pirate991 - re: DC-DC Step-Up Switching Regulator Using Transistors

Discussion in 'General Electronics Chat' started by SgtWookie, Jan 22, 2011.

  1. SgtWookie

    Thread Starter Expert

    Jul 17, 2007
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    You wrote in an E-mail:
    I see that you have not made a post yet, so I started this thread for you.
    I do not reply to E-mails from this forum.

    You should post a schematic of your existing circuit. .png image format is preferred, as they are small, not "lossy" like .jpg, and need no software other than a browser to view.

    Use the "Go Advanced" button and then "Manage Attachments" to upload your schematic to this website.

    Meanwhile, a common name for a "step-up" converter is a "boost" converter.
    Roman Black has some interesting 2-transistor "boost" step-up and "buck" step-down DC-DC converters on his site. Have a read through this page:
    http://romanblack.com/smps/smps.htm
     
  2. jiyluv

    New Member

    Aug 22, 2010
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    Thanks for the information, about the black regulator circuit.
    I am mainly looking for a low power consumption step down, and it seems this will be sufficient.

    How would I have to redesign the circuit, however, if I want it to step down in a different start and end voltage? (for instance, I am trying to design a 10 to 3 V buck converter instead of 15 to 5)

    Thanks for your help- if I posted in the wrong method or incorrectly, feel free to correct me. thanks!
     
  3. SgtWookie

    Thread Starter Expert

    Jul 17, 2007
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    You're welcome. You were not very specific in your E-mail, but decided to try to give you something to read.

    What range of current output do you need, minimum and maximum?

    Is your input voltage fixed at 10v, or could it vary by several volts?

    How much current is available at the input without it dropping significantly, or what is the maximum current available?

    Do you have Linear Technology's free LTSpice program installed? If not, you should do so, as I have some modified versions of Roman Black's regulators modeled in LTSpice.

    You're welcome.
    Having all replies going into the proper threads in the forums keeps things organized, and helps keep a "train of thought" going, so that one can see where things started off, and how well (and sometimes how poorly) things are progressing.

    It's important to have as many facts as possible in the early stages, as otherwise some detail may come up later on that makes the whole project impractical/impossible/unrealistic.

    If you wish to be notified via E-mail when someone posts a reply, then click on "Thread tools", then click "Subscribe to this thread".
     
  4. jiyluv

    New Member

    Aug 22, 2010
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    Thanks for the help!

    By the way, I wasn't the person that sent out the email to you.. I was just a member that found the information helpful :)

    Well, right now, the reason why I am asking about the regulator circuit is because I am trying to design a MOSFET-based switch that will switch based upon voltage produced from a charging source, such as a piezoelectric element attached to a capacitor (so like a battery charger- will constantly rise in voltage).

    My circuit system, needs to have low power dissipation as possible, and therefore, the current requirement should be as small as possible- I would like it to be tens to hundreds of microwatts, so the current will have to be in the range of hundreds of microamps (If such an application for the current requirement will be impractical, what power dissipation value should I expect? Maybe I can try to visualize and compensate for it if so)

    The reason I was looking into a regulator circuit is I am trying to design something that will detect a source charged up to 10V and will switch if it reaches 10V using a MOSFET switch. However, since a typical MOSFET switch's gate voltage is around 1.5V, I want to have the voltage source transformed down so that mosfet will turn on when the source charges 10V. (The 3 volts required that I mentioned earlier is for another switch)

    I am researching into different options for a power efficient buck converter for this application. I suspect the 10V doesn't have to be rock solid- it can sway around a volt or so give or take.

    My method for the step down was either:
    1. Regulator
    2. Resistor divider
    3. op amp with fractional gain

    I am using Orcad's Pspice program- I would imagine that is compatible with LTspice.

    Thanks for the help- please let me know if you need anything clarified.


    jiyluv
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
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    Could you please post a simple diagram of what you are trying to do. I would endorse the recommendation to describe your requirements in as much detail as possible.

    In particular, it would be helpful to know how the MOSFET switch is related to the input - is it connected to the high or low side of the input, or does it need to be isolated from it?

    Finally, does all the power for your circuit need to come from the piezo input, or can you use a separate supply to power it?
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I am unclear on how a piezo element fits into a charging circuit, other than as an alarm.
     
  7. SgtWookie

    Thread Starter Expert

    Jul 17, 2007
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  8. jiyluv

    New Member

    Aug 22, 2010
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    Alright- Lets scrap the piezo idea- my circuit just need to power off a battery source- lets say for a 9V battery.

    So therefore, I guess the first question I was curious will be for this question- just in general, not in a specific circuit-
    What configuration will be most power efficient to have a mosfet trigger based on a 10V voltage source? I was thinking of stepping down from 10V to the threshold voltage, so the mosfet will start conducting when my circuit reaches 10V.

    Therefore, what would be best in terms of charging sources?
    1. Regulator
    2. Resistor divider
    3. op amp with fractional gain

    thanks!

    And Thanks for the inputs, including the one for the linear IC chips- I will also look into that option in the future!
     
  9. SgtWookie

    Thread Starter Expert

    Jul 17, 2007
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    I don't know why new tinkerers like the 9v "transistor" battery so much.
    Sure, they have a higher voltage output for a single battery than most other common cells - but they have a very limited mAH capacity, and are expensive for what you get out of them. They're best used in transistor radios.

    What you wrote doesn't make much sense.
    In power supplies, MOSFETs are used as switches. Their gates are charged and discharged very quickly as possible to minimize their power dissipation. They are used as saturated electronic switches, rather than in the linear region where they would dissipate power as heat. Heat = inefficiency.

    If the regulator is linear, then all three of the above will dissipate power as heat. The resistive divider will have the disadvantage of high impedance and poor regulation. Switching supplies and DC-DC converters are generally far more efficient. Switched-capacitor DC-DC converters can be VERY efficient; near 99%. It's more of a struggle with inductive switchers, but they are still much more efficient than linear/resistive regulators.
     
  10. jiyluv

    New Member

    Aug 22, 2010
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    Thanks for all your input.

    My issue here is somewhat simple- just am designing a circuit that will start conducting after the input reaches a certain voltage. The input will gradually increase.

    As for the 9V battery- I have the necessary tools to implement the 9V battery, and therefore was using that.

    As for the questions involving using a MOSFET as a switch- what I was hoping to implement was a system where the MOSFET transistor will turn on and start conducting when the gate voltage reaches, lets say, 9V (battery value). Although there are two conduction mode of saturation and linear mode, isn't it that mosfet switches will start to conduct when the transistor surpasses its threshold voltage? I thought that this was the reason why people use it as a switch.

    For my application to start conducting after a certain voltage, would then using a mosfet be the right idea? Or would using a zener diode be a better one?

    thanks again.
     
  11. SgtWookie

    Thread Starter Expert

    Jul 17, 2007
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    The "threshold voltage" is where the MOSFET just begins to conduct; somewhere in the 250uA region. This threshold can vary somewhat from MOSFET to MOSFET. Datasheets usually just specify a maximum for the threshold; so for example if they specify Vth=2.5v, 100% of those MOSFETs you test should be conducting ~250uA from drain to source when Vgs >= 2.5v.

    If you're working in the uA range, a Zener would not be very practical, as most "hobby-type" Zeners require a significant current through them (10mA or more) in order to establish the rated breakdown voltage.

    Depletion-mode FETs might work, but they require a significant reverse bias on the gate to open up the drain-source connection, which complicates things.
     
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