How to prevent reverse current in diodes in medical electronic devices?

一, The physical mechanism of reverse current and medical risks
The reverse current of a diode is mainly composed of three parts:

Minority carrier diffusion: Under reverse bias, electrons in the P-type region and holes in the N-type region form a small current under the action of an electric field, and its magnitude is exponentially related to temperature.
Surface leakage current: Local electric field distortion caused by packaging defects or surface contamination, resulting in non ideal leakage paths.
Reverse breakdown: When the reverse voltage exceeds the critical value, the charge carriers increase sharply through avalanche multiplication or Zener tunneling effect, causing permanent damage to the device.
The harm of reverse current is particularly significant in medical equipment:

ECG monitor: Reverse current may introduce power frequency interference, masking the true electrocardiogram signal
MRI system: Reverse current in strong magnetic field environment may cause arc discharge, endangering patient safety
Implantable devices: Micro ampere level reverse current can interfere with neural stimulation signals, affecting treatment effectiveness
二, Core standards for selecting medical grade diodes
In response to the high reliability requirements of medical equipment, the selection of diodes should follow the following principles:

1. Material and structural optimization
Ultra fast recovery diode (FRD): using gold or platinum doped diffusion technology, the reverse recovery time is shortened to 20-50ns, suitable for high-frequency pulse medical equipment (such as ultrasound diagnostic instruments)
Schottky Barrier Diode (SBD): Utilizing the metal semiconductor interface characteristics, it achieves ultra-low forward voltage drop of 0.1-0.3V, reduces heat loss, and is particularly suitable for portable medical devices
High voltage rectifier diode: adopts a multi-layer diffusion structure to increase the reverse breakdown voltage to several thousand volts, meeting the requirements of high-voltage applications such as X-ray generators
2. Key parameter control
Reverse leakage current (IR): Medical grade devices require IR ≤ 1 μ A at 125 ℃, which is 10 times higher than the industrial standard
Thermal resistance (R θ JA): The thermal resistance is reduced to 2 ℃/W through copper clip bonding process, ensuring that the junction temperature does not exceed 150 ℃ at an ambient temperature of 40 ℃
Radiation resistance: For diodes in radiotherapy equipment, they need to pass a 100kGy gamma ray radiation test
三, Engineering Practice of Reverse Current Suppression
1. Circuit level protection design
Reverse parallel protection: Connect fast recovery diodes in parallel at both ends of the power diode to form a bidirectional conductive channel. For example, in defibrillators, a combination of 1N4148 and UF4007 is used to suppress the reverse spike voltage from 200V to within 50V.
RC absorption network: For high-frequency switch applications, a 0.1 μ F ceramic capacitor and a 10 Ω carbon film resistor are connected in parallel across the diode to effectively absorb reverse recovery charges. The measured data of a certain hemodialysis machine shows that this scheme reduces the peak reverse current from 1.2A to 0.3A.
Active clamp circuit: using a combination of MOSFET and Zener diode, it actively conducts when the reverse voltage exceeds the safety threshold. In implantable pacemakers, this technology limits the reverse current to below 10nA, meeting the medical safety standard IEC 60601-1.
2. Process and packaging innovation
Glass passivation technology: By forming a SiO ₂ passivation layer on the surface of the PN junction, the surface leakage current is reduced to 0.01 nA/mm ². After adopting this technology in a certain endoscopic CCD imaging system, the image noise was reduced by 3dB.
Ceramic packaging solution: Using Al ₂ O Ⅲ ceramic substrate and gold tin eutectic welding, the matching degree of thermal expansion coefficient is increased to 95%, and the parameters remain stable within the temperature range of -40 ℃ to+85 ℃.
3D integrated structure: Integrating diodes, temperature sensors, and ESD protection devices on the same silicon substrate to achieve real-time monitoring and dynamic compensation. In portable ultrasound probes, this scheme reduces the fluctuation range of reverse current from ± 15% to ± 3%.
四, Typical application cases in medical equipment
1. High precision monitoring of vital signs
In a multi parameter monitor, the BAS70-04 ultra-low leakage current diode array is used to achieve:

Input impedance:>10G Ω
Bias current:<50fA
Common mode rejection ratio:>120dB
This scheme successfully reduced the error of electrocardiogram signal acquisition from ± 5% to ± 0.2%, meeting the requirements of AAMI EC11 standard.
2. Minimally invasive surgical energy system
In high-frequency electric knives, the parallel combination of FRD and silicon carbide (SiC) diodes is used to achieve:

Reverse recovery time:<15ns
Positive voltage drop: 0.7V (@ 10A)
Surge current withstand capacity: 100A (10ms)
This design improves tissue cutting accuracy by 40% while reducing electromagnetic interference (EMI) by 20dB.
3. Portable insulin pump
The BAT54 series Schottky diode packaged in SOT-23 achieves:

Volume: 2.1 × 2.4 × 0.9mm ³
Reverse leakage current: 0.1 μ A (@ 25 ℃)
Start time:<1ns
This solution extends the device's battery life to 7 days while controlling the drug delivery error within ± 1%.