Selection of inter-line voltage IB and design of power supply system

XY·CN bus 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, it is necessary to pay attention to the power supply design method of each part. The system can work normally when the bus voltage is in the range of 12 to 36 V. Different operating 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 Effect

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 impedance is about 30Ω per kilometer. 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 at the i node.

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

(24-10.5)／(0.05*2C)00)=O. 135(A)=135(mA)

That is to say, the standard system can work at 2000 m in the monitoring state (130 mA). This calculation also shows that the system cannot be used in this way, because the static current will increase after the equipment is activated.

1.2 Impact of equipment power consumption

XY·CN bus is the host power supply mode, which provides great convenience for designing systems that need backup power. However, some systems have high power consumption, so the selection of bus voltage will fully consider the power requirements of the system. For example, canteens, fast food restaurant meal sales systems, fire alarm display systems, these systems all need to provide high 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, whose maximum driving current is 220 mA. The starting RF card can be solved 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 . Calculated based on an efficiency of 80%, at a 25 V input voltage (5 V is bus loss), the input current is 200/((25/3)*0.8)=30 mA. The total current is controlled within 600mA, and 20 rice vending machines can be installed on each bus.

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

If all the vending machines are at the end of the bus, the voltage should be kept at 25 V, and RV1.5 twisted pair 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 powered directly by the series regulator of the XY001 chip, with a static power consumption of about 0.6 mA and a maximum power consumption of about 1.5 mA after the action indication.

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 power supply. Therefore, its power calculation must take into account the current change caused by the long-distance bus voltage reduction. Calculated based on 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, calculated based on an 80% switching power supply efficiency, the bus power supply current IPOW is 3×200×80%/16=30 mA.

2.3 Time-sharing start-up equipment

Since the equipment is started in time under the control of the controller, only the total current n×ISIPTLME of the number n that can be started simultaneously is calculated.

2.3.1 Accidental start of equipment

For short-term power supply devices that use their own power, such as sound and light alarms, since the device 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 Calculation of total system power

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

Reserve Capacitor Quantity: Required static working time × static power + required maximum power working time after termination of work × maximum power.

3 Power System Design for Common Bus Voltages

The backup power design of the XY·CN BUS system is very simple. You only need 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. [page]

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 outputs are connected 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 lower than the output voltage, so a simple series switch constant current circuit can complete high-efficiency charging. And the backup power voltage regulation only requires a simple switch boost regulator, and the voltage system design is simple.

3.2 AC220V switching power supply

Complete the AC to DC power conversion, 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 failure signal to the control circuit, 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 uses the DC output of the main power supply as input and charges the battery with a constant current. The system can perform intermittent charging under the control of the MCU . It can also be designed to discharge and charge regularly to ensure the life of the lead-acid battery.

3.4 Switching Boost Circuit

The battery voltage is boosted and stabilized to the power supply output voltage to ensure the stability of the output voltage during backup power operation. 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 have high voltage requirements, and it can work normally as long as the ripple is less than 200 mV. Therefore, the use of the microcontroller's A/D sampling and output PWM control switch tube can meet the power system requirements. The battery boost and constant current switching 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 supply

The commonly used LM3478 is used to complete the isolated 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.

The power supply system of 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.