Electronic controls used in industrial motor drives must provide high system performance in harsh electrical environments. Power supply circuits can cause voltage spikes on motor windings, which can capacitively couple into low-voltage circuitry. Non-ideal behavior of power switches and parasitic components in the power supply circuits also generates inductively coupled noise. Long cables between the control circuitry, the motor, and sensors create multiple paths that can couple noise into the control feedback signal. High-performance drives require high-fidelity feedback control and signals that must be isolated from noisy power supply circuitry. Typical drive systems include isolating gate drive signals to drive inverter, current, and position feedback signals to the motor controller, as well as isolating communication signals between subsystems. Signal isolation must not sacrifice signal path bandwidth or significantly increase system cost. Optocouplers are a traditional method for achieving secure isolation across isolation barriers. Although optocouplers have been used for decades, their limitations can impact system-level performance.
The widespread use of variable speed motor drives in industrial applications is attributed to efficient power switches and cost-effective electronic control circuits. The design challenge lies in coupling high-power switching circuits with low-voltage control circuitry without sacrificing noise immunity or switching speed.
Figure 1. Inverter circuit including parasitic elements
Modern switching inverters typically achieve efficiencies exceeding 95%, and the power transistors used can connect the motor windings between the high-voltage DC rail and the low-voltage rail. This process reduces inverter losses because the power transistors operate in full saturation mode, which reduces voltage drop and power loss during conduction. Additional power transistor losses occur during switching because a significant voltage is present on the transistor during this period, while the load current switches between high- and low-power devices. Power semiconductor companies have designed transistors like IGBTs with shorter switching times to reduce these switching power losses. However, this higher switching speed also introduces some undesirable side effects, such as increased switching noise.
On the driver control side, continuous advancements in VLSI technology have improved the cost and performance of mixed-signal control circuits, paving the way for the widespread application of advanced digital control algorithms and increased efficiency in AC motors. The trade-off for this performance improvement is a reduction in IC operating voltage from 12V to 5V to the current 3.3V, resulting in increased sensitivity to noise. Traditional noise filtering methods are often less suitable because maintaining the bandwidth of the drive system is frequently necessary, and bandwidth is generally a critical performance parameter.
Motor drive inverter environment
A three-phase inverter is a power electronic switching circuit that controls the flow of power from the DC power supply rail to three AC motor windings. The inverter has three identical legs, each containing two IGBT transistors and two diodes, as shown in Figure 1. Each motor winding is connected to the same node that connects the high-side and low-side transistors via a shunt. The inverter switches the motor windings between the high-side and low-side rails of the DC bus to control the average voltage. The windings have extremely high inductance, which impedes current variations; therefore, when the power transistors are off, current begins to flow in the diodes connected to the opposite power rail. This ensures a continuous current flow to the motor windings even if there are discontinuous conductions in the inverter power devices and the DC link capacitors. The motor winding impedance acts as a low-pass filter for the high-voltage pulse-width modulated square wave output voltage from the inverter.
Connecting low-voltage control current to the inverter presents significant challenges. A fundamental issue is the switching of the high-side transistor emitter node between the high and low supply rails of the high-voltage bus. First, the high-side driver must be able to drive the gate signal relative to an emitter (potentially 300V or higher than the common input signal). Second, the motor current signal through the shunt (VSH) must be extracted from a common-mode voltage of 300V or higher. Other problems arise from parasitic elements in the power supply circuitry. Even a 10nH PCB trace inductance can cause significant voltages (>10V) when the switching frequency of the power transistors or diodes exceeds 1A/ns. Parasitic and component inductances can cause ringing, resulting in longer durations of noise pulses generated by device switching. Even the high-frequency impedance of the motor cables can pose problems, as the switchboard may be located far from the motor for safety reasons. Other effects include noise coupling from the motor into the feedback sensor signal due to rapidly switching winding voltage waveforms.
The problem becomes more severe because the power rating of the drive circuitry increases the physical size of the circuit board, resulting in a further increase in parasitic inductance and even a higher current and voltage switching rate. Eliminating noise coupling through isolated control and power supply circuitry is one of the main tools for addressing this issue. The performance of the isolation circuitry is a key factor determining drive performance. During shaft rotation, the shaft position encoder generates a digital pulse stream with a frequency of 100kHz or higher. However, in many cases, circuitry mounted on the encoder improves the device's accuracy and increases the data rate to over 10Mbps. Additionally, feedback signals across the shunt can also be isolated by first converting the data into a digital bit stream and then isolating that bit stream from the low-power circuitry. In this case, the data rate is 10Mbps to 20Mbps.
The switching performance required by the gate drive circuitry doesn't seem particularly high, as the switching rate of motor drive inverters rarely exceeds 20kHz. However, a dead time needs to be inserted between the switching signals of the high-side and low-side devices to prevent shoot-through. The dead time is a function of the uncertainty in the turn-on and turn-off delays of the power switches, as well as the delays caused by the isolation circuitry. Extending the dead time introduces more nonlinearity into the inverter's transfer function, resulting in unwanted current harmonics and potentially reduced drive efficiency.
Therefore, the method of transmitting data across the isolation barrier between the power supply circuit and the control circuit must not introduce timing uncertainties during the switching process and must have strong noise immunity.
Comparison of transmission rates of isolator technology
Isolation must not introduce any significant timing uncertainty or timing error into the overall system performance. The propagation delay of standard optocouplers is in the microsecond range and can vary depending on the device, temperature, and lifetime. Optocoupler technology has some fundamental shortcomings in terms of timing performance, while modern digital isolators employ completely different operating principles and offer much higher speeds.
The speed of an optocoupler can be increased with some compromises. An optocoupler works by sending light from an LED through an optically transparent insulating material, where it is detected by a photodiode at the other end. The speed of the optocoupler is directly related to the rate of the photodiode detector and the time it takes to charge its diode capacitor. One way to reduce propagation delay is to increase the amount of emitted light. By increasing the LED current, the delay can be reduced by 2 or 3 times, but this comes at the cost of increased power consumption, up to 50mW per data channel.
Another way to increase speed is to reduce optical transmission loss by using thinner isolation barriers. To maintain the same isolation capability, an additional layer of material is needed, but this comes at the cost of increased cost. Faster optical couplers are many times more expensive than standard, low-cost optical couplers.
In contrast, digital isolators utilize standard high-speed CMOS technology and incorporate isolated on-chip micro-transformers. Their transmission rates are naturally much faster than optocouplers. This higher speed is an inherent characteristic of the circuit and design, achieving higher speeds without requiring more complex and expensive isolation materials. The transformer can transmit data at rates up to 150 Mbps with propagation delays as low as 32 ns and low power consumption.
noise immunity of isolation
In motor drive systems, isolation also provides an opportunity to isolate noise sources by electrically isolating noise from power switching circuits and control circuits. Safety isolation is required between the following: high-voltage bus, line voltage, and user interface, to protect people and other equipment simultaneously. Functional isolation between high-side and low-side switches and control circuitry is also necessary. Isolation components must provide the necessary isolation while remaining insensitive to noisy environments.
The metric used to measure an isolator's ability to isolate high-speed noise between regions is generally called Common Mode Transient Immunity (CMTI). CMTI measures an isolator's ability to suppress voltage noise within the isolation barrier without interrupting data communication. Its unit is kV/μs transient.
Figure 3. Structure of a transformer-based digital isolator.
Voltage transient noise typically crosses the isolation barrier via parasitic capacitance. Optocouplers generally have a poor CMTI (Common Mode Insulation/Transient Noise) of 15 kV/μs. Some modern digital isolators employ capacitively coupled data isolation technology, where the signal and common-mode noise use the same path. Transformer-based isolators (such as ADI's iCoupler digital isolators) have a different signal path than noise path, and their CMTI values are typically 50 kV/μs or higher.
Isolation materials and reliability
Digital isolators are manufactured using wafer CMOS processes, which are limited to commonly used wafer materials. Non-standard materials complicate production, leading to decreased manufacturability and increased costs. Commonly used insulating materials include polymers (such as polyimide PI, which can be spin-coated into thin films) and silicon dioxide (SiO2). Both have well-known insulating properties and have been used in standard semiconductor processes for many years. Polymers are the basis of many optocouplers and have a long history as high-voltage insulators.
Table 1. Comparison of the performance of insulating materials
Safety standards typically specify 1-minute withstand voltage ratings (typically 2.5kVrms to 5kVrms) and operating voltages (typically 125Vrms to 400Vrms). Some standards also specify shorter durations and voltage surges (such as a 10kV peak lasting 50µs) as part of reinforced insulation certification requirements. Polymer/polyimide isolators offer the best isolation characteristics (see Table 1). Polyimide digital isolators, similar to optocouplers, have a service life exceeding that of motors at typical operating voltages, with a rated service life of 50 years. SiO2 isolators have a similar service life, but offer weaker protection against high-energy surges.
Under continuous high-temperature operation, the lifespan of an optocoupler may be affected not by the decomposition of the insulating material, but by LED wear. When the temperature is >85°C, the current transfer ratio (CTR) of the optocoupler will decrease by 10% to 20% after 10,000 hours of operation. After 100,000 hours, the CTR may decrease by half or more.
Integration Possibilities
Optocoupler LEDs and optimized optical detectors are incompatible with low-cost CMOS technology. Integrating gate drivers with desaturation detection, isolated current sensing using a Σ-Δ ADC, and other functions such as multi-directional data flow necessitates multi-chip solutions, making optocouplers with these functions very expensive. Digital isolators using CMOS technology and isolation transformers can naturally add these functions as integration increases. Since transformers can also be used to transmit isolated power, high-end power can be transmitted from the same package without the need for bootstrapping, which can cause problems in some applications. Currently, transformer-based digital isolators are available on the market that integrate a DC/DC converter, Σ-Δ ADC, gate driver, I2C, RS-485 transceiver, RS-232 transceiver, and CAN transceiver in a single package, enabling optimization of both size and cost in motor control systems.
Figure 4. Typical medium-sized industrial motor drive system.
Practical application circuits
Figure 4 illustrates a typical drive circuit with isolated gate drive, communication, and feedback signals. In this system, an isolated Σ-Δ ADC is used to measure the motor winding current, and the digital bit stream is processed by a digital filtering circuit on the motor control IC. The position encoder includes an ASIC that transmits position and speed data to the motor control IC via an isolated RS-485 interface. Other isolated serial interfaces include an I2C interface for connecting to the PFC and an isolated RS-232 link for connecting to the front panel. In this example, the PWM signal is isolated from the inverter module, and the IGBT is driven by a level-shifted gate driver embedded in the module.
