Wallwarts - SMPS & Transfomers

[ElectroEnthusiast]

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,

 

[enjunear]

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.

 

[ElectroEnthusiast]

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

 

[enjunear]

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.

You might start with the Wiki article to get your feet wet.
Switched-mode power supply - Wikipedia, the free encyclopedia

The difference between a simple AC transformer power converter and a SMPS is how the inductive element is used. A standard regulator feeds a full 110/220 VAC into the primary coil and uses the turns ratio of the transformer to step-down the voltage to a relatively close AC voltage, then uses diodes as a full or half-bridge rectifier to create the DC output voltage.

A switcher uses a duty cycle and the induced back-emf (voltage "kickback") from the inductor to either boost-up, or buck-down the output voltage, relative to the input voltage. Start working through some of the circuit tutorials and examples, and their operational concept should become more clear.

 

[ElectroEnthusiast]

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.
....

Thanks , I'll google to learn more.

 

[RetroTechie]

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 ).

 

[ElectroEnthusiast]

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.

 

[enjunear]

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.

Read through this Maxim tutorial and see if it helps clarify how a SMPS works. **broken link removed**
The schematics shown are for the DC-DC converter circuit, so between these circuits, and the AC voltage from the wall, you'd at least need a diode rectifier to get the AC signal turned into DC. Additionally, you could start with a small transformer to step-down the AC from 110/220V to something closer to 10-20VAC, which would then go into the rectifier (be converted to DC), then go into the SMPS DC to DC converter circuits from above.

A transformer based supply looks like the one pictured on this page: http://www.dxing.info/equipment/wall_warts_bryant.dx

As far as names, they will vary, but generally they'll contain the word "switch" and "supply" or "adapter".... like switching power adapter, switch-mode power adapter, switching-power supply, etc.

 

[ElectroEnthusiast]

... ... ... ...

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.

 

[enjunear]

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.

There are a limited number of ways to efficiently transform 110/220VAC to 5/9/12/etc VDC. You can implement the converter in a variety of ways, depending on how you are trying to satisfy your SWaP-C (Size, Weight, Power, Cost) requirements. You need to do two things, at a minimum... convert AC to DC and scale the output voltage. These steps can be done in any order, and with a variety of circuits. The exact configuration is up to the individual designer and the components he/she has available. What I've said in past posts are basic examples of how one might accomplish building these circuits.

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.

 

[ElectroEnthusiast]

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.

 

[enjunear]

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.

The Maxim article covers the basic concepts for the four major varieties, so I'd start there. Especially read and understand the Charge Phase, Discharge Phase, and Control Techniques sections. The stuff that follows is good for circuit designers, but might be confusing for a novice to SMPS circuits.

Google SMPS basics, SMPS tutorial, switcher basics, etc.... there are LOTS of pages with information. I'd steer towards webpages hosted by circuit vendors (Maxim, TI, Analog Devices, Linear, etc), schools (lecture notes), first... then electronic forums and hobby pages (sometimes they have inaccurate information... less likely to get bad info from a circuit vendor or professor).

 

[RetroTechie]

Personally I divide all power supplies in 3 categories:

1. Linear = 50/60 Hz transformer, with rectification / buffer capacitor / output smoothing etc all low voltage, on secondary side.
2. Secondary switching = 50/60 Hz transformer, and on secondary side rectifier + switching DC-DC converter.
3. Primary switching = rectification of mains voltage, with a circuit that switches that on/off at many kHz on primary side of transformer. Secondary is then basically rectify, buffer capacitor + a feedback signal to primary side (using an optocoupler for example).

That article describing wall warts shows type 1. Simple, easy to understand, but (like 2) with bulky transformer.

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.

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.

You put high voltage AC (or high-speed switched DC) into the transformer in both cases. But in a SMPS, the (rectify + electronic switch) on primary side serves to turn 50/60 Hz into many kHz, which makes the transformer much more effective & thus smaller / cheaper.

The other thing to understand is a transformer doesn't necessarily mean "high voltage -> low voltage", this depends on winding ratios & how it's used. You're putting an amount of power in on primary side, and (through magnetic coupling) that power comes out on secondary side. And: what's primary or secondary, can be reversed.

If you look at that Wikipedia photo of a small switch mode adapter, all the elements are recognizable: mains rectifier (4 diodes on right side) + buffer capacitor rated at 400V or so (bottom), electronic switch (below transformer, amazing that's a high voltage part 8-O or perhaps that's on solder side of circuit board), and rectification + buffer capacitor on secondary side (top). That square blocky thing on the right of transformer will be an 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:

 

[ElectroEnthusiast]

...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?

 

[enjunear]

TY...
Where(in ckt diagram) and Why is an optocoupler used there?

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.

 

[ElectroEnthusiast]

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.

Why(and where exactly) is it used in SMPS, Why isolation is required here? We are using transformer, that isolates nah?

 

[RetroTechie]

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:

1. You would lose the electrical (safety) isolation provided by the transformer. Read: mains AC voltage on secondary side, you might DIE. 8-O
2. 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.

 

[ElectroEnthusiast]

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:

1. You would lose the electrical (safety) isolation provided by the transformer. Read: mains AC voltage on secondary side, you might DIE. 8-O
2. 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.

 

[enjunear]

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.

If this were a simple linear power supply (which would need a BIG transformer), your comment about knowing the transformer properties would be correct. However, this is a switching power supply. The transformer is used as a gross power conversion stage, and the fine-tune voltage control is done by the switching mechanism. You MUST have a feedback loop that adjusts the PWM signal. This PWM signal drives one of the transistors, which varies the amount of energy being sent to the output, which in turn allows the circuit to maintain the desired output voltage.

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.

 

[ElectroEnthusiast]

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.

TY
I've tried measuring the loaded and unloaded voltage of a wall wart (maybe its a transformer type), and there was huge diff when measured unloaded. I don't know the theoretical reason, but practically i know it's because the wall warts are designed for that ampere rating. Please tell me theoretically why it happens. Do you mean there will be no problem in SMPS type(due to opto couplers-feedback).