Application of chip current sensor in solar inverter

In 2017, China's newly installed solar power capacity exceeded 53GW, ranking first in the world. Distributed photovoltaic power generation has received strong support, and the newly installed capacity has exceeded 19GW. At present, all links in the domestic photovoltaic industry chain are quite complete, with many manufacturers involved. The photovoltaic market is saturated. Under the situation of fierce competition, reducing the production cost of products while improving product reliability has become the main way for manufacturers to seize the market; the current sensor, as the core detection component in the photovoltaic grid-connected inverter, requires product stability while also taking into account high-precision electricity metering.

If the current sensor is classified according to its design principle, commonly used technologies include open loop, closed loop, flux gate, etc. Current sensors with different principles are usually selected according to different application scenarios to achieve corresponding functions.

The open-loop Hall current sensor is based on the direct measurement Hall principle. When the magnetic flux generated by the primary current is gathered in the magnetic circuit by the high-quality magnetic core, the Hall element is fixed in a very small air gap space at the magnetic circuit opening to perform linear detection on the change of the magnetic flux. After the Hall voltage output by the Hall device is processed by a special circuit, the secondary side outputs a follower voltage that is consistent with the primary waveform. This voltage can accurately feedback the change of the primary current.

The closed-loop Hall current sensor is based on the magnetic balance Hall principle, that is, the closed-loop principle (also called magnetic balance Hall). When the magnetic flux generated by the primary current on the primary side is concentrated in the magnetic circuit through the high-quality magnetic core, the Hall element is fixed in the air gap to detect the magnetic flux, and the multi-turn coil wound on the magnetic core outputs a reverse compensation current to offset the magnetic flux generated by the primary current, so that the magnetic flux in the magnetic circuit always remains zero; after processing by a special circuit, the output end of the sensor can output a signal (current output or voltage output) that accurately feedbacks the change in the primary current.

As we all know, the topology of a typical distributed photovoltaic inverter (as shown in the figure below) includes a DC input link (string input bus), a DC boost link (Boost MPPT line), a DC inverter AC link (DC/AC line), and an AC output link (leakage current detection). Current detection is essential in each link.

DC link open loop current sensor

At present, most manufacturers choose open-loop current sensors on the DC side (string current detection or DC/DC Boost line input current detection). Because the DC side current detection is only for measurement and does not participate in protection, the accuracy requirement is not very high. Usually, 1%-2% accuracy can meet the requirements. As for the shortcomings of temperature characteristics, the hard parameter indicators such as zero-point temperature drift and accuracy can be corrected and compensated through software algorithms. This helps the consistency of current sensors in use. In addition, the cost of open-loop current sensors is lower than that of closed-loop current sensors, so the advantages of open-loop sensors on the DC side are more obvious.

AC Link Closed Loop Current Sensor

At present, most domestic manufacturers use closed-loop sensors on the AC side, because the output of the AC side current sensor is generally used for software control. If the accuracy is too low, it will affect the detection and control of some key quantities. For example, the detection and extraction of DC components, although each country has different acceptance values ​​for DC components, it needs to be controlled at 0.5% or even 0.25% of the nominal output current, so only closed-loop sensors can meet the high-precision requirements.

At present, with the high integration of photovoltaic modules, the improvement of the process of new devices, and the progress of inverter manufacturers' R&D technology, the power of the single module of the photovoltaic inverter is getting bigger and bigger, and the power density is getting higher and higher. The selection of current sensors has also put forward higher requirements. In addition to having conventional electrical performance, it is also required to:

a) Small size, high insulation withstand voltage, high integration, easy for automated production

When the layout space for current measurement on the printed circuit board is relatively small, it is ideal to use a chip-type current measurement solution. The primary conductor is integrated and directly surface mounted on the printed circuit board, thereby reducing manufacturing costs and avoiding confusion among various welding processes. LEM's latest GO-SMS (left in the figure below)/HMSR-SMS (right in the figure below) series of current sensors are all chip-type current sensors in SMD packaging.

In addition to the small size, the primary and secondary pin design of GO-SMS also achieves 7.5mm creepage and electrical clearance distances, while the primary and secondary pin design of HMSR-SMS achieves 8.0mm creepage and electrical clearance distances. The package is built up with 600 CTI material, which gives it high isolation performance (test isolation voltage: 4.3kVrms/50 Hz/1min), and the HMSR chip current sensor is specifically used for solar energy systems with 1500Vdc DC input.

b) 10kA surge resistance

At present, the designs of inverter manufacturers generally have the risk of lightning surges on the lines directly connected to photovoltaic modules (PV panels) or power grids. In order to help manufacturers simplify the lightning protection design at the network port, HMSR-SMS fully considered the primary side surge resistance at the beginning of the design and designed a specially optimized primary conductor. When the primary side passes through a 10kA 8/20us lightning surge current, the chip can still work normally without any failure.

c) Built-in over-current protection alarm function

GO-SMS/HMSR-SMS chip current sensor can be used for peak current detection, for comparison between the actual value and the set point (protection point). The protection point can be set using built-in (factory default value) or external (user modified value), and outputs low-level effective alarm information through a dedicated OCD pin to notify the controller (DSP) of the generation of overcurrent signals so that the DSP can respond quickly to protect devices such as IGBT in the circuit.

In addition to the above features, LEM's chip current sensors can measure nominal AC, DC, pulse and mixed isolation currents with a measurement range as wide as ± 3 x Ipn and a bandwidth of 100kHz.

GO-SMS/HMSR-SMS chip current sensor design is based on the unique LEM open-loop Hall effect ASIC technology of the HG2 ORION platform. It is CE marked and complies with EN50178 standards. Compared with traditional discrete technology, it has a wider operating temperature range (-40 to +85°C), better offset and gain drift and linearity.

The GO-SMS/HMSR-SMS chip current sensor operates from a +5V power supply and provides a configurable reference voltage (2.5V). The gain and offset are fixed and set. The output voltage corresponding to the rated measurement current is equal to the input or output reference voltage ± 0.625 V. It can provide a rated current detection capability of 10-30A, which is very suitable for string current detection on the DC side and input current detection of DC/DC Boost circuits.