How Capacitive Isolation Addresses Key Challenges in AC Motor Drive Signal and power isolation helps ensure stable operation of AC motor drive systems and protects operators from high voltage hazards.
But not all isolation technologies meet all needs, especially in terms of device lifetime and temperature performance. To address alternating current (AC) motor design challenges, this white paper compares Texas Instruments (TI) capacitor-based isolation technology with traditional isolation techniques, including isolated gate drivers in power stages, isolated voltage, current feedback, or control modules. type digital input.
What is an AC Motor Drive System?

An AC motor drive is an induction motor that uses an AC input, as shown in Figure 1, and can drive large industrial loads such as heating, ventilation, air conditioning in commercial buildings, and the operation of 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 paper equipment.
Figure 1. Induction motor with AC motor drive in factory

An AC motor drive takes AC energy, rectifies it into a DC bus voltage, implements a complex control algorithm, and then converts the DC back to AC through a complex control algorithm based on the load demand.

Figure 2 shows the block diagram of the AC motor drive system, where the power stage and power supply are marked in green.

Isolation in AC Motor Drives

Motor drive systems such as AC motor drives contain high voltages and high power levels; therefore, measures must be taken to protect the operator 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 motor drives.

Insulation between circuits can be achieved by isolation at the component level by semiconductor integrated circuits (ICs), as defined by the International Electrotechnical Commission 61800-5-1 safety standard.

Isolation ICs transfer data and power between high voltage and low voltage units while protecting against any dangerous direct current or uncontrolled transient currents. Generally speaking, an isolator provides the required level of insulation within a circuit through an isolation barrier. An isolation barrier separates high voltages from parts accessible to humans.

Isolation in AC Motor Drives

Designers have several options for implementing isolation barriers in AC motor drives, but for the past 40 years, the most common device for implementing galvanic isolation in systems has been the optocoupler, also known as an optoisolator or optocoupler. Although cost-effective and ubiquitous, optocouplers do not provide the same level of temperature performance or device lifetime as the latest isolation methods.

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

The following sections discuss 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 AC motor driven power stages is a three-phase inverter topology for transferring power in the kilowatt to megawatt range. These inverters convert DC power to AC power. Typical DC bus voltage is 600 V-1, 200V . The three-phase inverter uses six isolated gate drivers to turn on and off power switches (usually a bank of insulated gate gate transistors [IGBTs] or IGBT modules). Because of their superior performance, designers are turning to wide-bandgap devices, such as silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) or modules.

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 current producing. A gate driver converts a pulse width modulation (PWM) signal from a controller into gate pulses for field effect transistors (FETs) or IGBTs. In addition, these gate drivers need to have integrated protection features such as desaturation, active Miller clamping and soft turn-off.

An isolated gate driver has two sides: the primary side (i.e. the input stage) and the secondary side (connected to the FET). There are two types of input stages on the primary side: voltage-based and current-based input stages. 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 current-based input stages are commonly used to 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 that use buffer stages also typically consume more power.

Conventional 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 lead to increased propagation delay times, which in turn affects system performance.

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

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

TI offers isolated gate drivers using capacitive isolation technology to help overcome some common design challenges in optocouplers.
Figure 3 compares a conventional optocoupler gate driver with TI's isolated gate driver using capacitive isolation. TI's capacitively isolated gate drivers feature higher CMTI ratings, a wider operating temperature range, and improved timing specifications such as part-to-part skew and propagation delay.

Figure 3. Comparison of optocoupler isolated gate driver (a) and capacitive isolated gate driver (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 needs to be measured on the high side, the signals must be isolated from the low voltage controller side.

The coaxial 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, delay, and CMTI of these coaxial phase current measurements directly affect 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 optocoupled counterparts.

Figure 4. Example of an isolated amplifier (a); and an isolated modulator (b)

Figure 5 shows a typical block diagram of a feedback sensing loop using an isolated amplifier for shunt-based current sensing and resistive divider-based voltage sensing. The measurement of the phase current is done through the shunt resistor RSHUNT.

Figure 5. Implementing Typical Current and Voltage Feedback

Compared to optocouplers, TI's isolated amplifiers support extremely 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 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, analog input and digital input of the position feedback module. These digital inputs are typically 24 V signals from field sensors and switches that convey emergency stop signals (such as Safe Torque Off (STO)) or information about motor operation (such as speed and position).

When used with control algorithms, 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 ground potential differences from causing communication errors.

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

Figure 6 shows a common solution for optocouplers for isolating digital inputs. The solution uses several discrete components (9 to 15) to implement current limit and controlled voltage threshold

Figure 6. Typical optocoupler isolated digital input solution

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

Figure 7. Isolated digital input solution using TI digital isolators

The advantage of capacitive-based digital isolation methods is their lower power consumption compared to optocouplers. The precise current limit of TI's digital isolators reduces the current drawn by the digital inputs by one-fifth, which greatly reduces power consumption and board temperature. Additional features include a dual-channel option with channel-to-channel isolation to help reduce board space, while also providing low propagation delay and 4 Mbps data rate to support STO inputs.

Supporting the STO input with an optocoupler requires a high-speed optocoupler. Such optocouplers are expensive and have a shorter lifespan than capacitive-based digital isolation techniques.