How to repair your switching power supply

Do you have a malfunctioning switch-mode power supply? In this article we will talk about it and tell you how you can repair a malfunctioning switching power supply yourself.

Introduction

Switch Mode Power Supplies (SMPS) are electronic devices that convert alternating current (AC) to direct current (DC) in an efficient and regulated manner. They are important because they allow powering different types of electronic devices, such as computers, televisions, chargers, etc., with the appropriate voltage and current. In addition, because they are smaller and lighter than linear power supplies, they take up less space and generate less heat.

Switching power supplies can fail for a number of reasons, such as component wear, surges, short circuits, dust or moisture. Some signs that a switching power supply is failing are: strange noises, flickering or blackouts on the powered device, excessive heating or smoke. If any of these problems are detected, it is recommended that the power supply be disconnected and taken to a specialised service centre for repair or replacement.

If you have some knowledge of electronics and you dare to do it, in this article I will give you some guidelines so that you can try to repair the power supply yourself.

If you want, you can watch this video in which I repair a 400W 65V switching power supply. In it, I show many of the techniques and tips that I have included in this article.

Structure of an SMPS

Below, I show you the generic and very summarised block diagram of a switching power supply and give you a basic description of its main components and how they work.

The main components and their operation are as follows:

• Rectifier and input filterconverts the AC input voltage to an unregulated DC voltage and filters it to eliminate noise and fluctuations.
• DC-DC converterSwitches the unregulated DC voltage at a high frequency (between 20 kHz and 2 MHz) by means of a transistor or an integrated circuit. This reduces the size and weight of passive components (transformer, inductors and capacitors) and improves the efficiency and performance of the power supply.
• Transformer and insulationDC-DC converter: transfers power from the DC-DC converter to the output circuit, adapting the voltage and current according to the needs of the load. It also provides electrical isolation between input and output, which increases the safety and reliability of the power supply.
• Rectifier and output filterDC-DC converter: converts the high-frequency AC voltage from the transformer into a regulated DC voltage and filters it to eliminate noise and ripple. Regulation is achieved by a feedback circuit that adjusts the duty cycle of the DC-DC converter according to the load.

In the following image you can see the block diagram, much more detailed, of a source Mean Well S-350It is very similar to the one I have repaired in the video, so you can get an idea of its complexity.

Note that some power supplies are much more complex, with additional output voltages and more stabilisation, safety and control circuits.

Here is also the schematic of the Mean Well S-350 source, so that you have all the details.

Safety first

Handling a Switched Mode Power Supply (SMPS) presents several risks due to the presence of high voltages and the complexity of its design. Here are some warnings and cautionary recommendations when working with SMPS:

Warnings:

1. High voltage: SMPS can operate at high voltages, even in the hundreds of volts. These voltages can pose an electrical shock hazard and can cause serious injury or death.
2. Stored energy: After switching off the power supply, capacitors may retain energy for some time. This energy can be dangerous and must be discharged before touching or handling components.
3. Short circuits: Accidental short circuits during handling can cause damage to the equipment and, in extreme cases, lead to fire. Use caution to avoid short circuits and check continuity before applying power.
4. Temperature: Some components of an SMPS can become significantly hot during normal operation. Avoid touching components without taking appropriate precautions.

Precautionary recommendations:

1. Power disconnection: Before starting any work on an SMPS, disconnect the power supply from the mains and make sure that there is no residual charge on the capacitors.
2. Capacitor discharge: Discharge the storage capacitors before touching any components. Use a discharge resistor or a tool designed for this purpose.
3. Personal protective equipment (PPE): Wear personal protective equipment, such as safety goggles and insulated gloves, especially if you are working on a high-powered SMPS.
4. Documentation and planning: Before you start, check the technical documentation, the schematic and the manufacturer's specifications. Plan your actions and be aware of critical points in the circuit.
5. Cold measurements: Perform cold checks and measurements whenever possible. Do not perform measurements or manipulations while the SMPS is energised.
6. Isolated tools: Use insulated tools to avoid accidental short circuits. Ensure that all tools and equipment are in good condition. If necessary, uses an isolation transformer (not to be confused with Variac).
7. Clean and dry work area: Make sure the work area is dry and free of conditions that may increase the risk of electric shock.
8. Replacement of components: When replacing components, use replacement components that meet the original specifications and follow the manufacturer's recommendations.
9. Training and experience: Do not venture into SMPS if you do not have the necessary training or experience. It is always advisable to seek the assistance of a qualified technician or electronics professional.
10. Monitoring of security standards: Follow applicable electrical safety standards and local regulations to ensure a safe working environment.

Remember that the handling of switching power supplies can be dangerous, and if you do not feel confident or comfortable doing this type of work, it is advisable to seek the help of a trained professional.. Safety must always be the top priority.

Visual inspection

1. Electrolytic capacitors
   • Look for signs of electrolyte leakage, which manifests itself as staining, corrosion or liquid around the base of the capacitor.
   • Look for swelling or deformation at the top of the capacitor, which could indicate an internal fault.
   • Check that there are no marks or cracks on the capacitor casing.
2. Resistors:
   • Look for burnt, charred resistors or signs of overheating. This may indicate excessive current through the resistor.
   • Examine for resistors with out-of-normal values or changes in colouring that could indicate damage.
3. Ferrite transformer:
   • Visually inspect the transformer for cracks, deformation or signs of overheating.
   • Make sure that there are no signs of liquid leakage (in case of oil-filled transformers) or winding damage.
4. Other components:
   • Look for any other abnormal-looking components, such as diodes, transistors, or other semiconductor devices. Look for signs of physical or thermal damage.

The appearance of components can provide clues to potential problems in the power supply. However, be aware that some problems may not be obvious to the naked eye and may require more advanced testing, such as resistance, capacitance and voltage measurements.

It is important to carry out these inspections with caution and to ensure that the power supply is in good working order. disconnected from the mains and capacitors discharged before touching any components. Also, keep in mind the safety precautions mentioned above and, if you are unsure how to proceed, seek the assistance of an electronics professional.

Do not discharge high-voltage electrolytic capacitors by short-circuiting them directly, a good spark may occur and damage the capacitor. If you don't have a tool to discharge them, you can discharge them through a light bulb.

No output, fuse OK

When a switching power supply (SMPS) does not produce any output, but the fuse is intact, there are several possible causes for this behaviour. Here are some possible reasons and how to check them:

1. Short-circuit at the output:
   • Check: Use a multimeter to check the continuity at the output of the power supply. If there is a short circuit at the output, the resistance will be very low.
2. Problems in the rectifier or output diodes:
   • Check: Check the rectifier diodes at the output. Use a multimeter in the diode configuration to check for direct voltage across the diodes in both directions.
3. Problems in the output filter:
   • Check: Examine the output capacitors and LC filter to make sure they are not shorted or open. Measure resistance and capacitance to check their condition.
4. Transformer failure:
   • Check: Perform a visual inspection of the transformer for signs of physical damage, such as deformation or burns. Use a multimeter to check continuity in the transformer windings.
5. Problems in the controller or feedback loop:
   • Check: Verifies the control and feedback circuit of the PWM controller. Examine components such as optocouplers, resistors and capacitors. Measure voltages at critical points to make sure they are within specifications.
6. Fault in the starter circuit:
   • Check: Examine the starter circuit to make sure it is working properly. Measure voltages and currents on components associated with the starter, such as resistors and capacitors.
7. Problems in the protection circuit:
   • Check: Some power supplies have built-in protection circuits. Check for damaged components in this circuit. Measure voltages and protection signals.

No output, fuse blown

When the fuse in a switching power supply (SMPS) is open, it indicates that there is a short circuit or overload somewhere in the circuit. Here are steps and hints for finding and replacing faulty components:

1. Power disconnection:
   • Before carrying out any inspections, be sure to disconnect the power supply from the mains and discharge the capacitors to avoid electrical hazards.
2. Visual verification:
   • Perform a detailed visual inspection of the circuit. Look for any signs of damage, such as burns, swollen or deformed components, traces of smoke, etc.
3. Resistance measurement:
   • Use a multimeter in the resistance measurement setting to check continuity in different sections of the circuit. Look for low resistance resistors or short circuits.
4. Inspection of components:
   • Examine diodes, transistors, and other semiconductor components for possible short circuits or visible damage. Pay attention to components near the area where the fuse is located.
5. Capacitor testing:
   • Measure the resistance and capacitance of the capacitors for possible shorts or leaks. Electrolytic capacitors that swell or leak are usually a sign of failure.
6. Transformer and coils:
   • Check the transformer and windings for shorts or damage. Measure the resistance of the transformer windings to make sure they are within specifications.
7. Protection circuits:
   • Some SMPS have protection circuits. Check for faulty components in these circuits, such as optocouplers, protection transistors, etc.
8. Review of the feedback loop:
   • Examine the feedback circuit and controller to ensure that they are operating correctly. Problems in this area can lead to malfunctions and short circuits.
9. Replacement of components:
   • Once defective components have been identified, replace them with new components and ensure that they meet the manufacturer's specifications.
10. Additional evidence:
    • After repairs are made, perform additional tests before fully energising the power supply. You can use a variac to gradually apply voltage and check for problems.

Incorrect supply voltage, intermittent output, etc.

Symptoms of an abnormally functioning switching power supply (SMPS) can include intermittent starts, incorrect voltages, excessive noise or even tripped protections. Here are some common symptoms and how you might diagnose problems:

1. Intermittent starts:
   • Symptom: Power supply intermittently turns on and off, or does not start up consistently.
   • Diagnosis:
     • Checks the continuity of critical components such as starting resistors, diodes, starting capacitors and the ignition switch itself.
     • Examine the feedback and control circuit to ensure that they are operating correctly.
     • Measures voltages and currents in the starter circuit to identify possible anomalies.
2. Incorrect voltages:
   • Symptom: Output voltages are not as expected, either too high, too low or fluctuating.
   • Diagnosis:
     • Use a multimeter to measure the output voltages at the designated points and compare them to the manufacturer's specifications.
     • Check the voltage regulators, diodes and capacitors in the output circuit.
     • Examine the feedback and control circuitry to make sure they are setting the voltages correctly.
3. Excessive noise:
   • Symptom: There is audible or visible noise or interference at the output of the power supply.
   • Diagnosis:
     • Use an oscilloscope to examine the output waveform and look for irregularities or unexpected spikes (watch out for the isolation transformer).
     • Check the integrity of output filters, such as capacitors and coils.
     • Examine the components of the control and feedback loop that could be generating interference.
4. Activation of protections:
   • Symptom: The power supply is automatically switched off due to activation of protections.
   • Diagnosis:
     • Identifies the specific protection that is activated (overload, short-circuit, over-temperature, etc.).
     • Check components associated with activated protection, such as current limiting resistors or temperature sensors.
     • It measures currents and voltages in different sections of the circuit to identify the cause of protection tripping.
5. Smell of burning or smoke:
   • Symptom: There may be a burning smell or the presence of smoke.
   • Diagnosis:
     • Stop operation of the power supply immediately and disconnect it from power.
     • Perform a visual inspection to identify burnt or damaged components.
     • Verify the integrity of the transformer, diodes, capacitors and other power components.

Capacitor testing

The verification of the electrolytic capacitors is crucial when diagnosing problems in switch-mode power supplies (SMPS). Electrolytic capacitors are common components and their failure can result in power supply malfunction. Here we highlight the importance of checking them and explain how to measure their capacitance, equivalent series resistance (ESR) and choose suitable replacements:

1. Importance of checking electrolytic capacitors:
   • Electrolytic capacitors can degrade over time due to temperature, humidity and electrical stress.
   • Their failure can result in a variety of problems, such as voltage fluctuations, output noise, intermittent starts or even short circuits.
2. Capacity measurement:
   • Use a multimeter with capacitor measurement capability.
   • Discharge the capacitor and connect it to the multimeter according to the correct polarity.
   • Read the capacitance in microfarads (μF) and compare it to the rated value of the capacitor.
3. Equivalent Series Resistance (ESR) measurement:
   • Use an ESR meter or a multimeter that has the capability to measure ESR.
   • Connect the meter to the capacitor according to the correct polarity.
   • Measure the ESR and compare the value with the manufacturer's specifications or with typical values for similar capacitors.
4. Choose suitable spare parts:
   • Capacity and voltage: Select capacitors with the same capacity (or slightly larger) and the same voltage rating as the original.
   • ESR: Look for capacitors with a similar ESR. ESR affects the efficiency and performance of the power supply.
   • Temperature: If possible, choose capacitors with a temperature rating suitable for the power supply environment.
5. Low impedance capacitors (low ESR):
   • In critical applications, consider the use of low impedance capacitors (low ESR) to improve efficiency and reduce heating.
6. Replacement of all capacitors at once:
   • If a particular capacitor has failed, consider replacing all capacitors in the power supply, as they may be subject to the same ageing and wear.
7. Visual inspection:
   • Perform a visual inspection of the capacitors for signs of leakage, swelling or damage. Faulty capacitors often show visual signs of failure.

In the following blog article you have an explanation of the ESR of capacitors with everything you need to know to repair a switching power supply.

The light bulb trick (dim-bulb)

Yes, the use of a bulb in series with the power line is a common and useful practice when diagnosing and repairing switching power supplies or other electronic equipment. This technique acts as a load limiting device and can help prevent further damage in the event of persistent problems. Here are some reasons why this practice is recommended:

1. Current limitation:
   • The bulb in series acts as a limiting resistor. If there is a short circuit or a problem in the circuit, the current through the bulb will be limited, preventing further damage to the equipment.
2. Short-circuit protection:
   • In the event of a short circuit, the bulb will glow brightly, indicating that there is a problem. This alerts the repairer to the presence of a short circuit before the power supply is fully connected.
3. Protection against intermittent faults:
   • If there is an intermittent problem causing intermittent starts or voltage drops, the bulb may dim or flicker, providing a visual indication of the problem.
4. Prevent further damage:
   • Current limiting helps to prevent further damage to other components in the circuit in case the problem is not completely solved.
5. Secure diagnosis:
   • The series bulb acts as a "fuse" that illuminates before significant damage occurs. This provides additional time to diagnose and address the problem before irreparable damage occurs.

To implement this technique, connect an incandescent bulb in series with the power line of the appliance you are repairing. The wattage of the bulb (in watts) will determine the maximum current it can pass. A 60 or 100 watt bulb is commonly used. If the bulb glows brightly, it indicates a problem in the circuit.

Note, however, that this technique is not a substitute for standard safety practices. Make sure that the bulb and other components are properly connected and that there is no risk of electric shock.

Laboratory equipment that I have used in the video and recommend

👉 Multimeter OWON XDM2041

👉 Multimeter UNI-T UT71C

👉 Thermal imaging camera VICTOR VC328B

👉 ESR Capacitor Tester MESR-100

👉 Soldering station T12

👉 TC1 Component Tester

👉 Wowstick 1F Pro electric screwdriver

👉 PCB holder (the 30 cm)

👉 Power supply MLINK DPS3005

👉 Adjustable electronic load 150W

👉 Variac (variable transformer) TDGC2-1KVA

👉 Rigol DS1054 oscilloscope at Amazon UK

Conclusion

Knowing how to repair switching power supplies provides great satisfaction because they are devices that fail a lot and are relatively easy to fix. If we know how to repair switching power supplies, we can repair a great many devices.

I have repaired many switching power supplies and I can tell you that, most of the time, they are very easy repairs, and in many cases you can see at a glance what is going on and you don't even have to measure.

The first culprits are undoubtedly the capacitors. I would tell you that, if I had replaced the capacitors in all the power supplies I have repaired, I would have replaced the capacitors, just like that, the 80% of them would have come out working without doing a single check or measurement.

Repairing a switch-mode power supply (SMPS) involves a careful and methodical approach. Here are the main ideas to keep in mind:

1. Safety first:
   • Disconnect the power supply from the mains before carrying out any inspections.
   • Discharge the capacitors to avoid electric shock.
2. Visual inspection:
   • Visually inspect the components for signs of damage, such as burns, swelling or deformation.
3. Fuse check:
   • Check if the fuses are intact. An open fuse may indicate short circuits or other problems.
4. Basic measurements:
   • Use a multimeter to measure voltages and resistances at key points in the circuit.
   • Measures capacitance and equivalent series resistance (ESR) of capacitors.
5. Oscilloscope for advanced diagnostics:
   • Use an oscilloscope to analyse waveforms and oscillation frequencies at critical points in the circuit. Use, if necessary, an isolation transformer.
6. Load testing:
   • Introduce a resistive load (such as a light bulb) in series with the power line to limit currents and prevent further damage.
7. Replacement of defective components:
   • Replace capacitors, resistors or other components showing signs of damage or out-of-specification values.
8. Step-by-step diagnosis:
   • It diagnoses problems in stages, from the input circuit to the output circuit, using specific measurements and tests.
9. Selection of suitable spare parts:
   • Choose capacitors and other replacement components with appropriate capacitance, voltage and ESR values.
10. Additional protection and testing:
    • Make sure that the power supply is protected against short circuits and overloads.
    • Perform additional tests before fully connecting the power supply.
11. Documentation:
    • Take detailed notes of your observations, measurements and actions taken during the repair process.
12. Professionalism and caution:
    • If you do not feel comfortable or confident performing certain tests, seek the assistance of an electronics professional.

Repairing an SMPS requires patience, technical skills and attention to safety. Always follow best practices and, if necessary, seek the help of trained professionals.

Finally, here is another article about repairing a 5V power supply: