Low-Power Design

Advanced Galvanic Isolation Technology Brings Efficiency and Reliability to Consumer Motor Control

CMOS-based isolation technology has given rise to isolated gate drivers, multi-channel digital isolators and AC current sensors that offer significant gains in performance and reliability compared to legacy galvanic isolation techniques.

Three-phase motors driven by pulse width modulation (PWM) techniques are used in many consumer appliances from air conditioners and garbage disposals to adjustable beds and pool pumps. These applications typically use brushless DC (BLDC) motors with power ratings between 0.25 HP (186 W) to 3 HP (2,238 W). Figure 1 shows an example motor circuit in which the AC line voltage is converted to a DC voltage by a rectifier and then applied to the input of an inverter. The inverter then converts its DC input voltage into three AC phases that sequence power switches G1 - G6 on and off, electronically commutating the motor. The controller regulates speed by comparing measured speed to the speed set point and adjusting the switch timing until the two parameters are equal.

Applications of this type require galvanic safety isolation to prevent high voltage from encroaching into user-accessible low voltage planes. Isolation also provides seamless level shifting for high-side gate drive, eliminating the need for high-voltage IC (HVIC) drivers.

Advanced CMOS Isolation Devices

Until recently, optocouplers and transformers have been the mainstays of galvanic isolation circuits, mostly because better technologies were not available. Designers were therefore forced to use optocouplers, which have poor timing performance and reliability issues, or bulky EMI-emitting transformers with the external reset circuits and duty cycle limitations that accompany them. But now, a relatively new breed of galvanic isolators fabricated in a standard CMOS process is displacing legacy isolation devices while offering significant advantages in performance, reliability, size and cost-per-channel. Unlike optocouplers and transformers, CMOS-based isolators are far easier to integrate into larger semiconductor ICs, enabling system-level IC functions with integrated isolation.

CMOS Isolated Gate Drivers

Three-phase motor systems require three high-side/low-side IGBT transistor pairs, as well as one motor brake IGBT for a total of seven isolated driver channels. CMOS isolated gate drivers, such as Silicon Labs’ Si823x ISOdrivers, are single-package isolated drivers that provide both isolation and high-side driver level shift functions. Figure 2 shows a three-phase AC motor drive where each IGBT pair is driven by high-side/low-side isolated CMOS drivers with integrated adjustable dead time generator. The dynamic brake IGBT is driven by a single-channel CMOS isolated driver. These single-package solutions offer advantages over legacy approaches including certified isolation ratings up to 5k Vrms, isolated floating output drivers with fast 60 nS input/output propagation delay, and built-in dead time generator and overlap protection. In addition, these isolation solutions exhibit none of the weakness of legacy technologies such as optocouplers and transformers.

CMOS Isolated AC Current Sensor

Legacy current sensing solutions include current-sense transformers (CTs), Hall Effect sensors and differential amplifier/resistive shunt combinations, again due to lack of a better solution. CTs need a cycle-by-cycle reset to prevent core saturation, and these circuits can be simple or complex, depending on the end application (Figure 3). CTs are also physically large and have relatively high series resistance and inductance that reduce efficiency and aggravate inductive ringing.

Hall Effect devices generate a voltage proportional to an applied perpendicular magnetic field, and without physical contact to the current-carrying conductor being sensed, providing intrinsic isolation. This attribute and the ability to measure AC and DC currents are the chief advantages of Hall Effect current sensors. But on the other hand, Hall Effect current sensors are easily corrupted by local external magnetic field interference, have large temperature drift issues creating offset errors at low current flows, and tend to have low output signal amplitude, and consequently low signal-to-noise. Closed-loop Hall Effect sensors may to remedy some of these issues, but at a substantially higher price.

The shunt plus differential amplifier is the most intuitive current sense solution, where a differential amplifier measures the voltage drop across the shunt and generates a current waveform proportional to measured current. The issue with this method is usually excessive power loss in the shunt and/or too low a measurement frequency range. In addition, these devices tend to have a low common mode voltage relegating them to lower voltage applications.

CMOS Isolated Current Sensor

The Rogowski coil, which is essentially a coupled inductor, has been used in AC current measurement applications of various types. The Rogowski coil primary-side current induces a voltage into the secondary-side equal to the mutual inductance times the rate of change of current (i.e. V2 = Lmxdt1/dt1). The current waveform is then “reconstructed” by an analog integrator connected to the secondary-side. Note the on-chip integrator (Figure 4) must be reset prior to the start of each current measurement cycle, which is accomplished using available local gate control signals.

CMOS Isolation Devices in Consumer Motor Control

Brushless DC (BLDC) motors, also called permanent magnet DC synchronous motors, have rapidly gained popularity because of their desirable characteristics. The BLDC behaves like a DC motor with linear relationships between current and torque and between voltage and rotational speed. However, BLDC motors offer advantages over brushed DC and induction motors that include better speed versus torque characteristics and faster dynamic response. Other advantages include high efficiency and reliability, long operating life, noiseless operation, higher speed ranges and reduced electromagnetic interference (EMI) emissions. Note also the ratio of delivered torque to the size of the motor is higher, making BLDCs useful in applications where space and weight are critical factors.

The BLDC speed controller shown in Figure 5 regulates BLDC speed by varying the average voltage across the motor phases using pulse-width modulation (PWM). This single-sided PWM, 120 degree conduction mode two-quadrant controller approach is simple and capable of driving the motor in both directions. The voltage switching scheme is simple as well. Only two of the six switches are on at any time, alternatively switching the voltage to motor phases. The voltage waveforms for all six gates of the controller are shown in Figure 6. The gate voltage timing sequence is: G1 & G2, G2 & G3, G3 & G4, G4 & G5, G5 & G6, G6 & G1. Note the use of the CMOS AC current sensor used to limit current.

As shown in Figure 5, the AC current sensor measures current in each BLDC motor phase. The required modulation frequency for the controller of Figure 5 is less than 70 kHz, and the maximum PWM duty cycle can be clamped to a maximum of 80 percent, allowing a more than ample amount of time to perform the cycle-by-cycle current sensor reset (see Figure 7). Figure 8 shows a simplified feedback torque controller that is only a slight variation of the speed controller shown in Figure 5.

Summary

Many appliances found in homes rely on low or fractional horsepower three-phase motors. These end applications frequently require galvanic isolation for safety, ground noise mitigation and/or voltage level shifting. Legacy technologies such as optocouplers and Hall Effect current sensors have traditionally been used for such applications. CMOS-based isolation technology has given rise to isolated gate drivers, multi-channel digital isolators and AC current sensors that offer significant gains in performance and reliability compared to legacy galvanic isolation techniques.

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