1 System Description

Isolated current sensing is vital in 110–690V_AC input 3-phase inverters for applications such as AC inverters and variable speed drives. The current is sensed in many subsystems, motor drive systems such as the motor phase currents, the DC-link current or the brake current as outlined in Figure 1-1. Accurate phase current sensing, for example, has a significant impact on the performance of vector-controlled three-phase inverters for industrial drives. The DC-link and brake currents are often measured to detect an overcurrent or short-circuit event due to a fault or miswiring in the system and a fast response is critical to turn off the related power switches and prevent further damage. Additional current sensing is important for diagnostics, monitoring, and predictive maintenance, such as analyzing the motor current harmonics for bearing deterioration or motor temperature estimation or input power monitoring.

Figure 1-1 Current Sensing Options in 3-Phase Inverters

For current ranges up to around 100A, in-package Hall-effect current sensor ICs, like the TMCS1123 and shunt-based designs using either isolated amplifiers like AMC1300 and isolated modulators like AMC1306 are often used.

In-line shunt-based designs offer a highly-linear, highly-accurate, and ultra-low-noise option to measure the motor currents with three-phase inverters. However, these designs require a high-side floating supply, which is not always available in the system and the power losses of the shunt limit the maximum continuous current range.

In-package Hall-effect current sensors do not need a high-side floating supply and offer inherent isolation. These sensors have very low conductor resistance such as 0.67µΩ with the TMCS1123, an as well as inherent isolation. Often Hall-effect current sensors like the TMCS1123 provide low propagation delay and include a very fast overcurrent protection; therefore, offering a single-chip analog current sense option. Conversely, the signal-to-noise ratio and effective number of bits achievable in the system is typically lower with Hall-effect sensors than with shunt-based designs.

In all three-phase inverter systems where the high-side DC-link current needs to be measured and monitored for overcurrent, the TMCS1123 device does not need an additional isolated high-side supply referenced on top of the VDC voltage nor require a shunt. Hence, using this device solves system cost and fast overcurrent detection with an overcurrent range up to 2.5-times the full-scale input current range.

In applications such as AC inverters and variable speed drives, the TMCS1123 reinforced isolated in-package Hall sensors help reduce system cost for phase current sensing and overcurrent detection. The ultra-low propagation (600ns) of the TMCS1123 enables higher bandwidth current control algorithms with faster torque response such as direct torque control, hysteresis control and fast current loop (FCL) as shown in Figure 1-2. The fast current loop (FCL) reduces the total current control loop delay by a factor of 3 thus enabling a higher bandwidth closed-loop current control with faster response times.

Figure 1-2 Fast Current Loop vs Traditional Current Loop Control With Double PWM Update