How to prevent current backflow in diodes in medical equipment?

一, Principle of anti backflow: Unidirectional conductivity of PN junction
The core structure of a diode is a PN junction, formed by the combination of P-type semiconductor (with many holes) and N-type semiconductor (with many free electrons). Its anti backflow function is based on the following physical mechanisms:

Positive conduction: When the P terminal is connected to the positive electrode and the N terminal is connected to the negative electrode, the applied voltage weakens the electric field inside the PN junction, causing carrier diffusion and forming a conduction current, allowing current to flow from P to N.
Reverse cutoff: When the N terminal is connected to the positive electrode and the P terminal is connected to the negative electrode, an external voltage is applied to enhance the internal electric field, widen the depletion layer, hinder the passage of charge carriers, and only allow extremely small reverse leakage current (usually nanoampere level).
This unidirectional conductivity makes the diode a natural 'current one-way valve'. For example, a Schottky diode (such as SS14) is connected in parallel to the power input of a portable ultrasound probe. When the power polarity is reversed, the diode cuts off in the opposite direction, blocking the current path and preventing the internal circuit from burning out.

二, Typical application scenarios in medical equipment
1. Portable medical devices: balance low power consumption and high reliability
In devices such as blood glucose meters and portable electrocardiographs, Schottky diodes are the preferred choice for preventing backflow 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 anti backflow solution: Collaborative design 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.
3. Ideal diode solution: integration and intelligence
With the popularity of wide bandgap materials, integrated ideal diodes (such as LM66100DCK) have become the preferred choice for high-end medical equipment. Its working principle is as follows:

Power adapter supply: Disconnect TYPE-C output through internal PMOS cutoff.
TYPE-C power supply: Output 5V voltage through internal PMOS conduction.
Battery powered: When both point A and point C have a potential of 0V, the internal PMOS conducts and the battery supplies power to the load.
This solution has the advantages of comprehensive protection, low pressure reduction, low internal resistance, and low heat generation, and is widely used in portable ultrasound, endoscope and other equipment.