SBOA536 December 2021 INA240
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:
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.
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.
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.