RESONANT PWM CIRCUITS ,ZVS and ZCS

Discussion in 'CAD' started by Vicky, Oct 11, 2003.

  1. Vicky

    Vicky Guest

    Hi,
    Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
    from an normal PWM circuit.I mean what is the basic concept of zero
    voltage switching and zero current switching techniques.Can anyone
    explain????
    Vicky
     
    Vicky, Oct 11, 2003
    #1
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  2. Vicky

    analog Guest

    Vicky wrote:

    > Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
    > from an normal PWM circuit. I mean what is the basic concept of zero
    > voltage switching and zero current switching techniques. Can anyone
    > explain????


    Just explaining it would be a no brainer. No brainstrain, no gain.

    So, then, let's pull out the brain teaser, shall we?

    Q: Why use PWM at all? Why not a simple series pass element?

    Q: In light of the answer to the prior question, how might ZVS and
    ZCS further illuminate this advantage?

    Good luck.
     
    analog, Oct 11, 2003
    #2
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  3. Vicky

    The Al Bundy Guest

    "Vicky" <> wrote
    news:...
    > Hi,
    > Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
    > from an normal PWM circuit.I mean what is the basic concept of zero
    > voltage switching and zero current switching techniques.Can anyone
    > explain????
    > Vicky


    I guess you meant a half or full bridge driven with PWM.
    Some very basically half bridge:

    --------
    |
    \ high
    |
    |---out
    |
    \ low
    |
    ------

    With the above circuit you can switch only 1 switch is on at a time,
    otherwise you will short the supply :) When the switches are ideal you can
    switch one off and directly the other one on, to get a square wave signal at
    the mid point and there is no power loss.

    When replacing these switches with MOSFET's you need to take care. Because
    of parasitic capacitance between the drain - source (basically the switch)
    you need some time to let those capacitors charge / discharge between
    switching. When you don't do this the MOSFET's will dissipate energy.

    Basically with ZVS you want to make sure the voltage across a MOSFET is
    zero before you turn it on. When doing this there is already a current
    flowing through the body diode of the MOSFET, which the Rds-on will take
    over.
    With ZCS is just the opposite, you want that no current will flow through
    the MOSFET when you turn it on. (Voltage is also near zero across the
    MOSFET)

    With P = V * I you can see that in both cases the MOSFET(s) wont dissipate.
    However due to the voltage drop of the body diode and Rds-on you always get
    some dissipation, but less then with no ZVS/ZCS.

    You need 2 things to have ZVS/ZCS, enough inductive load (never let switch a
    half/full bridge unloaded or only with a capacitor...) and some so-called
    deadtime between switching the MOSFETS.

    The inductive load will charge/discharge the parasitic capacitance of the
    MOSFETS and so creating ZVS/ZCS when running the half/full bridge with a
    deadtime.
    The term 'resonant' comes from that the inductive load charges / discharges
    the capacitance of the MOSFET's.

    PWM is just the modulation. However you need to take more care with PWM to
    switch ZVS/ZCS then with a 50% duty-cycle.

    Do some search with google and you will find more information.
    Hope it helps a bit.

    Al
     
    The Al Bundy, Oct 11, 2003
    #3
  4. On 11 Oct 2003 02:38:36 -0700, (Vicky) wrote:

    >Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
    >from an normal PWM circuit.I mean what is the basic concept of zero
    >voltage switching and zero current switching techniques.Can anyone
    >explain????


    Have a search of the TI site (www.ti.com). AFAIR they have the
    Unitrode lecture notes in pdfs. That will give you loads of
    information. But to sum up, you add a tuned circuit to the mosfet.
    Typically this uses parasitic elements like mosfet capacitance and
    transformer leakage inductance although you can pad these with real
    components too. When you switch off a tuned circuit it will ring.
    That is it will rise up in sine wave fashion and then come back down
    again. Turn on when it is at the bottom and you have losses switching
    since you turn off at 0V and on at 0V and since P=I*V then P=0 (more
    or less).

    There are variations on this and I suggest you look at the unitrode/TI
    notes.

    Malcolm

    --

    ....malcolm

    Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
    (, or ).
    Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
    electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

    http://www.fullcircuit.com or http://www.fullcircuit.co.uk

    NEW - Desktop ToDo/Reminder program (free)
     
    Malcolm Reeves, Oct 13, 2003
    #4
  5. Vicky

    Vicky Guest

    Hi ,
    I have typed down the following points as per the brasin teaser
    asked and some answers i recieved in the groups.Please add to or
    correct the points
    if they are not perfect:

    > Q: Why use PWM at all? Why not a simple series pass element?

    PWM's are used because of the higher effieciency they offer in
    power supply design.The pass device will be operated in either cutoff
    or saturation thus converting input DC to a high frequency
    square/pulse waveform.But if we go for a series or linear regulators
    there the pass device operates in linear region and efficiency is
    lesser.


    Q: In light of the answer to the prior question, how might ZVS and
    ZCS further illuminate this advantage?

    So main loss in a smps would occur when the device(MOSFET) is
    actually switching.So if we have make the voltage across the switching
    device or current throuh it actually zero before the actual switching
    happens then the power losses are further minimised.
    I hope this is the basic concept.
    But how do we choose a topology when we start the actual design.Also
    what exactly is the role of transformers in the SMPS.

    Regards
    Vicky
     
    Vicky, Oct 14, 2003
    #5
  6. Vicky

    analog Guest

    Vicky, analog wrote:

    > I have typed down the following points as per the brasin teaser
    > asked and some answers i recieved in the groups. Please add to or
    > correct the points if they are not perfect:
    >
    >> Q: Why use PWM at all? Why not a simple series pass element?

    >
    > PWM's are used because of the higher effieciency they offer in
    > power supply design. The pass device will be operated in either
    > cutoff or saturation thus converting input DC to a high frequency
    > square/pulse waveform. But if we go for a series or linear
    > regulators there the pass device operates in linear region and
    > efficiency is lesser.
    >
    > Q: In light of the answer to the prior question, how might ZVS and
    > ZCS further illuminate this advantage?
    >
    > So main loss in a smps would occur when the device(MOSFET) is
    > actually switching. So if we have make the voltage across the
    > switching device or current throuh it actually zero before the
    > actual switching happens then the power losses are further minimised.
    > I hope this is the basic concept.


    Very good. You seem to understand the motivation behind ZVS and ZVC
    designs. By the way, the main loss in a smps (even a hard switching
    type) is not necessarily switching (although it may well be).

    > But how do we choose a topology when we start the actual design.


    I know of no simple answer to this question. It all depends on your
    design constraints. For example, some of the simplest and best
    performing lossless switching designs operate with a line and load
    dependent variable frequency. This may be incompatible with your
    design constraints.

    > Also what exactly is the role of transformers in the SMPS.


    How were you able to answer the first two questions and not know
    the answer to this much more basic question? Perhaps you are a
    student and have no practical experience? Also, if you don't mind
    my asking, what country are you from?

    A transformer is most often used to provide two properties. One is
    galvanic isolation. What the other is, is given away by its name.
    Care to guess what it may be?
     
    analog, Oct 14, 2003
    #6
  7. Vicky

    Vicky Guest

    >> Also what exactly is the role of transformers in the SMPS.
    >
    > How were you able to answer the first two questions and not know
    > the answer to this much more basic question? Perhaps you are a
    > student and have no practical experience? Also, if you don't mind
    > my asking, what country are you from?
    >
    > A transformer is most often used to provide two properties. One is
    > galvanic isolation. What the other is, is given away by its name.
    > Care to guess what it may be?


    Hi,
    I do know that two main functions of a transformer are :
    1>Electrical isolation
    2>Stepup/stepdown voltage.

    But if we go through the smps reference manual(page22) from ONSEMI it
    is mentioned as below:

    "In forward mode topologies, the transformer is only used for stepping
    up or down the AC voltage generated by the power switches. The output
    filter (the output inductor and capacitor) in forward mode topologies
    is used for
    energy storage. In boost mode topologies, the transformer is used both
    for energy storage and to provide a stepup or stepdown function."
    I did not get the point here.that was the reason behind asking the
    role of transformer.i mean how does it differ in the various modes.
    Regards
    vicky
     
    Vicky, Oct 15, 2003
    #7
  8. Vicky

    analog Guest

    Vicky, analog, Vicky wrote:

    >>> Also what exactly is the role of transformers in the SMPS.

    >>
    >> How were you able to answer the first two questions and not know
    >> the answer to this much more basic question? Perhaps you are a
    >> student and have no practical experience? Also, if you don't mind
    >> my asking, what country are you from?
    >>
    >> A transformer is most often used to provide two properties. One is
    >> galvanic isolation. What the other is, is given away by its name.
    >> Care to guess what it may be?

    >
    > I do know that two main functions of a transformer are :
    > 1>Electrical isolation
    > 2>Stepup/stepdown voltage.


    This is a good start, but always bear in mind that current is
    transformed (stepped down and up) inverse to that of voltage. An
    ideal transformer stores no energy, only transforming it in such a
    way that the product of instantaneous voltage and current on the
    secondary winding is always equal to that of the primary winding
    (at least for a transformer with only two windings). A more
    general observation would be that net ampere-turns must always
    equal zero (do you know what this means?).

    > But if we go through the smps reference manual(page22) from ONSEMI it
    > is mentioned as below:
    >
    > "In forward mode topologies, the transformer is only used for stepping
    > up or down the AC voltage generated by the power switches. The output
    > filter (the output inductor and capacitor) in forward mode topologies
    > is used for energy storage. In boost mode topologies, the transformer
    > is used both for energy storage and to provide a stepup or stepdown
    > function."


    > I did not get the point here.that was the reason behind asking the
    > role of transformer.i mean how does it differ in the various modes.


    This manual is confusing coupled inductors with transformers. At least
    for the sake of understanding, it is probably best to think of coupled
    inductors (such as used in flyback converters) as a combination of an
    idea transformer with an ideal inductor connected in parallel with its
    primary winding.
     
    analog, Oct 16, 2003
    #8
  9. Vicky.
    If you go to Google and search RESONANT PWM CONVERTERS, IONVERTERS AND
    COMBINATIONS OF THESE WORDS you'll get mountains fo information in the form,
    in many cases of MSc and PhD thesis. IEEE is closed if your'e not a member
    and os are some fo the universities aroung the world but, in the main you'll
    find some very interesting but VERY technical materil with schematincs and
    the rest.
    Happy searching!
    ROBIN


    "Vicky" <> wrote in message
    news:...
    > Hi,
    > Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
    > from an normal PWM circuit.I mean what is the basic concept of zero
    > voltage switching and zero current switching techniques.Can anyone
    > explain????
    > Vicky
     
    ROBIN MARSTRAND, Oct 19, 2003
    #9
  10. To All on this thread.

    I saw from other posting below that there could be some confusion.
    I am learnign but would like to clarify on some points:-

    PWM = Pulse Width Modulation (everybody knows this)
    ZVS = Zero Voltage Switching (as applies to PWM power converters and
    Inverters)
    ZCS = Zero Current Switching (as applies to PWM power converters and
    Inverters)

    These techiniques allow for less stress in the switching devices and rely
    upon a whole variety of circuit-additional features which add complexity but
    improve performance in may respects.
    The subject is very deep and a rapdily growing and developing technolgy,
    especially as semiconductors are now capable of massive power handling at
    reasonably high frequencies - megawatts of power, indeed.

    Best. ROBIN.


    "Vicky" <> wrote in message
    news:...
    > Hi,
    > Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
    > from an normal PWM circuit.I mean what is the basic concept of zero
    > voltage switching and zero current switching techniques.Can anyone
    > explain????
    > Vicky
     
    ROBIN MARSTRAND, Oct 19, 2003
    #10
  11. Vicky,

    You have grasped the concept.
    I suggest you follow my suggestions an earlier posting on this thread, re
    searching for info.

    Best, ROBIN

    "Vicky" <> wrote in message
    news:...
    > Hi ,
    > I have typed down the following points as per the brasin teaser
    > asked and some answers i recieved in the groups.Please add to or
    > correct the points
    > if they are not perfect:
    >
    > > Q: Why use PWM at all? Why not a simple series pass element?

    > PWM's are used because of the higher effieciency they offer in
    > power supply design.The pass device will be operated in either cutoff
    > or saturation thus converting input DC to a high frequency
    > square/pulse waveform.But if we go for a series or linear regulators
    > there the pass device operates in linear region and efficiency is
    > lesser.
    >
    >
    > Q: In light of the answer to the prior question, how might ZVS and
    > ZCS further illuminate this advantage?
    >
    > So main loss in a smps would occur when the device(MOSFET) is
    > actually switching.So if we have make the voltage across the switching
    > device or current throuh it actually zero before the actual switching
    > happens then the power losses are further minimised.
    > I hope this is the basic concept.
    > But how do we choose a topology when we start the actual design.Also
    > what exactly is the role of transformers in the SMPS.
    >
    > Regards
    > Vicky
     
    ROBIN MARSTRAND, Oct 19, 2003
    #11
  12. Vicky

    ddwyer Guest

    > I did not get the point here.that was the reason behind asking the
    >role of transformer.i mean how does it differ in the various modes.
    >Regards
    >vicky

    At the risk of being obvious the various characteristics of switched
    mode escaped me initially, this may also help:
    Energy can be stored in an inductor in a similar but different manner to
    a capacitor.
    Line (mains) PSU generally rectify the mains to DC and then chop it up
    by charging an inductor for a time that determines how much energy is in
    the inductor the energy is released in one of a number of ways including
    the switching element going into the off state providing the flyback
    condition to boost the voltage.
    Transformers (with switching circuits) with mutual inductance less than
    1 can also be used simultaneously to store energy transform the voltage
    and limit ripple.
    If you get into the practical realms I found that unlike signal and
    signal analogue circuits switched mode prototypes can generate heaps of
    blown IC s and Power transistors.
    LT spice is most helpful but does not include breakdown in its models
    perhaps this helps sell ICs!



    ,


    through a
    --
    ddwyer
     
    ddwyer, Oct 19, 2003
    #12
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