Utilize digital isolators to simplify design and ensure system reliability

Brief introduction

The electronic controls used in industrial motor drives must provide high system performance in harsh electrical environments. The power supply circuit causes a surge in voltage along the motor windings, which can be capacitively coupled into the low-voltage circuit. In the power supply circuit, the non-ideal behavior of the power switch and parasitic components can also produce inductive coupling noise. The control circuit forms a variety of paths with the long cable between the motor and the sensor that couples noise to the control feedback signal. High-performance drivers require high-fidelity feedback control and signals that must be isolated from the high-noise power supply circuitry. In a typical drive system, isolating gate drive signals is included to drive inverter, current, and position feedback signals to the motor controller and to isolate communication signals between subsystems. When signal isolation is realized, 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 safe isolation across the barrier. Although optocouplers have been used for decades, their shortcomings can also affect system-level performance.

The widespread use of variable speed motor drives in industrial applications is due to efficient power switches and cost-advantageous electronic control circuits. The design difficulty is to couple a low-voltage control circuit with a high-power switching circuit without sacrificing noise immunity or switching speed.

The efficiency of modern switching inverters is generally more than 95%, and the power transistor switches used can also connect the motor windings between the high and low rails of the high-voltage DC rail. This process reduces inverter losses because the power transistor operates in a fully saturated mode, which reduces 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, and at the same time, the negative load current switches between high and low power devices. Power semiconductor companies design transistors with short switching times, such as IGBTs, to reduce this switching power loss. However, this higher switching speed also comes with some useless side effects, such as increased switching noise.

On the drive control side, continuous advances in the VLSI process have improved the cost and performance of mixed-signal control circuits, creating conditions for the widespread use of advanced digital control algorithms and the improvement of AC motor efficiency. The cost of improved performance is that the IC operating voltage has been reduced from 12 V to 5 V to 3.3 V today, resulting in increased sensitivity to noise. This traditional approach to noise filtering is often less applicable because it often needs to maintain the bandwidth of the drive system, which is generally a critical performance parameter.

Motor drives inverter environment

A three-phase inverter is a power electronic switching circuit that controls the flow of power from the DC supply rail to the three AC motor windings. The inverter has three identical legs, each of which includes two IGBT transistors and two diodes, as shown in Figure 1. Each motor winding is connected to the same node that connects the high-end and low-end transistors via a shunt. The inverter switches the motor windings between the high and low voltage rails of the DC bus to control the average voltage. The windings are extremely inductive and will block changes in current, so when the power transistor is turned off, current will begin to flow through the diode connected to the opposite power rail. In this way, even if there is intermittent conduction in the inverter power equipment and DC link capacitors, there will be a continuous flow of current into the motor windings. The motor winding impedance acts as a low-pass filter that modulates the square wave output voltage by the high voltage pulse width from the inverter.

Figure 1. Inverter circuit including parasitic elements.

There are significant difficulties in connecting low-voltage control current to the inverter. A basic problem is that the high-end transistor transmitter node switches between the high-voltage bus high-supply rail and the low-supply rail. First, a high-end driver must be able to drive a gate signal relative to a transmitter (which may be 300 V or more higher than the common input signal). Second, the motor current signal through the shunt (vsh) must be extracted from a common-mode voltage of 300 V or more. Other problems will be caused by parasitic components in the power supply circuit. When the switching frequency of a power transistor or diode exceeds 1 A/ns, even a PCB trace inductor of 10 nH can cause significant voltages (> 10 V). Parasitic inductance and component inductance can cause ringing, resulting in a longer duration of noise pulses from device switches. Even the high-frequency impedance of the motor cable can be problematic because the switchboard may be far from the motor for safety reasons. Other effects include noise coupling from the motor to the feedback sensor signal, which is caused by the fast-switching winding voltage waveform. The problem will become more serious because the power rating of the drive circuit will increase the physical size of the board, which in turn will further increase the parasitic inductance and even increase the current and voltage switching rate.

Eliminating noise coupling through isolation control and power supply circuitry is one of the main tools to deal with this problem. The performance of the isolation circuit is a key factor in determining the driving performance. As the shaft rotates, the shaft position encoder will generate a digital pulse stream with a frequency of 100 kHz or more. However, in many cases, the circuitry installed on the encoder increases the accuracy of the device and increases the data rate to more than 10 Mbps. In addition, feedback signals across the shunt can also be isolated by converting the data into a digital stream and then isolating the bit stream from the low-power circuit. In this case, the data rate is 10 Mbps to 20 Mbps.

The switching performance required for gate drive circuits does not appear to be high, as the switching rate of the motor drives the inverter rarely exceeds 20 kHz. However, a dead zone needs to be inserted between the switching signals of high-end and low-end devices to prevent pass-throughs from occurring. The dead zone is a function of the uncertainty of the delay caused by the power switch on and off and the delay caused by the isolation circuit. Extended dead zones introduce more nonlinearity to the inverter transfer function, resulting in unnecessary current harmonics and potentially reducing drive efficiency.

Therefore, the method of transmitting data across the isolation barrier between the power supply circuit and the control circuit must not bring timing uncertainty during the switching process, and must have strong noise immunity.

Isolator technology transmission rate comparison

Isolation must not introduce any significant timing uncertainty or timing error to the overall system performance. Standard optocouplers have a propagation delay in the microsecond range, which can vary from device to device, depending on temperature and lifetime. Optocoupler technology has some fundamental shortcomings in timing performance, while modern digital isolators use completely different computing principles and are at higher speeds.

The rate of the optocoupler can be increased with compromises. Optocouplers work by sending light from LEDs to an optically transparent isolation material and detecting the light with a photodiode on the other end. The speed of an optocoupler is directly related to the rate of the photodiode detector and the time it takes to charge its diode capacitors. One way to reduce propagation delay is to increase the amount of light emitted. By increasing the LED current, the latency can be reduced by a factor of 2 or 3, but at the cost of increased device power consumption, which can be up to 50 mW per data channel.

Figure 2. Optocoupler internals.

Another way to increase speed is to reduce light transmission losses by using thinner barriers. To maintain the same isolation capacity, an additional layer of material is required, but at the cost of increasing costs. Faster optocouplers are many times more expensive than standard low-cost optocouplers.

相反，数字隔离器则是采用标准的高速CMOS工艺，并 搭载隔离式片内微变压器。其传输速率自然比光耦合器 快很多。较高的速度是电路和设计与生俱来的特点，不 需要更复杂、成本更高的隔离材料也可实现更高的速 度。变压器可以以最高150 Mbps的传输速率传递数据， 传播延迟低至32 ns，功耗

Figure 3. Structure of digital isolators based on transformers.

Isolated noise immunity

In motor drive systems, isolation also provides an opportunity to isolate the noise source by using an electric current to isolate the noise from the power switching circuit and the control circuit. There is a need for safety isolation between the high-voltage bus, line voltage, and user interface to protect people and other equipment at the same time. It is also necessary to functionally isolate the high-end switches and low-end switches from the control circuit. The isolation element must provide the necessary isolation while also being insensitive to noisy environments.

The index that measures the ability of the isolator to isolate high-speed noise between regions is generally called common-mode transient immunity (CMTI). CMTI is designed to measure the ability of an isolator to suppress voltage noise in the isolation gate without interrupting the isolator data communication. Its unit is kV/s transients.

The path of voltage transient noise across the isolation gate is generally parasitic capacitance crossing the isolation barrier in the separator. Optocouplers generally have a poor CMTI of 15 kV/s. Some modern digital isolators employ capacitively coupled data isolation technology, where signals and common-mode noise use the same path. Transformer-based isolators, such as ADI's iCoupler digital isolators, have a signal path that differs from the noise path, and their CMTI values are typically 50 kV/s or more. partition

Isolation materials and reliability

Digital isolators are manufactured using the wafer CMOS process and are limited to commonly used wafer materials. Non-standard materials can complicate production, leading to poor manufacturability and higher costs. Commonly used insulation materials include polymers such as polyimide PI, which can be spin-coated into thin films, and silica (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.

Safety standards typically specify a 1-minute withstand voltage rating (2.5 kV rms to 5 kV rms typical) and an operating voltage (125 V rms to 400 V rms typical). Some standards also specify shorter durations, voltage surges (e.g., 10 kV peak and 50 μs for 50 μs) as part of the enhanced insulation certification.

Polymer/polyimide isolators provide the best isolation properties (see Table 1). Polyimide digital isolators are similar to optocouplers in that they have a working life that exceeds the motor at typical operating voltages and are rated for 50 years. The working life of SiO2 separators is close to this, but the protection against high-energy surges is weak.

In the case of continuous use at high temperatures, the life of the optocoupler may not affect the decomposition of the isolation material but the wear of the LEDs. When the temperature > 85°C, the current transfer ratio (CTR) of the optocoupler will drop by 10% to 20% after 10,000 hours of operation. At 100,000 hours, the CTR may drop by half or more.

Integration possibilities

Optocoupler LEDs and optimized detectors are not compatible with low-cost CMOS technology. Integrating other features such as gate drivers with desaturation detection, isolated current sensing with ADCs, and multidirectional data streaming requires a multi-chip solution, which can make optocouplers with these features very expensive. Digital isolators with CMOS technology and isolated transformers can naturally add these functions as integration increases. Because transformers can also be used to transmit isolated power, high-end power can be emitted from the same package without the bootstrap that can cause problems in some applications. Currently, there are transformer-based digital isolators on the market that integrate dc/dc converters, ADCs, gate drivers, I2C, RS-485 transceivers, RS-232 transceivers, and CAN transceivers in a single package, enabling the motor control system to optimize size and cost at the same time.

Practical application circuitry

A typical drive circuit for gate drive, communication, and feedback signal isolation is shown in Figure 4. In this system, an isolated ADC is used to measure the motor winding current, and the digital bit current is handled by a digital filter circuit on the motor control IC. The position encoder contains an ASIC that sends position and speed data to the motor control IC via an isolated RS-485 interface. Other isolated serial interfaces include an I2C interface with a PFC and an isolated RS-232 link to the front panel. In this example, the PWM signal is isolated from the inverter module, and the IGBT is driven by a level-shifting gate driver embedded in the module.

Figure 4. Typical medium-sized industrial motor drive systems.