AC motor drives use digital isolators to distance themselves from traditional solutions

Optocouplers use light from an LED to transmit data across an isolation barrier to a photodiode. When the LED turns on and off, logic high and low signals are generated on one side of the electrically isolated photodiode. The speed of an optocoupler is directly related to the speed of the photodiode detector and the time it takes to charge its diode capacitance. One way to increase speed is to increase the LED current, but this comes at the expense of increased power consumption.

Transformer-based digital isolators use transformers to magnetically couple data across the isolation barrier. Transformer current pulses through one coil, creating a small local magnetic field that induces current in the other coil. Transformers are naturally much faster than optocouplers. And transformers are differential, giving them excellent common-mode transient immunity. Also, because digital isolators are transformer-based and optocouplers are LED-based, digital isolators have much better reliability/MTTF than optocouplers.

Isolation in Motor Drive Design

Figure 1 shows a block diagram of a high voltage FlexMC motor control drive developed by Boston Engineering Corporation . It accepts a universal AC input, provides a power factor correction (PFC) front end, drives a permanent magnet synchronous motor (PMSM), and provides the necessary feedback conditioning for a sensored or sensorless control. In the middle is an isolation barrier between the high voltage power electronics and the controller. The motor power electronics float with a high voltage potential, while the controller is referenced to earth ground, so isolation is required.

In a closed-loop motor control design, two key hardware components are the pulse width modulation (PWM) controller output and the motor phase current feedback. These signals (as shown in the block diagram) pass through the isolation barrier. In addition, the use of isolators can also benefit several other functions, including digital communication and low voltage, low power and isolated dc-dc conversion.

PWM Isolation

Pulse width modulation of the power stage is at the heart of any motor drive. The switching frequency range is typically 10 kHz-20 kHz. Precise control of pulse width, dead time, and inter-channel delay is critical to optimizing control performance. When choosing the appropriate isolation device for the PWM control signal, digital isolators are far superior to comparable optocoupler options in both performance and cost (see comparison in Table 1).

For example, the controller will introduce dead time between switching signals to prevent any high-side and low-side transistor pairs from being on at the same time (i.e., shoot-through). Dead time is a function of the turn-on and turn-off delays of the power switches and the uncertainty of the delays introduced by the isolation circuit. The channel-to-channel matching time of the ADuM1310 iCoupler digital isolator is only 2ns, while the optocoupler is up to 500ns. The use of digital isolators can significantly reduce dead time, thereby improving the performance of the power inverter. In addition, as shown in the comparison table, in addition to performance, the ADuM1310 is also a more integrated solution, which can reduce component count and bill of materials cost.

Motor phase current

Most advanced motor drives use motor phase current as the primary feedback. To provide continuous feedback, ultra-low resistance shunt resistors are connected in series with the motor phases. However, this increases the complexity of the circuit because it is necessary to measure millivolt-level signals and common-mode voltage swings in the hundreds of volts that switch at high frequencies with fast dv/dt. For this design, two isolated _-_ modulators (i.e., AD7401A) are used to measure the motor winding currents, and the digital bit stream is processed by digital filtering circuits on the motor control IC. The third phase current can be mathematically calculated based on the other two phase currents to reduce power consumption and component cost. The AD7401A integrates a differential sample-and-hold stage, a modulator, and digital isolation in a single package. The analog signal on the high voltage side is converted into a digital serial data stream and then transmitted across the isolation barrier to the low voltage side. The AD7401A also contains a clock input pin, which only requires a single clock source to measure each device simultaneously. As shown in Table 2, there are optocouplers on the market with similar levels of integration and cost; however, digital isolator technology still excels in terms of power consumption, speed, and reliability, which is related to the basic structure of the device, not to mention the excellent modulator performance of the AD7401A.

Digital Communications

I2C is a two-wire, multi-drop communication interface that is typically used to provide digital or analog I/O expansion capabilities to a controller. This approach is typically reserved for “general housekeeping” type functions that are periodically monitored or updated. The FlexMC high voltage board uses an I2C interface to communicate with the PFC controller while simultaneously monitoring the bus voltage, bus current, and IGBT temperature with an ADC. The ADuM1250 allows the controller to monitor all of these functions on the high voltage side using only a two-wire peripheral interface through an isolator. In contrast, no optocoupler can provide I2C isolation capability on its own. As a result, as shown in Table 3, the ADuM1250 is a more advantageous I2C isolation option than optocouplers in terms of cost, size, component count, and performance.

Isolated Power Supply

Another benefit that digital isolator technology brings to this design is the ability to generate very low levels of isolated power. Two ADuM5000 devices are used to generate 5V isolated supplies with a maximum power output capability of 500mW. These are used to drive the analog side of the _-_ converter, which is floating relative to the rapidly changing motor voltage. These isolated supplies use the same technology as the data isolators, so they all have a built-in transformer with a switching frequency of 180MHz. This frequency is three orders of magnitude higher than standard DC-DC converters, allowing for significant size reductions. The ADuM5000 devices are packaged in SOIC-16 and are a simple solution for providing low power isolated voltages.