How to Extend the Performance of Low-Voltage Precision Op Amps for High-Voltage High-Side Current Sensing Applications
Foreword
High-side current sensing applications often require specialized devices capable of handling extended common-mode voltages. However, these dedicated solutions come with their own limitations. What happens when the common-mode voltage exceeds 100V? Can accurate current measurement still be achieved? At first glance, classic 5V op amps seem unsuitable for such tasks. But with just a few external components, it's possible to use low-voltage amplifiers effectively for high-precision current detection without any common-mode voltage restrictions.
Schematic and Description
The main objective of this application is to measure the current controlled by an industrial motor operating at 150V, as shown in Figure 1. To accurately measure low currents, a 5V precision op amp is used.
Figure 1: Typical Application
Will the 150V input damage the op amp? If the V1 voltage is used to generate the positive supply (Vcc_H) for the first operational amplifier (OP_A), it won't. A Zener diode (BZT52C4V7S) with a breakdown voltage of 4.7V can be used to generate the negative power supply (Vcc_L) for OP_A. In this case, the voltage across OP_A is 4.7V, ranging from Vcc_L = 145.3V to Vcc_H = 150V.
Resistor Rz is used to bias the Zener diode (~5mA) and provide a path for the op amp’s bias current (~40μA). The voltage Vsense is generated by the current flowing through Rsense and is then amplified by resistors R1, R2, R3, and R4.
The precision output current from the P-MOSFET (BSP2220) is proportional to the current through Rsense. The voltage Vo relative to ground, generated by resistor R4, is proportional to the high-side current. The first-stage voltage output can be calculated using Equation 1:
Equation 1
The second operational amplifier (OP_B) is used to buffer the Vo voltage. Resistor R5 is added to protect OP_B’s inherent ESD diodes from potential high-current damage during startup.
The maximum current consumed by the motor control is 100A. With a shunt resistor of 100μΩ, the maximum Vsense is 10mV. The maximum output voltage Vo depends on the Vsense and the current through R4. Since the signal is processed by the microcontroller’s ADC, the maximum output voltage must not exceed 3.3V.
To ensure proper system operation, component values must be carefully selected. The primary goal is to maintain a low |Vgs| to prevent the output of OP_A from saturating. Keeping the drain current (Ids) low, we chose a high value for R4.
To avoid op-amp saturation, the gain associated with OP_A, given by the R2/R1 ratio, should not be too high. A balance must be struck when choosing component values, following Equation 2:
Equation 2
Where Vgmax is the gate-source voltage required for the transistor, and Vzener = Vcc_H - Vcc_L.
Now, let's consider the system's accuracy. Inaccuracy mainly stems from resistor mismatch and amplifier offset.
Error Analysis
Resistance Mismatch
Assuming perfectly matched resistors, Equation 1 gives the output voltage. However, real-world resistors have tolerances. The gain error due to resistor mismatch is given by:
Equation 3
Where εR is the precision of any resistor, and εRshunt is the accuracy of the shunt resistor. From Equation 3, it's clear that R2 has the most significant impact on error. Therefore, R2 should be as low as possible (e.g., 10kΩ). Additionally, to achieve the desired gain, the sum of R1 and R3 should be high and unbalanced, while R1 should be kept low to minimize noise.
Influence of Vio
Another source of error is the input voltage offset. In this application, the TSZ121 chopper amplifier was chosen due to its very low Vio, only 8μV over temperature variations. This becomes particularly important when measuring small currents.
Considering the transfer function, Vio can be expressed as:
Equation 4
Where Vio1 is the input offset of the first op-amp (OP_A), and Vio2 is the input offset of the second op-amp (OP_B). Since the TSZ121 has a very low input offset voltage, Vio2 is negligible.
Total Error
To understand the total output error, both resistor mismatch and op-amp offset must be considered. Finally, the output voltage can be expressed by Equation 5:
Equation 5
The graphs in Figures 2 and 3 show the expected maximum error as a function of temperature, considering the shunt resistor's accuracy.
Conclusion
Dedicated amplifiers are typically used for high-side current sensing. However, in applications where the common-mode voltage exceeds 70V, conventional 5V op amps can also be effective. We've demonstrated that a precision op amp like the TSZ121 can detect high-side currents when combined with a Zener diode and level-shifting transistors.
We've accounted for errors caused by resistors and amplifiers. To improve current measurement accuracy, we recommend using resistors with a precision of 0.1%.
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