smps with 250v output

Discussion in 'Electronic Design' started by jean-marc MERCY, Sep 12, 2003.

  1. I am starting to work on smps and my goal is to achieve an output
    ranging 250~300 volts. Power would be 250w or less. Are there any
    (non-obvious) pitfalls I should be aware of with such a voltage
    level. One of my concerns is rectifiers, the other is insulation.
    Would anyone out there have any expertise/advice ? General discussion
    can be carried out here, private one to my email.

    Thanks
    --
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    jean-marc MERCY, Sep 12, 2003
    #1
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  2. "jean-marc MERCY" <> wrote in message
    news:...
    >
    > I am starting to work on smps and my goal is to achieve an output
    > ranging 250~300 volts. Power would be 250w or less. Are there any
    > (non-obvious) pitfalls I should be aware of with such a voltage
    > level. One of my concerns is rectifiers, the other is insulation.
    > Would anyone out there have any expertise/advice ? General discussion
    > can be carried out here, private one to my email.
    >
    > Thanks
    > --
    > Ce message a ete poste via la plateforme Web club-Internet.fr
    > This message has been posted by the Web platform club-Internet.fr
    >
    > http://forums.club-internet.fr/




    Without more details about topology and whatnot you plan to use, there are
    two general hints I might suggest.

    Absolutely positively make certain that you use very effectual fast (IE,
    pulse by pulse, something that can respond in say less than 10us and keep
    the duty cycles thereafter to around 1% or less thereafter) current limiting
    scheme capable of providing full protection from indefinite output short
    circuits. Include this current limiting right from the very beginning,
    don't wait until you've blown up several iterations of prototypes before
    doing this.

    The other thing I would suggest is to not get carried away with seemingly
    good diode ratings. Some device like the UF4007 (1A 1000V very fast
    recovery) is NOT appropriate for your output rectifier even though your
    output voltage and currents are well within the datasheet ratings of the
    device.

    As you increase the blocking voltage of the diode, the reverse recovery
    charge (don't worry too much about the "reverse recovery time" figure,
    mainly it is the reverse recovery charge figure that determines the losses)
    goes up quite a bit. Ultrafast diodes rated 200V and below can have some
    pretty good reverse recovery charge figures, but when you start playing with
    higher voltage rated parts like beyond 300V, the reverse recovery charge
    gets rather substantial. Complicating this fact further is the increased
    voltage stress caused by the higher blocking required in your high voltage
    output application.

    If you can figure out the reverse recovery charge of your diodes under your
    operating conditions (use figures of 125 deg. C, the 25C values are much
    smaller than real world will likely be), which will be measured in coulombs,
    then multiply by the frequency they will see (measured in inverse seconds)
    and you will get a product with the units (coulombs/second). You will find
    those units are the same units for current, so you effecively get a somewhat
    equivalent rms reverse recovery current given your switching frequency.
    Then multiply this rms reverse recovery current figure by the reverse
    applied voltage (in your case probably say 30% greater than 300V or 600V
    depending upon your circuit topology and input voltage variation margin) to
    find a power loss figure due to rectifier reverse recovery. The power will
    be dissipated in both the power MOSFET(s) in your circuit as well as the
    rectifier itself.

    So as you might be able to see from the above, when you try to make SMPS
    devices with high output voltage you end up with a double whammy in terms of
    diode losses getting dramatically worse when compared against lower output
    voltage switch mode power supplies.

    As a result you need to operate at a lower frequency than powersupplies with
    5V or 12V outputs (using nice schottky diode technology) to obtain best
    efficiciency (or perhaps in some cases just to stay within the power
    dissipation ratings of the output rectifier). Given your conditions I
    suggest the output rectifiers should see a frequency of say something in the
    range of 50kHz to 100kHz (so in say a half bridge the main MOSFETs should be
    switching at 25kHz to 50kHz). This is unfortunate in that it makes your
    magnetics and perhaps capacitors bigger, but is necessary to avoid
    overburdening your output rectifier. Use nice TO-220 or TO-247 packaged
    diodes with adequate heatsinking. The biggest problem with little devices
    like DO-41 and DO-201 packages is their lack of easy to use heatsink tab,
    despite their seemingly good current, voltage, and recovery time ratings.
     
    Fritz Schlunder, Sep 13, 2003
    #2
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  3. Fritz Schlunder wrote...
    >
    > Absolutely positively make certain that you use very effectual fast (IE,
    > pulse by pulse, something that can respond in say less than 10us and keep
    > the duty cycles thereafter to around 1% or less thereafter) current limiting
    > scheme capable of providing full protection from indefinite output short
    > circuits. Include this current limiting right from the very beginning,
    > don't wait until you've blown up several iterations of prototypes before
    > doing this.
    >
    > The other thing I would suggest is to not get carried away with seemingly
    > good diode ratings. Some device like the UF4007 (1A 1000V very fast
    > recovery) is NOT appropriate for your output rectifier even though your
    > output voltage and currents are well within the datasheet ratings of the
    > device.
    >
    > As you increase the blocking voltage of the diode, the reverse recovery
    > charge (don't worry too much about the "reverse recovery time" figure,
    > mainly it is the reverse recovery charge figure that determines the losses)
    > goes up quite a bit. Ultrafast diodes rated 200V and below can have some
    > pretty good reverse recovery charge figures, but when you start playing with
    > higher voltage rated parts like beyond 300V, the reverse recovery charge
    > gets rather substantial. Complicating this fact further is the increased
    > voltage stress caused by the higher blocking required in your high voltage
    > output application.
    >
    > If you can figure out the reverse recovery charge of your diodes under your
    > operating conditions (use figures of 125 deg. C, the 25C values are much
    > smaller than real world will likely be), which will be measured in coulombs,
    > then multiply by the frequency they will see (measured in inverse seconds)
    > and you will get a product with the units (coulombs/second). You will find
    > those units are the same units for current, so you effecively get a somewhat
    > equivalent rms reverse recovery current given your switching frequency.
    > Then multiply this rms reverse recovery current figure by the reverse
    > applied voltage (in your case probably say 30% greater than 300V or 600V
    > depending upon your circuit topology and input voltage variation margin) to
    > find a power loss figure due to rectifier reverse recovery. The power will
    > be dissipated in both the power MOSFET(s) in your circuit as well as the
    > rectifier itself.
    >
    > So as you might be able to see from the above, when you try to make SMPS
    > devices with high output voltage you end up with a double whammy in terms of
    > diode losses getting dramatically worse when compared against lower output
    > voltage switch mode power supplies.
    >
    > As a result you need to operate at a lower frequency than powersupplies with
    > 5V or 12V outputs (using nice schottky diode technology) to obtain best
    > efficiciency (or perhaps in some cases just to stay within the power
    > dissipation ratings of the output rectifier). Given your conditions I
    > suggest the output rectifiers should see a frequency of say something in the
    > range of 50kHz to 100kHz (so in say a half bridge the main MOSFETs should
    > be switching at 25kHz to 50kHz). This is unfortunate in that it makes
    > your magnetics and perhaps capacitors bigger, but is necessary to avoid
    > overburdening your output rectifier. Use nice TO-220 or TO-247 packaged
    > diodes with adequate heatsinking. The biggest problem with little devices
    > like DO-41 and DO-201 packages is their lack of easy to use heatsink tab,
    > despite their seemingly good current, voltage, and recovery time ratings.


    Thanks for the interesting and useful info, Fritz. The issue of diode
    reverse recovery charge is a bit messy, because this is generally not
    given by the manufacturer. While one can check the reverse capacitance
    curves when attempting to evaluate one diode vs another, the efficiency
    calculation that you suggested requires a charge value that's painfully
    determined, I suppose, by integrating the diode's nonlinear capacitance
    vs voltage curve.

    Do you have any low reverse-recovery charge diodes to suggest for us?
    For example both the MUR160 (ON Semi) and UF4005 (Vishay) data sheets
    show about 8pF of capacitance at 50V, but we know the capacitance of
    high-voltage diodes is generally less than low-voltage parts in that
    same series (e.g. see the Fairchild UF4007 data sheet - we'd really
    like to see more detailed data).

    (BTW, doesn't the latter observation mean that using higher-voltage
    diodes than necessary can reduce rather than worsen reverse-recovery
    charge, when evaluated at a fixed voltage? If one finds some charge
    "specs" to the contrary, this could be because the higher-voltage
    diodes are specified for higher voltage operation, duh, so Q = CV is
    higher because even though C is slightly lower, V is much higher.)

    Thanks,
    - Win
     
    Winfield Hill, Sep 13, 2003
    #3
  4. > Thanks for the interesting and useful info, Fritz. The issue of diode
    > reverse recovery charge is a bit messy, because this is generally not
    > given by the manufacturer. While one can check the reverse capacitance
    > curves when attempting to evaluate one diode vs another, the efficiency
    > calculation that you suggested requires a charge value that's painfully
    > determined, I suppose, by integrating the diode's nonlinear capacitance
    > vs voltage curve.



    Indeed the issue is extremely messy. Most diode manufactures don't ever
    bother to include reverse recovery charge data at all, and those that do
    usually provide extremely skimpy data considering how important the
    information is.

    To their credit though I guess it is a difficult device parameter to
    characterize. The reverse recovery charge is a strong function of numerous
    variables: applied reverse voltage, forward current waveshape, foward
    conducting current magnitude, temperature, and the rate of change of current
    during reverse recovery if I'm not forgetting anything. While it is
    relatively easy to hook a device up to a curve tracer and figure out its
    characteristics as a function of one variable, five variables is a whole
    nother beast. Similarly it would also be difficult to display the results
    since a normal graph usually only has two axis (axises?).

    The diode's nonlinear capacitance curves are easier to characterize, but
    nevertheless not all manufacturers even provide consistent data here either.
    Unfortunately the device's capacitance isn't an especially important figure
    for determining the switching loss of most diodes used in many high
    frequency switch mode power supply applications. Usually the diode loss due
    to device capacitance in a high output voltage SMPS is much smaller than
    either the forward conduction loss or the reverse recovery charge loss
    (sometimes as much as an order of magnitude smaller).

    Sometimes manufacturers will provide a value for the Irrm (peak reverse
    recovery current) under some single static condition. This information
    along with the device's reverse recovery time specification can sometimes be
    useful as well as shown by this Maxim document:

    http://www.maxim-ic.com/appnotes.cfm/appnote_number/849/ln/en



    > Do you have any low reverse-recovery charge diodes to suggest for us?
    > For example both the MUR160 (ON Semi) and UF4005 (Vishay) data sheets
    > show about 8pF of capacitance at 50V, but we know the capacitance of
    > high-voltage diodes is generally less than low-voltage parts in that
    > same series (e.g. see the Fairchild UF4007 data sheet - we'd really
    > like to see more detailed data).



    The best high voltage diodes for use in high frequency high voltage SMPS
    type applications that I can find are the International Rectifier "hyperfast
    recovery" and "hexfred" type devices. Not all of the devices in these
    product lines appear to be equally well performing, so it might be wise to
    be careful.

    Devices like the HFA04TB60:

    http://www.irf.com/product-info/datasheets/data/hfa04tb60.pdf

    Or perhaps the 8ETH06:

    http://www.irf.com/product-info/datasheets/data/8eth06.pdf

    And maybe the MUR1620:

    http://www.irf.com/product-info/datasheets/data/mur1620ct.pdf

    Seem to offer about the best performance I can find. Perhaps someday when
    high voltage schottky diode technology becomes cheaper and more available
    better performance can be practically obtained.


    > (BTW, doesn't the latter observation mean that using higher-voltage
    > diodes than necessary can reduce rather than worsen reverse-recovery
    > charge, when evaluated at a fixed voltage? If one finds some charge
    > "specs" to the contrary, this could be because the higher-voltage
    > diodes are specified for higher voltage operation, duh, so Q = CV is
    > higher because even though C is slightly lower, V is much higher.)



    I'm not sure I fully understand what you are saying here, but I don't think
    so. Generally lower voltage rated diodes have lower reverse recovery charge
    figures than higher voltage versions even when everything else is held
    contstant. In addition to reduced recovery charge, they usually also offer
    lower forward conduction voltage. Therefore for best efficiency it is
    usually best to use the lowest possible voltage rated diode for a high
    frequency SMPS (even if it has a higher capacitance associated with it).
    Generally the diode's capacitance and Q=CV formulas won't be very applicable
    to finding the total diode switching power loss since the reverse recovery
    charge (not a capacitive charge element, a different phenomena) dominates.
     
    Fritz Schlunder, Sep 14, 2003
    #4
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