Foreword: A frequency converter is a device that outputs any desired frequency value. Frequency conversion is usually implemented in an AC-DC-AC manner. Voltage and current sensors are an important "sensing organ" function in a frequency converter. They can monitor and feedback key parameters in real time, and are collected and calculated by the CPU. Finally, precise control is made according to preset conditions. Current and voltage sensors play an important role in fault protection and equipment protection. This system describes the application scenarios of voltage and current sensors in frequency converters and the selection strategy of devices. CHIPSENSE voltage and current sensors will be a part of them.

First. Overview of frequency converter system and the core role of sensors
The frequency converter is through the high-speed switching action of the power electronics, the low-voltage direct current/or the direct current of the energy storage battery is finally converted into a specific frequency of alternating current. This frequency can be set by the user at will on site, and is generally supplied to the motor load to achieve the purpose of controlling the speed and saving energy. Its typical topology includes lightning protection, rectification and filtering, DC/DC boost, DC/AC frequency converter, and waveform shaping AC output. Voltage and current sensors can play an important role in each link, mainly depending on the goal pursued by the designer. The overall block diagram is roughly as follows:

Voltage and current sensors play a central role in these aspects:
Real-time monitoring:
Real-time, accurate acquisition of various parts of the current, voltage parameters, passed to the CPU, CPU high-speed A/D, the analog signal into a digital signal for the program control algorithm to provide data basis;
Closed-loop control, process:
Through the monitored voltage and current data, the program performs data operation and logic analysis and judgment, and then realizes the precise control of power devices (SiC /MOSFET/IGBT) to achieve the purpose of closed-loop control;
Fault protection and device protection:
The CPU detects the signals output by the voltage and current sensor devices, and makes corresponding judgments on abnormal states such as overcurrent, short circuit, and leakage of the equipment, and finally triggers the protection mechanism.

Second. Practical Application of Voltage and Current Sensors in Each Link of frequency converter
1.DC input link:
The battery pack itself usually has a BMS system that includes battery charging protection and low-voltage protection for low discharge, so it generally does not need to be tested. From the perspective of safety regulations, it is usually only necessary to consider the leakage current monitoring of photovoltaic cells/energy storage batteries. In terms of hardware circuit reliability, it can be considered that the leakage current sensor is arranged here. Therefore, it is a low-voltage area with less interference and easy handling, which will be an ideal location;

2. DC/DC boost and lightning protection:
The DC/DC boost link usually relies on a complete circuit to increase from a low voltage such as 48V to about 300V. Therefore, it is generally not necessary to detect the voltage after boosting; if necessary, add lightning protection and surge protection in the front stage to prevent damage to the back-end components and cause equipment abnormalities. The lightning protection part of the following circuit can meet the test of anti-2000V surge/EFT, etc. The circuit is shown below:

3. DC/AC frequency converter link: IGBT protection, bridge arm control and fault protection
Compared with MOS transistors, IGBT are much more expensive. In order to protect the power transistor from damage as much as possible; or, in order to protect the upper and lower bridge arms from burning or even electrical fires due to damage to the power transistor, it is necessary to place a device to monitor the working current on the + 300V bus. In the early days, it was customary to use shunts + isolated OP amplifiers to achieve.

The use of shunts to detect current is easy to apply in low-voltage circuits. If it is a high-voltage circuit, such as + 300V, the problem of insulation withstand voltage is not well handled, and some protection circuits should be added. In addition, the larger the current carrying capacity, the larger the size, which also limits its application. But CHIPSENSE current sensor is unlimited. 
High-precision current sensors are recommended: consider using high-precision, fast response time closed-loop Hall current sensors, such as CHIPSENSE CNxx series current sensor, which have a response time of less than 0.5us and an accuracy of about 0.2%.

The response time of CHIPSENSE current sensor is less than 0.5 us.

This high-precision closed-loop Hall sensor from CHIPSENSE is placed here to serve two main functions: 1) to detect the total output current, which is used to calculate and display the total output power; 2) to protect it like a fuse; once the MOS/IGBT of a bridge arm is damaged, it will inevitably cause + 300V to pass through to the ground, causing a short circuit. Such faults will be quickly detected.

Although the response time of the closed-loop current sensor of current sensor is less than 0.5us, considering that the waveform data output by the frequency converter is generated by the calculation of the internal program of the CPU, which leads to a large amount of running, the CPU may not be able to quickly match the response time of 0.5us. Increasing the working frequency of the CPU or upgrading the CPU is certainly a way, but it is best to add a maximum value detection circuit: the analog quantity is then converted into a high and low level digital signal through a window comparator, and finally sent to the interrupt port of the CPU to achieve a fast response, quickly cut off the output of the IGBT, and play a protective role.

Considering that the response speed of the hardware circuit will be much higher than that of the program, then this protection signal can also be introduced into the enable terminal of the driver IC of the MOS/IGBT, which can be turned off faster than the program.
If it is a price consideration, cheaper open-loop Hall devices can also be used, such as CHIPSENSE AN3V/AN1V/AS1V series current sensors, which have a response time of about 4us.CHIPSENSE current sensors have advantages in this regard.

4.AC output link: synchronous grid connection and power quality

The load of the frequency converter is usually used to supply power directly to electrical equipment, power supplies, etc. Therefore, an inductor needs to be added to reduce the PWM to an approximate sine wave to improve the power quality of the power supply.

III. Application challenges and coping strategies in special environments
Try to choose high-precision, low-temperature drift closed-loop Hall devices. The temperature drift of the device is less than 50ppm/℃, which can effectively avoid some false protection caused by temperature drift. A more reliable approach is to introduce the silicone rubber heating film/PTC heater commonly used in BMS systems. The program is used as the main control to control the start and stop of the heating film, and the mechanical thermostat is used as the secondary protection, which can effectively improve the operating reliability of the entire frequency converter device under the whole temperature environment. CHIPSENSE current sensors do well in these aspects.

Packaging: Devices are usually designed to meet IP67 requirements to better meet insulation voltage requirements

High current application scenario
The copper thickness of the PCB is commonly used as 0.5oz, 1oz, etc. Obviously, in order to flow through a large current, a larger copper cross-sectional area is required, so either widen the wiring or thicken the copper layer thickness. Or widen the line width and thicken the copper layer at the same time. For example, using 2oz and 5oz PCBs not only increases the cost significantly, but even if it is handled in this way, it is difficult to meet the reliability requirements by relying on the PCB's wiring to go up to 100A: the heat generated by the copper layer cannot be dissipated under the wrapping of the solder mask.
In these cases, Hall current sensors that can be threaded can be considered. Up to 10mm2 of copper wire/copper bar can easily flow through 100A, and only the corresponding pad vias need to be opened on the PCB. This is still a low-cost solution.

Conclusion
Voltage and current sensors are an important measurement element infrequency converters, and another measurement element is temperature sensors, whose importance is constantly being recognized by engineers. With the improvement of its topology, AC-AC frequency conversion is currently being experimentally applied, and silicon carbide devices (SiC) power devices are gradually introduced into thefrequency converter industry. The continuous improvement of switching frequency is bound to push the sensor industry to a new level. CHIPSENSE current sensors will keep growing.

CHIPSENSE is a national high-tech enterprise that focuses on the research and development, production, and application of high-end current and voltage sensors, as well as forward research on sensor chips and cutting-edge sensor technologies. CHIPSENSE is committed to providing customers with independently developed sensors, as well as diversified customized products and solutions.

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