smps and power supply system?

Discussion in 'General Electronics Chat' started by simpsonss, Jan 11, 2010.

  1. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
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    hi all,
    i'm just start working on understanding SMPS. when i google around and the questions just boost up. What is the different between smps and power supply unit for example in a DVD player.
    Is it the power supply unit in a dvd player is also consider as a SMPS?

    When i read on the net, everyone says that SMPS is different with the linear regulator. So what is it actually makes SMPS a switching mode power supply? Which part of the SMPS shows that it is in switching mode?

    hope someone else can help me on this.

    thank you.
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Regulated power supplies are basically two types:-

    Switch Mode (SMPS) and Linear.

    The basic difference is that in a SMPS the controlling device (transistor) is either ON or OFF. The regulation is done by varying the ratio of ON time to OFF time. Therefore, very little power is dissipated in the transistor. Energy storage (using capacitors and inductors) is required to supply power while the transistor is OFF.

    In a linear power supply, the controlling device varies its resistance and drops voltage as the current passes through it. Power is lost in the transistor. No energy storage components are required.

    Advantages of SMPS:-

    • More efficient - less lost power, runs cooler.
    • Can be made smaller
    • Output voltage can be higher than input voltage.
    • Isolation between input and output possible.
    • Multiple outputs possible
    Disadvantages of SMPS

    • More complex
    • Generates electrical and magnetic interference.
    Advantages of Linear Power supply:-

    • Simple
    • Low noise
    Disadvantages of Linear Power Supply:-

    • Low efficiency - Cooling required
    • Output voltage must be less than input voltage.
    Due to integration and miniaturisation of electronic components, most regulated power supplies are now Switch Mode. Operation at high frequencies allows the use of smaller inductors and capacitors and makes filtering easier.
     
  3. S_lannan

    Active Member

    Jun 20, 2007
    247
    2
    I would also add the main disadvantage of most linear power supplies is size and weight. A 100 watt linear power supply in a slimline dvd player just wouldn't work out.
     
  4. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    [​IMG]hi,

    first of all thanks for the replies.
    ok,after understand all the theories part, now i go for practical part.
    [​IMG][​IMG] ABCD0005.jpg
    As the figure above,i already square up all those section that i recognize in a power supply system. Anything wrong with my labeling?
    Upon JDT's explanation, i think i'm sure that this is a SMPS, because i found a smps primary switcher on the PCB.Am i right?:D
    But i'm not sure with rest of the part esp the light blue square box that i label in the figure.Anyone can help me on this?

    thank you.
     
  5. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Definitely a SMPS. Labelling OK.

    Note the yellow box capacitor and the dual inductor in your un-labelled cyan box - these are the RFI filtering components to reduce interference feed back to the supply.

    The thermistor has a high resistance when cold. This reduces the current surge when the mains supply is first connected and the filter (reservoir) capacitor is charged up from zero. The resistance of the thermistor drops to a low value when warm.

    Also notice the big fat diode - output rectifier. This is probably the part that causes most of the power loss (efficiency loss) in the power supply.

    This is a "flyback" type of SMPS so the transformer doubles as output isolation and energy storage.
     
  6. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    hi JDT,
    thanks again for the reply.
    Is it the RFI filtering u mention is also call EMI filtering? Do the dual inductor has any other name?because when i google it, it seems like not much info on it.how is the symbol looks like in a schematic diagram?

    Another question is what is the blue color looks like capacitor component which stated near the transformer? what is the purpose of it?

    and also what is the purpose of the big resistor(should be bigger watt) on the right side of the filter cap.

    thank you.
     
    Last edited: Jan 13, 2010
  7. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
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    still waiting..thanks.
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    It is also known as a "Common mode choke". Just Goggles it.

    It is just a capacitor. Notice how thick is another blue capacitor right at the bottom of the big transformer. This is the bridging/coupling capacitor which connects primary live circuit to the secondary low voltage circuit to bypass the interference caused by the switching current.

    Because the failure of this capacitor could led to possible electrocution, this capacitor is special and could be rated for several kilovolts. Do not substitute it with any other types of capacitor, even one rated at 600V.

    If you meant the bigger gray(grey) resistor, it could either be a light load on the SMPS output to prevent it from shutdown or just some forms of filtering.

    You have missed labeling another vital component, the secondary rectifier. This is not normal AC rectifier like 1N400x or 1N5401, but have to be really fast in turn off speed in order to work correctly.
     
  9. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    hi eblc,
    thanks for the reply.
    the vital component that u mentioned is it the one i just highlighted it with a green box?
    ABCD0005.jpg
    thanks.:D
     
  10. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Yes, it is.

    To help you understand the circuit better, I've attached a typical schematic diagram I taken from page13 of ST Application note AN1735, a 10W off-line switcher.

    Judging by the look of your SMPS, I'm 99.9%(up from 80%) certain that it is using the Viper22A chip. Can you readout any marking on the IC?

    The attached circuit is for single output while yours has multiple outputs. The circuit component arrangement will be nearly identical, especially those on the primary side.

    [​IMG]
     
    Last edited: Jan 13, 2010
  11. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    yeah~you're right.the IC that u mentioned is viper22A. For the L1 in the schematic that u attach,is it the dual inductor/common mode choke?

    i feel like the SMPS i used is a bit complex for me.so i decided to use a more simple SMPS which is just a single output which is 5V.I have a few question on that SMPS.
    From the schematic below(which i draw it), for the MBRF10150,if i'm not wrong it should be a 10.0amps schottky barrier rectifier. The purpose for this section is it for overload protection?:confused:

    And the big blue cap that i label is it the one that u mentioned which is a bridging/coupling capacitor which connects primary live circuit to the secondary low voltage circuit to bypass the interference caused by the switching current?:confused:
    ABCD0006.jpg
    thank you.:D
     
    Last edited: Jan 13, 2010
  12. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Yes, it is.

    No. It is the main rectifier for the transformer secondary. The vital component that I have mentioned before. It is not there for protection of any sort.

    Yes.

    Simpsonss, it seems to me that you are now moving into dangerous territory of actually working on a SMPS with mains connected. Since this is a public forum, I feel obliged to place the following warning message:

    Please be reminded that all suggestions and comments are based on the quest of knowledge on how a SMPS circuit board works, which does not require the board be powered up or connected to the mains.

    In fact I would now explicitly warn you about the danger of connecting any SMPS to the mains.

    If you ever connect it to the mains then the board can easily kill someone. No one besides you should take responsibility if such thing happens.
     
  13. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    thanks for the reply and advices.

    For the MBRF10150 component, u mentioned that it is the main rectifier for secondary part.But isn't it rectifier is from AC to DC? How actually it work as a rectifier on the secondary section? Can you explain abit more on it?

    thank you.
     
  14. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Transformer does not work with DC.

    The voltage at the transformer secondary is high frequency AC. It must be rectified into DC before you can use it.

    The main rectifier MBRF10150 does exactly that.
     
  15. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    hi eblc,
    ok.Since u mentioned that transformer does not work with DC right? but isnt it the main AC after pass through the bridge rectifier will become DC?then the DC will connect through a filter CAP and pass through the transformer?isnt it in this way the power supply works?
    I just confuse on what u just mentioned with the theories. :confused:

    or is it the switching power IC which connect to the transformer generates high frequency AC?and then the AC need to be rectified into DC? but the main AC after rectified by the bridge rectifier isn't it a DC?wow~so confusing.hope u can help me on this.haha..

    Below is the new SMPS that i bought which has a 5Vdc output. From the schematic, there is a MBRF10150CT which is the component that we discussed. From the circuit, why at the MBRF10150CT section there are a seems like a loop back to the secondary transformer part?How actually this component works?

    For the primary to secondary part, how can we find out it is a forward mode or a flyback mode? is it depends on the inductor that connected after the secondary rectifier?

    ABCD0001.jpg
    thank you.
     
    Last edited: Jan 15, 2010
  16. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Exactly. This is the essence of "SWITCH MODE" power supply. There must be switching going on and it happens on the primary current.

    Told you this is necessary because the transformer cannot pass DC current therefore the primary DC has to be switched ON/OFF to create a change of flux in the core of the transformer. This flux changes will then be coupled into the secondary winding as AC current and the rectifier changes it into DC current.

    Look carefully, it is just two diodes wired in parallel, with output at the middle junction.

    If you wants to learn the theory and really understand SMPS, it will take quite sometime. The internet is full of material that can explain SMPS in more details than the space of a forum post. I would suggest you google them and read them first.

    No, it does not.

    If one takes a look at the primary and secondary current, then it will be obvious which mode the SMPS is operating. There are other tell tale signs like capacitors arrangement etc.. As forward mode design is slightly more involved than flyback mode, small SMPS of less than a few tens of Watts will normally be of flyback mode design.
     
  17. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    The "two diodes wired in parallel and with a output at the middle junction", actually i draw this by referring to the datasheet of MBRF10150CT. what inside the MBRF10150CT is the one i draw out.

    yea! it is in parallel.duh~now just i realized.so if the two diodes is in parallel it is just like a full wave rectifier which using two diodes right?

    Basically i plan to learn SMPS by google it.but somehow all the results are just explaining by just using block diagram or just the rectification part. So i decide to ask, to have a more interactive and more practical learning. thats why i bought a simple SMPS.haha..:D

    By the way hope u can still help me on my learning.:D:D:D

    The question is about the shunt regulator component at the secondary part. As what i see, the shunt regulator is working with the opto-isolator to generate the feedback for the switching IC. Am i right? Is it the shunt regulator is the one to let the output maintain at +5v dc?From what i google for shunt regulator, it just like a regulator. Am i right too?

    thank you.
     
  18. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Wrong.

    Two diodes in parallel is just like one diode, but with higher current rating.

    The shunt regulator does not regulate the output directly. It just forms a stable working reference point for the opto-isolator so any increase/decrease in ouput voltage is communicated back to the primary via the opto-isolator. The primary side switching IC then adjust the switching waveform timing and this results in the correction of the output voltage.
     
  19. simpsonss

    Thread Starter Active Member

    Jul 8, 2008
    173
    0
    ok.for higher current rating,so if i have a look on the waveform before and after the MBRF10150C, i'll get an AC signal before the MBRF10150CT; get a DC signal waveform which just like a half wave rectification waveform?

    thank you.
     
  20. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You will only get a half-wave rectification waveform if there are no other smoothing components(inductor, capacitors) after the diode.

    This isn't the case in the schematic.
     
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