Why Industrial SMPS Fail
Switched-mode power supplies convert AC mains voltage to regulated DC (typically 24VDC for industrial controls). They work harder than any other component in the cabinet — running at high frequency, switching thousands of times per second, managing heat, and absorbing every voltage transient from the mains. Failure modes are well-understood and almost always repairable.
1. Electrolytic Capacitor Failure
This is the single most common cause of SMPS failure, accounting for over 60% of all units we repair. Electrolytic capacitors degrade over time due to heat and ripple current. Signs of failure: bulging or leaking tops, output voltage ripple exceeding specification, instability or shutdown under load. In industrial supplies running 24/7 at elevated temperatures, capacitors typically need replacement after 7–10 years regardless of visible symptoms — this is preventive maintenance, not reactive repair.
2. Primary-Side MOSFET Failure
The primary switching MOSFETs are the most electrically stressed components. They fail due to: mains voltage surges that exceed the MOSFET's drain-source breakdown voltage, current overloads that trigger secondary-side protection too slowly, or gate drive failures that leave the MOSFET in a linear (not switching) state, causing thermal runaway. A failed primary MOSFET typically takes out the primary-side fuse and sometimes the gate driver IC as well.
3. Opto-Isolator and Control IC Failure
The feedback loop in an SMPS passes through an opto-isolator (typically PC817 or TLP181) that separates the primary and secondary sides. When the opto degrades (CTR drops with age), the output voltage rises uncontrolled. Control ICs (UC3842, NCP1252, etc.) also fail and must be replaced as part of a thorough repair.
Diagnosis Without a Schematic
Professional SMPS diagnosis does not require the original schematic. Standard test procedures: check primary fuse, measure DC bus voltage (should be ~310V from 230VAC input), check primary capacitor ESR, probe gate drive waveform with an oscilloscope. A systematic approach isolates the fault to primary side, secondary side, or control circuit within 30 minutes.
Repair Economics
A Siemens SITOP PSU8200 24V/20A unit costs €580 new. A complete recap and MOSFET replacement typically costs €180–250 with a 3–4 day turnaround. For multi-output supplies powering entire control systems (e.g., Siemens power module with 5V, 15V, 24V outputs), repair is the only option when the unit is discontinued — replacement may not exist.