SBOA536 December   2021 INA240

 

  1.   Trademarks
  2. 1Introduction
  3. 2Grounding in DC Circuits
  4. 3Grounding in Isolated Current Sensing Applications
  5. 4Working Principle of Non-isolated Current Sense Amplifiers
    1. 4.1 Single or Multi-stage Difference Amplifier
    2. 4.2 Current Feedback
    3. 4.3 Switched Capacitor
    4. 4.4 Input Stage and Input Bias Current
  6. 5Grounding in Non-isolated Current-Sensing Applications
  7. 6Level Shifting for High-Voltage Current-Sensing Applications
  8. 7Grounding in Motor Current-Sensing Applications
    1. 7.1 Common-Mode Voltage of Motor Current Sense Amplifiers
    2. 7.2 Directionality of Motor Current-Sense Amplifiers
    3. 7.3 PCB Design for High-Performance Motor Drive
  9. 8Summary
  10. 9References

7 Grounding in Motor Current-Sensing Applications

Accurate phase current sensing is critical in motor control applications. A poor current sensor may lead to large torque ripple, audible noise, and inefficiency. This section explores the origin of input voltage spikes experienced by CSA in typical motor applications. Explanations of some of the commonly-asked questions are provided, such as why the CSA needs to be able to withstand negative common-mode voltage even when it is used in the low side of the inverter; or why the CSA needs to be configured as bidirectional. Due to switching noise, component placement and layout is an integral part of a good motor drive design, an example of how to minimize interference and contain switching noise is provided.

Motor phase currents are measured in the following ways:

  • DC link current, either high side or low side
  • Inverter leg currents, either high side or low side
  • Direct inline with the motor windings

Due to relaxed requirements for the CSA, low-side current sensing has been the most widely adopted topology, whether it is to sense the DC link (VBus) current or the inverter leg current. The current sensor can be either isolated or non-isolated. The selection is based on parameters such as performance, cost, and applicable end equipment standards. DC link voltage plays a large role in high-side current sensing, which ranges from a few volts to hundreds of volts. Figure 7-1 shows common current-sensing topologies in motor drive. Switches Q1 though Q6 make up the inverter output stage.

Figure 7-1 Common Motor Current-Sensing Topologies

Trapezoidal control of BLDC is a popular commutation method due to its simplicity. In trapezoidal control, the combination of the six switches defines six conduction zones. In each conduction zone one pair is turned on and rotates in the following order: (Q5, Q4), (Q1, Q4), (Q1, Q6), (Q3, Q6), (Q3, Q2), (Q5, Q2). The motor winding current in phase U is shown in Figure 7-2 for a complete electrical cycle. The other two phases are identical, except they are successively 120° out of phase.

Figure 7-2 Ideal BLDC Motor Phase Current

In conduction zones 2 and 3, phase U current flows through Q1 and can be measured by high-side CSA; in conduction zones 5 and 6, phase U current flows through Q2 and can be measured by low-side CSA. An inline CSA can measure the current in both cases.

Figure 7-2 shows the DC current in its ideal form, ignoring the effect of motor inductance and switching. In reality, the instantaneous change of current is impossible, instead the vertical lines will resemble ramps. There are also glitches due to commutation and PWM duty cycle if the phase current is being modulated. For the purposes of this article, such ideal approximation is sufficient.