Abstract: This paper mainly introduces the structural principle and characteristics of closed-loop current sensors based on ASIC technology and analyzes and explains the application advantages and examples.
Keywords: ASIC technology, closed-loop current sensor, magnetic field compensation, suppression characteristics, electrical isolation, monitoring and regulation
1. Best solution for popular current isolation measurement
Due to its innovative design concept, the closed-loop current sensor based on ASIC technology has become the best solution for current isolation measurement in the industry, and is widely used in drivers, high-frequency and high-current measurement, UPS and power electronics industries, making it a product that is in sync with the development of the power electronics industry and the needs of users. Here, we take LEM's new LTSR product series as an example to analyze and introduce the characteristics and applications of its closed-loop current sensor based on ASIC technology.
The LTSR product series allows for the measurement of currents rated at 6A for the LTSR-6-NP, 15A for the LTSR-15-NP, and 25A for the LTSR-25-NP. The "R" in LTSR stands for reference terminal.
The goal of the LTS25-NP series is to find a good method for isolated measurement in the field of power electronics. Since digital devices and processors are widely used in the field of power electronics, and these devices are powered by single-ended 5V, LTSR is also powered by single-ended 5V. Therefore, with the LTSR sensor, we can get closer to the field of power electronics.
LTSR can provide reference voltage for surrounding components, and can also accept external voltage as reference voltage, such as DSP, ADC and many very common electronic components.
2. Basic structure
LTS-25-NP is a very small current sensor designed for PCB installation. From the perspective of new technology and new process, the development and production of this product is a challenge.
The LTSR series closed-loop current sensors are developed based on the basic structure and principle of closed-loop principle sensors (see Figure 1). Therefore, to analyze whether LTSR is a closed-loop principle sensor, it is only necessary to use Figure 1 as an illustration of the basic structure and principle.
LTSR is a closed-loop sensor based on the principle of primary and secondary magnetic field compensation. That is to say, the magnetic flux generated by the measured current is sensed by the HALL element in the sensor, and the HALL element generates an induced current, which is supplied to the secondary coil, and then generates an opposite magnetic flux in the magnetic core to compensate for the magnetic flux generated by the primary. The numerical value of the primary current is reduced in proportion to the actual value of the current in the coil. The basic structure and principle of the closed-loop sensor are shown in Figure 1.
The design concept of the product is based on the following standards, that is, small size, 5V single-ended power supply sensor. The product is small in size, and the corresponding magnetic material is also relatively small, which leads to a relatively short distance between the primary and secondary sides of the sensor. This structure is conducive to the sensor's ability to resist high dv/dt. This high-frequency suppression characteristic dv/dt (see curve 2-purple line in Figure 2) is very suitable for motor control applications. Because in motor control, in the three-phase load of the motor, high dv/dt will be generated due to the continuous conduction and shutdown of the bridge arm (see curve 1-yellow line in Figure 2).
Electrical components with high insulation between the primary and secondary sides can couple and isolate the potential. In high-frequency switching applications, there will be very high-frequency and very steep spikes (such as high-speed changes in the primary voltage), which will cause unnecessary electromagnetic interference (EMI). In this case, an interference signal will be generated on the secondary output components. For example, a 10kV/μs voltage change will generate a parasitic current of 100mA through a 10pF coupling capacitor. For the LTSR series, this is 8 times the rated output current.
Note that the peak value in the figure is IPN l5.3% interference (purple line 2), which is mainly caused by the wiring of the test device. Note that the disturbance delay is less than 800ns in the figure, which can be filtered out (the repetitive settling time is 1.6μs), which is very beneficial for PWM modulation of digital control circuits. In this case, a small filter can achieve the purpose without reducing the dynamic characteristics.
It can be seen that LTS25-NP is the first current sensor to use ASIC (Application Specific Integrated Circuit) technology. Its unique feature is that the magnetic field sensing element is integrated in one layer, so that the change of the Hall lattice due to temperature drift can be partially compensated. The working principle diagram of the closed-loop current sensor LTSR series based on ASIC technology (taking LTSR25-NP as an example) is shown in Figure 3 (a) and the appearance is shown in Figure 3 (b).
3.1 Has better temperature drift characteristics
The new generation of ASIC technology is based on silicon technology, which is different from the first generation of ASIC technology products LTS. This technology has smaller temperature drift characteristics than the first generation. This is a very important factor for the control loop built into the sensor: the smaller the drift, the better the stability of the control loop.
The temperature range of LTS25-NP is -40℃ to +85℃. Within this range, the maximum bias drift we can accept is 100ppm/℃. In the same temperature range, the structure of LTSR can reduce the bias drift to 37.5ppm/℃.
The new ASIC technology can also improve the current capability of the drive coil and prevent the output voltage from dropping to the short-circuit detection area. For example, 
if the primary current is 10 times the rated current and the reference voltage is 2.5V, the output voltage will not drop by 2V and will be lower than 0.5V, 
thus not triggering the short-circuit detection.
3.2. The reference voltage pin (REF) of LTSR has two basic operating modes.
LTSR is a closed-loop sensor, but it has an additional reference voltage pin (REF). This pin is the reference voltage channel of the ASIC and is generally set to 2.5V (see Figure 3(a)). The reference pin has two basic operating modes.
The first mode is the reference output mode (see Figure 6). In this mode, the primary current is 0A and the output voltage is consistent with the reference pin voltage (with a maximum deviation of ±25mV between the output and the reference point). Changes in the primary current will not cause changes in the reference voltage.
The second mode is the reference input mode (see Figure 7). In this mode, an external voltage can be provided to the reference pin as a reference and drive the internal reference. The allowed voltage is between 1.9V and 2.7V. The source can sink or source a minimum of 1mA. This is necessary to ensure that the external reference voltage drives the internal reference voltage.
When the temperature is 25°C and the reference current Iref is equal to 0, the internal reference voltage is equal to 0. The reference voltage is related to the following conditions:
*In reference output mode, the reference current Iref is determined by the connected load. To ensure the reference point is 2.5V±25mV, we apply a minimum load of 220kΩ to ensure Iref, minimum. In reference output mode, the source current should be higher than -125pA. Voltage drop will occur below this value.
*When the current is provided by an external reference (reference input mode). In reference input mode, the external reference source can absorb or provide a current that matches the voltage, Vref=f(Iref). (When the external reference is greater than the internal reference voltage Vref=2.5V±25mV or the external reference voltage is less than the internal reference voltage Vref).
Using
the external reference mode, interconnection of the sensor with other devices (such as ADC) is very simple (see Figure 7). Based on the ASIC structure, this feature will have some impact on the measurement range. For example, if the reference point is 1.9V, the measurement range of the reverse current will be 0.6V less than provided, and the same is true for the forward current.
3.3. Wide frequency band
The excellent coupling characteristics are also reflected in the bandwidth characteristics, see the curve in Figure 4. The bandwidth variation of LTSR is about 0.3dB. Due to the influence of the device and Hall components, the response bandwidth variation reaches a peak when the input current frequency is 100kHz. Until now, the bandwidth variation of LEM closed-loop current sensor is 3dB from 100kHz to 200kHz.
3.4. Accurate and fast response characteristics
Fast power switching device: IGBT requires very fast overcurrent detection as protection for IGBT. For a current change of 30A/μs, the secondary output will not actually have any delay in the primary current change. Due to the excellent 
coupling , the LTSR has such a good performance.
3.5. Excellent accuracy and temperature stability
The overall accuracy of the LTSR series current sensors is better than ±0.2% at 25°C. The overall accuracy includes all tolerances of the sensor, such as linearity error, coil error and long-term stability.
Compared with most closed-loop current sensors that output current values, the LTSR series current sensors are current sensors with built-in measuring resistors. The built-in resistors are selected to have an accuracy of ±0.5% and a temperature drift of up to 50ppm/K.
The internal reference point has a stable temperature performance of 50ppm/K at (-10℃+85℃) and 100ppm/K at (-40℃…-10℃). In most cases, the absolute accuracy of the reference point is not very important for the LTSR series because of external auxiliary circuits or digital processor control (reference output mode). (Similar situation also lies in the zero drift indicator)
Although the external reference voltage is provided by the user's external circuit, the voltage drift is controlled by the processor. In this case (reference input mode), you can provide the sensor with a reference voltage between 1.9 and 2.7V, but this voltage is lower than the control capability of the processor, so the voltage offset must be corrected, and the thermal offset must also be corrected.
3.51 For LTSR25-NP, the output voltage Vout has the greatest temperature stability at -10℃…+85℃
*Vout is biased at 37.5ppm/K relative to Vref over the operating temperature range.
* Relative to 0V, Vout is biased at 87.5ppm/K over the operating temperature range and 137.5ppm/K from -40°C to -10°C.
87.5ppm/k is due to the deviation of Vout from Vref (37.5ppm/k) plus the deviation of Vref from 0V, 50ppm/K.
137.5ppm/K is because the Vref bias is equal to 37.5ppm/K in the operating temperature range, and 100ppm/K between -40℃ and -10℃.
As explained before if the controller can compensate for the drift of Vref, the offset and the initial offset of Vout at Ip = 0, you can improve the accuracy. Increasing the tolerance over the operating temperature range will give you a higher accuracy.
4. Main parameter characteristics
Table 1 shows some main parameter characteristics of LTSR current sensor.
The power supply voltage is 0 to 5V, which matches the supply voltage of many controllers. Compared with previous closed-loop current sensors, the measurement range of LTSR can be 3 times the rated current. This improvement is very beneficial to the application. The maximum measurement range of the LTSR25-NP series is 80A at a rated current of 25A. The reference point is 2.5V (reference input mode and reference output mode), which is exactly half of the power supply rail voltage. The amplifier changes the range: 0.625V/IPN, so the output voltage is 4.5V at +80A and the output voltage is 0.5V at -80A (in reference output mode). The current sensor meets the relevant standards in the field of power electronics.
5. Application of LTSR series closed-loop current sensor based on ASIC technology
5.1 Wide application range and flexible device selection
The LTSR series sensors have more additional topological functions than the previous generation products. In many application environments, the output of the sensor can be easily connected to the ADC and the data can be input into the DSP or other microcontroller.
LTSR is designed to be directly connected to a 5V powered DSP, but currently some DSPs or ADCs are powered by 3.3V. The internal reference point voltage of these 3.3V powered chips is 1.8V. In this case, the internal reference voltage of the DSP can be used as the reference voltage of the sensor. This connection method can eliminate reference temperature drift.
In addition, the reference voltage inside the sensor can be used to provide a reference voltage to the ADC. That is, it is also possible to use three sensors to connect the reference pins together. The reference point voltage will take the average value of the three internal reference voltages. See Figure 5 for the reference output mode of the LTSR, which shows a typical application diagram of the sensor in the reference output mode, providing a reference signal to the ADC as a reference signal.
This connection method can also be applied to the reference input mode. See Figure 6 for a schematic diagram of the LTSR reference input mode and ADC connection. It demonstrates that the reference input function of the applied sensor can be used to synchronize several sensors so that the reference voltage is at the same level. Since the 47nF load capacitor at the end of the LTSR may cause the voltage of the external amplifier to rise, the output resistor of the amplifier in Figure 6 is 10Ω to avoid the increase of the amplifier output voltage.
Figure 7 shows the reference output connected to a differential amplifier to eliminate the output offset. In addition, the circuit zero reference output is also very convenient for connecting to a double-ended power supply processor.
5.2. Various input methods of primary circuit
LTSR has three U-shaped primary connection terminals and an additional primary threading hole, which provides designers with a flexible and diverse range of options for current measurement. Figure 8 shows the internal connection methods of LTSR. The first mode: parallel connection. This can measure the maximum primary current.
The second mode: series connection. Although the measurement range is reduced, the accuracy is improved by three times when measuring small currents.
The third mode: micro-current measurement. Measure different currents I1-I2. I2 is the current flowing through the hole, and there is a certain gap distance from the printed circuit board. The induction measurement value is different depending on the current direction.
5.3 Application Examples
5.31. Electrical isolation current measurement in frequency converters
LTSR is typically used in classic frequency converters. Due to its excellent accuracy and dv/dt suppression, LTSR is also very suitable for servo drive applications. Figure 9 shows the application of electrical isolation current measurement in frequency converters. Its advantages are: excellent linearity, very suitable for motor current measurement; fast response of short-circuit protection, can protect short circuit and leakage; good temperature stability, suitable for accurate and repeatable measurement and strong capacitance current change suppression ability for long-distance motor wiring.
5.32 General current monitoring and regulation applications:
*The LTSR series can be used wherever current needs to be accurately monitored and regulated. This application is suitable for AC measurement systems.
*Non-linear loads generate non-sinusoidal waves including square waves.
The LTSR series sensor is also suitable for this application because it can measure AC or DC. LTSR can also be used in DC devices such as power supplies, battery devices or DC drives.
5.33 It should be noted that the LTSR series has the following advantages over shunts: lower energy loss in high-frequency and high-current measurements; electrical isolation; and better EMI characteristics.
6. Conclusion
6.1 Summarize the advantages of all LTSRs
Single-ended power supply 0/5V, can measure positive and negative current; can output internal reference voltage Ref output mode; external power supply provides reference voltage to the sensor, that is, Ref input mode; high temperature stability and small temperature drift; multi-range measurement concept can make the same sensor device cover more primary current measurement spans; low energy loss; closed-loop principle provides excellent linearity, wide frequency range, fast response time, wide measurement range and the ability to measure high-frequency current pulses; easy to install; competitive cost control solution.
6.2 Application
LTSR can be applied to low power electronic systems such as inverters, industrial heating drives, ventilation and air conditioning units and many other industrial applications such as servos, small UPS, power supply and amplifier energy management systems, forklifts and general current monitoring.
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