Hi,
I brought a AC Adapter for my electronic device rated for 5.5V 500mA. Since, i couldn't find a 5.5V Adapter, i brought a 5V 1A adapter. While i was in shop, the person showed me a two type of ac adapters:
1) Transformer AC Adapter.
2) SMPS Adapter.
I don't know how they work differently. Especially the SMPS type. Can anyone explain me the SMPS adapter with a diagram? I also found that the SMPS adapter is light weighted compared to the other type(why?). But, i found there's a slight noise in the output of the adapter.
Thank You,
The transformer-based power adapter is simply a large AC-to-AC transformer that converts 110/220V to a smaller voltage using a step-down winding scheme. That fundamental stage may be followed by a simple linear regulator... maybe a simple zener diode, to give a flat output voltage. These are often bulky and heavy, due to the iron-core transformer that does the brunt of the voltage transformation. When you look at the output signal from these transformers, they are quite clean (in frequency content) and low in noise.
SMPS stands for Switch-Mode Power Supply. These type of transformers use an inductive element (inductor or transformer) and a switch-like element operating at a high frequency (kHz) to boost (increase) or buck (reduce) the output voltage, relative to the input voltage. Since these type of transformers need smaller inductive elements, they are often smaller than a similarly capable (volts & amps) transformer-based power adapter. However, you get the reduction in size and weight by trading off signal cleanliness. Since you have added a high frequency switching element to boost/buck the voltage from input to output, you add in frequency content at/around the switching frequency and it's harmonics (2*Fs, 3*Fs, 4*Fs, etc). Also, there can be an increase in the noise level of the output; moreso than with the transformer adapter.
If your circuit can handle a electrically "noisy" power source, then a switching power adapter may work well. However, if you plan to power a radio or amplifier with it, I'd seriously think twice, since the signals coming out of the power supply can get right into your radio or amp and contaminate the signals you're trying to listen to (or amplify), as the case may be.
Hi, thanks for the reply. It helped me.
I'm aware of working of transformer type. but donno about switching type / SMPS type. Can you please suggest me a website for this? Yes, i used it for Panasonic Cordless Phones, and i hear a bit noise during the signal reception. IMO, a transformer is also an inductive element... i didn understand that point.
TY
Start with your favorite search engine and look for "switched mode power supply" and "tutorial", "guide", etc. There a LOTS of websites out there on the topic.
....
Both types of wall adapters use transformers.IMO, a transformer is also an inductive element... i didn understand that point.
Both types of wall adapters use transformers.
In one case the transformer operates on the mains frequency (50/60Hz), this makes the transformer heavy/bulky for a given power rating. Such an adapter may output AC at same 50/60Hz frequency (but lower voltage, and electrically isolated from mains), unregulated DC (that is, with a large 50/60 Hz 'hum' on the output), or may include a regulation circuit that smooths the DC to an exact voltage.
In the other case the transformer operates on a much higher frequency (say, around 50~100 kHz or so), this makes the circuit more complex but the transformer much smaller for same power rating. Output will usually be regulated DC (with a small, high-frequency ripple). Measured over the entire AC mains -> regulated DC conversion, this type of transformer is usually more efficient than 1st type.
What's better for a specific application depends on many factors, but the 2nd (switch mode) type has become more popular lately because of technology advances, size, efficiency ('green'), and cost reasons (big/bulky transformer = more copper, which is relatively expensive metal these days! 8-O ).
Hey, Thanks, Can please give me the diagrammatic representation of both the types? What are SMPS type of adapters called? I have understood the transformer type of adapters clearly, but found the other type bit diff to understand.
... ... ... ...
Was quite helpful, but that is very much different compared to SMPS type of adapter. Whats there in the link is DC to DC convertor, but in a AC to DC converter we don't convert the AC to DC and then again convert DC to DC. I'm little confused about this. We don't use a transformer to directly step down 110/220v AC, so we never step-down ac using a transformer in my opinion. I've googled, but he results are more of what i dont wanted.
Edit: https://en.wikipedia.org/wiki/Switched-mode_power_supply#Explanation ; Here's an explanation which wasn't that clear to me.
As far as understanding how a switching power supply works, you'll have to push a pencil and think about the implementation (maybe even build a couple up to experiment with, or simulate them) to further your understanding. I can't do much more to explain broad topics, so please ask specific questions and I will try to help fill in the blanks.
Yeah, Thanks for the explanation, that really helped me a bit. To get started - May i get any link that start from basics of SMPS? / if not... i'll try to search. TY Again.
Actually we do, have a look at fig. 4 in that **broken link removed**. Now imagine +Vin to be rectified AC mains voltage (~300V DC if you start with 220V AC). On secondary side you will have low-voltage AC -> rectify + small buffer capacitor, done. So you have in fact high voltage AC -> high voltage DC -> electronic switch -> transformer -> low voltage AC -> low voltage DC.Was quite helpful, but that is very much different compared to SMPS type of adapter. Whats there in the link is DC to DC convertor, but in a AC to DC converter we don't convert the AC to DC and then again convert DC to DC. I'm little confused about this. We don't use a transformer to directly step down 110/220v AC, so we never step-down ac using a transformer in my opinion.
TY......optocoupler, and together with the other transistor + a few passives form a feedback circuit that controls the whole thing. Looks rather cheapo but does the job... :razz:
TY...
Where(in ckt diagram) and Why is an optocoupler used there?
Why(and where exactly) is it used in SMPS, Why isolation is required here? We are using transformer, that isolates nah?Optocouplers are used so you can physically disconnect the input and output circuits from one another (but they remain electrically connected, just isolated). A design like that lets you "float" the output signal, so you can tie it to whatever "ground" reference point you want.
Usually in a SMPS, circuitry on primary side needs to know what happens on secondary side (for example whether desired output voltage is reached), but can't 'read' this directly through the transformer. So you need a separate signal path. If you would run such feedback signal directly from secondary back to primary side, then:
- You would lose the electrical (safety) isolation provided by the transformer. Read: mains AC voltage on secondary side, you might DIE. 8-O
- Secondary side wouldn't be 'floating' anymore. Which for most applications, would make that adapter useless.
So that optocoupler provides a way to run a signal from secondary back to primary side circuitry, while keeping intact the transformer-provided electrical isolation.
Why is that feedback required? Because what i see is that the feedback can't change the output of secondary. EX: if the desired output is 6V and output is 5V, then how will the feedback help in increasing the output(so that 6v is achieved). Moreover In my humble opinion, a transformer's inductive property can be used to know what the output will be.
SMPS's are often much more accurate than a linear power supply, which will vary in voltage depending on the load presented. Try measuring a high-current capable wall wart, say 1A. One that I have nearby is a 12VDC @ 1A transformer-only (not switching) wall wart. It reads 17.38V with no load applied. Most supplies like that will operate at their RATED voltage, when loaded with their RATED current. At lighter loads, the voltage will rise because it's not having to overcome losses to maintain Vrated @ Irated.
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