How to use diodes in medical power isolation protection?

一, The core principle of isolation protection is to block electrical pathways and suppress interference propagation
The core goal of medical power isolation is to cut off the direct electrical connection between the patient's circuit and the mains power, preventing leakage current from passing through the human body and forming micro electric shocks (allowable value ≤ 10 μ A). The diode achieves isolation protection through the following mechanism:

Unidirectional conductivity blocking reverse current: In isolated power supply design, diodes are combined with components such as transformers and optocouplers to form a unidirectional current path. For example, in a flyback converter, the output diode only allows current to flow from the secondary of the transformer to the load, blocking reverse current and avoiding faults on the mains side from affecting the patient circuit.
Clamp voltage suppression transient overvoltage: Transient voltage suppression diodes (TVS) can respond to overvoltage events in nanoseconds and clamp the voltage to a safe level. At the power input end of medical equipment, TVS diodes can absorb interference such as lightning strikes and electrical fast transients (EFT), protecting the downstream circuit from high voltage shocks.
Optoelectronic isolation for signal transmission: The light-emitting diode and photodiode in the optocoupler transmit electrical signals through optical signals, completely isolating the electrical connections between input and output terminals. For example, in an isolated electrocardiogram machine, the optocoupler converts the patient's surface electrical signal into an optical signal, which is then restored to an electrical signal to eliminate ground interference.
二, Typical application scenario: Full scene coverage from operating rooms to portable devices
1. Isolation power supply system for operating room
Class II medical facilities such as operating rooms must adopt IT systems (ungrounded power supply mode) and build a secure power supply network through isolation transformers, insulation monitoring devices, and alarm terminals. The diode plays a dual role in this system:

Flyback converter isolation: In an isolated DC/DC converter, diodes and transformers work together to achieve input-output isolation. For example, the Lingli Te LT8300 chip infers the output voltage by detecting the main terminal voltage of the transformer, and can achieve isolation feedback without the need for optocouplers, simplifying the design and improving reliability.
Insulation fault protection: When a ground insulation fault occurs on the secondary side of the isolation transformer, the diode clamp circuit limits the leakage current to microampere level to avoid the risk of electric shock to the patient. At the same time, the insulation monitoring device triggers an alarm by monitoring changes in insulation resistance, ensuring surgical safety.
2. Power protection for portable medical devices
Portable monitors, infusion pumps, and other equipment need to work stably in complex electromagnetic environments, and diodes provide protection through the following methods:

ESD protection: In the ECG signal acquisition path, the SMBJ5.0CA TVS diode clamps the electrostatic discharge voltage to a safe level, protecting high input impedance instrumentation amplifiers from damage. Its response time reaches picosecond level, which can effectively suppress human static electricity (± 15kV) and static electricity generated by equipment friction.
Reverse voltage protection: In the solar photovoltaic auxiliary power supply system, diodes prevent the reverse flow of grid current into the photovoltaic panel, avoiding equipment damage. For example, at night or on cloudy days, the diode cutoff characteristic can block reverse current, ensuring the safety of the inverter.
3. Energy control of high-frequency surgical equipment
High frequency electric knives, laser therapy devices, and other equipment require precise control of output energy, and diodes achieve protection through the following mechanisms:

Fast Recovery Diode (FRD) freewheeling: In motor drive circuits, FRD provides a reverse current path when the relay is powered off, absorbing the energy of the motor's back electromotive force and protecting power devices from voltage surges. Its reverse recovery time has been shortened to 20ns, which is 30% more efficient than traditional silicon diodes.
Laser power regulation: By using a fast recovery diode and MOSFET to form a switch circuit, the conduction angle of the diode is adjusted to control the driving current of the laser diode, achieving continuous adjustable output power while preventing overcurrent damage to the laser.
三, Technology selection and optimization strategy: balancing performance, cost, and reliability
1. Device parameter matching
TVS diode selection: Parameters such as maximum clamping voltage (VC), minimum breakdown voltage (VBR), and peak pulse power (PPP) need to be considered. For example, in a 220V AC input circuit, TVS diodes with Vbr ≥ 600V and PPP ≥ 600W should be selected to ensure equipment safety under lightning surge (8/20 μ s waveform).
Isolation diode withstand voltage: In an isolated power supply, the reverse withstand voltage of the diode needs to be 1.5 times higher than the secondary voltage of the transformer. For example, in a 48V output system, Schottky diodes with reverse withstand voltage ≥ 100V should be selected to avoid voltage breakdown.
2. Topology innovation
Multi level protection architecture: adopting "GDT+MOV+TVS" three-level protection, GDT absorbs primary surge energy, MOV suppresses intermediate overvoltage, TVS clamps residual voltage, and achieves energy attenuation step by step. For example, in medical X-ray machine power supplies, this architecture can reduce lightning surge voltage from 8kV to a safe level.
Integrated protection module: Choose TVS diode array or ESD protection module to reduce PCB layout space. For example, the Littelfuse SP1003 series TVS array can integrate four signal protections on a single chip, reducing the impact of parasitic capacitance on high-speed signals.
3. Thermal management and reliability improvement
Heat dissipation design: In high-power applications, diodes need to be equipped with heat sinks or heat sinks. For example, in medical magnetic resonance imaging (MRI) gradient amplifiers, SiC Schottky diodes dissipate heat through a copper substrate to ensure a junction temperature below 150 ℃.
Redundant design: Parallel multiple diodes in critical circuits to improve system fault tolerance. For example, in the high-voltage capacitor charging circuit of a defibrillator, dual TVS diodes are connected in parallel to prevent equipment failure caused by single point failure.