Why SMPS fail

home
back to learn
back to power supplies

post separation
Why SMPS often fail after just a few years of service

As explained in the power supply section SMPS (Switch Mode Power Supply) work at high frequencies.
Usually around the 100kHz range.
The problem here is that capacitors have their lowest impedance at higher frequencies (between 100kHz and 1MHz).
This resistance is called the ESR (Equivalent Series Resistance) and can be found in the datasheets of those capacitors.
The lower a resistance is the higher the current through it will be.
The failure of an SMPS, in most of the cases, is because capacitors lose their properties over time.
High (local) temperatures and high output current capabilities make the degradation happen even sooner.
The ESR of the capacitor increases over time when the circuit is active.
As the ESR increases the ‘losses’ inside the capacitor become greater and thus the temperature in the capacitor also rises causing the effect to accelerate.
It’s the cheaper SMPS that use capacitors with a low temperature range (85oC) and maybe even capacitors that aren’t that well suited (not being low ESR caps to save production costs).
SMPS in DVD players (and other ‘wide AC input range’ appliances) and smaller wallwarts are notorious for this.
All electrolytic capacitors have a limited lifespan and certainly the cheaper ones that are specified to just 1000 hrs @ 85oC.
The higher the temperature the shorter it’s lifespan.
The lower the voltage rating (diameter) the shorter the lifespan.
The closer the actual voltage is to the voltage rating of the capacitor the shorter the lifespan.
The lower the temperature rating the shorter the lifespan.
A 1,000hrs 105oC or even 125oC rated capacitor will thus last longer than a 1,000hrs 85oC cap.
A 35V rated cap will have a longer lifespan than a 16V rated cap (with say 12V on it).
Size, capacitance and voltage have a fixed relationship. Manufacturers can play with diameter and height a bit.

The temperature of individual components inside may locally be higher than the temperature of the wallwart itself on the outside.
A cheap 85oC capacitor may last only 20,000hrs. This seems a lot when you say it but there is 24 hours in a day so only 833 days. That’s just over 2 years.
Some capacitors do not even reach that age. One would think that this time would be the time the device it feeds is actually used, which would mean tens of years of service as a device isn’t really used 24hrs a day.
Alas the SMPS are always on as soon as they are connected to mains, even equipment with a ‘power’ switch (that can also  be switched on using a remote control) are ALWAYS active.
The reasons above all contribute to the SHORT lifespan of most SMPS. Yes … even the ones in high-priced ‘good brand’ equipment…

Another quirk of SMPS which makes them to become defective in some cases is the fact that some of these power supplies need to see a minimum load. The stand-by state of some equipment may be too small for some external power supplies to be seen as a proper minimum load. In these cases the SMPS sometimes may give up pretty quickly. If you encounter an external SMPS that failed pretty quickly … look for another brand/type that is suited to be connected to mains without a load.

a word of CAUTION !

NEVER open up or operate/measure an SMPS when it is still connected to mains. Wait at least 1 minute before touching the circuits after it has been disconnected for mains.
And even then it is still recommended to bleed the reservoir cap with a low value resistor before working on it.
The DC voltage on the big primary capacitor is very high and LETHAL and can still be high when just disconnected from mains.
The primary DC voltage is also connected directly to mains which adds to the dangerous character.
Also NEVER short that input capacitor with a wire or pliers… the flash that follows is fierce and dangerous and can even damage the capacitor itself. It is wise to drain the reservoir cap empty using a resistor or at least measure the DC voltage levels on it BEFORE touching that circuit.
NEVER play with these SMPS power supplies unless you really KNOW what you are doing.

When these power supplies fail (do not start up any more after disconnected from mains) or a device starts to behave ‘strange’ the SMPS is often the only part that needs repair. The easiest way to fix these power supplies is to simply replace all the electrolytic capacitors that are present on the SECONDARY side of the SMPS.
There is NO need to replace the BIG reservoir cap on the primary side.
Just ALL the secondary ones PLUS a small capacitor of between 10uF and 100uF on the primary side.

That capacitor is usually located NEAR a component that generates HEAT.
This could be the heatsink of the switching device or a big resistor that get’s hot.
This is placed there intentionally and is in essence a small ‘time/temperature bomb‘ that, because of the relatively high temperature it reaches being close to a heat generator, degenerates faster than the secondary side capacitors do.
This small capacitor on the primary side is the start-up capacitor.
When your SMPS does not work any more after mains has been switched off earlier (cord unplugged or power failure) THIS capacitor failing is most likely the cause.
Why would one plant a ‘time bomb’ inside a power supply ?
One might suspect to increase sales of these devices as they break down fast and the price is low, and thus one simply buys another one…
The real reason is because the secondary caps degenerate rather quickly and at the end of it’s lifespan start leaking or can even go out with a literal ‘bang’.
To prevent that a time bomb prevents the SMPS to start-up when powered up after a certain time period that is shorter than the ‘expected’ lifespan of the secondary side capacitors.
Works great, but NOT when mains power is always present because that start-up capacitor is only active during start-up.
So when power has been left on continuously the secondary caps may still fail.

In short: To repair a faulty SMPS simply replace all the secondary side electrolytic capacitors PLUS the small electrolytic capacitor(s) on the primary side close to heatsink/resistor that gets hot.
Replace the caps with higher VOLTAGE rating types (IF the physical size allows it).
Also make sure they are at least 105oC or even better 125oC.
Keep the capacitance values the same, do NOT increase them, regardless how tempting that may be !
If there is enough space on the bottom of the PCB you could add 100nF ceramic multilayer (X7R dielectric) capacitors in parallel to the secondary electrolytic capacitors that have just been replaced.
This lowers the ESR of the circuit when electrolytic capacitors age.
I usually mount the ‘time bomb’ capacitor on the bottom side of the PCB (if possible) or mount it further away from the heat generating devices.
Ah… yes… do not forget to check or preventively re-solder the pins of components that have been hot. This shows itself as dis-coloration of the PCB or dull looking solder joints, sometimes you can even see a small ‘ring’ in the solder joint. re-flow the solder by adding some new solder. In most cases the SMPS will work again and may last many TIMES longer than it did before.

a word of CAUTION again… to remind you!

NEVER open up or operate/measure an SMPS when it is still connected to mains. Wait at least 1 minute before touching the circuits after it has been disconnected for mains.
The DC voltage on the big primary capacitor is very high and LETHAL and can still be high when just disconnected from mains.
The primary DC voltage is also connected directly to mains which adds to the dangerous character.
Also NEVER short that input capacitor with a wire or pliers… the flash that follows is fierce and dangerous and can even damage the capacitor itself. It is wise to drain the reservoir cap empty using a resistor or at least measure the DC voltage levels on it BEFORE touching that circuit.
NEVER play around with these SMPS power supplies unless you really KNOW what you are doing.

post separation

home
back to learn
back to power supplies

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s