How capacitive isolation solves key challenges in AC motor drives

Signal and power isolation helps ensure stable operation of AC motor drive systems and protect operators from high voltage hazards.

But not all isolation technologies meet all needs, especially when it comes to device lifetime and temperature performance.

To address alternating current (AC) motor design challenges, this article compares capacitor-based isolation technology from Texas Instruments (TI) with traditional isolation technologies, including isolated gate drivers in the power stage, isolated voltage, current feedback or isolated digital inputs in control modules.

What is an AC motor drive system?

An AC motor drive is an induction motor that uses AC input, as shown in Figure 1. It can drive large industrial loads such as heating, ventilation, air conditioning in commercial buildings, pumps and compressors. AC motors can also drive factory automation and industrial device loads that require speed regulation, such as conveyor belts or tunneling, mining and papermaking equipment.

Figure 1. Induction motor with AC motor drive in a factory

AC motor drives take AC energy, rectify it to a DC bus voltage, implement complex control algorithms, and then convert the DC back to AC via complex control algorithms based on load demand.

Figure 2 shows a block diagram of an AC motor drive system, with the power stage and power supply highlighted in green.

Isolation in AC Motor Drives

Motor drive systems such as AC motor drives contain high voltages and high power levels; therefore, steps must be taken to protect operators and critical components of the overall system.

Additionally, critical system components, such as controllers and communication peripherals, need to be protected from the high-power and high-voltage circuits in the motor drive.

As defined in the International Electrotechnical Commission 61800-5-1 safety standard, insulation between circuits can be achieved by isolating at the component level using semiconductor integrated circuits (ICs).

Isolation ICs transfer data and power between high-voltage and low-voltage units while preventing any dangerous direct current or uncontrolled transient currents. Typically, isolators provide the required level of insulation within a circuit through an isolation barrier. The isolation barrier separates the high voltage from human-accessible parts.

Implementing Isolation in AC Motor Drives

Designers have several options when implementing an isolation barrier in an AC motor drive, but for the past 40 years, the most common device used to implement galvanic isolation in the system has been an optocoupler, also known as an optoisolator or photocoupler. Although optocouplers are cost-effective and ubiquitous, they do not offer the same level of temperature performance or device life as the latest isolation methods.

TI's capacitive isolation technology integrates enhanced signal isolation in a capacitive circuit using silicon dioxide (base on-chip insulation) as the dielectric. Unlike optocouplers, it can integrate isolation circuits with other circuits on the same chip. Isolators manufactured by this process have reliability, shock resistance and enhanced isolation, equivalent to two basic isolation levels in a single package.

The following sections explore three key design challenges related to isolation in AC motor drive designs, while also highlighting the advantages of capacitive isolation over optocouplers.

Gate Drivers in Isolated Power Stages

The power converter topology used in the power stage of an AC motor drive is a three-phase inverter topology for transmitting power in the kilowatt to megawatt range. These inverters convert DC power to AC power. Typical DC bus voltages are 600 V-1,200 V. The three-phase inverter uses six isolated gate drivers to turn the power switches (usually a bank of insulated gate transistors [IGBTs] or IGBT modules) on and off. Designers are starting to use wide-bandgap devices such as silicon carbide (SiC) metal oxide semiconductor field effect transistors (MOSFETs) or modules due to their superior performance.

Each phase uses high-side and low-side IGBT switches, typically operating in the 20kHz to 30kHz range, to apply positive and negative high-voltage DC pulses to the motor windings in an alternating pattern. Each IGBT or SiC module is driven by a single isolated gate driver. The isolation between the high-voltage output of the gate driver and the low-voltage control input from the controller is the current generation. The gate driver converts the pulse width modulation (PWM) signal from the controller into gate pulses for field effect transistors (FETs) or IGBTs. In addition, these gate drivers need to have integrated protection functions such as desaturation, active Miller clamping, and soft shutdown.

An isolated gate driver has two sides: the primary side, which is the input stage, and the secondary side, which is connected to the FET. There are two types of input stages on the primary side: voltage-based and current-based. Through the input stage, the gate driver can be connected to a controller that can tell the gate driver to turn on or off at a specified time.

Optocoupler gate drivers using a current-based input stage typically drive IGBTs in motor drive applications. Current-based input stages tend to have better noise immunity, so a buffer stage is required between the controller and the optocoupler. Current-based input stage drivers using a buffer stage also typically have higher power consumption.

Traditional optocoupler gate drivers do present some challenges:

The performance of the LEDs in the input stage degrades over time, which affects device lifetime and can cause propagation delay times to increase, affecting system performance.

Their low common-mode transient immunity (CMTI) limits the switching speed of power FETs.

They typically support only lower operating temperature ranges, making it difficult to create more compact designs.

TI offers isolated gate drivers that use capacitive isolation technology to help overcome some common design challenges with optocouplers.

Figure 3 compares a traditional optocoupler gate driver to TI’s isolated gate driver using capacitive isolation. TI’s capacitively isolated gate drivers have higher CMTI ratings, wider operating temperature ranges, and improved timing specifications, such as part-to-part skew and propagation delay.

Figure 3. Comparison of optocoupler-isolated gate drivers (a) and capacitor-isolated gate drivers (b)

Isolated current and voltage feedback

AC motor drives use a closed-loop control system consisting of voltage and current feedback measurements to control the speed and torque of the AC motor. Since the voltage and current feedback are measured on the high voltage side, the signals must be isolated from the low voltage controller side.

The in-line phase currents measured on each of the three phases of the motor are used to derive the optimal PWM pattern for controlling the IGBTs. The accuracy, noise, bandwidth, latency, and CMTI of these in-line phase current measurements directly impact the torque and speed output curves of the motor.

As shown in Figure 4, capacitively coupled isolated amplifiers and modulators have less signal propagation delay, better CMTI, and longer lifetime and reliability than their optically coupled counterparts.

Figure 4. Examples of isolated amplifiers (a); and isolated modulators (b)

A typical block diagram of a feedback sensing loop using an isolated amplifier for shunt-based current sensing and resistor-divider-based voltage sensing is shown in Figure 5. The measurement of the phase current is accomplished through the shunt resistor, RSHUNT.

Figure 5. Typical implementation of current and voltage feedback

Compared to optocouplers, TI's isolated amplifiers support very small bidirectional input voltage ranges with high CMTI and overall accuracy. These features enable reliable current sensing in noisy motor drive environments. The high impedance input and wide input voltage range of these devices make them ideal for DC bus voltage sensing.

What is an AC motor drive system?

The control module in the AC motor drive is responsible for the signal processing and overall control algorithm of the motor drive system based on the input of the position feedback module, analog input and digital input. These digital inputs are usually 24 V signals from field sensors and switches, which can convey emergency stop signals (such as safe torque off (STO)) or information about the motor operation (such as speed and position).

When used with the control algorithm, these digital signal inputs will make any necessary adjustments to the power stage to achieve the target output. Isolating the control module from the digital inputs prevents communication errors caused by ground potential differences.

Although optocouplers have been used to isolate digital inputs, recent developments in digital isolator technology have revolutionized the way system designers approach digital inputs.

A common optocoupler solution for isolating digital inputs is shown in Figure 6. This solution uses several discrete components (9 to 15) to implement the current limit and controlled voltage threshold.

Figure 6. Typical optocoupler isolated digital input solution

Using this complex solution, the current limit can be much higher than the target current limit of 2 mA and may be as high as 6 mA over the entire temperature range (depending on the design). In addition, the Schmitt trigger buffer after the optocoupler provides hysteresis for noise immunity. Figure 7 shows a simplified solution, a dedicated digital isolator designed for digital input applications. The device using TI's capacitive isolation technology can achieve a current limit of <2.5 mA. This solution does not require a Schmitt trigger for noise immunity and only requires two resistors (RSENSE and RTHR) to set the selected current limit and voltage threshold.