PCB Repair Guide: Diagnosing Voltage Regulator Failures with FADOS

Voltage Regulators: Where Power Faults Begin

Voltage regulators, LDOs, DC-DC converters, and PMICs, are among the most failure-prone components on any PCB. They sit in the direct current path between the power source and every downstream circuit, absorbing transients, handling inrush current, and dissipating heat. When a regulator fails, everything downstream stops working.

The failure modes vary by regulator type, but the diagnostic approach using VI curve tracing is consistent: compare the regulator’s pin signatures against a known-good reference and interpret the deviations.

LDO Regulator Fault Signatures

Linear regulators (LDOs) have a simple architecture, input pin, output pin, ground pin, and sometimes an enable pin. Their VI curve signatures are correspondingly straightforward to interpret.

Healthy LDO Signatures

• Input to GND: Shows a diode-like characteristic from the input protection diode, combined with any input capacitor signature
• Output to GND: Shows the semiconductor junction characteristic of the pass transistor plus the output capacitor
• Enable to GND: Typically shows a high-impedance CMOS input signature

Common Failure Signatures

Short circuit (input to output): The pass transistor fails short, creating a direct connection between input and output. The VI curve between input and output shows a near-vertical line (very low resistance) instead of the normal semiconductor characteristic.

Short circuit (output to GND): The pass transistor fails to ground. Output-to-GND measurement shows a steep resistive line. This failure mode causes the input supply to source maximum current into the short, often triggering upstream overcurrent protection.

Open circuit: The pass transistor fails open. The output pin shows only the signature of the output capacitor and load components, without the expected semiconductor contribution from the regulator itself. Compare carefully against the reference, the difference may be subtle if large output capacitors dominate the signature.

Switching Regulator Fault Signatures

DC-DC converters (buck, boost, buck-boost) have more complex internal circuitry, but VI curve tracing remains effective because each pin still connects to identifiable circuit elements.

Key Test Points

• VIN to GND: Input filter capacitors plus input protection
• SW (switch node) to GND: The power MOSFET junction, this is the most diagnostic pin for switching regulators
• VOUT to GND: Output inductor and capacitor network plus load
• FB (feedback) to GND: Resistor divider network, its signature directly reflects the divider ratio
• EN (enable) to GND: CMOS logic input

Interpreting Switch Node Signatures

The switch node is connected to the internal power MOSFET drain. A healthy MOSFET shows a characteristic diode curve (the body diode) between the switch node and GND. Deviations from this signature are highly diagnostic:

• FET failed short: SW-to-GND shows near-zero impedance
• FET failed open: SW-to-GND shows only the inductor and output capacitor path, missing the diode signature
• Gate-drain short: SW-to-VIN shows lower impedance than expected

Inductor and Capacitor Verification

While testing the regulator, also verify the output inductor and capacitors. A cracked inductor (open circuit) prevents the switching regulator from delivering power even if the IC itself is functional. The VI curve across the inductor should show a very low-impedance characteristic (nearly a short at the FADOS test frequencies); an open inductor produces a flat line.

PMIC Fault Diagnosis

Power Management ICs (PMICs) integrate multiple regulators and control functions into a single package. These devices often have 20–48 pins, making systematic VI curve testing essential.

Approach

1. Record all pins against a golden reference, the FADOS software can store multi-pin reference sets
2. Compare all pins on the suspect board, the overlay display highlights which pins deviate
3. Group deviations by power domain: a PMIC fault on one regulator block typically affects all pins associated with that domain while leaving other domains with normal signatures

Using the FADOS 9F1 Programmable Supply

For PMICs, the FADOS 9F1’s built-in power supply adds diagnostic depth:

1. Apply the PMIC’s input voltage with current limiting
2. Monitor which power domains activate and which remain off
3. A domain that draws excessive current points to a failure in that specific regulator block
4. Combine with IR thermal scanning to identify the specific failed component

The FADOS 7F1 handles power-off VI curve testing equally well for all regulator types, the 9F1’s advantage is the powered testing capability for cases where power-off signatures are inconclusive.

Building a Regulator Signature Library

For repair workshops handling recurring board types, building a library of regulator signatures dramatically speeds diagnosis:

• Record signatures for each regulator type you commonly encounter (LM1117, AMS1117, TPS62130, MP2315, etc.)
• Document the expected failure modes and their corresponding VI curve patterns
• Train technicians to recognize the common patterns without needing reference boards for every repair

The FADOS software’s reference database supports this workflow, signatures are stored, labelled, and instantly recallable during diagnostic sessions.

Why Buy FADOS from GSAS

GSAS Micro Systems is an authorized ProT Ar-Ge partner and ODM assembler. Every FADOS unit shipped in India is assembled and tested at our Bengaluru facility.

• Demo units at offices in Bengaluru, Hyderabad, Chennai, Pune, Mumbai, and Delhi NCR
• Technical training covering regulator diagnostics, signature interpretation, and reference library management
• INR invoicing with GST-compliant documentation

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