To correctly drive motor system speed and torque, it requires feedback accuracy and signal integrity to provide stability inside the motor’s dynamic range. This article compares three motor control measurement strategies — low-side, high-side, and in-line current sensing — to accurately capture motor phase data and recommends the appropriate approach to preserve the phase relationships of the three legs of the AC motor.
Modern motor control systems require feedback accuracy and signal integrity to correctly drive system responses such as speed and torque and provide stability inside the motor’s dynamic range (Fig. 1).
Fig. 1: AC brushless three-phase motor
When capturing motor information, there are three motor control measurement strategies: low-side, high-side, or in-line (also direct winding) current sensing (Fig. 2).
Fig. 2: The low-side, high-side, and in-line circuit options for motor speed and torque sensing
Let’s take a look at each of the three motor control approaches.
Low-side motor current sensors
The low-cost, low-side current-sensing application uses an amplifier and sense resistor (RL) at the bottom of the gate-driving FET stack. The low-side sensing system keeps the amplifier operation near ground and captures the ampere activity of each leg in succession.
As an advantage, the sense resistor (RL) and amplifier are close to ground, simplifying the circuit design. Additionally, a noninverting amplifier input at the top of the sensing resistor presents limited interference with its high non-inverting terminal input-impedance.
As a disadvantage, the current-sensing resistor, between ground and the motor legs, adds an undesirable dynamic voltage drop and disrupts the ground path. This added resistor, RL, produces an offset voltage that can cause electromagnetic interference (EMI) noise problems.
If the load becomes shorted to ground, the RL resistance removes the ability to detect faults in the load. The combination of the sense resistor and amplifier captures motor flyback and return currents. Unless a sense resistor and amplifier are used in each leg, the only way to calculate the winding currents is with the microcontroller (MCU) algorithm.
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High-side motor current sensors
The high-side resistor, directly connected to the FET driver positive supply source, minimizes the resistor’s dynamic AC voltage impact on the entire system. This configuration is less intrusive, generating minimal EMI characteristics. Additionally, a high common-mode-rejection-ratio (CMRR) differential amplifier filters pulse-width–modulator (PWM) switching noise.
The high-side approach requires that the circuity copes with large common-mode signals at the FET network supply voltage. This requirement necessitates a robust amplifier that handles potentially high voltages. Finally, the circuit position of RH makes flyback and winding current measurements impossible.
In-line motor current sensors
The in-line (or direct winding) is the preferred approach because it provides the true, current-phase information. In the direct winding configuration, the system processor knows the current in each phase of the three-phase motor at any given time, resulting in a more efficient motor operation (Fig. 3).
Fig. 3. In-line current sensing for motor control
In Fig. 3, all three legs are simultaneously sampled by the MCU to preserve the phase relationships between each leg excitation.
An ideal amplifier for in-line measurements amplifies each motor leg’s differential signal, with a rejection of the PWM common-mode transients. Amplifiers with a fast settling time and high bandwidth can reject common-mode transients. Strong PWM rejection facilitates the fastest settle time and higher accuracy and enables the customer to minimize the PWM duty cycle as close to 0% as possible.
For rugged motor control systems, the recommendation is to use an in-line, fast-settling-time, high-bandwidth amplifier that rejects common-mode transients. One example is the 300-kHz MAX40056, a bidirectional current-sense amplifier with a high-AC PWM edge, 60-dB rejection at 50 V, and ±500 V/µs (Fig. 4).
Fig. 4. Current-sense amplifier (CSA) with patented PWM rejection circuitry
In Fig. 4, there are multiple gain options of 10 V/V, 20 V/V, and 50 V/V. The internal reference provides a 1% bidirectional offset.
This device has a 500-ns PWM edge recovery from 500-V/μs and higher PWM edges. The input common-mode voltage range is –0.1 V to 65 V, and the amplifier is protected from –5 V to 70 V. The MAX40056 and competitor bench data illustrate a significant difference in the PWM common-mode immunity (Fig. 5).
Fig. 5: PWM edge rejection of a 50-V PWM cycle.
In Fig. 5, the MAX40056 CSA’s analog output shows a minor bump and recovers within 500 ns, while the competing device requires approximately 2 µs to recover. The CSA’s patented PWM rejection input suppresses transients and provides a clean differential signal measurement.
Conclusion
After comparing three motor control strategies, the recommended approach for motor control sensing is in-line current sensing. It provides robust current-sensing circuitry that preserves the phase relationships of the three legs of the AC motor.
About the authors
Maurizio Gavardoni is a senior principal member of technical staff at Maxim Integrated. He has many years of experience as a product definer and business manager. He has defined and brought to market several successful products in signal chain, sensors, and timing. Prior to Maxim Integrated, he worked in similar positions at Texas Instruments and Microchip. He has an MSEE from the University of Milano, Italy.
Bonnie Baker is a principal writer at Maxim Integrated. She is a seasoned analog, mixed-signal, and signal-chain professional and electronics engineer. Baker has published and authored hundreds of technical articles in industry publications. She is also the author of “A Baker’s Dozen: Real Analog Solutions for Digital Designers,” as well as coauthor of several other books. In past roles, she worked in modeling, strategic marketing, as an IC architect, and as a designer engineer. Baker has a master’s degree in electrical engineering from University of Arizona, Tucson, Arizona, and a bachelor’s degree in music education from Northern Arizona University (Flagstaff, Arizona). She has also planned, written, and presented hotel and online courses on engineering topics, including ADC, DAC, operational amplifier, instrumentation amplifier, SPICE, and IBIS modeling topics.
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