Isolators
Isolators are key components used to achieve electrical isolation in electronic circuits. Their main function is to block direct electrical connections between different circuits while allowing unidirectional transmission of signals or energy, thereby improving the safety and anti-interference ability of the system. The following is a detailed description of its core features and application scenarios:
1. Isolators Overview
1)Basic composition
Usually composed of optical couplers or transformers, and some high-frequency scenarios use RF-specific structures.
There are various packaging forms, such as direct plug-in type, surface mount type (SMD), etc. Some high-end models use hermetic packaging to enhance weather resistance.
2)Core functions
Electrical isolation: Isolate high-voltage and low-voltage circuits to prevent abnormal current flow from causing damage to equipment or personnel.
Signal transmission: Realize cross-isolation transmission of signals through electromagnetic induction or photoelectric conversion to avoid ground loop interference.
Decoupling and filtering: Separate DC components and AC noise in the power supply circuit to improve system stability.
2. What are Isolators Used for?
Power supply and industrial system
Decoupling circuit for power module to suppress high-frequency noise interference.
Isolate control circuits and execution circuits in industrial automation equipment to prevent high-voltage shock from damaging sensitive components.
Communication and RF field
RF isolators (such as MACOM MADL series) are used in high-frequency systems such as wireless communication and radar, supporting 1 GHz to 2 GHz frequency bands, with an impedance of 50Ω to ensure the stability of signal transmission.
Protect the front-end circuit of the receiver to avoid equipment damage caused by signal reflection.
Medical and precision equipment
Isolate the patient side from the main control circuit in medical equipment to ensure safety and compliance.
Block common-mode interference in precision measuring instruments to improve signal acquisition accuracy.
3. How to Choose Isolators?
1)Key parameters
Frequency range: For example, the RF isolator needs to match the system operating frequency band (such as 1.3 GHz). Insertion loss: Typical value is as low as 0.35 dB, reducing signal attenuation.
Temperature resistance: The operating temperature range is usually -65°C to 125°C.
2)Classification and selection recommendations
Photoelectric coupling type: Suitable for low-frequency signal isolation, and low cost.
Transformer coupling type: Supports high frequency and energy transmission, but has a larger volume.
RF dedicated type: Optimized for high-frequency scenarios, attention should be paid to impedance matching and packaging form.
4. Typical Brands for Isolators
Infineon
TOSHIBA
Onsemi
TI
VISHAY
LITEON
5. Isolators FAQs
1) How do Isolators work?
Photocoupler: The input LED emits light, and the photosensitive element (such as a phototransistor) receives the light signal and converts it into an electrical signal, realizing no direct electrical connection between the input and the output.
Capacitive Isolator: Through high-frequency signal modulation, energy or data is transferred using the capacitor medium, blocking DC and low-frequency interference.
Inductive Isolator: Energy or signal is transferred using magnetic field coupling, isolating the primary and secondary circuits.
2) What are the common failure mechanisms of Isolators and how to avoid them?
Optical Attenuation of Optocouplers: LED aging causes signal transmission failure, so it is necessary to select high-reliability models and avoid long-term overload.
Capacitive Isolation Breakdown: Overvoltage or dielectric aging causes short circuits, so it is necessary to strictly limit the operating voltage and add protection circuits.
Inductive Isolation Saturation: The magnetic field is too strong, resulting in a decrease in efficiency, and the core material and drive current need to be optimized.
3) How to determine whether the isolation level of Isolators meets the requirements?
The isolation level needs to be comprehensively evaluated based on the system's maximum operating voltage, transient overvoltage (such as lightning strikes or switching surges), and safety standards (such as IEC 61010). For example, medical equipment usually requires reinforced isolation (such as 5000Vrms), while industrial control scenarios may only require basic isolation (2500Vrms).
4) What factors affect the life of isolators?
Optocoupler: LED light decay is the main factor. Long-term high temperature or overcurrent will accelerate aging. It is recommended to control the forward current within 60% of the rated value.
Capacitive Isolator: The aging of the dielectric material or moisture penetration may cause capacitance drift. Moisture-resistant packaging (such as ceramic dielectric) is required.
Solid-State Relay: The number of switching times and load type (capacitive/inductive) affect the contact life. The life under resistive load can reach more than 10^7 times.
5) What are the common design issues of isolators in I2C communication?
Bidirectional Signal Conflict: Direct use of unidirectional isolators will cause bus latch failure. It is necessary to select a dedicated isolation I2C chip (such as ISO1540) or implement bidirectional isolation through an external logic circuit.
Signal Delay: The transmission delay of the isolation device must be less than the minimum clock cycle of the I2C bus. For example, a 100kHz bus requires a delay of ≤ 1μs.
Power Consumption Matching: The pull-up resistor values on both sides of the isolator must match the bus driving capability to avoid abnormal logic levels due to impedance mismatch.
6) What is the difference between isolators and other protection devices (such as TVS diodes)?
Functional Difference: Isolators achieve electrical isolation and block common-mode interference and ground loops; TVS diodes are mainly used to suppress transient overvoltages (such as ESD) and belong to clamping protection.
Application Scenarios: Isolators are suitable for long-term isolation needs (such as power systems), while TVS diodes are used for short-term pulse protection (such as communication ports).
Cooperative Use: In a high-voltage environment, isolators and TVS diodes can be connected in series, the former isolating continuous interference, and the latter absorbing transient energy.
7) How to test the isolation performance of isolators?
Withstand Voltage Test: Use a withstand voltage tester to apply isolation voltage (such as 3000Vrms/1 minute) to detect whether the leakage current exceeds the standard (usually ≤ 1mA).
Transmission Characteristics Test: Input a square wave through a signal generator, and observe the waveform distortion and delay at the output end with an oscilloscope to verify whether the bandwidth meets the standard.
Temperature Rise Test: Continuously operate at the highest operating temperature to monitor whether the isolation medium has a breakdown or parameter drift.
6. Summary
Isolators ensure circuit safety and signal integrity through physical isolation and are indispensable components in power supply, communication, industry, and medical fields. When selecting, it is necessary to consider the frequency requirements, isolation strength, and packaging form.
