flyback using NE5555

Discussion in 'The Projects Forum' started by mtLg, Nov 15, 2013.

  1. mtLg

    Thread Starter New Member

    Oct 16, 2013
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    Hello,

    I've been a reader of this forum for a while now, thanks to everyone who makes all the information in here available!

    Now to my problem, I'm trying to build a wireless battery charger based on a flyback converter design. I've designed the modules and separately everything works, but when together, they dont:

    1) DC power supply as source
    2) Oscillator circuit - NE555 - tested and behaves I wish on a frequency close to what I want, about 43 kHz.
    3) "flyback" converter - Coupled inductive coils + Ac-DC converter - tested and works successfully from 40-50 kHz with square, triangle, sine, offset, etc waves.

    When I try to power up circuit (3) with the output of the 555 timer, nothing works, I have no idea why.

    I also attempted driving a mosfet (irf5400 - no success at all) and a bjt(2n5401 - drives the transistor, but again as soon as the load is connected, nothing happens).

    I think the problem might be related to waveform spikes in voltage generated at the receiving circuit (see oscilloscope photo).

    I'm stuck at the moment, not sure what to do to successfully get the square waveform at some voltage to my flyback converter circuit.

    All helps is appreciated.
    Thanks
     
  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
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    How pure is the signal coming from the power supply??
    I have no Idea but just trying to help and learn something ..
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    The 555 oscillator circuit is faulty. When the discharge pin 7 goes low it can only discharge C2 to half of Vcc!

    It needs to discharge C2 to less than 1/3 of Vcc to trigger the internal comparator which oscillates between 2/3 Vcc and 1/3 Vcc.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    For that circuit to work the transformer primary must be connected between the power supply and the collector/drain of the transistors without any resistors. You can't drive the primary from the output of a collector resistor as you (apparently) show.

    A flyback circuit works by turning on the transistor to build up a current in the primary which stores energy in the primary inductance. This energy is then transferred to the secondary by transformer action when the primary current is abruptly terminated by the transistor turning off, which generates current in the secondary.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    I believe that is a standard astable configuration for a 555.
     
  6. mtLg

    Thread Starter New Member

    Oct 16, 2013
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    Thanks for the quick replies.

    The power supply I'm using for testing is from an electronics laboratory, so I'm be confident thats not the issue.

    As crutschow mentioned, the circuit is based on a standard 555 astable configuration. I tested this circuit separately and the 555 outputs an offset (positive only) square wave as desired.


    Ok, so based on what you recommended, I drew a new schematic for the whole circuit (attached).
    I'm curious though, why can't I drive the primary from the transformer straight from the 555 output and ground without the use of a transistor?


    I'll test this out as soon as I can access the lab again, in the meantime any other advice or solutions are most welcome.


    Thanks for all the help and replies so far, keep advices coming! :D
     
    Last edited: Nov 16, 2013
  7. iimagine

    Active Member

    Dec 20, 2010
    129
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    A transistor is needed to provide additional current beyond the 200 mA limit of the 555 timer.

    Why did you reversed Q1 CE? :confused:
     
  8. mtLg

    Thread Starter New Member

    Oct 16, 2013
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    0
    Hmm, now I remember reading something about the 200mA limit. Thanks.

    What do you mean by reversed Q1 CE? It's still the same orientation as in the original diagrams.
    As it is at the moment, the inductive coil is connected between the DC power supply and the Collector of the transistor. Isn't that the correct orientation?

    Cheers
     
  9. iimagine

    Active Member

    Dec 20, 2010
    129
    9
    That transistor is a PNP, not an NPN, therefore its E (emitter) should be connected to the coil, and C (collector) to ground.
    The way that you have it connected right now is what i believe, called reversed bias, it would still function, but it will not provide much gain
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Your circuit is oscillating at about 49khz, so the transformer should also be tuned with the capacitor to give that resonance to give out max voltage. L1/C3, L2/C4 and drive the transformer with a mosfet for more power
     
    Last edited: Nov 16, 2013
  11. crutschow

    Expert

    Mar 14, 2008
    13,033
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    Your circuit looks okay expect for a couple things.

    I didn't notice your circuits show a PNP transistor, which won't work since the voltages are the wrong polarity. You need an NPN.

    What is the purpose of C3 and C4?

    You may want a add a zener across the transistor collector-emitter to suppress high voltage spikes from the parasitic inductances. Its voltage should be about 75% of the transistor voltage rating.
     
  12. iimagine

    Active Member

    Dec 20, 2010
    129
    9
    You better of using a NPN transistor, because the high output level of the 555 timer is 1.7 volts less than the supply voltage, therefore the need of adding two diodes (or an LED) in series with R3 to increases the forward voltage required for the PNP transistor to about 2.1 volts so that the 1.7 volt difference from supply to the output is not enough to turn on the transistor.

    In other word, even if you correct the polarity of the PNP, It will stays on forever without the 2 diodes or LED mentioned :)
     
    Last edited: Nov 16, 2013
  13. mtLg

    Thread Starter New Member

    Oct 16, 2013
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    Hmm, I'll look into this. Thanks

    Based on calculations of the inductance and capacitor, they were meant to resonate at around 48 kHz. The thing is all the components errors are giving me a lot of "distortion", during testing the 555 circuit output was at about 43 kHz. I also didnt do much investigation the resonance itself, just relied on the face values of inductance and capacitance.
    I just tested the resonant circuit from 40-50 kHz and power was going through ok.
    I guess its something to keep in mind when optimising the circuit.

    I'll keep the mosfet idea in mind next time I visit an electronics shop.

    Thanks
     
    Last edited: Nov 16, 2013
  14. mtLg

    Thread Starter New Member

    Oct 16, 2013
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    C3 and C4 I'm using to increase the coupling of the inductors, define a resonant frequency and run the circuit at around that frequency. Im not using a real transformer, just two co-planar coils at a short distance. Did I do something wrong here?


    [/QUOTE]

    Honestly it has been a while since I really studied all of this, a few mistakes to correct I guess. I'll keep the idea of the NPN transistor as an improvement and something to test later in the week, however I wanted to have this running by Monday before I have the chance to visit an electronics shop again.


    I have a spare N-channel MOSFET, VN10KM (the name on the diagram attached is wrong), as a bonus this mosfet already seems to have a zener diode across it. Would replacing the transistor with this mosfet yield the desired results? Is the diagram below the correct way of going about this?

    Again, thanks for all feedback!
     
    Last edited: Nov 16, 2013
  15. iimagine

    Active Member

    Dec 20, 2010
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    It should work, Although I would add a pull down resistor at the gate of the MOSFET to help it turns off quicker, a value of 1k would do
     
  16. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    The 555 has a push-pull output so a pull down resistor is not needed.
     
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  17. crutschow

    Expert

    Mar 14, 2008
    13,033
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    I didn't know you were using resonant coupling and not a regular transformer. That's a different animal.

    That MOSFET is connected correctly but its maximum current rating is only 0.31A. Is that sufficient for your circuit?

    The zener in that MOSFET protects the gate, not the drain-source. But a MOSFET is more tolerant of drain-source voltage spikes so likely will be okay in your circuit without additional protection.
     
  18. mtLg

    Thread Starter New Member

    Oct 16, 2013
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    For the purposes of the experiment, yes. It is only meant to take a very light load.
    I'm hoping the 10k resistor on the secondary is enough to reduce the current through the mosfet. Do you think I need to add anything to circuit to ensure the current remains low?

    Off to the lab in a couple of hours, lets see how everything goes!

    Thanks
     
  19. iimagine

    Active Member

    Dec 20, 2010
    129
    9
    You got it all wrong. What you'd get through the MOSFET everytime it turns off is 'voltage spike', not current, google 'flyback diode' :D
     
  20. mtLg

    Thread Starter New Member

    Oct 16, 2013
    8
    0
    Shocking!

    So from what I read, a Schottky diode will do the trick (or am I better off with a normal diode?).
    I happen to have a 40V rated one. Added to the diagram attached. The model I have is actually the 1N5819. I do have normal diodes available too in case it fails.

    I guess this would also help reducing the voltage spikes I used to see on the oscilloscope, right?

    Thanks
     
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