What’s The Purpose Of Two Different Type Of Capacitors In Parallel?
Repairing Switch Mode Power Supplies (SMPS) can be much fun, interesting but sometimes you may get frustrated too if it can’t be repaired. There are simple and complex designs of SMPS in the market. Problems of SMPS can be:
- No Power
- Low Output Voltage
- Too high output voltage
- Shutdown Upon Turning On (due to too low or too high the output voltages)
- Power cycling/pulsating
- Run for sometimes (probably from few seconds to couple of minutes) and then shutdown
- No Standby Voltage
- Burnt components in either primary or secondary side
Before you start to repair any SMPS, obviously understanding how a SMPS work will give you a good advantage in tackling the problems. You will know which area to start checking for the faulty parts instead of wasting time checking on all the components. If the component’s density is high, it will waste you lots of time trying to find out the bad components espeically in a compact area. So, understanding each section of the function of SMPS or to be more specific if you could understand each component’s function would be even better. Why? Because if there is a problem in the SMPS, you will know where to start troubleshooting it instead of guessing where and how to start finding the bad component.
In the example below, I will just explain one of the functions of component in SMPS. Ever wonder why certain SMPS have two different type of capacitors in the DC output filter circuit and to some are using only one type? If you compare both schematic diagrams in figure 1 and 2 below, you will find that in figure 1, beside the electrolytic capacitor (C906 2200 uF 16V) you will also see another capacitor (non-polar 0.22uf) that is connected parallel in the same DC output line. Look at the small arrows:
If you look at figure 2, you can’t find any non polarity capacitor in the DC output line. You could only see two electrolytic capacitors connected parallel but separated by the inductor L201.
Why not all DC output have the same common designs which is to have one electrolytic type and one non polarity type of capacitor? Well, it depends on the manufacturers, by right a good and reliable SMPS should have two different types of capacitors in the DC output line. For your information, the non polarity capacitor can also be installed in the load/mainboard area. Now, you may ask, what’s the different between the two capacitors since the common function of capacitors is to remove the ripples and to store current?
Here is the reason why:
- Electrolytic capacitor- handle low frequency ripple and mains noise and major output load changes (act as reservoir to provide load current during the negative cycle of rectification process)
- Non-Polarity capacitor (usually is ceramic type)- handle noise (high frequency) and fast transients. That means it reduce electromagnetic interference (EMI) that result from switching noise.
Now you know that both capacitors serve different functions in the DC output filter circuit. What about the common problems that these two capacitors can cause in the power supply?
Electrolytic capacitor– Usually bulged, leaked and have high ESR ohm. The capacitance value will run also. It can go shorted too but quite rare. These problems can cause the power supply to produce lower than normal DC voltage on that particular DC line, the power supply will pulsate, shutdown and etc. If it is shorted, the power supply can’t be turned on due to large current draw by the capacitor to ground.
Non Polarity Capacitor– Hardly fail. Even if it fails, the capacitance value would be out and have very less effect (impact) to the power supply due to that it may not be a significant component. If it is shorted then it would have the same implication like what a shorted electrolytic capacitor can do to a power supply mentioned above.
Conclusion- Every component in an electronic circuit be it in SMPS or motherboard/mainboard has their own functions. This is why, when you want to find replacement, the best is to get back the same type or part number to avoid unnecessary weird symptom in the device you are repairing. Removing a particular component from a working circuit may or may not have a direct impact on the circuit. It is all depends on how significant or the role the particular component is playing. For example, if you remove the electrolytic capacitor from the DC output line, the effect will be the DC line voltage will become low (because there is no storage current (reservoir) to supply to the load) or even will cause the power supply to shutdown (because abnormal voltage will be detected by the optoisolator IC through the feedback/regulation circuit).
However, if you remove the non polar capacitor from the DC output line, it does not affect the DC output voltage at all because the function of the non polar capacitor is not to store the much needed current by the load, in fact the function is more toward to help in reducing the noise in the DC output line. Being said that, if the load itself already have a built in non polar capacitor then without the non polar capacitor in the DC output line will not affect the performance of the power supply at all. I hope you have gain some new knowledge in this article and you are welcome to comment related to this topic. Ok will see you again in the next repair article.
This article is brought to you by Jestine Yong. He is from Kuala Lumpur Malaysia and he loved electronics repair and blogging about electronics repair information. He is the author of the famous SMPS repair ebook and also a lecturer and conduct electronics repair courses at Noahtech Electronics Training Center.
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