rectification in smps

[Ashroyer05]

it seems ideal to have a step down transformer and then have a rectifier in a ps. Why then do i see smps's with filter then rectifier and then transformer? And then 2 main caps?

[Ashroyer05]

Why would we convert to dc before ac operated transformer? Just to power a transistor?

[Ashroyer05]

do we power the transformer with pulsing dc in smps?

[370forlife]

Many simple power supplies do simply rectify the input AC voltage into a DC voltage, then run it through a transformer to step it down to the correct DC voltage. The problem with this is you need a huge power supply to get a small amount of current and they cannot regulate themselves. What this means is that a 12V supply would probably put out 16V with no load, and under full load may only put out 10V or 11V.

DC transformers that can handle the same amount of current as an AC transformer are substantially bigger. We can solve this with an SMPS, or Switched Mode Power Supply.

In a common SMPS like a computer power supply, the input AC voltage is filtered by simple filtering components like X caps and Y caps and coils. The input filtering is mostly to keep the power supply from putting interference back into the wall. It is then rectified by a bridge rectifier or a 4 diode setup into DC and stepped up to around 320V DC, this is, as you would say, "pulsed". Because of this "pulsing", the primary capacitors store and filter that. The primary switching transistors then convert that back into a high frequency AC voltage to be able to take advantage of the smaller, more efficient AC transformer. The out put of the transformer is then rectified back into DC using diodes. The output of the diodes is "pulsed", and smoothed out using capacitors and coils.

One of the best things about this is that the primary transistors frequency of how fast they switch "on and off" is controlled by a controller. When the power supply comes under a load and the voltage starts to drop, the controller can increase the frequency that the transistors switch in order to keep the voltages within their set limits.

[mariushm]

mains voltage is varying ac voltage between 90v and 250v and a with a frequency between 45 and 65 Hz, depending on the country you're in.

Were you to convert this to a lower voltage using a step down transformer, you would need a very large and heavy transformer to achieve 500-1000w, ratings that are common for power supplies.

So you rectify this to dc voltage and maybe use an active power factor circuit to prepare the input for the transformer, up to 400-420v dc voltage smoothed out by those large capacitors on the input side.

Then you use mosfets or transistors or whatever floats your boat to send pulses through the smps transformer, at a frequency that depends on the amount of load on the power supply... but usually the frequency is up to 100-150 kHz... as you ca see much higher than 45-65 Hz, which explains why smaller specialized transformers can be used.
The output voltage is then rectified and capacitors are used to smooth out the output and then an optional PI filter (inductor plus capacitor) to smooth out the output even better.


PS. Read these applications notes to understand better switching power supplies (click on the PDF icons in the page, if you don't get a pdf directly)
AN1114 Switch Mode Power Supply (SMPS) Topologies (Part I)
http://www.microchip.com/stellent/id...pnote=en532115

AN1207 Switch Mode Power Supply (SMPS) Topologies (Part II)
http://www.microchip.com/stellent/id...pnote=en536097

Power Semiconductor Applications - chapter 2 CHAPTER 2 Switched Mode Power Supplies

http://www.nxp.com/documents/applica...te/APPCHP2.pdf

[budm]

"Many simple power supplies do simply rectify the input AC voltage into a DC voltage, then run it through a transformer to step it down " You do not feed the transformer with DC, you feed the transform with AC, in case of linear power supply for 120VAC, the primary side is fed with 120VAC 60Hz (US), if it is step down transformer, then the output voltage will be lower, that out put is then fed into rectifier, it can be half-wave, full-wave with center tap transformer, or full-wave bridge (no center tap).

https://en.wikipedia.org/wiki/Linear...d_power_supply

https://en.wikipedia.org/wiki/Switch...e_power_supply
https://en.wikipedia.org/wiki/Rectifier

[Ashroyer05]

How could a transformer run off dc since the ac is rectified first.

[cheapie]

Originally Posted by Ashroyer05

How could a transformer run off dc since the ac is rectified first.

In a linear power supply, the transformer is fed from the incoming power supply (AC). In a switching power supply, the AC is rectified to DC, then it is turned back into AC with a much higher frequency, and that goes into the transformer. That way, a smaller transformer can be used.

[rievax_60]

Transformers dont mind having pulsed Direct Current flowing through their windings, It is only important that the voltage averages zero to avoid magnetic saturation.
This applies mainly to single ended Flyback and Forward designs.

[Per Hansson]

Ashroyer05, you're creating allot of threads on the theory of how a SMPS works.
I don't mind but maybe these links can give you a more thorough understanding than simply shooting out questions as they pop up?

http://www.badcaps.net/forum/showthread.php?t=1238

http://www.learnabout-electronics.org/PSU/psu30.php

http://www.hardwaresecrets.com/article/327

http://en.wikipedia.org/wiki/Switche...y_of_operation

[Ashroyer05]

What is a single ended flyback?

[Ashroyer05]

Hansson, i post because people give answers and don't check back or they give answers like my past instructor who was an engineer and couldnt comprehend my simplistic questions. I have already read most of the links you sent me (thanks by the way) but they all lack the answer im looking for.

[Khron]

Single-ended = only one switching element (transistor) is used
Flyback = simplest(?) form of isolated switching PSU - the transformer is actually a coupled inductor.

[mariushm]

Originally Posted by Ashroyer05

Hansson, i post because people give answers and don't check back or they give answers like my past instructor who was an engineer and couldnt comprehend my simplistic questions. I have already read most of the links you sent me (thanks by the way) but they all lack the answer im looking for.

Did you read the application notes and the chapter in that Phillips books I posted above? Those explain in pretty simple terms how switching power supplies work.

[Ashroyer05]

Marius, I just read back through the comments here and noticed them. I will read them and thank you for the info.

[budm]

and more post here:
http://www.badcaps.net/forum/showthr...717#post425717

I think he is skipping the basic electronics if you have to ask about the grounds.
You have to understand the basic first instead of keep creating lots of threads.

[mmartell]

FWIW I've read through some threads and faqs herw as well and agree with Ash that there are far too many gaps to connect the dots for a beginner.

I too will check those links above. Thanks.

[paulnetv]

Can I try?

I think you are asking WHY do we do it rather than HOW does it work? If so then putting it simply, you get 45 - 60Hz from your wall outlet. (Depending on which country you live in.)

The lower the frequency you switch at, the larger the transformer needs to be.

So, if you want smaller component size, less weight etc, you switch at higher frequency.

To do this, you firstly have to rectify your 50Hz (Here in UK) and smooth it giving you DC, this then allows you switch it very rapidly allowing for smaller transformers.

I hope that helps WHY we have SMPS.

[Ashroyer05]

Its mainly this power supply that's confusing me. I've worked on a few but this one is like a city. All the transformers confuse me. Say i needed 50dc output and 12dc and the typical standby of 3.3 or 5. Does each transformer supply each of the voltages? I took electronics but i grew up in a mechanics home so yes i have a lot of basics to learn or relearn in electronics but im willing. In college my professor had a hard time explaining things to me as well As he understands electronics to the nth degree and its something i came into blindly. I have learned a lot here though and hope to be able to help others one day.

[domas]

a single transformer can have more than one output winding