Buck-boost converter

Discussion in 'General Electronics Chat' started by Trilochan Penthia, Nov 17, 2014.

  1. Trilochan Penthia

    Thread Starter New Member

    Nov 17, 2014
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    Why we use MOSFET and variable resistor in a buck boost converter ?
    What are the benefits by using these components ?
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Banana cream pie is delicious, what is the benefit of using butter in this pie?

    Is your question any more answerable than mine?
     
    KJ6EAD and Robin Mitchell like this.
  3. Robin Mitchell

    Well-Known Member

    Oct 25, 2009
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    I really dont like these questions because it seems that this is some kind of work or assessment for school/uni. Questions that people usually ask are: I cant get this circuit to work or why does this circuit only do this.

    Trilochan Penthai, try understanding the function of a buck -boost before asking questions:

    http://en.wikipedia.org/wiki/Buck–boost_converter
     
  4. Trilochan Penthia

    Thread Starter New Member

    Nov 17, 2014
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    I got you..MITECH electronics..thnx
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    Several manufacturers publish application notes and books on such topics - obviously each manufacturer wants to steer you in the general direction of their products, but you can still get a general overview of the topology.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    MOSFETs are used because they generally make more efficient switches than bipolar transistors.
    Variable resistors are used whenever you want to vary a voltage, such as the output voltage of the regulator.
     
  7. Trilochan Penthia

    Thread Starter New Member

    Nov 17, 2014
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    Thank you very much buddy..
     
  8. ian field

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    Oct 27, 2012
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    You have to qualify that statement somewhat - early MOSFETs had high RDSon and wasted more power when switched on hard, than did transistors.

    In the interim, they developed the IGBT, it has a MOSFET input and a bipolar output, its basically a hybrid Sziklai pair and the bipolar output is an emitter follower - that makes them relatively slow, so you're basically exchanging conduction losses for switching losses.

    MOSFETS can be fast to minimise switching losses - if you drive them properly. The higher the drain current rating; usually the higher the gate capacitance. At high switching speed, you need a driver capable of high peak currents to charge/discharge the gate capacitance quickly.
     
  9. crutschow

    Expert

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    Not sure how early obsolete high resistance MOSFETs are pertinent to this discussion. Modern MOSFETs can be readily found with a few milliohms of ON resistance, giving much less ON voltage drop as compared to BJTs.

    IGBT's are not only slow, but have a high ON voltage drop, similar to a Darlington, thus aren't efficient for switching low voltages.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Many of the mosfets that people new to the forum want to use have pretty high Rds/on values. They are in the IRF640, IRF740 ..... family, high by today's standards.
     
  11. ian field

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    Oct 27, 2012
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    Its not easy to make high Vds MOSFET with low RDSon - so the most evolved parts will still be more pricey.

    A while back I tried to find logic level MOSFETs that can handle the back emf from an ignition coil - apparently there's no such thing!

    That was some time ago, so things might have changed - but don't bet on them being cheap.
     
  12. crutschow

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    If you check suppliers, such as Digikey, you should find many MOSFETs with ON resistance of less than 100 milliohms for less than $1US.
     
  13. ian field

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    The technology has made leaps & bounds - but high voltage ones are still more expensive.
     
  14. crutschow

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    Which is generally true for any transistor type.
     
  15. ian field

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    Oct 27, 2012
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    Your point?
     
  16. Fuji

    Member

    Nov 8, 2014
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    I think the answer is because it helps the booster to save you power?

    Bucks in general should give you around a 94% efficiency.

    Shutdown current < 1 μA for extended battery life if your application requires a battery.

    They are mainly used where the input voltage can change from VIN - VOUT > VIN to VIN - VOUT < VIN

    Definately are being used in smart phones. DC/DC converters are the way to go for batteries in general.
     
  17. crutschow

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    You stated that "high voltage ones" (meaning MOSFETs) are more expensive. My "point" was that it's not unusual for transistors, whether MOSFETS, IGBTs or BJTs to increase in price with voltage. :rolleyes:
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    When CRTs reigned supreme, sheer volume of production made bipolar transistors up to about 2kV as cheap as chips.

    Its when you also want a collector current rating of 12 - 15A that prices start to climb.

    There may well be by now, but I've yet to actually see a MOSFET with that voltage rating - and if there is, the RDSon won't be anything to shout about.
     
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