BJT vs Mosfet Boost Regulator

Discussion in 'General Electronics Chat' started by KCHARROIS, Jun 24, 2013.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Hello,

    Been working on a boost regulator circuit using a 555 timer and a comparator and such to get my required output. Yes I have only simulated the circuit but before purchasing the parts I'd like to have some opinions on wether to use a mosfet or BJT for the switching parts. I noticed with mosfet that I get big voltage spikes but with a BJT I dont. The schematic and LT spice file is attached.

    Thanks for the opinions
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  3. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Yes I have, I need a boost regulator to power VFD tubes which are similar to Nixie tubes but require only a constant voltage of 30V. Alot of BJT's can support that much voltage and current that I require. So is there truly an advantage from BJT to mosfet for my requirement?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    The main advantage of MOSFETs is that they a usually more efficient in switch-mode supplies. If efficiency is not particularly important than you can use a BJT.

    What is the purpose of the emitter resistor in you circuit? That's not normally used in a switching circuit and just reduces the efficiency.

    U2 applies -12V to the 555 input which is not allowed. You need to add a resistor and clamp diode to limit the negative voltage.

    You need look at all the voltages in a simulation to determine if any of them exceed the limits of the components. A simulation does not usually explicitly indicate such abuse of a part so you have to determine that by observation.
     
  5. BobTPH

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    Jun 5, 2013
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    Voltage spikes are a bad thing in a boost converter?

    Bob
     
  6. crutschow

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    They are if the exceed the voltage rating of any of the components subjected to the spike.
     
  7. BobTPH

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    Jun 5, 2013
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    A boost converter will need a sufficient load to keep it from spiking too high.

    Bob
     
  8. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Ahh very good, thanks everyone. The emitter resistor is there because it limits the current going through the inductor which I believe acts as a current limiter for the the output.
     
  9. crutschow

    Expert

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    OK as long as you don't mind a big hit in efficiency.
     
  10. BobTPH

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    Jun 5, 2013
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    But the whole purpose of the inductor in a switching power supply is to limit the current WITHOUT resistive losses. Using a resistor defeats this purpose.

    Bob
     
  11. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Well I could get rid of it but does it not help me so that if theres a short at the output that the current is limited and doesn't damage anything?
     
  12. BobTPH

    Active Member

    Jun 5, 2013
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    Do you understand what an inductor does?

    Bob
     
  13. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Yes I understand that an inductor opposes change in current when discharging and that IL = V * dt/L. I know this is frustrating to you but what is it that you see that I'm not seeing.

    Thanks
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I read every post in this thread twice and never saw anyplace where it specified the load current/power output.
     
  15. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    I require a maximum of 100mA and if my output is 30V power will there for be 3W which means that my 12V supply requires a minimum current of 250mA. Is this what you wanted to know?
     
  16. BobTPH

    Active Member

    Jun 5, 2013
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    I think that point that you are missing is that as long as the inductor is being switched, the current is limited. Now, if whatever is switches it freezes with it on, then the current would be only limited by other factors. A short in the output would just cause the inductor to be fully discharged with each cycle, then it would charge up again with the on cycle, but unlimited current would never occur as long as it is switching.

    Bob
     
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  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Yes, that tells me you don't need to worry about efficiency because power is low. It also means a BJT is a better choice than FET because the base drive current to the BJT will be low because the inductor current is not that much.
     
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  18. crutschow

    Expert

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    That's not always the whole purpose. It's also used for the conversion of a low voltage to a higher voltage which a resistor can't do.
     
  19. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    Ok I see what you guys are saying, aslong as my circuit is switching, my current limit is determined by I = V * Dt/L. Dt being Ton of the switching On/Off. Thanks guys
     
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