How can diodes achieve overcurrent protection in medical equipment?

一, The basic principle of diode overcurrent protection
The core goal of overcurrent protection is to limit the damage of abnormal currents to circuit components, and diodes achieve this function through the following characteristics:

Unidirectional conductivity: A diode only allows current to flow from the anode to the cathode, blocking reverse current. This feature makes it a core component for anti reverse protection. For example, in the power input terminal of a portable ultrasound probe, parallel Schottky diodes (such as SS14) can prevent current backflow caused by reverse polarity of the power supply and avoid internal circuit burnout.
Forward voltage drop characteristic: When current passes through a diode, a voltage drop of approximately 0.2V (Schottky diode) to 0.7V (silicon PN junction diode) is generated at both ends. By utilizing this characteristic, a current limiting circuit can be designed. For example, in the power module of a surgical robot, multiple 1N4007 rectifier diodes are connected in series to limit the maximum output current through voltage drop superposition and prevent motor overload.
Reverse breakdown characteristic: When the reverse voltage exceeds its breakdown voltage, the Zener diode will enter a stable state, clamping the voltage within a safe range. This feature is widely used in the overvoltage/overcurrent composite protection of medical equipment. For example, in the gradient magnetic field amplifier of magnetic resonance imaging (MRI), the Zener diode works in conjunction with the fuse. When the current exceeds the threshold, the Zener diode breaks down and conducts, triggering the fuse to melt and cut off the circuit.
二, Typical application scenarios in medical equipment
1. Portable medical devices: balance low power consumption and high reliability
Schottky diodes are preferred in devices such as blood glucose meters and portable electrocardiographs due to their low forward voltage drop (0.15-0.45V). For example, a certain model of blood glucose meter uses BAT54S dual Schottky diode array to achieve the following functions:

Anti reverse connection protection: parallel connected to the power input terminal. When the power polarity is reversed, the diode will reverse and cut off, blocking the current path.
Power path selection: In a dual battery power supply system, the main and backup power sources are automatically switched through diodes to ensure continuous power supply.
Current limiting protection: connected in series with the motor drive circuit, using voltage drop to limit the starting current and prevent a surge in current when the motor is locked.
2. High power medical equipment: impact resistance and stability optimization
In devices such as defibrillators and high-frequency electric knives, it is necessary to deal with transient high current surges. At this point, fast recovery diodes (FRDs) and silicon carbide (SiC) diodes become key components:

Defibrillator charging circuit: MBR30200PT Schottky module (30A/200V) is used, with a reverse recovery time (trr) of less than 5ns, which can prevent voltage spikes caused by diode switch delay during charging and protect high-voltage capacitors from overvoltage breakdown.
High frequency electric knife output stage: using C6D10065A SiC Schottky diode (100A/650V), its low forward voltage drop (1.5V) and high temperature resistance characteristics (175 ℃ junction temperature) ensure that the diode's own power consumption is reduced by 60% during 1MHz high-frequency cutting, while avoiding performance degradation caused by overheating.
3. Precision medical instruments: signal integrity and anti-interference design
In devices such as electrocardiographs and electroencephalographs, the acquisition of weak bioelectric signals requires strict suppression of noise. At this point, the collaborative design of photodiodes and protective diodes becomes crucial:

Optoelectronic coupling isolation: In the signal input channel, a 6N137 optocoupler is used to achieve electrical isolation and block common mode interference through the photoelectric conversion of diodes.
ESD protection: At the sensor interface, parallel ESD5D150TA Schottky diode with low leakage current (<0.1 μ A) and high reverse withstand voltage (150V) can effectively discharge the transient current generated by electrostatic discharge (ESD) and prevent sensor damage.
三, Innovative design: Collaborative protection of diodes and other components
1. Composite protection circuit: diode+TVS diode
In the image transmission module of medical endoscopes, a composite protection scheme of "Schottky diode+TVS diode" is adopted to prevent transient overvoltage caused by lightning strikes or static electricity:

Schottky diode: parallel connected to the power input terminal, providing daily anti reverse protection.
TVS diode: series connected to the signal line, its ultra fast response time (<1ps) and low clamping voltage (such as SMAJ5.0A's clamping voltage of 7.8V) can limit overvoltage within a safe range in nanoseconds, protecting the downstream ADC chip from damage.
2. Self recovery protection: diode+PTC thermistor
In the charging circuit of wearable medical devices (such as smart bracelets), a self recovery protection scheme of "Schottky diode+PTC thermistor" is adopted:

Schottky diode: prevents battery reverse connection while utilizing its low voltage drop characteristics to reduce charging losses.
PTC thermistor: series connected to the charging path, when the current exceeds the threshold, the PTC resistance value sharply increases, limiting the current; After troubleshooting, PTC automatically returns to a low resistance state without the need to replace components.