As long as the power supply circuit has the potential to interact with other circuits, hardware, infrastructure, or human users, destructive overvoltage situations may occur. Physical or electronic isolation (commonly referred to as electrical isolation) between current and potential interaction points is crucial for the safety and continuous operation of circuits. Isolation can also reduce unnecessary noise in the output signal.
Isolation requirements are very common in robots, high-voltage power grid equipment, factory workshop equipment, automotive applications, and consumer products. When designing isolation systems, it is also necessary to consider the specificity of the application, such as variable input voltage, use of battery power, or the need for compact packaging.
To select the correct isolation components, designers need to understand the advantages, disadvantages, and composition of various isolator structures. With this understanding, they can adopt the most effective, reliable, and space saving isolators in electronic design.
Get to know the isolator
Electrical isolation can be achieved in various ways, but they all have a common basic principle: the high voltage input on the primary side is isolated from the low voltage, low current secondary side through some physical barriers. The details of the isolation barrier and the method of transmitting power, signals, or both through the isolation barrier depend on the type of isolator.
The optocoupler uses LEDs to convert the signal on the primary side from electrical pulses to photons. On the secondary side, photosensitive elements such as phototransistors, photodiodes, or photoelectric field-effect transistors receive photons and convert them into electrical signals. In addition to physically isolating the primary and secondary circuits, optocouplers can also automatically eliminate unnecessary noise in the output signal and prevent grounding loops.
In a magnetic coupler, the voltage on the primary winding of the transformer generates a magnetic field. This magnetic field will generate induced voltage on the secondary winding, thereby transmitting electrical signals while maintaining electrical isolation. Transformers can have two independent windings on a single iron core, or they can be two inductors, each with a winding wound around its own iron core, separated by insulation material. The reason why designers choose magnetic coupling is because it has high voltage capability, relatively fast response time, and the ability to filter out signal noise. However, the size of the isolator, the possibility of heat generation, and the generation of electromagnetic interference should also be considered.
The capacitive coupler uses a capacitor consisting of two electrodes separated by a dielectric material. The input voltage will accumulate charges on the primary side electrode. This generates an electric field and induces a voltage on the secondary electrode. Capacitive couplers are known for their small size, low power consumption, and rapid response to input changes, making them convenient and efficient for transmitting electrical signals across isolation gates. Designers must take measures to protect capacitive couplers from the effects of input voltage, environmental humidity, and dielectric breakdown that exceed their capabilities.
Deploy digital isolators
Any of the above types of isolators can be integrated into a digital isolator system on an integrated circuit (IC). These topological structures can be further integrated with power modules or signal transmission components to form a complete digital isolation system on a single chip. The common topology structures of digital isolator systems include flyback, half bridge, and push-pull.
The flyback power supply adopts a magnetic isolation form, which combines a shunt inductor with a buck boost converter to construct a transformer, thereby increasing or decreasing the voltage of the direct current (DC) input to match the required output. The feedback of the buck boost converter is provided by a three-stage inductor winding or optocoupler. It is recommended to use flyback power supplies in low-power applications, but designers must be aware that unnecessary EMI may be generated.
The half bridge (H-bridge) design includes an H-bridge square wave generator, a resonant circuit consisting of two inductors and one capacitor (LLC), and two rectifiers that can provide the required DC output voltage. Compared to certain designs, rectifiers can achieve higher output power, and it is recommended to use H-bridge isolation design for medium power applications.
The push-pull isolated power supply uses two transformers for magnetic coupling. Two switches alternately switch the transformer to receive input voltage. The two full bridge rectifier diodes on the secondary side can predict voltage changes and regulate them to symmetrical outputs.
To enhance control, designers can choose to add transformer drivers to the push-pull device. This driver integrates an oscillator, a frequency divider, and a logic controller to coordinate the opening and closing of switches in a BBM mode. This mode can generate a relatively constant output signal while protecting internal and downstream components from damage caused by connecting two switches simultaneously.
Systems with transformer drivers can also use low dropout linear regulators (LDOs) to control the output, replacing rectifier diodes or enhancing their functionality. Voltage difference is the minimum difference between the input voltage and the output voltage, below which the circuit cannot fully regulate the output. In LDO, this difference is extremely small, ensuring reliable operation over a wide input voltage range.