Design of intrinsically safe power supply for mines based on STM32

Intrinsically safe power supply is an important part of intrinsically safe electrical equipment. Due to the mechanization of coal mines, the power supply, communication and alarm of underground sensors, bend alarms and other equipment all require power supply equipment. However, based on the complex environment underground, the requirements for power supply equipment are also very strict. It requires that the internal and lead-out wires of the power supply circuit are safe in both normal operation and fault conditions, and the generated electric sparks will not ignite the explosive mixture in the surrounding environment. Intrinsically safe power supply has two levels: "ia" and "ib". The intrinsically safe power supply introduced in this article is the "ia" level commonly used underground, which means that it can continue to work even if a fault occurs and a set of protection circuits are damaged.

1 Composition and working principle of intrinsically safe power supply

1.1 Power Supply

The input +Vin and -Vin of the intrinsically safe power supply in this system are 48 V DC power supplies, which output 12 V DC power through the DC/DC power conversion module (U1). U2 is a 7805 voltage regulator module, which stabilizes the 12 V DC power and outputs 5 V voltage to provide power for the comparator. In Figure 1, R1 is the first-level overcurrent protection sampling resistor, and R2 is the second-level overcurrent protection sampling resistor. The sampling resistor converts the current value into the voltage value GND1 and GND2 so that the STM32 can be used for overcurrent monitoring.

Figure 1 Working principle

1.2 Main circuit

Since this system is based on the monitoring of the intrinsically safe power supply of STM32, Figure 2 is the main circuit, in which UrU1, UrU2 and UrUrout

It is the sampling voltage. STM32 can determine whether the intrinsically safe power supply is over-voltage by monitoring these three voltage values. When BH1 is low, Q4 is not turned on and Q3 is turned off, so the intrinsically safe power supply has no output; when BH1 is high, Q4 and Q3 are turned on, Q1 and Q2 are turned off.

Also turned on. Since Q1 and Q2 are turned on, there is a sampling voltage on UrU2, and the level of BH2 determines whether the voltage on R24 is zero. Therefore, the intrinsically safe power supply has output only when BH1 and BH2 are both high level, and if any of them is low level, the intrinsically safe power supply enters the protection state, which is the double protection of the main circuit.

Figure 2 Main circuit

1.3 Protection Circuit

The protection circuits in this system include overcurrent protection and overvoltage protection and their recovery circuits. This system is an "ia" level intrinsically safe power supply. All protection circuits have 2 levels, and this article only introduces level 1. The circuits of level 2 and level 1 are the same.

(1) Overcurrent protection

The sampling resistor in Figure 1 converts the current value into the voltage value GND1, GND2. In Figure 3, the variable resistor R32 is adjusted to set the overcurrent protection

The value is 800 mA. The sampling voltage GND1 is amplified by U3, and then compared with the set voltage value by comparator U4 to control BH1. When GND1 is greater than the set value, the comparator output is low level, and the intrinsically safe power supply has no output. At the same time, the amplified GND1 is connected to the AD conversion pin of STM32 through the voltage follower U5 so that it can be monitored by software. The second level overcurrent protection is the same.

Figure 3 Overcurrent protection

(2) Overvoltage protection and recovery circuit

Adjust the variable resistor R5 in Figure 4 to set the overvoltage protection value to 12.5 V. The output Urout of the intrinsically safe power supply passes through the voltage divider of R44 and R47.

The voltage is compared with the set value through U7. When the voltage division value is greater than the set value, BH1 outputs a low level and the intrinsically safe power supply output is zero. Adjust R37 so that when the intrinsically safe power supply output is zero, the voltage at the reverse input terminal of U6 is greater than the voltage at the same input terminal. When Urout=0, D7 and D8 are turned on, and the output of the comparator BH1 is high level, then the intrinsically safe power supply has output. If Urout is still greater than the protection value, U7

Output BH1 is zero, D7 and D8 are turned on, and U6 output BH1 is high level, and this is repeated until the output of the intrinsically safe power supply is within the normal range. The second level overvoltage protection and its recovery circuit are the same.

Figure 4 Overvoltage protection

2 Software Protection

In the main circuit of Figure 2, the two pins BPC1 and BPC2 are connected to the ordinary GPIO pins of STM32.

The pin is the monitoring pin for the first level overcurrent protection, and AD1 is the monitoring pin for the second level overcurrent protection. AD2, AD3, AD4, and AD5 in Figure 5 are overvoltage monitoring pins. They are connected to the AD conversion pins of STM32 respectively. After AD conversion, it is determined whether the intrinsically safe power supply is overvoltage or overcurrent, and then the two pins BPC1 and BPC2 in the main circuit are controlled to ensure that the intrinsically safe power supply works within a safe range.

Figure 6 is a flowchart of the software protection program. Because AD conversion is multi-channel, DMA transmission is used during data transmission. In the process of STM32 initialization, in addition to the initialization of the system clock RCC, general input and output port GPIO, nested vector interrupt controller, timer, serial port, AD conversion, DMA transmission initialization is also included. After the initialization is completed, the AD conversion is started, and the conversion result is processed accordingly to determine whether it exceeds the overvoltage and overcurrent limit value. If it exceeds, BPC1 and BPC2 are cleared. When the software protection sets BPC1 and BPC2 to zero, recovery can only be completed by the hardware circuit.

Figure 5 Software protection

Figure 6 Software protection flow chart

3 Conclusion

The intrinsically safe power supply is introduced from two aspects: software and hardware. The hardware aspect introduces the main circuit, overcurrent protection, overvoltage protection and its recovery.

The working principle of the circuit, the software is mainly controlled by STM32, and the communication module ADM2483 can be added to transmit the AD acquisition results to the host computer through the serial port through MODBUS communication, so that the output of the intrinsically safe power supply can be observed in real time, and the pins of the chip can be fully utilized. The test shows that the intrinsically safe power supply works normally and is protected in time, and is used as the working power supply of the sensor and the curve alarm.