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Posted on 01 January 2019

Motor Control Current Sensing Applications

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Isolation Amplifiers with Shunt Resistors vs. Hall-Effect Devices

The current sensor is an essential component in a motor control system. Recent progresses in sensor technology have improved the accuracy and reliability of sensors, while reducing their cost. Many sensors are now available that integrate a sensor and signalconditioning circuitry into a single package

By Chew, Ming-Hian, Applications Engineer, Optical Communication Solutions Division, Avago

 

The three most popular isolated current sensors for feeding current information to a microcontroller or digital signal processor in motor control applications are:

• Isolation amplifier and shunt resistor
• Hall effect current sensor
• Current-sensing transformer

In this article, we’ll be discussing and comparing the first two types.

Isolation Amplifier and Shunt Resistor

Shunt resistors are the prevalent current sensors because they provide an accurate measurement at a low cost. The voltage drop across a known low-value resistor is monitored to determine the current flowing through the load. A circuit, such as an optically- coupled amplifier or level-shifting high voltage IC is then used to translate the differential voltage to a usable level while providing any required galvanic isolation between the resistor and control circuitry.

Figure 1 is a typical motor control block diagram using low-value shunt resistors for current sensing and a high-value parallel resistor as a voltage sensing element.

Typical motor control block diagram using resistors for both current and voltage sensing elements

One of the more difficult problems in designing a current shunt sensing circuit is providing either galvanic isolation or dynamic level shifting of a precision analog signal in an extremely noisy environment such as that found in a motor phase current. The difficulty arises from the large common mode voltage, the high degree of variability of the common mode voltage, and the transients that are generated by the switching of the IGBT inverter transistor. These transients are equal in amplitude to the DC supply voltage or higher, and can exhibit extremely fast rise times (greater than 10 kV/µs), making it extremely difficult to sense the current flowing through each of the motor phases.

Optical isolation amplifiers are not affected by external magnetic fields, and do not exhibit the residual magnetization effects that can affect offset in Hall effect current sensors. Optical isolation amplifiers can be easily mounted on a printed circuit board and can be very flexible in performance, allowing the same circuit and layout to be used to sense different current ranges simply by substituting shunt resistors.

Amplifiers using linear optocouplers can suffer from linearity drift over the operating temperature range and reduced operating life because of current transfer ratio (CTR) degradation over time. This can be overcome by using negative feedback circuits consisting of matched photodiodes in the input and output side of the amplifier to make the transfer function is virtually independent of any degradation in the LED output as long as the two photodiodes and optics are closely matched. Another approach uses a voltage-to-frequency converter (analog-to-digital circuit or ADC) at the input, with the optically-isolated path transmitting digital rather than analog information. On the other side of the isolation, a digitalto- analog converter (DAC) develops the proportional analog output signal.

A sigma-delta (also known as oversampling, or 1-bit) architecture consists of a 1-bit ADC and filtering circuitry, which oversamples the input signal and performs noise shaping to achieve a high-resolution digital bit stream, the average of which is directly proportional to the input signal. The sigma-delta converter is comprised of two op-amp integrators and a clocked comparator, driven by a high frequency non-overlapping two-phase clock at about 6 MHz. The 6 MSPS (million sam- ples per second) inherent in the operation of the sigma-delta converter eliminates the need for input sample\hold or track\hold circuits. A significant benefit of this coding scheme is that any non-ideal characteristics of the LED (such as nonlinearity and drift over time and temperature) have little, if any, effect on the performance of the isolation amplifier.

The advantage of using an Σ-Δ converter for analog-to-digital conversion is two fold:

1. The conversion accuracy is achieved mainly by virtue of the high sampling rate and is not very dependent upon IC process device matching.

The  Σ-Δ  modulator shapes amplifier noise to allow it to be efficiently filtered out.

Understanding Isolation Amplifier Parameters

Isolation amplifier specifications which are key for motor drive current sensing applications include:

Input-Referred Offset Voltage – this is the input required to obtain a 0 V output. All isolation amplifiers require a small voltage between their inverting and non-inverting inputs to balance mismatches due to unavoidable process variations. The required voltage is known as the input offset voltage (VOS).

Data sheets for  Σ-Δ amplifiers indicate another parameter related to VOS: the average temperature coefficient of input offset voltage. This parameter, | ΔVOSΔTA|, expressed in µV/°C specifies the expected input offset drift over temperature. VOS is measured at the temperature extremes of the part, and | ΔVOSΔTA| is computed as ÄVOS/µ°C.

Gain Tolerance – this is especially important especially in multiplephase drives where accurate gain tolerance is required for ensuring that precise phase-to-phase accuracy is maintained. For an isolated modulator such as the HCPL-7860/786J/7560, the important specification is the reference tolerance of the D/A, VREF.

Avago’s data sheets show another parameter related to G: the average temperature coefficient of gain. The average temperature coefficient of G, | ΔVOSΔTA|, with units of V/V/°C, specifies the expected gain drift over temperature. G is measured at the temperature extremes of the part, and | ΔG/ ΔTA| is computed as  ΔG/ Δ°C. For isolated modulators, it will be | ΔVREFΔTA|, with units of ppm/°C.

Nonlinearity – this gives an indication of the device’s accuracy over the input current range. It is the deviation of the device output voltage from the expected voltage expressed as a percentage of the fullscale output range. A smaller percentage is better (closer to perfectly linear).

Avago data sheets show another parameter related to NL: the average temperature coefficient of nonlinearity. The average temperature coefficient of nonlinearity, |ΔNL/ ΔTA|, with units of %/°C, specifies the expected nonlinearity over temperature. NL is measured at the temperature extremes of the part, and | ΔNL/ ΔTA| is computed as  Δ%/ Δ°C.

Common-Mode Rejection (CMR) – in electronic motor drives, there are large voltage transients generated by the switching of the inverter transistors. These transients are at least equal in amplitude to the DC rail voltage, and can exhibit extremely fast rates of rise (as high as 10 kV/µs), making it difficult to sense the current flowing through each of the motor phases.

Propagation Delay and Bandwidth – device speed should be fast enough to ensure that the input signal is accurately represented and system stability is not compromised. The device should also be fast enough to protect against a short circuit.

Accuracy of an Isolation Amplifier

The typical isolation amplifier has an overall accuracy of a few percent. There are a number of error terms that combine to create this error, both at a nominal temperature (+25°C) and across the operating temperature range.

The accuracy is limited by the combination of:
• DC offset at zero current
• Gain error
• Linearity
• Bandwidth limitations

Temperature changes also create drift in:
• DC offset
• Gain
• Linearity

Tables 1 through 3 demonstrate the performance of three  Σ-Δ isolation amplifiers (Avago parts are used as examples) and shunt resistors.

HCPL-7860 Isolation Amplifier and Shunt Resistor Performance

HCPL-7800A Σ-Δ Isolation Amplifier and Shunt Resistor Performance

HCPL-7510 Σ-Δ Isolation Amplifier and Shunt Resistor Performance

A Note on Shunt Resistor Selection

The selection criteria for a shunt current resistor requires the evaluation of several trade-offs, including:

• Increasing RSENSE increases the VSENSE voltage, which makes the voltage offset (VOS) and input bias current offset (IOS) amplifier errors less significant.
• A large RSENSE value causes a voltage loss and a reduction in the power efficiency due to the I2R loss of the resistor.
• A large RSENSE value will cause a voltage offset to the load in a low-side measurement that may impact the EMI characteristics and noise sensitivity of the system.
• Special-purpose, low inductance resistors are required if the current has a high-frequency content.
• The power rating of RSENSE must be evaluated because the I2R power dissipation can produce self-heating and a change in the nominal resistance of the shunt.

In order to maximize the accuracy of current measurement with isolation amplifiers, it is important to choose a shunt resistor with good tolerance, low lead inductance, and low temperature coefficient. Many resistor manufacturers offer such resistors.

Choosing a particular value for the current resistor us usually a compromise between minimizing power dissipation and maximizing accuracy. Smaller-value current-sense resistors decrease power dissipation, while a larger-value current-sense resistance can improve accuracy by utilizing the full input range of the isolation amplifier.

Two-terminal current-sense resistors are usually appropriate for lower-cost applications, while precision applications are better served with four-terminal resistors. Four-terminal current-sense resistors provide two contacts for current flow and two sense contacts for measuring voltage by making a Kelvin connection from the sense terminal to the isolation amplifier input. With a four-terminal current-sense resistor the voltage that is sensed is the voltage appearing across the body of the resistor (and not across the higher-inductance resistor lead.) Furthermore, four-terminal current-sense resistors typically have very low-temperature-coefficient and thermal resistance.

Hall Effect Current Sensors

Hall effect current sensors measure current flowing in a wire by measuring the magnetic field created by that current with a Hall effect IC and produce an output voltage (known as the Hall voltage). Hall effect current sensors are widely used because they provide a nonintrusive measurement. Several vendors offer devices that combine the magnetic sensor and conditioning circuit in a single package. These IC sensors typically produce an analog output voltage that can be input directly into the microcontroller’s ADC.

Generally, Hall effect current sensors can be classified as either open-loop or closed-loop. Open-loop Hall effect current sensors consist of a core to magnify the magnetic field created by the sensed current, and a Hall effect IC, which detects the magnetic field and produces a voltage linearly proportional to the sensed current. Like all ferromagnetic material, open-loop Hall effect current sensors have hysteresis error, which contributes significantly to offset error.

Closed-loop Hall effect current sensors integrate additional circuitry and a secondary winding nulling the flux and improving the accuracy significantly, but at a higher cost than open-loop versions. They also tend to consume substantial current from the secondary power supply (which must provide the compensation and bias current).

In general, the comparatively large profile and footprint of Hall effect current sensors poses a challenge for incorporation onto high-density circuit boards. The larger profile also means that auto-insertion is difficult or impossible with standard pick-and-place machines. A second disadvantage of Hall effect sensors is that their accuracy varies with temperature.

Accuracy of Hall effect Current Sensors

The typical Hall effect current sensor has an overall accuracy of a few percent. There are a number of error terms that combine to create this error, both at nominal (+25°C) temperature and across the temperature range.

The accuracy is limited by the combination of:
• DC offset at zero current
• Tolerance of measuring resistor, RIM (for closed-loop Hall effect current sensors)
• Gain error
• Linearity
• Bandwidth limitation

Temperature changes also create drift in:
• DC offset
• Gain
• Drift of measuring resistor, RIM (for closed-loop Hall effect current sensors)
• Linearity

Generally,  Σ-Δ isolation amplifiers and open-loop Hall effect current sensors are comparably priced and closed-loop Hall effect current sensors are relatively more expensive. The higher cost of closed-loop Hall effect current sensors is due primarily to the additional core winding and the flux-nulling servo-amplifier.

At room temperature, both closed- and open-loop Hall effect current sensors have better accuracy than isolation amplifiers. A comparison of over-temperature accuracy between Hall effect current sensor and isolation amplifiers reveals a pronounced performance difference.

Open-Loop Hall Effect Current Sensor Typical Performance

This is because isolation amplifiers do not share the same sensitivity to temperature that affects Hall effect current sensors.

With calibration, isolation amplifiers show a clear accuracy advantage. Hysteresis error on Hall effect current sensors is always present and cannot be calibrated.

 Closed-Loop Hall Effect Current Sensor Typical Performance

 

 

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