Application advantages of digital isolators in industrial motor drives
Electronic controls used in industrial motor drives must provide high system performance in harsh electrical environments. The power supply circuit causes voltage spikes on the motor windings, and these voltage edges can be capacitively coupled into the low voltage circuit. In power circuits, the non-ideal behavior of power switches and parasitic components can also generate inductively coupled noise. The long cable between the control circuit and the motor and sensor forms a variety of paths that couple noise into the control feedback signal. High-performance drivers require high-fidelity feedback control and signals that must be isolated from high-noise power circuits. In a typical drive system, an isolated gate drive signal is included to drive the inverter, current and position feedback signals to the motor controller, and to isolate communication signals between the various subsystems. When signal isolation is achieved, the bandwidth of the signal path must not be sacrificed and the system cost must not be significantly increased. Optocouplers are the traditional method of achieving safe isolation across the isolation barrier. Although optocouplers have been in use for decades, their deficiencies can affect system-level performance.
The widespread use of variable speed motor drives in industrial applications is due to efficient power switches and cost-effective electronic control circuits. The design difficulty is to couple high-power switching circuits with low-voltage control circuits without sacrificing noise immunity or switching speed.
Modern switching inverters typically have efficiencies in excess of 95%, and the power transistor switches used can also be connected to the motor windings between the high and low rails of the high voltage DC rail. This process reduces the loss of the inverter because the power transistor operates in full saturation mode, which reduces the voltage drop and power loss during conduction. There is also additional power transistor loss during the switching process because during this time there is a large voltage on the transistor while the load current is switching between high and low power devices. Power semiconductor companies have designed transistors with shorter switching times, such as IGBTs, to reduce this switching power loss. However, this higher switching speed also introduces some useless side effects, such as increased switching noise.
