Simplify design and ensure system reliability with digital isolators

Brief introduction

Electronic controls used in industrial motor drives must provide high system performance in harsh electrical environments. Power circuitry causes voltage edge spikes on the motor windings that can be capacitively coupled into the low-voltage circuit. Inductive coupled noise can also be generated by the non-ideal behavior of power switches and parasitic components in power circuits. The long cable between the control circuit and the motor and sensor forms multiple paths that couple noise into the control feedback signal. High-performance drivers require high-fidelity feedback control and signals that must be isolated from noisy power circuitry. In a typical drive system, isolating gate drive signals to drive inverter, current, and position feedback signals to the motor controller, and isolating communication signals between subsystems. Signal isolation must not be achieved at the expense of the bandwidth of the signal path or significantly increase system cost. Optocouplers are the traditional method of achieving safe isolation across the isolation 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 high-efficiency power switches and cost-effective electronic control circuits. The design challenge is to couple high-power switching circuits with low-voltage control circuitry without sacrificing noise immunity or switching speed.

Modern switching inverters typically have efficiencies of more than 95%, and the power transistor switches used can also be connected to motor windings between the high and low rails of the HVDC rail. This process reduces inverter losses because the power transistors operate in a fully saturated mode that reduces voltage drop and power loss during conduction. There are additional power transistor losses during switching, during which a large voltage is applied to the transistor while the load current switches between high and low power devices. Power semiconductor companies design transistors with short switching times, such as IGBTs, to reduce such switching power losses. However, this higher switching speed can also have some unwanted side effects, such as increased switching noise.

On the drive control side, continued advances in the VLSI process have improved the cost and performance of mixed-signal control circuits, paving the way for the widespread use of advanced digital control algorithms and improved AC motor efficiency. The cost of improving performance is that the IC operating voltage has been reduced from 12 V to 5 V to 3.3 V today, resulting in improved sensitivity to noise. This traditional method of noise filtering is often not suitable because it is often necessary to maintain the bandwidth of the drive system, which is generally a critical performance parameter.

The motor drives the inverter environment

A three-phase inverter is a power electronic switching circuit that controls the flow of power from the DC rail to the three AC motor windings. The inverter has three identical legs, each consisting of two IGBT transistors and two diodes, as shown in Figure 1. Each motor winding is connected to the same node where the high-side transistors and low-side transistors are connected via shunts. The inverter switches the motor windings between the high-voltage rail and low-voltage rail 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 device and the capacitance of the direct link, there will be a continuous flow of current into the motor windings. The motor winding impedance acts as a low-pass filter for the high-voltage pulse-width modulated square-wave output voltage from the inverter.

Figure 1. Inverter circuits including parasitic elements.

There are significant difficulties in connecting the low-voltage control current to the inverter. A basic problem is that the high-side transistor transmitter node switches between the high and low rails of the high-voltage bus. First, the high-side driver must be able to drive a gate signal relative to one 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 circuit. When the switching frequency of a power transistor or diode exceeds 1 A/ns, even a 10 nH PCB trace inductance can cause significant voltages (>10 V). Parasitic inductance and component inductance can cause ringing, which results in longer duration of noise pulses generated by device switching. Even the high-frequency impedance of the motor cable can be problematic, as the distribution board may be far from the motor for safety reasons. Other effects include noise coupling from the motor into the feedback sensor signal due to the winding voltage waveform that switches quickly. The problem will become even more acute because the power rating of the drive circuit will increase the physical size of the board, resulting in a further increase in parasitic inductance and even higher current and voltage switching rates.

Eliminating noise coupling by isolating control and power circuitry is one of the main tools to address this problem. The performance of the isolation circuit is a key factor in determining the performance of the drive. As the shaft turns, the shaft position encoder generates a stream of digital pulses at frequencies of 100 kHz or more. However, in many cases, the circuitry mounted on the encoder improves the accuracy of the device and increases the data rate to more than 10 Mbps. In addition, the feedback signal across the shunt can be isolated by converting the data to a digital bit stream and then isolating the bit stream from the low-power circuitry. In this case, the data rate is 10 Mbps to 20 Mbps.

The switching performance required for the gate drive circuit does not appear to be high, because the switching rate of the motor driving inverter rarely exceeds 20 kHz. However, a dead zone needs to be inserted between the switching signals of the high-end device and the low-end device to prevent pass-through. The dead time is a function of the power switch on and off delay and the uncertainty of the delay caused by the isolation circuit. Extended dead time introduces more nonlinearity to the inverter transfer function, resulting in unwanted current harmonics and potentially reduced drive efficiency.

Therefore, the method of sending data across the isolation barrier between the power supply circuit and the control circuit must not introduce timing uncertainty during switching and must be highly noise resistant.

Isolator technology transfer rate comparison

Isolation must not introduce any significant timing uncertainty or timing errors into overall system performance. Standard optocouplers have propagation delays in the order of microseconds and can vary from device to device, temperature to temperature, and lifetime. Optocoupler technology has some fundamental shortcomings in timing performance, while modern digital isolators use completely different arithmetic principles and have higher rates.

The rate of the optocoupler can be increased with some compromise. Optocouplers work by sending light from an LED to an optically clear isolation material and detecting the light with a photodiode at 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 capacitance. One way to reduce propagation delay is to increase the amount of light emitted. By increasing the LED current, the delay can be reduced by a factor of 2 or 3, but at the cost of increased power consumption of the device, up to 50 mW per data channel.

Figure 2. Optocoupler internal structure.

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

In contrast, digital isolators use standard high-speed CMOS processes with isolated on-chip microtransformers. Its transmission rate is naturally much faster than optocouplers. Higher speeds are inherent in circuits and designs, and higher speeds can be achieved without the need for more complex and costly isolation materials. The transformer can transfer data at transmission rates up to 150 Mbps with propagation delays as low as 32 ns and power consumption

Figure 3. Transformer-based structure of digital isolators.

Isolated noise immunity

In motor drive systems, isolation also provides an opportunity to separate noise sources by galvanically isolating noise from power switching circuits and control circuits. There is a need for safe isolation between high-voltage buses, line voltages, and user interfaces to protect people and other devices at the same time. It is also necessary to functionally isolate the high-side switches and low-side switches from the control circuit. Isolation components must provide the necessary isolation while also being insensitive to noisy environments.

A measure of an isolator's ability to separate high-speed noise between regions is commonly referred to as common-mode transient immunity (CMTI). The purpose of the CMTI is a measure of an isolator's ability to reject voltage noise in an isolation barrier without interrupting the isolator data communication without being interrupted by noise. Its units are kV/s transients.

The path of voltage transient noise across the isolation barrier is typically the parasitic capacitance across the isolation barrier in the isolator. Optocouplers typically have poor CMTIs of 15 kV/s. Some modern digital isolators use capacitively coupled data isolation techniques where the signal and common-mode noise use the same path. Transformer-based isolators such as ADI's iCoupler digital isolators have different signal paths than noise paths, typically with CMTIs of 50 kV/s or more. partition

Isolation materials and reliability

Digital isolators are manufactured using a wafer CMOS process and are limited to commonly used wafer materials. Non-standard materials complicate production, resulting in poor manufacturability and higher 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 for 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 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 continuous) as part of the certification of reinforced insulation.

Table 1. Comparison of isolation material properties

	Polymer-based optocoupling	Polyimide-based digital isolators	SiO2-based digital isolators
Withstand voltage [1 min]	7.5 kV rms	5 kV rms	5 kV rms
Life at 400 V rms operating voltage	50 Years	50 Years	50 Years
Enhance the surge level of the rating	20 kV	12 kV	6 kV
Isolation distance	400 µm	20 µm	8 µm

Poly/polyimide isolators provide the best isolation characteristics (see Table 1). Polyimide digital isolators are similar to optocouplers in that they exceed motor operating life at typical operating voltages and are rated for 50 years. The operating life of SiO2 isolators 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 be affected by the decomposition of the isolation material but by LED wear. When the temperature > 85°C, after 10,000 hours of operation, the current transfer ratio (CTR) of the optocoupler will decrease by 10% to 20%. At 100,000 hours, CTR may drop by half or more.

Integration possibilities

Optocoupler LEDs and optimized light detectors are not compatible with low-cost CMOS technology. Integrating other functions such as gate drive with desaturation detection, isolated current sensing with an ADC, and multidirectional data flow requires a multichip solution, which can make optocouplers with these features very expensive. Digital isolators using 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 delivered from the same package without bootstrapping that would pose a problem for some applications. Transformer-based digital isolators are available today, integrating dc/dc converters, ADCs, gate drivers, I2C, RS-485 transceivers, RS-232 transceivers, and CAN transceivers in a single package, enabling motor control systems to be both size and cost optimized.

Practical application circuits

A typical drive circuit showing 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 stream is processed by a digital filter circuit on the motor control IC. The position encoder consists of an ASIC that sends position and speed data to the motor control IC through an isolated RS-485 interface. Other isolated serial interfaces include an I2C interface for PFC connections 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 system.