A leakage protector circuit design

With the promotion of the use of leakage circuit breakers and the improvement of people's living standards, the number of household appliances and other equipment has increased. Household appliances generally have inductive loads and capacitive loads. These loads are prone to produce induced electromotive force, surge voltage and impulse current during use, thus requiring The ability of leakage circuit breakers to withstand interference such as surge voltage and impact current is getting stronger and stronger, allowing leakage circuit breakers to be used reliably in various situations and ensuring that leakage circuit breakers do not trip accidentally or fail.

1. Leakage chip power supply part

The latest national standard GB16917.1-2014 (Standard for leakage circuit breakers with overload current protection) stipulates that for leakage protectors used in three-phase four-wire systems, when any two phases have a phase failure fault and at this time There is leakage current in the line. If the leakage current reaches the set operating current value IΔn of the leakage protector, then the leakage protector must trip reliably and be able to perform its protective function. For this standard, the rectifier part of the leakage protector usually uses full-bridge rectification to meet the requirements, as long as the reverse withstand voltage value of the rectifier diode can meet the EMC test requirements. If an integrated chip such as the 54123 solution is used, tantalum capacitors and X7R package capacitors need to be connected in parallel to pin 8 of the chip to play the role of post-rectification filtering and anti-high-frequency interference, so that the chip 54123 can work in an ideal state. At the same time, the national standard GB16917.1-2014 stipulates that if the set leakage operating current of the leakage protector is less than or equal to 30mA, the leakage protector needs to meet the 50V operating characteristics.

50V action characteristics: When the leakage protector RCBO is in the closed position and the power is cut off, the RCBO should not be disconnected; then apply a 50V voltage to the power supply end and suddenly apply IΔn to a certain pole of the RCBO, and the RCBO should trip. In order to meet this action characteristic, it is necessary to appropriately select the voltage-reducing resistor of the power pin of the chip 54123, so that when the power terminal voltage is AC 50V, the power supply voltage of the chip 54123 after rectification and filtering is within the range specified in its data sheet (minimum value is 12V, usually 16V is required). After these two steps, it should be said that the power supply end of the leakage integrated chip is ideal. If the PCB space is sufficient, a voltage regulator tube can be connected in parallel between the power pin and the ground.

2. SCR trigger part

When the three phases of the circuit breaker are energized at the same time, the DC voltage applied to both ends of the thyristor after full-bridge rectification exceeds 500V. The parameter requirements for a single thyristor are very high. Currently, mainstream products use dual silicon series circuits. Reduce the DC voltage across a single thyristor and improve the reliability and stability of the thyristor trigger circuit. The dual silicon series circuit used in the existing technology is shown in the figure below. It simply connects two thyristors in series. When the leakage IC chip detects that the leakage signal reaches the trip threshold, the trip signal output pin outputs a high level, which can The silicon-controlled SCR2 triggers, and then the silicon-controlled SCR1 triggers, causing the leakage circuit breaker coil to trip. It can be seen from Figure 1 that when the circuit breaker is normally powered on, almost all of the rectified DC voltage is applied to the thyristor SCR1, and SCR2 does not play a role in sharing the voltage. If the thyristor SCR1 has poor performance parameters and is broken down, the high DC voltage in the line will easily break down the thyristor SCR2 again, causing the leakage circuit breaker to jump abnormally. This kind of line still has high requirements on the parameter performance of a single thyristor, and does not fully utilize the reliability of dual silicon series connection.

Therefore, through the above analysis, the most important thing in the design of the thyristor triggering part of the leakage protector is to make the withstand voltage of the two series connected thyristors relatively uniform when the leakage protector is normally energized, so that the two thyristors can be fully utilized. The function of silicon control improves the overall voltage resistance level of the line. The easiest thing to think of is to divide the voltage through resistors as shown in the figure below. The resistance values ​​of R3 and R4 should be the same. Select the appropriate resistance values ​​of R5 and R6 according to the specific parameters of the thyristor SCR1 to ensure that the thyristor SCR2 is triggered. The thyristor SCR1 can be reliably triggered. When the circuit breaker is powered on normally, the rectified DC voltage passes through the equalizing voltage divider circuit composed of R3 and R4. The silicon-controlled SCR2 is regarded as a resistor with a very large resistance, so that the voltage applied to the cathode of the silicon-controlled SCR1 is almost It is the same as the trigger electrode, and it is half the anode voltage. At this time, the anode voltage of the thyristor SCR2 is half of the anode voltage of the thyristor SCR1, so that the two thyristors can share the voltage equally. Even if the withstand voltage parameter of a single thyristor is not high, it can still work reliably. When the leakage IC chip detects that the leakage signal reaches the trip threshold, the trip signal output pin outputs a high level, and the silicon-controlled SCR2 is triggered. At this time, the anode voltage of the silicon-controlled SCR2 is close to 0, and the anode voltage of the silicon-controlled SCR1 passes through the resistor. The voltage dividing circuit composed of R3, R4, R5 and R6 applies a suitable voltage to the trigger pole, thereby causing the leakage circuit breaker coil to trip.