Advantages and disadvantages of three commonly used isolation technologies

The three commonly used isolation technologies are photoelectric isolation, electromagnetic isolation and capacitive isolation. The three usually cannot achieve the best in terms of power consumption, speed and isolation voltage, and each has its own advantages and disadvantages.

Photoelectric isolation uses "electric-optical-electrical" conversion to achieve signal isolation and transmission. This type of product appeared the earliest, but due to the existence of the photoelectric conversion link, its power consumption is much higher than the other two, and the transmission rate is low, and the LED is prone to aging. It is gradually being replaced by other products. Electromagnetic isolation uses a transformer for isolation, and uses the changing magnetic field between the two coils to achieve data communication on the isolation layer. It can transmit both data and power. Capacitive isolation uses capacitor charging and discharging for isolation. It has strong anti-electromagnetic interference ability, low speed and power consumption, but it is difficult to transmit power.

The transformer uses the electromagnetic coupling effect between coils to complete energy conversion. When two coils are close to each other, one coil is passed through by current, and part of the magnetic flux it generates is linked with the other coil. When the current in the energized coil changes, a changing magnetic flux is generated. Under the influence of the current, the magnetic flux passing through the other coil also changes, so an induced current is generated in the coil. The coil connected to the power supply is called the main coil or primary coil, and the other coil is called the secondary coil. The circuit model is shown in Figure 1.

Figure 1 Transformer coupling circuit model

The terminal voltage and current of an ideal transformer have the following relationship, where Lp and Ls are the self-inductance of the primary and secondary windings, and M is the mutual inductance.

Figure 2 is a schematic diagram of transformer magnetic coupling isolation. Magnetic coupling isolation uses the changing magnetic field between two coils to achieve data communication on the isolation layer. When the voltage or current in the primary coil changes, the magnetic flux passing through the coil changes, affecting the change of the magnetic flux in the secondary coil. According to Faraday's law, the secondary coil generates an induced electromotive force, and the AC signal is transmitted from the primary coil to the secondary coil, realizing the transmission of electrical signal-magnetic signal-electrical signal. Among them, the DC signal is isolated because the current is constant.

Figure 2 Transformer magnetic coupling isolation

There are two methods of signal transmission: one is pulse encoding and decoding, and the other is on-off keying (OOK). Figure 3 shows the principle diagram of digital signal isolation based on pulse encoding and decoding. The input digital signal is first encoded. When the rising edge of the input signal is encountered, it is encoded as two consecutive pulses, and when the falling edge of the input signal is encountered, it is encoded as a pulse signal. The signal generated by the encoding is passed to the decoder through an isolated on-chip transformer. The decoder is responsible for restoring the initial signal and generating a stable output. Figure 4 shows the principle diagram of digital signal isolation based on OOK. In OOK transmission, one of the logic input signals is represented by the transmitted carrier frequency, while the other logic input has no signal transmission.

Figure 3 Digital signal isolation based on pulse encoding and decoding

Figure 4 Digital signal isolation based on OOK

In many isolation applications, power isolation and digital signal isolation are often used together to achieve dual isolation of signal and power. The digital signal isolation transformer and the power isolation transformer are made on one chip, and the signal transmitter chip and the signal receiver chip are packaged together. In this way, a chip with dual isolation of digital signal and power is formed.