High-precision isolated measurements: LEM LA 25-NP current sensors in photovoltaic inverters

This article provides a comprehensive expansion of the LEM LA 25-NP current sensor in PV inverters, covering its technical features, high accuracy and wide bandwidth advantages, signal conditioning and system integration, EMC and safety compliance, and a typical three-phase measurement solution, showing how the LA 25-NP meets the needs of high-efficiency MPPT, fault protection and grid-connection safety. protection and grid safety.

Product overview

The LEM LA 25-NP is a closed-loop Hall effect based PCB in-line current sensor that supports multiple current ratings of 5 A, 6 A, 8 A, 12 A and 25 A with a measuring range of ±36 A and an instantaneous output signal of 25 mA in a ratio of 1:1000. The devices operate over a temperature range of -40 °C to +85 °C, are supplied with a voltage of ±15 V, have a ±0.5% total error and very low temperature drift, have been tested with a 2.5 kV isolation voltage, and are UL 94-V0 rated for flame retardancy.

Principle of operation

The LA 25-NP utilises compensated closed-loop Hall technology: the magnetic field generated by the primary conductor is detected by the Hall element and then eliminated by the feedback coil to achieve a zero-field operating point, resulting in an excellent linear response and a very fast response time of less than 1 µs. This structure effectively eliminates core saturation and hysteresis, and at the same time provides a high degree of immunity to external disturbances, ensuring a high degree of immunity in high-frequency PWM environments. This structure effectively eliminates core saturation and hysteresis, and is highly immune to external interference, ensuring measurement accuracy and stability in high-frequency PWM environments.

PV Inverter Current Monitoring Requirements

PV inverters require real-time DC bus current monitoring to support the MPPT algorithm and closed-loop control of AC output current to ensure grid security and power quality. In transformerless inverter architectures, islanding and ground fault currents need to be detected, and fast response can improve system reliability and meet international standards.

Signal Conditioning and System Integration

Since the LA 25-NP output is a current signal, the system design needs to choose the appropriate load resistor to convert it to a voltage signal, and configure the RC low-pass filter network before ADC sampling to suppress high-frequency switching noise. It is recommended that the primary and secondary side alignments be routed separately, and that ground partitioning and shielding be added to the PCB to further reduce the impact of electromagnetic interference on measurement accuracy.

EMC and Safety Standards

The plastic parts of the LA 25-NP case meet UL 94-V0 certification and have passed the 2.5 kV isolation test, which meets the safety isolation requirements of the PV inverter for the high-voltage DC side and the low-voltage control side. The design should leave enough creepage distance and air gap, and follow the safety design specification of IEC 60664-1 and IEC 61010-1, to ensure the long-term reliable operation of the system under extreme working conditions.

Three-phase measurement programme

In three-phase grid-connected PV inverters, three LA 25-NPs are often used to detect the output current of A, B and C phases respectively, together with the corresponding LV 25-P voltage sensors, to achieve complete power, power factor and harmonic analysis. Synchronised sampling of current and voltage signals through the DSP platform enables multi-loop control and fault detection, and supports grid switching and harmonic suppression.

Typical Application Examples

In a three-phase PV inverter prototype based on TMS320F28335, the combination of LA 25-NP and LV 25-P is used to perform closed-loop measurements on the DC and AC sides, and experiments have confirmed that the scheme performs well in scenarios such as MPPT tracking, overload protection, and grid-connected fault switching, and that the system stability and conversion efficiency are significantly improved. The system stability and conversion efficiency have been significantly improved.

The LEM LA 25-NP, with its multi-range, high accuracy, wide bandwidth, fast response and high electrical isolation, provides an ideal hardware foundation for current monitoring and protection of PV inverters. Through reasonable signal conditioning and EMC design, it can ensure the long-term stability of the measurement performance in complex electromagnetic environments, helping to improve the efficiency and safety of PV inverter systems.

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