How do diodes prevent current surges in vital sign monitors?

一, Current surge risk in vital sign monitoring devices
The core modules of the vital sign monitoring device include photoelectric sensors, bioelectric potential acquisition circuits, and power management units. The current surge risk mainly comes from the following scenarios:

Power input terminal: During the operation of mains waves or battery charging and discharging, transient voltage spikes may exceed the equipment's withstand voltage value, leading to circuit breakdown.
Sensor interface: Photodiodes (PPG sensors) are susceptible to environmental light interference or electrostatic discharge when receiving reflected light signals, causing signal overload.
Bioelectric potential acquisition: When ECG electrodes come into contact with the skin, transient high voltage may be generated by human static electricity or electromyography interference, which can damage the preamplifier.
High frequency switching power supply: Internal DC-DC converters may cause voltage ringing and electromagnetic interference (EMI) due to poor diode reverse recovery characteristics during switching.
二, The core technical principle of diode to prevent current surge
1. Transient Voltage Suppression (TVS): A "safety valve" with nanosecond level response
TVS diodes quickly conduct when the voltage exceeds the breakdown voltage (Vbr) through nonlinear volt ampere characteristics, clamping the overvoltage to a safe range. Its response time is as low as 1ps, which can effectively absorb transient energy such as lightning strikes and ESD. For example, in the LED driver circuit of a pulse oximeter, a bidirectional TVS diode (such as SMAJ5.0A) can simultaneously suppress positive and negative voltage spikes, protecting the photodiode from electrostatic shock.

2. Reverse recovery feature optimization: eliminate switch noise
Fast recovery diodes (FRDs) and Schottky diodes reduce voltage ringing in high-frequency switching power supplies by shortening the reverse recovery time (TRR). For example, in the DC-DC converter of ECG acquisition circuit, MBR30200PT Schottky diode (trr<5ns) is used to avoid the superposition of reverse current and MOSFET turn off process, reduce EMI interference, and ensure the purity of ECG signal.

3. Current limiting protection: soft start and thermal management
Connect an NTC thermistor in series with the power input terminal, and use its cold high resistance value (such as 5 Ω for the 5D-9 model at 25 ℃) to limit the surge current when powered on. After the current heats up, the resistance value drops to<1 Ω, achieving soft start. For example, a certain model of portable monitor adopts an NTC+relay combination scheme to suppress the impulse current of 220V AC input from 300A to 60A, with a reduction of over 80%.

4. Clamping and voltage stabilization: protecting sensitive circuits
Zener diodes provide a stable reference voltage for bioelectric potential amplifiers through the voltage constancy in the reverse breakdown region. For example, the AD8233 chip series integrates a voltage regulator diode internally, which can compress the input voltage fluctuation range of the ECG signal amplification circuit from ± 20% to ± 1%, ensuring the accuracy of heart rate detection.

三, Typical application scenarios and solutions
1. Protection of photoplethysmography (PPG) sensor
In the oximeter, the red LED (660nm) and the infrared LED (940nm) alternately emit light, and the photodiode receives the reflected light and converts it into an electrical signal. To prevent ESD impact, a bidirectional TVS diode (such as P6SMB15CA) should be connected in parallel at the sensor interface, with a clamping voltage of 15V, which can absorb ± 8kV of electrostatic discharge energy. At the same time, Schottky diodes (such as BAT54S) are connected in series in the LED driving circuit to reduce power consumption by utilizing their low forward voltage drop (0.15V) and avoid LED lifespan degradation caused by excessive current.

2. Protection of ECG bioelectric potential acquisition circuit
When ECG electrodes come into contact with the skin, human static electricity may generate thousands of volts of transient voltage. By paralleling TVS diodes (such as SMAJ12CA) at the electrode input, the voltage can be clamped to below 12V, protecting the preamplifier (such as AD8221) from damage. In addition, a bridge rectifier circuit+NTC thermistor combination is used at the power input end to suppress the surge current caused by the fluctuation of the mains wave and ensure the stability of ECG signal acquisition.

3. Portable device power management
In portable monitors such as smart bracelets, the battery charging and discharging circuit needs to cope with transient high current surges. By using SiC Schottky diodes (such as C6D10065A) instead of traditional silicon diodes, the reverse recovery time is close to zero, which can reduce switching losses by 60% and withstand transient currents of 100A, avoiding hardware damage caused by battery overcharging or short circuits. In addition, an ESD protection diode (such as ESD5D150TA) is integrated at the USB charging interface to discharge ± 15kV static electricity and protect the internal charging chip.