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Buck Boost with 555

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by polashd, Nov 27, 2016.

  1. spec

    spec Well-Known Member Most Helpful Member

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    Yes, my error. The inductor needs to have a secondary winding. I had transposed from an inverting buck/boost inverter, using a PMOSFET, without realizing the consequences.

    As it stands, the circuit of post #4 is a boost controller only.

    spec
     
    Last edited: Nov 30, 2016
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Modifying Mr. Spec's drawing:
    If you want the output voltage to be more/equal/less than the input voltage then:
    L1 kicks up through D1 to produce a voltage on the LEDs. (note the LEDs are not on ground but the supply)
    When the LED current reaches a point it will turn on Q-PNP. (Base is on supply. When the Emitter is above supply by 0.65V the transistor will turn on.)
    When Q-PNP is on it will turn on Q1 and turn 555 off.
    ---edited---
    Probably should make R4 (bottom right) a 10k resistor. OR Make R4 (top right) a 100 ohm resistor.
    upload_2016-11-29_22-48-49.png
     
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  3. spec

    spec Well-Known Member Most Helpful Member

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    Pretty neat Ron:cool:

    That approach would have been so useful for previous designs I have messed with.:)

    I also updated the circuit of post #4 to make the converter buck/boost (I hope).

    spec
     
    Last edited: Nov 29, 2016
  4. dave

    Dave New Member

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  5. dr pepper

    dr pepper Well-Known Member

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    You could configure the 555 as a monostable, and use the reset i/p to form an astable oscillator based on current sensed on the o/p, similar to a current mode switching controller.
     
  6. spec

    spec Well-Known Member Most Helpful Member

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    Yes DP, you mean do a proper job- but I am too lazy for that.:)

    spec
     
    Last edited: Nov 29, 2016
  7. spec

    spec Well-Known Member Most Helpful Member

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    Below is the outline circuit I started with (timing components and decoupling capacitors not shown), but then got a bit careless transposing from a PMOSFET to an NMOSFET.:banghead:

    spec

    2016_11_30_Iss1_ETO_555_INVERTING_BUCK_BOOST_CONVERTER.jpg
     
    Last edited: Nov 30, 2016
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  8. spec

    spec Well-Known Member Most Helpful Member

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    Hi all,

    I think the circuit below may do the job (buck/boost with NMOSFET)

    spec

    2016_12_01_Iss1_ETO_555_BUCK_BOOST_INVERTER_CONSTANT_I_VER4.png .​



     
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  9. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    So this project is to make a 555 do a job it was never intended to do. Thanks to Mr. Spec's cleverness it works.
    A buck/boost is hard. The idea of any input voltage and any output voltage is what makes it hard(er).

    Here is a boost I have used in several projects. I used it as a back light driver in several pieces of test equipment. Vin must be smaller than Vout.
    Vin 2.7 to 16V, Vout <30V. Small SOT23 6-pin IC. Notice the low part count when you use a IC built to do the job.
    upload_2016-12-1_17-53-29.png
    Here is a buck. Again all in a SOT23 5-pin IC.
    Vin<Vout
    Vin 4.5 to 40V 1.0 and 1.5A versions.
    I have not used this AL8860 but we are shipping millions a year of its older brother. The entire circuit is small so it fits inside a flash light bulb.
    upload_2016-12-1_18-0-49.png
    Here is a picture of a flashlight bulb. You can see the SOT23 IC + external transistor + diode + cap. The other side the board has a coil + cap. This is a "boost/buck" in that the input and output voltages do not have restrictions. The transistor and diode could have been saved if this was boost or buck only.

    upload_2016-12-1_18-20-56.png
     
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  10. john046

    john046 New Member

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    Great circuit, using a well known 555 timer chip. Can you give some instructions (calculation formula) how to calculate the inductor value? I'd like to drive 4 CREE XML T6 (~12V output) with 210-300 mA. The input voltage varies between 6-12 Volts. Thanks.
     
  11. dr pepper

    dr pepper Well-Known Member

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    Ron Dekkers site shows you how to design a very similar circuit, only he uses the control input instead of the reset, the circuit doesnt have the constant current driver as its a high voltage supply, but it can easily be adjusted for lower voltages.
    http://www.dos4ever.com/flyback/flyback.html
    Note Ron' s circuit is a boost, I've used this circuit for either buck or boost.
     
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  12. int main(){return 0;}

    int main(){return 0;} New Member

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    Noo Joizee
    I just registered to say thank you for that very useful write up!

    I had a breadboard with a half finished boost converter to power an NE-2 clone neon glow lamp from a 9 V battery when I found that web page via this forum. If I have questions, do you have a preferred thread to ask them in?

     
  13. dr pepper

    dr pepper Well-Known Member

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    Pleased to help.
    I've forwarded loads to rons page, I should contact him.
    I built one of his inductor test jigs years back, I modded it to work better on an old 'scope, and I use it a lot.
     
  14. ci139

    ci139 Active Member

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  15. int main(){return 0;}

    int main(){return 0;} New Member

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    The boost converter I'm building is similar to the one in Fig. 3 on the web page you linked to ("Simple 12-180V boost converter using the 555 as controller"). However, since my input voltage is provided by a 9V battery, there is no voltage ripple and I assume the C1 and C2 caps would serve no purpose?

    Also, I will only need about 100V output voltage and very low current (about 0.1mA). Do I really need a power transistor, or could I get away with using a regular high-voltage NPN transistor, such as A42 (rated for 300V) or even 2N5551 (160V)? I have both the latter available.

     
  16. dr pepper

    dr pepper Well-Known Member

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    100v at 0.1ma is about 10ma plus inefficiencies at 12v, so an mpsa42 might do the trick, the thing to take note of is the output cap, if its only small then the mpsa42 ought to cope.
    There will be some heating of the device due to base current and base emitter capacitance, also take care of the switch off ringing this can destroy the switching device by over volting, a vdr helps here maybe a little lower voltage rating than the vce max of the transistor, lash it up & try it.
     
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