Details
Humid environments exert multi‑dimensional and progressive impacts on power‑supply internal circuits, ranging from insulation degradation and metal corrosion to parameter drift and ultimate catastrophic failure. Such destructive effects are often concealed and long‑lasting. This paper systematically elaborates on relevant mechanisms and consequences as follows.
1. Degraded Insulation Performance and Increased Leakage Current
Power supplies contain abundant insulating materials, including PCB substrates (FR‑4, CEM‑3), magnet wire insulation of transformers, plastic capacitor encapsulants, connector housings, and applied conformal coatings. When ambient relative humidity rises (typically above 60 % RH), water molecules form a continuous adsorbed water film on insulator surfaces. Water has a dielectric constant of approximately 80, far higher than air (≈ 1), and contains trace ionic impurities. This substantially reduces both surface resistivity and volume resistivity of insulating materials.
On high‑voltage sides (e.g., the primary side of AC‑DC power supplies with voltages exceeding 300 V), insulation degradation triggers sharp growth of leakage current. Leakage current not only introduces extra power loss but also causes mis‑operation of protection circuits, forcing the power supply into hiccup mode or complete shutdown. More critically, leakage current through accessible paths poses direct hazards to personal safety. This explains why power‑supply safety tests (e.g., dielectric withstand and insulation resistance tests specified in IEC 60950 / IEC 62368) frequently fail under humid conditions.
2. Short‑circuit Risks: From Dendrite Growth to Electric Arcing
This represents one of the most fatal threats posed by humidity. PCBs inside power supplies feature densely routed traces with spacing as narrow as 0.2‑0.3 mm. When a water film covers adjacent pads or traces and dissolves conductive ions originating from flux residues, dust and salt deposits, originally insulating gaps turn into conductive paths.
Under electric‑field stress, Electrochemical Migration (ECM) occurs: anode metals (primarily copper or silver) oxidize into ions, migrate across the water film toward the cathode, and then reduce and deposit to form dendritic metal growths. Dendrites keep propagating until they bridge opposing electrodes and produce micro‑short circuits. Initial short‑circuit current may be low, yet continuous heating accelerates water evaporation and ion concentration, further triggering severe short‑circuits or even arcing. For high‑power power supplies, such faults can burn PCBs, blow fuses within seconds, and potentially ignite fires.
3. Corrosion of Metallic Components
Power supplies incorporate numerous metallic parts: copper PCB traces, component leads (copper, iron, nickel, tin), aluminum heat sinks, transformer cores and windings. Humid conditions provide essential prerequisites for electrochemical reactions between water molecules and metals.
Copper corrosion: Subjected to water and oxygen, copper traces generate basic copper carbonate (verdigris), reducing conductor cross‑sectional area, raising resistance and aggravating local overheating. For high‑frequency switching power supplies, elevated copper losses directly degrade conversion efficiency.
Lead corrosion: Terminations of chip resistors and capacitors (usually nickel‑tin plating) are susceptible to tin‑whisker growth or galvanic corrosion under combined humidity and electrical bias, resulting in cold solder joints, open circuits or parameter drift.
Aluminum heat‑sink corrosion: Once surface oxide films are damaged in humid surroundings, aluminum undergoes sustained corrosion. Heat‑dissipation efficiency drops, and detached corrosion by‑products may contaminate PCBs and introduce new short‑circuit hazards.
4. Parameter Drift of Magnetic Components and Semiconductor Devices
Transformers and inductors are core magnetic components of power supplies. Though ferrite cores themselves resist moisture absorption, winding bobbins and magnet‑wire insulation layers absorb moisture. Moistened insulation increases dielectric loss, intensifies heating at high frequencies, and may eventually cause insulation breakdown and inter‑turn short‑circuits. Inter‑turn short‑circuits alter transformer turns‑ratio, induce abnormal output voltage, surge primary‑side current, and may destroy power switching transistors secondarily.
For semiconductor devices including MOSFETs, IGBTs and diodes, plastic encapsulants isolate leads from silicon chips. Encapsulation compounds are not fully hermetic, so water molecules permeate gradually. If micro‑cracks exist in chip passivation layers (SiO₂, SiN), invading moisture modifies surface potential distribution, raising leakage current, shifting threshold voltages and slowing switching speed. In high‑voltage power devices, this may provoke gate‑oxide breakdown or secondary breakdown and lead to catastrophic failure.
Aluminum electrolytic capacitors are extremely humidity‑sensitive. Long‑term humid erosion can produce micro‑cracks on aluminum casing seals. Ingressed moisture reacts with electrolytes, generating hydrogen gas and causing bulging, or sharply increasing ESR (Equivalent Series Resistance). Capacitors lose filtering capability and bring excessive output ripple in power supplies.
5. Deteriorated Thermal Management
Humidity does not generate heat directly, yet it disrupts power‑supply thermal balance via multiple mechanisms. Firstly, corrosion residues and mould form thermal‑insulating layers on heat‑sink surfaces and degrade convective heat exchange. Secondly, moisture‑soaked insulators show higher dielectric loss converted into extra heat. Most importantly, faults such as short‑circuits and leakage current release energy in the form of Joule heating. Rising internal temperature raises saturated water‑vapor capacity of internal air. When temperature drops abruptly (e.g., unit shutdown overnight), condensation takes place. Condensation is more hazardous than plain high humidity, since condensed liquid water films possess far higher conductivity than adsorbed moisture films and can trigger large‑scale short‑circuits instantaneously.
6. Mould Propagation and Biological Contamination
At temperatures of 20‑35 °C and relative humidity above 75 %, mould readily grows inside power supplies. Mould feeds on organic substances, including plasticizers in PCB solder masks, organics within transformer impregnating varnish, and skin debris in dust. Conductive mould mycelia create leakage paths across component leads. Metabolically‑secreted organic acids (citric acid, oxalic acid etc.) accelerate metallic corrosion. Accumulated colonies also block air ducts and impair heat dissipation. For sealed power supplies deployed in medical and food‑processing industries, mould contamination creates hygiene‑compliance risks.
7. Long‑Term Reliability: Cumulative Failure Effects
Moisture‑induced damage to power supplies is seldom instantaneous but cumulative. Standards such as IPC‑SM‑840 indicate that electronic‑product failure rate varies exponentially with humidity. Temperature‑Humidity‑Bias (THB) testing under 85 °C / 85 % RH or Highly Accelerated Stress Test (HAST) demonstrate that unprotected power supplies suffer insulation failure within hundreds of operating hours. In practical industrial sites such as outdoor base stations, coastal salt‑fog zones, underground equipment rooms and wash‑down environments, power supplies may operate under high humidity for years, and their MTBF (Mean Time Between Failures) decreases significantly.
8. Brief Overview of Mitigation Measures
Targeted protection measures can be formulated after understanding failure mechanisms:
Structural design: Adopt sealed enclosures (IP65 or higher) equipped with desiccants or molecular sieves; install breather valves to balance pressure while blocking moisture ingress.
Circuit design: Increase creepage distance and electrical clearance within high‑voltage zones; select PCB substrates with high CTI (Comparative Tracking Index).
Process protection: Apply conformal coatings (acrylic, polyurethane, silicone) or perform vacuum impregnation; implement conformal coating for critical solder joints.
Material selection: Deploy hermetically packaged components; replace aluminum electrolytic capacitors with metallized polypropylene film capacitors where feasible.
Environmental control: Fit cabinets with dehumidifiers or heaters, maintain internal temperature 5‑10 °C above dew‑point to prevent condensation.
Humid environments introduce systematic threats to internal power‑supply circuits by degrading insulation, inducing short‑circuits, corroding metals, shifting component parameters, impairing thermal performance and fostering mould growth. These hazards couple and amplify each other: insulation degradation produces leakage current, leakage current generates heat, heat accelerates corrosion, and corrosion residues initiate new short‑circuits. Consequently, power supplies deployed in high‑humidity surroundings require comprehensive protection covering materials, mechanical structures, manufacturing processes and ambient‑condition control rather than isolated countermeasures. For designers and maintenance engineers, full awareness of humidity‑driven failure mechanisms constitutes a prerequisite for securing long‑term reliable power‑supply operation.
