Scope of this technical record
Auxiliary SMPS route that supplies the keypad, CPU, I/O, fan control, relays and often local or isolated driver rails.
Auxiliary power work still begins at the hazardous input and DC-link boundary. Do not treat low-voltage symptoms as low-risk symptoms.
Auxiliary SMPS route
A weak rail may be a failed regulator or a downstream load dragging the regulator down.
Auxiliary SMPS control-power image
The SMPS is both a source and a victim
A VFD auxiliary supply can fail because its own startup parts, controller, switching device, electrolytic capacitors or feedback path aged or burned. It can also appear failed because a fan, I/O branch, driver supply or control-board section is shorted downstream.
A no-display page becomes useful when it separates those two cases: failed regulator versus regulator pulled down by load.
Control-power path table
The page should produce a repair shop request that includes rail behavior, not only a photo of a dark keypad.
SMPS boundary map
| Evidence | Likely boundary | Repair implication |
|---|---|---|
| No startup pulse | Startup feed / controller | Inspect feed resistor, controller and switch |
| Pulsing rail | Overload or weak feedback | Separate supply from loads |
| One rail missing | Secondary branch | Inspect rectifier/regulator/capacitor branch |
| Rail stable, no boot | CPU/keypad interface | Move to control-board interface |
| Fan short drags rail | Load fault | Repair load before supply replacement |
Field record checklist
- Keypad behavior
- Relay/fan behavior
- Rail pulsing/stable/missing
- Visible SMPS components
- Downstream loads attached
- Storage/moisture/contamination history
Technical basis and reference documents
This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.
Board-area record for SMPS repair boundary.
Turn this record into a qualified service request
A repair decision is much more reliable when the request includes the exact identity of the drive, the first fault evidence and the machine condition when the symptom appeared.
- Complete drive type code / MLFB or nameplate model
- Fault code, fault value and first event before reset
- When the event appears: power-up, enable, ramp, run, decel or stop
- Motor/cable connected or isolated during the symptom
- Visible board, option-card, module and connector identifiers
- Previous repair history, replacement parts and repeat-failure pattern