Techniques for Signal and Power Isolation

Electronic isolation is a method of preventing direct current (dc) and unwanted alternating current (ac) transfer between two parts of a system while still allowing signal and power transfer between those two parts. This type of isolation is necessary in a variety of situations, including:

  • High-voltage protection for industrial workers.
  • High voltage protection for costly processors and related circuits.
  • In communication networks, ground loops must be avoided.
  • Enhancing noise resistance.
  • Interacting with high-side devices in motor drive or power-converter systems.

PLCs, motor drives, medical equipment, solar inverters, electrical vehicles (EVs), and some specialty power supplies are examples of industrial equipment that requires isolation.

There is a need for an industrial isolation solution that is efficient, affordable, and compact. A newly available fully integrated signal and power isolation product provides a number of advantages to system design, including reduced board space, ease of certification, and simplified design.

Isolation Methods

The use of a transformer is one common method of electrical isolation. The primary and secondary windings are electrically isolated, and power is transferred solely through magnetic induction rather than current flow.

A transformer is still an important component of isolation methods, and it is the method of choice for dc power isolation. While transformers can be used for signal transfer, they are slow, bulky, and expensive. However, there are other options.

An infrared LED and phototransistor, usually a BJT with an open collector, are used in optical integrating signal and power isolation, which has been around for years. A photodiode detector is used separately in some devices (Fig. 1).

When there is no input signal, the LED is dark and the phototransistor is turned off, resulting in a high output from the external pull-up resistor. When an input signal is applied, the LED turns on, the base of the phototransistor illuminates, and the bias that turns the transistor on is produced. As a result, the output is reduced.

Optocouplers perform well and provide adequate high-voltage isolation up to 10 kV. Their main disadvantage is the slowness with which some digital systems operate. A newer type of isolator that uses capacitive connectivity is now available.

The isolation method used in digital isolators is silicon-dioxide dielectric capacitors. However, because capacitance is limited by the physical limitations of an integrated circuit, special techniques are used to ensure rapid energy transfer. The first is edge-based, while the second is based on on-off keying (OOK) modulation.

A typical digital isolator consists of a transmitter (TX) and a receiver (RX) section. Capacitive coupling isolates them.

A single-ended input signal is converted into a balanced signal before being routed through the isolation capacitors. The received signal is divided into narrow pulses in the lower path.

These pulses are processed by comparators and a flip-flop before being fed into a multiplexer (MUX), which provides the output. It’s worth noting that the input signal is also routed to the upper low-speed path, where its pulse width modulates a higher-frequency oscillator. The pulse-width-modulation (PWM) signal is converted to a balanced signal before being processed back into pulses by the isolation capacitors.

The high-frequency PWM is removed by a low-pass filter (LPF), and the original signal is recovered. A decision logic circuit (DLC) detects the situation and switches the multiplexer to the low-frequency path if the input signal is too low in frequency.

To modulate the input, the single-ended signal is sent to an AND gate along with an oscillator via spread spectrum techniques. The resulting higher-frequency signal is balanced and passed through the isolation capacitors.

The signal is conditioned and amplified on the IC’s receiving side. The original signal is then extracted and output by an envelope detector.

Isolation Examples

A PLC in a factory-automation setup is a good example of a need for isolation. PLCs are widely used in industry to monitor and control a wide range of machines. Sensors that measure temperature, pressure, position, and other physical properties provide input to the PLC.

These sensors are frequently located far away from the PLC, requiring lengthy cable runs. This situation frequently results in ground potential differences, which can skew sensor data and introduce errors. To ensure accuracy, some kind of isolation is required.

The dc power to signal conditioning circuits are the other component of the isolation. The dc power for the signal conditioning circuits and ADC is drawn from the PLC’s 24-V rail. This is converted into 5 V by a dc-dc converter, which produces pulses that are transferred by an isolation transformer to a dc supply and LDO, which provides the voltage for the signal-conditioning circuits.

Another example of an application is the requirement to isolate one system from another while still allowing communication between the two. Automation necessitates the communication of PLCs, computers, special controllers, and other equipment. For these types of communications, the RS-485 interface standard is commonly used.

This application employs a differential twisted-pair cable in runs up to 4000 feet long. Such long runs are susceptible to noise pickup and can disrupt operations. The differential signals help to reduce noise, but it is still an issue. Another issue is ground loops. The solution is digital, and the RS-485 interface’s power isolation protects the processors from noise and high-voltage signals.