Design of Power Supply System for XY·CN Bus Communication System

XY·CN bus It is a low-cost, point-to-multipoint field bus communication system. One of the advantages of this system is its unparalleled power saving advantage. To give full play to this advantage, we must pay attention to the power supply design method of each part. The system can work normally in the bus voltage range of 12 to 36 V. Different working voltages can be selected for different applications. This article will comprehensively discuss the power supply design principles and methods of its system.

1 Selection of bus voltage

1.1 Distance influence

The limit working voltage of the slave receiving circuit composed of standard XY001 is 10.5 V. The bus uses RV1.5 twisted pair as the medium, and its kilometer impedance is about 30Ω. We only need to consider that the terminal voltage of the system is greater than 10.5 V at the maximum working current. If the power supply time ratio is 3:5 (3/5 of the time is powered), the bus voltage calculation formula is as follows:





N is the number of nodes, L is the length of the twisted pair, and Ii is the current of the i node.

According to the above formula, if all node devices are installed at 2000 m, the maximum output current of the bus is:

(24-10.5)/(0.05*2C00)=0.135(A)=135(mA)

That is to say, the standard system can work at 2000 m in the monitoring state (130 mA). It can also be seen from this calculation that the system cannot be used in this way, because the static current will increase after the device is in operation.

1.2 Impact of device power consumption

XY·CN bus is a host power supply method, which provides great convenience for designing systems that require backup power. However, some systems have high power consumption. At this time, the selection of bus voltage will fully consider meeting the system's requirements for power supply power. For example, canteens, fast food restaurant meal sales systems, fire alarm display systems, these systems all need to provide large voltage power for the equipment.

To meet the power requirements of this system, we can choose a 36V system. The design method of the power system is explained by taking the rice vending system as an example: the main power consumption of the rice vending machine is to start 16 medium-sized digital LEDs, and the maximum driving current is 220 mA. The radio frequency card can be started by time-sharing driving with the digital tube, so it is calculated based on an average current of 200 mA. The power supply part uses a low-cost switching power supply composed of MC34063 chips. According to the efficiency of 80%, the input current is 200/((25/3)*0.8)=30 mA at a 25 V input voltage (5 V is bus loss). The total current is controlled within 600 mA, and 20 rice vending machines can be installed on each bus.







Note: The bus inlet commutation and isolation diodes use 1N5819 Schottky diodes.

If all rice vending machines are at the end of the bus, the voltage must be maintained at 25 V, and the RV1.5 twisted pair cable is used. The communication distance of the system bus is 5/(0.05*0.6)=160 m. [page]

2 System power calculation

2.1 Input devices

Most input devices are directly powered by the series regulator of the XY001 chip. The static power consumption ISTAICT/O is about 0.6 mA, and the maximum power consumption IMAXI/O after the action indication is about 1.5 mA.

2.2 High-power control equipment

Since this type of node equipment has high power consumption, it mostly uses a 3 V system and a switching regulated power supply. Therefore, its power calculation must take into account the current change caused by the long-distance bus voltage reduction. Calculated with a 24 V system, the maximum power supply voltage drop of 2 km is 12 V, and the average voltage drop is 6 V. Therefore, its average switching power supply input voltage is 24-2 (circuit loss)-6=16 V. If the 3 V system needs to use a 200 mA display current, the bus power supply current IPOW is calculated based on the switching power supply efficiency of 80% to be 3×200×80%/16=30 mA.

2.3 Time-sharing start-up equipment

Since the equipment is time-sharingly started under the control of the controller, only the total current n×ISIPTLME of the number of devices that can be started at the same time is calculated.

2.3.1 Occasionally started equipment

For short-term power supply equipment using self-powered power, such as sound and light alarms, since the equipment does not consume bus energy when it is in operation, it is only necessary to calculate its average pulse charging power consumption ILITTIME.

2.3.2 System total power calculation

Main power = bus operating voltage × bus maximum operating current + controller maximum power + charging power.

Backup power capacity = required static working time × static power + required maximum power working time after termination of work × maximum power.

3 Design of power supply system for common bus voltage

The backup power design of XY·CN BUS system is very simple. It only needs to design the backup power on the controller according to the total power capacity requirements of the system. Therefore, low-cost large-capacity lead-acid batteries are the first choice for backup power.

3.1 System block diagram





Use a single-output AC switching power supply in parallel with the switching boost power supply output as the power output. Because the output is in parallel, a synchronous rectifier cannot be used to avoid mutual influence between the two outputs.

The maximum charging voltage of the lead-acid battery must be less than the output voltage. In this way, a simple series switch constant current circuit can complete high-efficiency charging. The backup power voltage regulation only requires a simple switching boost regulator, and the voltage system design is simple.

3.2 AC220V switching power supply

Complete the power conversion from AC to DC, the voltage is equal to the bus voltage, and the current is equal to 1.2 times the rated output current. When the AC input voltage is lower than 154V, the voltage will output the main power fault signal to the control circuit to control the backup power boost circuit to work, and turn off the backup power boost when the main power is normal, and charge the battery according to the battery condition.

3.3 Switching constant current charging circuit

The power supply takes the main power DC output as input and charges the battery with constant current. The system can be intermittently charged under the control of the single-chip microcomputer. It can also be designed to discharge and charge regularly to ensure the life of the lead-acid battery.

3.4 Switching boost circuit

Boost and stabilize the battery voltage to the power supply output voltage to ensure the stability of the output voltage when the backup power is working. When the battery voltage is lower than the minimum output voltage of the battery, the control circuit automatically turns off the boost circuit to ensure that the battery will not be damaged by over-discharge.





3.5 Power Control

The bus power supply system does not require high voltage, and it can work normally if the ripple is less than 200 mV. Therefore, the A/D sampling and output PWM control switch tube of the microcontroller can meet the power supply system requirements. The battery boost and constant current switch power supply of the system will not work at the same time, so the use of PICl6F690 can meet the power management requirements.

3.6 Controller Control Circuit Power

The commonly used LM3478 is used to complete the isolation power supply design, and the reference circuit is shown in Figure 4. After selecting different MOS tubes, the output power of the system can reach more than 10 W.





Conclusion

The power supply system of the XY·CN bus is an important guarantee for the stable operation of the bus. For such a complex field control system, the design of its backup power system is relatively simple, which is a major advantage for you to choose this system design.