Analysis of the method of using an ultra-low-power voltage monitor or Reset IC as a leakage current detector

When an alternate path to the ground is established, a residual current operated protector (RCD) or residual current circuit breaker (RCCB) will detect the leakage current. The RCD isolates the power supply from the leakage path by breaking the circuit. Unlike fuses, these types of circuit breakers can be reset and reused, and they play an important role in protecting personnel and equipment.
In this article, I will review the requirements of RCD leakage current detection and tripping, and how to use an ultra-low-power voltage monitor or Reset IC as a leakage current detector. It will also explain how voltage monitors can benefit medium voltage circuit breakers, such as air circuit breakers (ACB) and molded case circuit breakers (MCCB) using microcontrollers.
Use a voltage monitor as a leakage current threshold detector
When a faulty device or electric shock occurs, leakage current will occur. The purpose of RCD is to respond to a current of 5 mA-500 mA. As stated in the International Electrotechnical Commission TS60479-1 standard, a person will experience muscle contraction within 50 mA for 200 ms, and exposure time longer than 3 seconds will increase the possibility of ventricular fibrillation.
RCD detects the leakage current by sensing the difference between the active line and neutral line current. If the line current and the neutral current are unbalanced and the leakage current exceeds a predetermined threshold, the circuit breaker trips, interrupting and isolating the power supply. RCD equipment can be divided into three main stages. The first is the sensing phase, during which the leakage sensor senses the leakage current. In the second stage, the detection circuit sets the leakage current threshold. In the third stage, the relay trips to isolate the source of leakage.
In the past, a relay was used as a detection circuit to set the leakage current threshold. Modern RCDs use integrated circuits such as voltage monitors to improve the accuracy and response time of detecting leakage currents and driving solenoid relays.

Figure 1 shows how TI's TPS3840 voltage monitor detects leakage current.
In this example, the leakage sensor (such as a differential current transformer or a zero-phase current transformer) is represented by a current source. For the current detection circuit, a resistor divider is used to convert the input current into a voltage, which is then detected by the TPS3840. TPS3840 integrated reference voltage band gap and voltage comparator. The trigger point is programmed through non-volatile memory (OTP) at the factory, and the voltage threshold can be set in the range of 1.6 V to 4.9 V, with a typical accuracy of 1%.
When the voltage on the VDD pin rises above the threshold, the RESET pin will be pulled high to interrupt the microcontroller or drive an electromagnetic relay. In addition, a single external capacitor can also be used to extend the RESET response time to meet the varying RCD response time according to the magnitude of the leakage current.
This reference design focuses on general use cases for low-power reset ICs and watchdog timers. The TPS3840 device has a current consumption of less than 1μA, and it can be configured to be used as a comparator (through a simple 3-pin configuration) or as a general-purpose power monitor, which can be cascaded as a sequencer in a daisy-chain fashion. In addition, the response and delay time can be configured via external capacitors. For applications that need to run a watchdog, you can use the TPS3431 device, which is an independently programmable watchdog timer with an accuracy of ±2.5% (typical value).
characteristics
Threshold detection (TPS3840): A self-powered threshold detector that uses a voltage monitor as a comparator (through 3-pin configuration, no external power supply).
Very low quiescent current (<700nA) architecture, with a set threshold range of 1.6V-4.9V (0.1V step); user-controllable response time (80μs to 600ms through an external capacitor) fast Electricity (start delay <250μs).
Power monitoring and sequencing reset (TPS3840): It can be used for multi-track reset monitoring and start sequencing.
Provide low-level effective and high-level effective configuration, the threshold range is 1.6V-4.9V, and the TD range is 80μs-600ms.
Watchdog Timer (TPS3431): User adjustable timer with enable pin and wide input range.
Suitable for the low current consumption (20A) of the watchdog function and the user can configure the timeout function using external resistors and capacitors.
This reference design focuses on general use cases for low-power reset ICs and watchdog timers. The current consumption of the TPS3840 device is less than 1A, and it can be configured as a comparator (with a simple 3-pin configuration), or as a general-purpose power monitor, and can be connected as a sequencer in a daisy chain. In addition, external capacitors can be used to configure the response and delay time. For applications that require watchdog operation, you can use the TPS3431 device, which is an independently programmable watchdog timer with ±2.5% accuracy (typical value).
The voltage monitor in this reference design meets three key specifications:
Fast power-on and programmable response time. When the voltage monitor rises from the zero input voltage to the trip point or above the threshold voltage, it takes a certain amount of time to start and react. Circuit breakers need to quickly detect the leakage current level and have the flexibility to configure the trip response time according to the leakage level and duration to avoid false trips caused by transients. The startup time of TPS3840 is 200s, which can respond quickly.
Ultra-low input current. In Figure 1, the power supply pin (VDD) of the monitor IC is the same as the input signal monitoring pin (SENSE). Since it is powered by the input signal, the voltage monitor should have a high input impedance to reduce the error (IIN) on the voltage divider (IDIV). The TPS3840 voltage monitor consumes ultra-low current, with a typical IQ of 350 nA,
Low VPOR can achieve low VOL. VPOR is the input voltage required for the controlled output state. When VIN <vpor, the output tracks the input and may trigger a relay. vpor should be as low as possible to provide a margin between the enable voltage level of the relay and the low output voltage level (vol) of the voltage monitor. ="" tps3840 has an active low configuration, with low vpor and a vol of 300="" mv to avoid false reset.
If the voltage monitor is integrated in the microcontroller, it is recommended to use an external watchdog timer. The external watchdog can ensure that the microcontroller will not latch up by periodically detecting the pulses sent by the general-purpose input/output pins of the microcontroller. If the software malfunctions and misses the pulse, the external watchdog timer can reset the microcontroller.
TPS3430 programmable watchdog timer is a good choice, because it provides a programmable watchdog, responsible for monitoring timeout and reset delay, can meet the timing requirements of any microcontroller. If higher reliability is required, voltage monitor and watchdog should be used at the same time. TPS3823 integrated watchdog and voltage monitor are a good choice, providing fixed thresholds and watchdog timeout options.
Voltage monitor can not only monitor the voltage supply of the microcontroller to ensure normal operation, but also can be used as a leakage current detector to help enhance your circuit breaker design.
The reference design uses a signal processing front end and a self-powered module for the electronic trip unit (ETU) used in the circuit breaker. This design uses an FRAM-based microcontroller to process the current input from the signal conditioning amplifier to obtain a three-phase neutral ground current. Two gains are used to expand the phase current measurement range.
This reference design can also use rectified current input for self-powered. TIDA-00498 is designed to achieve rapid repetitive trips within a wide current and temperature range (within 30mS).
characteristics
The measurement input range is (0.2 to 12 INOMINAL) three currents (phase) and the input range is (0.05 to 2 INOMINAL) two currents (neutral, ground), the accuracy is ±3%.
Using TI's MSP430 FRAM MCU for fast startup, the startup time including single-cycle RMS compensation is less than 30ms.
Under-voltage detection provides about 200s of reset time after power-on.
Self-powered power supply (based on rectified current input), with MOSFET-based shunt regulation.