Isolating users and sensitive electronic components is an important consideration in motor control systems. Safety isolation is used to protect users from harmful voltages, and functional isolation is designed to protect equipment and devices. The motor control system may contain a variety of isolation devices such as isolated gate drivers in the driver circuit; isolated ADCs, amplifiers and sensors in the sense circuit; and isolated SPI, RS-485 in the communication circuit, Standard digital isolator. Careful selection of these devices is required for safety reasons or to optimize performance.
While isolation is an important system consideration, it also has the disadvantage of increasing power consumption, delaying the transmission of data across the isolation barrier, and increasing system cost. System designers have traditionally resorted to optical isolation schemes, which have been the natural choice for system isolation for many years. In the last decade, digital isolators based on magnetic (transformer transmission) methods have provided a viable and often superior alternative; from a system perspective, it also has advantages that system designers may not have realized.
This article will discuss these two isolation solutions, focusing on the improvements in delay timing performance due to magnetic isolation and the benefits at the system level for motor control applications.
Isolation methodThe optocoupler uses light as the primary transmission method, as shown in Figure 1. The transmitting side includes an LED, a high level signal turns on the LED, and a low level signal turns off the LED. The receiving side converts the received optical signal back to an electrical signal using a photodetector. Isolation is provided by a molding compound between the LED and the photodetector, but can also be enhanced with an additional barrier layer (usually based on a polymer).
One of the biggest drawbacks of optocouplers is that LED aging can cause transmission characteristics to drift; designers must consider this additional problem. LED aging causes timing performance to drift with time and temperature. As a result, signal transmission and rise/fall times are affected, complicating the design, especially considering the issues to be addressed later in this article.
The performance extension of optocouplers is also limited. In order to increase the data rate, the inherent parasitic capacitance problem of the optocoupler must be overcome, which leads to an increase in power consumption. Parasitic capacitance also provides a coupling mechanism that results in CMTI (Common Mode Transient Immunity) performance of optocoupler-based isolation devices that are inferior to competing solutions.
Magnetic isolators (based on transformers) have been used on a large scale for more than a decade and are an effective alternative to optocouplers. These isolators are based on standard CMOS technology and use the principle of magnetic transmission. The isolation layer consists of polyimide or silicon dioxide, as shown in Figure 2. A low level current is transmitted through the coil in a pulsed manner, creating a magnetic field that passes through the isolation barrier and induces a current in the second coil on the other side of the isolation barrier. Due to the standard CMOS structure, it has significant advantages in power consumption and speed, and there is no lifetime deviation problem associated with optocouplers. In addition, transformer-based isolators offer better CMTI performance than optocoupler-based isolators.
Transformer-based isolators also allow the use of conventional signal processing modules (to prevent transmission of spurious inputs) and advanced transmission codec mechanisms. This enables bidirectional data transmission, using different coding schemes to optimize the relationship between power consumption and transmission rate, and to transmit important signals more quickly and consistently to the other end of the isolation barrier.
Comparison of delay characteristicsAn important but often despised feature of all isolators is their transmission delay. This characteristic measures the time it takes for a signal (which can be a drive signal or a fault detection signal) to cross the isolation barrier in either direction. The technology has different transmission delays. Typical delay values ​​are usually provided, but system designers pay special attention to maximum delay, which is an important feature to consider when designing a motor control system. Table 1 gives examples of propagation delay and delay offset values ​​for optocouplers and magnetically isolated gate drivers.
As shown in Table 1, magnetic isolation has significant advantages in terms of maximum delay and delay repeatability (deviation). In this way, the motor control designer will have more confidence in the design without increasing the timing margin to meet the gate driver characteristics. This is very important for the performance and safety of motor control systems.
System impact on the motor control systemFigure 3 shows a typical three-phase inverter used in AC motor control applications. The inverter is powered by a DC bus, which is typically generated directly from an AC source via a diode bridge rectifier and a capacitive/inductive-capacitive filter. In most industrial applications, the DC bus voltage is in the range of 300 V to 1000 V. The power transistors T1 to T6 are switched at a typical frequency of 5 kHz to 10 kHz using a pulse width modulation (PWM) scheme to produce a variable voltage, variable frequency three-phase sinusoidal AC voltage at the motor terminals.
PWM signals (such as PWMaH and PWMaL) are generated in a motor controller (typically implemented in a processor and/or FPGA). These signals are typically low voltage signals that are common to the processor. In order to properly turn the power transistor on and off, the voltage level and current drive capability of the logic level signal must be amplified, and level shifting must be performed to emit an extremely ground reference with the associated power transistor. Depending on the location of the processor in the system, these signals may also require secure insulation.
The gate driver (such as GDRVaL and GDRVaH in Figure 3) performs this function. Each gate driver IC requires a primary side supply voltage referenced to the processor ground and a secondary side supply that is highly referenced by the transistor. The voltage level of the secondary side supply must be able to turn on the power transistor (typically 15 V) and have sufficient current drive capability to charge and discharge the transistor gate.
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