How to repair/troubleshoot your SMPS?

A switched-mode power supply or SMPS or sometimes called the switcher, is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. An SMPS transfers power from a source, like any other power supplies, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics.

So what if your SMPS got broke? With a little electronics know-how and some guides, it should be simple.  Knowledge is power. Don’t forget that the SMPS repairing guide is packed with invaluable step-by-step instructions presented by a reputable and highly experienced electronics engineer.

Some say that it’s extremely difficult to repair a switch more power supply. No! It’s not. The first thing to do is to take a closer look at the SMPS structure. It would be best to look at the generic block diagram of a SMPS. The image below is a simple structure.

Safety is our primary concert.

SMPS are dangerous circuits, so before starting to troubleshoot, safety should come first. Just reminders, half of the components are directly connected to the mains voltage, so voltage shock would be painful. A large storage capacitor is charged at high voltage and can be dangerous even when the mains supply is disconnected. The capacitors could stay charged for a long time since not all SMPS include bleeding resistors or even some might be broken. If you don’t know exactly what you’re doing, might as well call an experienced technician.

It’s good to do a visual inspection

It is always good to do a visual inspection. First, disconnect the SMPS and make sure all capacitor are discharged. Burned resistor can be spotted by their black colour and bad smell. It is important to always check the ferrite transformer, if it looks burned and smells badly, we can give it up because it may sometime have shorted turns and it will be a nightmare to repair or find a replacement part. If the transformer if faulty, it would be best to replace the whole SMPS and will save us a lot of time.

No output, good fuse

SMPS can fail in many different ways, but the most common would be no output power at all. With this, start by checking the input fuse. If the fuse is good but there is no output, probably all the semiconductors are good and it could be easy to fix.

No output, blown fuse

If the fuse is open, then something went really wrong in the circuit. Although don’t replace the fuse yet, as it would just blow again; you have to fix first a short circuit somewhere. Typical problems are blown up power transistors or rectifier diodes, especially on the primary side.

SMPS partially working

There are times when the SMPS is only partially working; it may start for a fraction of a second then shuts down or it may pulsate trying to start every few seconds and shutting down after a fraction of second, or it may producing a wrong output voltage. Probably all power semiconductors are good, so the first thing to check are the capacitors.

Check the capacitors

Often the cause of SMPS problems is the electrolytic capacitors. In what we call cheap designs, where thermal dissipation is a bit too close to the limit, and a choice of components a bit too cost-oriented, electrolytic capacitors are oftentimes real time-bombs that will eventually fail or sometimes explodes. The liquid electrolyte inside these components tends to evaporate and dry out completely altering the characteristics.

Here's the full article.