DC Link Undervoltage
DC link undervoltage occurs when the intermediate DC bus voltage in the switch-mode power supply drops below the minimum threshold required for proper regulation, typically below 300VDC on 480VAC systems. This condition affects the PFC stage's ability to maintain stable DC voltage for the main switching converter, resulting in output voltage instability or complete shutdown.
Error Code
F908
Severity
Fault
Equipment
Power Supply
Manufacturer
ABB
Red fault LED illuminated continuously with possible slow blinking pattern
Output voltage measures 15-20% below rated specification (e.g., 22VDC instead of 24VDC)
DC link voltage measures below 300VDC when input is 480VAC (should be 650-680VDC)
Audible relay clicking every 3-5 seconds as unit attempts auto-restart
Connected equipment experiences intermittent resets or brown-out conditions
Input current draw measures 20-30% higher than nameplate rating during operation
Degraded electrolytic capacitors in DC link with ESR above 0.5 ohms or capacity loss exceeding 20%
Failed PFC boost diodes (typically 1N5408 or equivalent) showing forward voltage drop above 1.2V
Input voltage sag below 85% of nominal due to utility supply issues or inadequate wire sizing
PFC control IC failure (UC3854 or similar) with damaged voltage reference or PWM output
Main PFC inductor with shorted turns causing reduced boost capability and core saturation
Input EMI filter capacitor failure creating high-frequency noise affecting PFC regulation
Isolate power supply using LOTO procedures and verify zero energy state with CAT III multimeter
Discharge DC link capacitors using 10k ohm 10W resistor and verify voltage below 42VDC
Measure DC link capacitors with ESR meter - replace if ESR >0.5 ohms or capacity <80% of rating
Test PFC boost diodes in-circuit with diode test function - replace if Vf >1.2V at 1A test current
Verify input voltage stability under load using oscilloscope - coordinate with facilities if below 85% nominal
Replace PFC control IC if output PWM duty cycle exceeds 85% or voltage reference drifts beyond ±2%
Perform full load test at rated output current for 30 minutes and verify DC link voltage remains above 90% of nominal throughout test cycle
Safety First: Always isolate power and follow LOTO (Lockout/Tagout) procedures before working on electrical components. If unsure, contact a certified repair technician.
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