EMC circuit design of intelligent residual current protector

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

LC03-3.3 SOP8 TVS Static Protection 3.3V

Overview:
This paper presents an intelligent residual current protector based on single-chip design. The EMC design method of the intelligent residual current protector is introduced from various aspects, and the influence of high frequency radiation on the system is realized by the cooperation of software and hardware.

1 energy type residual current protector system introduction
EMC is the ability of a device or system to operate in its electromagnetic environment without posing an unacceptable electromagnetic disturbance to any object in the environment. The residual current protector must be satisfied as the power supply line protection device (below 400V) at the end of the grid.
The EMC national standard GB/T17626.5-1999 requires that 3C certification can be put into the grid operation. Figure 1 shows the block diagram of the intelligent residual current protector system designed with P87LPC767 microcontroller. The EMC design is considered in circuit design, software design and PCB board design. The residual current protector is a kind of low-voltage electrical equipment. There is no high-power high-frequency circuit inside, and the electromagnetic radiation is weak. The electromagnetic disturbance generated by it has little influence on other equipments. This is not the focus of EMC design. The focus of the residual current protector EMC design is its ability to maintain stable operation, ie, immunity to interference, when subjected to electromagnetic interference generated by other equipment. The residual current protector is mainly from the power grid itself, mainly including unipolar surges (shocks) caused by sudden line breaks or lightning transient overvoltages, and signal parameters caused by lightning, ground faults or switching inductive devices. The generation of instantaneous disturbances is the focus of EMC design and the difficulty of design. The EMC design method of the intelligent residual current protector is introduced from various aspects below.

Figure 1: Block diagram of the intelligent residual current protector system

2 Anti-interference design of the system The residual current protector adopts P87LPC767 single-chip microcomputer. The important work such as sampling detection, calculation, display, action criterion and protection action of residual current are all completed by single-chip microcomputer. The anti-interference ability of the single-chip system itself is directly determined. The anti-jamming capability of the entire protector. The clock signal, reset circuit, interrupt signal and sampling signal of the MCU itself are easily affected by electromagnetic interference, and it is very important to eliminate or suppress the influence of the electromagnetic interference signal on the MCU.
Specifically designed from the following aspects:
The working power of the single-chip microcomputer and the other circuit power supply of the system are designed separately to avoid the influence of other circuits on the working power of the single-chip microcomputer. The power supply design of the single-chip microcomputer has sufficient margin to prevent the fluctuation of the power supply from affecting the operation of the single-chip microcomputer. When designing the PCB board, connect the capacitor and the transient voltage suppressor (TVS) to the power supply pin of the microcontroller, as shown in Figure 2. The energy stored in the 100 μF electrolytic capacitor is released (lower) when the power supply fluctuates, keeping the power supply stable; the 0.1μF high-frequency capacitor can absorb the high-frequency interference on the power supply; the TVS absorbs the transient surge power, and the voltage between the two poles is clamped. Located at a predetermined value, keep the power supply of the microcontroller stable.

Figure 2: The power supply pin design reset circuit of the MCU uses the internal power-on reset circuit of the P87LPC767 MCU to avoid the direct influence of the interference signal on the reset circuit. As long as the working power of the MCU is stable, there will be no malfunction caused by the false reset. Interrupt and other I/O port capacitances are filtered to reduce the effects of interference.
System software design, enable the P87LPC767 internal watchdog to prevent the PC from being disturbed and out of control, causing the program to fly, enter the "infinite loop". Set software traps in the non-program area of ​​the program storage space, when the operating system is out of control due to interference When entering a non-program area, the boot command is redirected to a program that specifically handles program errors, putting the program on the right track.
(Please read the PDF for details)