How can medical wearable devices protect their batteries through diodes?

1, Core protection mechanism of diodes
1. Reverse current blocking: prevents battery short circuit and energy loss
The battery output terminal of medical wearable devices (such as smart bracelets and continuous blood glucose monitors) must strictly limit the direction of current. If the current flows in reverse due to circuit failure, it may cause battery short circuit, heating, or even explosion. At this point, Schottky diodes (such as SS14, SS110) become the preferred choice for reverse current blocking due to their low forward voltage drop (0.2-0.3V) and fast switching characteristics. Its working principle is:

Forward conduction: When the battery is discharged normally, the diode is in a low resistance state and the current passes smoothly;
Reverse cutoff: If the current attempts to flow in reverse, the diode quickly enters a high impedance state, blocking the current path.
For example, a certain model of smart bracelet uses SS14 diode in parallel with the battery output terminal. In the reverse current test, the current was successfully limited to below 0.1 μ A, far below the battery safety threshold.

2. Overvoltage protection: Suppress charging surges and ESD impacts
Medical equipment has strict requirements for electromagnetic compatibility (EMC), and charging interfaces or human static electricity may generate transient high voltage (up to thousands of volts), which can penetrate battery management chips (BMS). Transient voltage suppression diodes (TVS) such as SMBJ5.0CA and SLESD5V0LED02 can clamp the voltage to a safe range within ps time through the Zener breakdown effect

Clamping voltage: When the TVS diode experiences reverse breakdown, it limits the voltage to a preset value (such as clamping the diode to 10V in a 5V system);
Low dynamic resistance: The typical dynamic resistance (RDYN) is below 0.5 Ω, ensuring controllable voltage drop under high current;
Low junction capacitance: For example, the junction capacitance of SLESD5V0LED02 is only 0.28pF to avoid high-frequency signal distortion.
A portable electrocardiograph adopts SMBJ5.0CA diode protection charging interface, and successfully withstands 30A peak current surge in IEC 61000-4-2 ESD test without any functional abnormalities.

3. Overcharge/Overdischarge Protection: Extend Battery Cycle Life
Overcharging (voltage>4.2V) or overdischarging (voltage<2.5V) of lithium-ion batteries can accelerate electrode material aging and even cause thermal runaway. Zener diodes (such as BZX85C series) can be used in conjunction with MOSFETs to construct precision protection circuits:

Overcharge protection: When the battery voltage rises to the threshold, the voltage regulator diode conducts, triggering the MOSFET to cut off the charging circuit;
Overdischarge protection: The voltage is monitored through a voltage divider resistor. When the voltage is below the safe value, the voltage regulator diode conducts and the open circuit is disconnected.
After adopting this solution, the battery cycle life of a certain brand of insulin pump was increased from 500 times to over 2000 times, and the failure rate was reduced by 80%.

2, Typical application scenarios and circuit design
1. Wearable device charging interface protection
Medical wearable devices (such as smart patches) typically use micro USB or magnetic charging interfaces, which are susceptible to ESD and surge voltage. During design, TVS diodes need to be connected in parallel on the data/power lines, for example:

D1/D2：SMBJ5.0CA, Protect the 5V power cord;
D3/D4：SLESD5V0LED02, Protect data transmission lines (such as I2C, SPI).
This type of design ensures that the equipment can still operate stably in humid and sweaty environments, meeting the IEC 60601-1 medical electrical safety standard.

2. Reverse protection of battery pack
In scenarios where multiple batteries are connected in series (such as defibrillators), if one battery is reversed, it may cause a short circuit in the entire battery pack. At this time, Schottky diodes (such as BAV21W) need to be connected in parallel at both ends of each battery, with a reverse withstand voltage of up to 200V and a forward withstand voltage reduced to 0.3V, which not only avoids energy loss but also prevents thermal runaway caused by reverse connection.

3. Low power standby protection
Medical wearable devices require long-term standby, and battery self discharge and circuit leakage current may shorten the battery life. By connecting a low leakage current diode (such as BAS70) in series with the battery output, the standby current can be reduced from 10 μ A to below 0.1 μ A, significantly extending the device's usage time.

3, Industry Trends and Challenges
1. Application of wide bandgap materials
Gallium nitride (GaN) based diodes have begun to be applied in medical wearable devices due to their high frequency and efficiency characteristics. For example, GaN Schottky diodes have a 90% shorter reverse recovery time (trr) than silicon-based devices, which can reduce energy loss in charging circuits and improve device endurance.

2. Integrated design
To reduce the size of the device, diodes are being integrated with BMS chips and power management units (PMUs). For example, a single-chip solution launched by a certain manufacturer integrates TVS diodes, voltage regulator diodes, and MOSFETs into a 0.8mm × 0.8mm package to meet the needs of ultra small devices such as smart rings.

3. Balance low power consumption and high reliability
Medical equipment is sensitive to power consumption, but at the same time needs to meet high reliability requirements. Future diodes need to break through in the following directions:

Lower forward voltage drop: such as using Super Junction technology to reduce the voltage drop of Schottky diodes to below 0.1V;
Higher reverse withstand voltage: Develop micro diodes with a withstand voltage of over 100V to meet the needs of high-power medical equipment;
Intelligent protection function: Combining sensors and algorithms to dynamically adjust diode parameters and optimize protection effects.