supply for the PC, the current ripple on the inductor L must be within the allowable range of the PC. Here, the ripple is 50mA, so I (peak-peak) = 100mA. Since f = 100KHz, T = 10μs, the relationship between the output input voltage of the boost chopper circuit and the on-off time of the switch tube is shown in formula (3).

3.4 Power supply time calculation

For UPS systems, we are also concerned about the time the load can maintain normal power supply after switching to UPS. After the battery is selected, its ampere-hour is certain, so the power supply time is determined by the current required by the load. For this system, the current required by the load is 3A, and the ampere-hour of the battery is 150mAh, so the power supply time is 3 minutes. That is to say, within 3 minutes of power outage, the PC can save and process the data, ensuring normal operation after the mains power is restored.

4 Experimental results analysis

4.1 UPS main circuit test results

Figure 5 shows the experimental waveforms when the UPS is working. 1 is the driving signal waveform of the MOSFET in the Boost circuit (5V per grid), 2 is the drain waveform of the MOSFET in the Boost circuit (2V per grid), 3 is the input voltage (and battery voltage) waveform of the Boost circuit (2V per grid), and 4 is the output of the Boost circuit, i.e. the final output voltage waveform of the UPS (5V per grid). It can be seen from the figure that the UPS system can still provide a stable 5V voltage to the load after switching.

Figure 6 shows the waveform of the switching unit when switching. The waveform from 0 to 42ms in the figure is when the switch is connected to the switching power supply end, and the waveform after 42ms is when the switch is connected to the UPS end. 1 is the waveform of the chip selection signal of MAXIM668, that is, the output out of the detection unit, 2 is the control signal waveform output by MAXIM668 to the gate of the switch tube, and 3 is the final output voltage Vout waveform of the UPS system.

As can be seen from the figure, the AC power is interrupted before 42ms. The system can quickly detect the interruption signal to enable MAXIM668 to work and generate a driving signal, and the UPS system starts immediately with a fast response speed, and the output DC voltage is a stable 5V.

5 Conclusion

This paper designs a micro UPS system based on the MAIXM668 boost chopper circuit. The control circuit uses the MAXIM company's MXAIM668 chip. The entire system circuit is simple and has a fast response speed. The experiment verifies the effectiveness of the scheme.