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PWM in a switching power supply

Discussion in 'Electronic Basics' started by BradBrigade, May 27, 2005.

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  1. BradBrigade

    BradBrigade Guest

    Hi,

    First of all, I'm trying to figure out how switching power supplies
    work (the ones in PCs). I've found very basic info, but I want more
    technical stuff. If anyone has some good links please let me know.
    These are questions I have yet to find an answer for.

    Anyway, here's my question. One thing I read was that the output
    voltage of the supply is fed back to the PWM which changes it's duty
    cycle accordingly to keep the output voltage constant. But I thought
    that the input-to-output ratio of a transformer is fixed. If the PWM
    is outputting 100V at 20KHz to a 10:1 transformer, you get out 10V at
    20KHz, right? What does it matter what the duty cycle is? It's still
    100V at 20KHz. What am I missing?

    Second, why does a switching power supply break without a load?

    Third, in all my years in electronics, I have never used a choke, now I
    see them all over these power supplies. Can someone clue me in about
    what they do, and why they are in these things?

    I appreciate any info at all. Thanks a lot.
     
  2. w_tom

    w_tom Guest

    Duty cycle changes the energy of frequencies (sine waves of
    different frequencies) that, together, form a square wave.
    Learn about Fourier series (from math books) to better
    appreicate the concept. As duty cycles change, then more
    energy may appear in a higher frequency sine wave. Those
    higher frequencies dissipate more energy in the transformer.
    Best to have only a frequency at the ideal transformer
    frequency. But then duty cycle would not change to adjust
    output voltage. Less efficiency for better voltage
    regulation. This and other compromises are why the switching
    power supply cannot have 100% efficiency.

    Unloaded switching power supplies do not break. Some
    switching power supplies do not operate well in a no load
    condition; so power supply shuts down without damage.
    Characteristics of each design. Important is even how the
    transfomer is designed. Numerous design compromises are
    involved. Many switching power supplies work just fine under
    no load. Which ones? The long list of numeric specs should
    be provided for each switching power supply model.

    Chokes permit energy at some frequency to be converted to
    energy at other frequencies. Chokes permits the design to
    intergrate filters with sharper cutoff frequencies. Chokes
    are essential to better EMC solutions. Chokes can store
    energy at certain key operational points. But chokes cost
    money. Some power supplies are designed to cut costs rather
    than meet standard criteria. To better appreciate the value
    and energy conservation of chokes, learn fundamentals of
    filter design. Concepts that also better explain what happens
    as duty cycle changes.
     
  3. If the transformer is a voltage output (produces some ratio of the
    primary voltage when the switches are on) and zero the rest of the
    time), then, yes, the peak output voltage is essentially independent
    of the duty cycle. but those kind of transformers also require an
    additional LC filter that outputs a voltage about equal to the average
    input voltage, not the peak. Holding the peak voltage for a smaller
    part of the cycle lowers the average voltage.

    In supplies, where the transformer acts as an energy storage device
    (apply input voltage, till the primary current ramps up to some value,
    then cut the primary current, forcing the stored energy to reverse the
    winding voltage and go up till an output rectifier connects the
    transformer secondary to some storage capacitor). the average energy
    throughput depends on how high the energy each charge-discharge cycle,
    and how many cycles per second, and if the stored energy is all dumped
    each cycle, or only some of it (whether or not the primary switch is
    left off till the transformer dumps all its magnetic energy, or is
    turned back on while the dump is in progress).
    Break? As in Kablooie? I don't know about that, but many
    malfunction, because the control loop gain is dependent on the load
    current. That is, the gain goes up as the load decreases. Stability
    requires that the loop gain fall as frequency rises, so that before
    the frequency is reached where the loop phase shift swings by 180
    degrees (compared to low frequencies) the gain has fallen below 1, so
    that the negative feedback (converted to positive feedback by the
    extra phase shift) cannot generate a self sustaining echo.
    There are just inductors. They store energy proportional to the
    inductance and proportional to the square of the current passing
    through them. Once you get up into significant currents, they become
    as useful and necessary for energy storage as capacitors. Whereas
    capacitors pass current in order to control the rate of change of
    their voltage, inductors generate voltage across them to control the
    rate of change of the current through them. If you want to absorb
    current pulses and stabilize voltage, you use a capacitor. If you
    want to absorb voltage pulses and stabilize a current, you use an
    inductor. And we are back to that averaging filter that is needed to
    smooth out the current from that pulsing voltage, variable duty cycle,
    constant peak voltage rectified transformer so that it can be
    connected to a storage capacitor where the voltage is to be regulated.
     
  4. Joel Kolstad

    Joel Kolstad Guest

    Hi BradBrigade,

    Yes, the _instantaneous_ output voltage of the transformer is still 10V.
    The output of the transformer is feed to some form of filtering -- usually a
    series inductor and parallel capacitor -- and the output voltage then found
    across the capacitor is the _average_ of the input voltage. So... 10V
    instantaneous output from the transformer at 100% duty cycle gets you 10V
    across the capacitor... 50% duty cycle would get you 10V instantaneous output
    from the transformer for half the time and 0V for the other half of the time,
    so this averages to 5V across the capacitor... etc...

    (This is somewhat simplified; in actuality the voltage change a little due to
    diode drops, active device losses, etc. -- the feedback loops jiggles the duty
    cycle until the right regulated output voltage appears, though.)
    Because the designers are either (1) ignorant or (2) cheap. :)

    OK, actually, not all switchers have no-load problems. What happens -- and
    this is for the most basic example you could come up with, something like a
    simple buck converter -- is that during the time that the transformer (or
    inductor) is being fed current, flux (current) builds up in it. When the
    switch controlling this current is turned off, the current (flux) in the core
    starts dropping. The rate at which it drops is proportional to the load...
    bigger load (lower impedance), faster current drop. With a "big enough" load,
    the current goes all the way to zero. On the next switching cycle, the
    current ramps up again to some current, then ramps down to zero, etc. -- this
    repeats forever.

    Now, with a small load, while the current does drop, it doesn't go all the way
    to zero before the next switching cycle. Now current ramps up again and -- if
    one isn't careful in design -- the current ends up higher than it was at the
    turn-off point of the last cycle. It drops a little again (but not to zero),
    and now at the next turn on the current is driven even higher. Sooner or
    later, the inductor saturates, which tends to look almost (but not quite) like
    a short circuit to the driver. That driver starts having massive current run
    through it, heats up, and sooner or later dies.

    Hence, there's some minimum load that causes the switcher to change from
    discontinuous to a continuous mode of operation, and for loads lighter than
    this you can get into trouble.

    If you think about it for a moment, it's clear you could simply detect the
    current in the core and quit driving it when you start to approach
    saturation -- this simple solution is what "current mode" switchers do, and
    they usually don't have no-load problems. In fact, if you think about it even
    more, even in the case with regular "voltage mode" feedback, since the current
    in the core is increasing, the output voltage will as well, so the feedback
    regulator should keep clamping down on the duty cycle so as to avoid
    saturating the core. The problem here is that the 'transfer function' of the
    power supply from input to output is different when it's operating in this
    'continuous' mode rather than 'discontinuous' mode, and it takes more effort
    to build a feedback network that can keep the entire 'loop' stable in both
    modes (and without starting to become slightly sophisticated in your feedback
    network, making a power supply no-load stable often degrades the step
    response, which isn't desirable). Hence, for both the sake of cost (the extra
    feedback network circuitry) or merely ignorance on the part of the designer,
    some power supplies turn over and die when run without a minimum load.
    Ummm... you know what capacitors do, right? You 'feed' them a current and
    this causes charge to accumulate within them such that a voltage appears
    across them? Inductors are their 'dual' -- you 'feed' them a voltage and this
    causes flux to accumulate within them such that a current flows throughout
    them. Hence, just like a capacitor, inductors can be used to store energy.
    With an inductor, by varying the duty cycle of a voltage across it, you can
    'charge up' the inductor to some arbitrary average current and then 'dump'
    this into a load to get a corresponding voltage. This makes it easy to make
    regulated _voltage_ power supplies, whereas a similar approach with capacitors
    would get you a regulated _current_ power supply.

    I'd suggest checking out Abraham Pressman's switching power supply book. It's
    not cheap, but it's written by a guy who seemed far more intent on building
    working power supplies than doing more theoretical research. It does of
    course have some math in it, but even someone with one semester of caclulus
    will probalby be able to follow it.

    ---Joel
     
  5. Joel Kolstad

    Joel Kolstad Guest

    W_Tom,

    Uh, no, it doesn't! If I give you a periodic signal of a certain frequency,
    it doesn't matter WHAT it 'looks' like, its Fourier spectrum will ONLY contain
    the fundamental frequency and harmonics. Changing what it 'looks' like (e.g.,
    the duty cycle of a square wave) only changes the magnitudes of the harmonics.
    Overall most switchers lose far more efficiency in the switches than they do
    in the core.
    Some do. :) Although you could convince me that such switchers are, by
    definition, poorly designed.
    Where do you get this stuff? Some generic "everything you wanted to know
    about electronics" encyclopedia? A lot of what you say is factually correct,
    just not at all relevant to the discussion at hand.
    Someone who's an expert at filter design could very well not know the first
    thing about switcher design. They're pretty disparate areas of design, and
    really only start to overlap somewhat when you discuss output filtering,
    resonant designs, etc.

    ---Joel Kolstad
     
  6. BradBrigade

    BradBrigade Guest

    OK, I got it, the output is filtered to create a stable voltage that is
    the average of the duty cycle. But now I'm wondering, what is the
    purpose of the transformer? If you want to convert 100V to 10V, why
    not filter the output straight from a PWM with a 10% duty cycle?
    What's the difference?

    Thanks a lot.
     
  7. w_tom

    w_tom Guest

    A switching power supply could easily provide those voltages
    without the expensive transformer. But a power supply must
    also perform many other functions including galvanic
    isolation. For computers, this breakdown voltage must exceed
    1000 volts. Therefore the transformer (to send power to the
    load) and the feedback circuit (to regulate output voltage and
    other functions) must both provide that galvanic isolation:
    First and foremost for human safety reasons. Second for
    transistor safety.
     
  8. w_tom

    w_tom Guest

    ... its Fourier spectrum will ONLY contain the fundamental
    That's correct and consistent with what I had posted. Those
    harmonics are the different frequencies. As those harmonics
    increase magnitude (contain more energy), then the transformer
    is confronted by other frequencies with more power. As noted
    earlier, this can cause increased energy dissipation in the
    transformer and elsewhere.

    Meanwhile I did not even try to say where most energy is
    lost. Why do criticize me for something that was not even
    posted? To argue semantics?

    An unloaded power supply must not be damaged by no load.
    Any power supply that is damaged by a no load condition is
    typical of something bought by a bean counter - the enemy of
    innovators, responsible manufacturers, and those educated in
    computer electronics. No load must not damage a properly
    designed switching power supply.
    So what? That is not what I said and is completely
    irrelevant to what I did say. A filter expert need not know
    anything about switching power supplies. But a power supply
    designer better damn well understand the principles of filter
    design. Why does Joel read reverse logic into a post? Why
    post what is both irrelevant and not even stated? Joel is
    arguing semantics rather than trying to help or answer the
    OP. Don't let him confuse you.
     
  9. Jamie

    Jamie Guest

    if you very the duty cycle alway from 50%, you get an un-even charge and
    discharge in the coils of the transformer. this interaction produces is
    low output on the secondary.
    its gets much deeper than that, but it's just a simple starter for
    you to think about.
    just think of charging a cap for 10 secs, but only let it discharge
    for 1 sec. you will see that the charging current duration is very short
    after the first initial charge.
     
  10. Jamie

    Jamie Guest

    high freq is much easier to filter, uses much smaller caps..
    also using a switching supply allows you to put the driving
    components in saturation or even used things like Power fets to
    put the path in near 0 ohm thus generating very little heat and
    making the power efficiency much higher and smaller components.
     
  11. Energy is passing from transformer to filter only during the "on" part
    of the duty cycle, The larger the % on time, the lower peak energy
    that must pass during the peak to satisfy the average energy flow.

    You could also move an automobile by setting sticks of dynamite off
    behind it a small percentage of the time, but the peak forces would
    have to be pretty high to make up for the average force needed to keep
    the car moving. The windings in the transformer and the switch on the
    primary side are also affected by the peak verses average energy
    flow. For this reason, one normally wants to have the duty cycle just
    hit 100% at full output load and minimum input voltage, to keep the
    minimum duty cycle under other conditions as high as possible.

    Good question.
     
  12. PeteS

    PeteS Guest

    There are excellent resources for switching power supplies at all the
    major manufacturers (TI, Linear Tech, Maxim and others).

    One of my favourite design notes is from Linear Tech:
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1142,C1114,P1134,D4162

    (AN-73 [pdf] at http://www.linear.com/ should the link not work). This
    shows the basic principle of the Switchmode power supply using a
    specific device as an example, with the coil used as (as noted) an
    energy storage device.

    As to why some switching power supplies 'break' with no load, I would
    agree it could be poor design, although to be fair to the designers
    they may be designed for a specific load. Much depends on the specifics
    of the type :
    Topologies:
    Buck (Step down)
    Boost (Step up)
    Buck-boost (inverting, usually)
    SEPIC (step up and step down - for isntance, generate +5V from a
    nominal 6V battery that may have an actual range of 4V to 7V)

    Mode:
    Current. Inductor (or switch) current is controlled directly
    Voltage. Output voltage is controlled directly
    Generally, current mode controllers are insensitive to *input voltage*
    variations and voltage mode controllers are insensitive to *output
    current* variations.

    A switching power supply (actually, any regulated power supply) is a
    closed loop system that has various (and numerous) filter elements in
    the loop. To get regulation employs negative feedback (i.e. an output
    variation causes a change at the input such as to [partially] negate
    the output variation).

    What makes negative feedback negative is the effective phase of the
    feedback signal. The filters in the loop add their own phase
    characteristics, and if not carefully considered cause sufficient phase
    shift in the loop to make the negative feedback positive - giving an
    oscillator if it happens at unity gain. This is one of the [many
    possible] things that can happen at no load.

    Feedback loops of this type have many analogies - the most basic
    principles are found in servo theory.
    For an excellent app note on loop compensation (the art of keeping
    negative feedback negative) for a current mode controller, see
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1143,C1083,P1735,D4165
    (AN-76, once more from http://www.linear.com/ )
    For the filters, the relevant equations

    Capacitive filters:
    Fx = 1/(2*pi*R*C) where R is the equivalent resistance of the ffilter
    and Fx is the Frequency at which the difference between input and
    output is 3dB, which is also the point at which the phase difference
    between input and output is 45 degrees. The phase and relative
    amplitudes may be either leading or lagging depending on the filter
    configuration (a leading phase filter is known as a zero, a lagging
    phase filter is known as a pole)

    Forr voltage mode controllers, there is a 2-pole filter at the output,
    given by 1/(2*pi* [sqr(LC)]) where L is the output inductor and C the
    effective output capacitance. At this frequency, there is 180 degrees
    of phase shift at the output.

    Each pole (or zero) has a phase response of 45 degrees per decade, and
    an amplitude response of 20dB per decade (alternatively, 6dB per
    octave). (Note to others - I realise the filters may be -45 or +45 and
    amplitude response could be rising or falling)

    So there's a lot of terminolgy and a lot of fundamentals to learn to
    understand these things.
    I think there's plenty of reading noted here to be getting on with if
    you want to understand the subject :)

    Cheers

    PeteS
     
  13. w_tom

    w_tom Guest

    Peter discusses (among many good points) the feedback
    circuit in switching power supplies.

    A power supply failed repeatedly in the field (I later
    learned was also failing in the shop; but the failure was
    ignored). The feedback optocoupler required a gain of 150. I
    also learned that finding gains of 150 was all but
    impossible. So the bean counters installed a lower gain
    optocoupler. Therefore the power supply would sometimes -
    rarely - but sometimes not power up and sometimes just
    shutdown arbitrarily. IOW the power supply was 100% defective
    - but was shipped because it usually passed tests. With
    insufficient feedback, it was unstable - failed
    intermittently. Cost thousands of dollars to replace that
    defectively manufactured power supply.

    Power supplies break for many reasons. This manufacturer
    blamed insufficient load to avoid admitting the real problem.
    I'll never forget nor forgive that company. They tried to
    claim it was due to insufficient load when even their own spec
    sheets said that load was sufficient.

    Feedback is a concept taught in control systems. Like
    filter design, the power supply designer must also have
    fundamental comprehension of feedback control loops.

    The user need not understand these technical issues. This
    is where manufacturer spec sheets, manufacturer reputation,
    and workers with power to flag problems are so much a part of
    the system design process. But a power supply design is quite
    complex - for something regarded by users as so trivial.
     
  14. BradBrigade

    BradBrigade Guest

    Awesome! That really answered my question. Thanks for all the help
    guys, and the links!
     
  15. Joel Kolstad

    Joel Kolstad Guest

    Your "100V input" is typically going to vary between, say, 95-105V.
    Similarly, depending on just how heavy the load on the "10V" side is, the
    extra current will cause various losses so that your output would tend to vary
    between, say, 9.5-10.5V even with a perfect 100V input. (How well a power
    supplies copes with the former problem is called "source regulation" and how
    well it copes with the laters is "load regulation.")

    You do occasionally see some really cheap 12V->24V or 12V->6V DC/DC converters
    out there that use a fixed ~50% duty cycle and figure the output will be
    "close enough."

    ---Joel Kolstad
     
  16. Joel Kolstad

    Joel Kolstad Guest

    Hi W_Tom,

    Hmm... if you say so. That's sure not how I read your post.
    Yes, certainly true, but not really relevant to what a beginner needs to know.
    For a bench supply or even a PC power supply, I'm tempted to agree with you.
    For embedded applications in cut-throat markets (TVs are a very good example),
    it's hard to ignore the cost savings that can be achieved by not bothering to
    make your power supply unconditionally stable when you 'know' there will
    always be a minimum load around.
    It's the consumer who pushes the bean counters to skimp on parts count and/or
    quality. Although the US is a wealthy country that can readily afford to pay
    a couple bucks extra for a better computer power supply, this isn't true in
    all parts of the world (e.g., China). It's not the kind of engineering work I
    want to do, but I can see the justification for designing these really awful
    PC power supplies that cost literally no more than $10 but can readily blow-up
    if you look at them crosseyed.
    Mnay people get along just fine with being able to analyze single section
    filters (both the main L-C 'power' filter and something cheesy like an R-C
    feedback loop filter). Now, you may consider than the "principles of filter
    design," but personally I would say that someone well versed in such
    "principles" is more like to be able to spout off about Chebychev filters,
    group delay, Butterworth pole positions -- that sort of thing -- than just
    what simple LC and RC filters do. (I realize many power supplies do have
    "fancy" filters in the feedback path, I'm just saying plenty of them don't and
    I don't think you really need to know that much about filter design to make a
    workable switcher.)

    ---Joel
     
  17. w_tom

    w_tom Guest

    Somewhere was an 85(?) page introduction (from linear.com)
    to switching power supply design. Up front was a flow chart
    that asked if one wants to design a supply? The Yes path lead
    to a message that asked, "Are you Nuts?" Power supply design
    is complex. As demonstrated here, one must have knowledge of
    filters, feedback systems, principles of power systems, EMC,
    UL approval, FCC regulations, etc. A beginner must first
    decide how much will be learn before his eyes disappear into
    the back of his head.

    If consumer pushed bean counters to reduce costs, then Honda
    and Toyota would not be dominant and growing car companies.
    Bean counters typically increase product costs - at least in
    the long term. It explains why GM has so many problems. Cost
    controls increase costs. To reduce costs, obtain more
    customers, create new markets, increase market share, create
    jobs, create wealth, increase product efficiency, etc... all
    require the most important thing - innovation. The US is a
    miser as are most every other nation. To sell in the US or
    anywhere else, the power supply manufacturer must innovate -
    not cost control. Meanwhile the US does have standards that
    some other nations do not have - for power supply performance,
    reliability, safety, emissions, and .... Europe has even
    tougher standards.

    Most supplies, to be profitable, must be designed to operate
    anywhere in the world - either as a universal supply or with
    different options for different regions. Just another example
    of innovation; the alternative being bankruptcy. Just another
    reason why (outside of niche markets), the power supply
    designs must meet fairly universal world standards.

    Tell us about harmonics? Do we solve this problem with
    filters, or what else? Will a power supply create too much
    harmonics AND will it operate when line harmonics are high?
    I recall the Intel spec that even demands output transient
    response - another factor in a feedback control system
    design. Power supply design is not about costs. It is about
    innovation - where costs are only one small part of a
    profitable design. The only way to cut costs and remain
    competitive - innovation.

    A large market for inferior supplies exists that create
    problems such as computer system damage and intermittent
    failures. All power supply outputs must even be shorted and
    still not damage the supply. This too has been defacto
    standard for many decades.

    We have demonstrated how much is basic information on power
    supplies. IOW, "your nuts" in that flow chart should be
    appreciated. 85(?) pages in that introduction paper
    demonstrates how complex a switching power supply really is
    and why a properly constructed supply selling for only $65
    retail is a marvel of free market economics.

    However when purchasing a $40 retail supply, then ask what
    critical functions were forgotten to sell that supply only on
    price. Yes, some sell power supplies only using cost
    controls.
     
  18. The following thread I started a couple of days ago may also be
    helpful.

    From: Terry Pinnell <>
    Newsgroups: sci.electronics.design
    Subject: Rating of PC power supplies?
    Date: Sun, 29 May 2005 07:10:38 +0100
    Message-ID: <>

    In particular, Franc Zabkar posted this link to a detailed schematic:
    http://www.pavouk.comp.cz/hw/en_atxps.html
     
  19. PeteS

    PeteS Guest

    Regarding Joel's notes on filter sections, it is true that in many
    instances a relatively simple filter calculation may be done for a
    workable switcher, but that depends on a number of things.

    For a voltage mode controller, a widely varying input makes life
    difficult and is where a multipole (4 or even 5 filter sections) may be
    necessary, with the same provision applicable to large load steps for a
    current mode controller.

    Where one has both (the usual situation in a lot of embedded systems
    nowadays), then a thorough knowledge of filter theory is certainly an
    advantage when one must do their own switcher, even though one may use
    spice programs; the issue is to use the program effectively, a
    knowledge of what one is doing helps :)

    There are a number of reasons for designing one's own, including space,
    efficiency, unusual output voltages (althoguh there are 'adjustable'
    bricks out there) and Vin / Vout functions that have a particularly
    wide range. As an example, I had to do one that had 10-14V in nominal,
    1.2V out, at loads from ~0 to 45A, with load steps of <30A in
    <2millisec, and efficiency was required to be >90% across the load and
    input range. That was quite a challenge. I managed to achieve >94% with
    the assistance of the vendors involved.

    I deliberately did not even attempt to cover everything (that's a
    subject in it's own right that many spend entire careers on, and I
    thank them for their assistance :), but merely try to point out that
    the loop filter is a critical issue in the design of a switcher
    (although one may use the 'suggested application' in many cases) that
    requires some attention, and is critical to understanding failure
    causes.

    Cheers

    PeteS
     
  20. w_tom

    w_tom Guest

    To demonstrate the complexity of power supplies - the so
    many functions that must be part of a minimally sufficient
    supply: Take that example supply from www.pavouk.comp.cz.
    Notice the feedback and voltage divider circuit as discussed
    by Franc using equations. That supply does not provide
    sufficient galvanic isolation. All appliances must have
    internal transient protection. Intel specs demand that
    computers be even more robust. But that feedback circuit does
    not provide thousands of voltages of isolation. That's right -
    thousands of volts. An optoisolator for galvanic isolation
    should have been located where those feedback resistors are
    located.

    Does your power supply for the 'who-dad device' require
    galvanic isolation? Maybe. Maybe not. But galvanic
    isolation is but another function in power supply design.
    This noted because so many buy power supplies on price that
    are then missing essential internal functions. Does your
    clone computer power supply provide galvanic isolation? If
    not, then internal transient protection has been compromised.

    This posted just to demonstrate but another complication of
    power supply design.
     
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