SCR firing in an inverter

Discussion in 'General Electronics Chat' started by recca02, Oct 14, 2007.

  1. recca02

    recca02 Thread Starter Senior Member

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    I am supposed to give a presentation on VFD
    so i thought of having a Load commutated inverter circuit simulated.
    so i first tried to make an inverter circuit using matlab simulink
    the problem that i am encountering is when i turn on a SCR it is supposed to remain on until it gets reverse biased (right?)
    so how do i turn it off in case of an inverter which has constant polarity DC at its input?

    i gave firing using a pulse generating circuit which had an on for 10 % of cycle and i kept the firing pulse period = o/p voltage frequency?
    but i only got the desired o/p for first cycle(only monitored one phase )
    after which the SCR remained on, any ideas on what i might be doing wrong?
    did i set the firing times incorrectly?(i'll post all details if required -amplitude,Ton
    circuit i designed and waveform that i obtained )

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  2. pebe

    pebe Distinguished Member

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    If you are going to give a presentation, take a tip.
    Red lines on a blue background looks awful! Stick to a white background.
  3. recca02

    recca02 Thread Starter Senior Member

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    oh they are not the ones i'll use for presentation
    they were taken from another presentation though ;)
    anyways thanks for the tip will keep that in mind.
  4. beenthere

    beenthere Retired Moderator

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    Once triggered, SCR's will remain in conduction under DC conditions (assuming foreward bias). I recall a Coufelt VVP-15 electrofishing controller that used a second SCR to pull the main one out of conduction for pulsed operation. The secondary SCR pulled charge onto a capacitor that in turn dropped the voltage across the conducting SCR and dropped it out of conduction.

    This is a very kludgy way to do things, but it worked. I don't have the schematics available to see just how the arrrangement was made.

    It would be helpful to see the details of the trigger circuits to see if they are keeping the gate voltage up.
  5. recca02

    recca02 Thread Starter Senior Member

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    here is my pitiful try,
    i will be able to get thru with the presentation since there is a million times better 'premade' circuit with a customized pulse generator to do the trick in the matlab library but i wud like to get mine working as well.
    if any other detail is reqd please ask there seem to be many parameters i have no idea how to configure :(.

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  6. SgtWookie

    SgtWookie Expert

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    Recca, have you considered MOSFETS?

    There are some mighty beefy ones out there nowadays, with amazingly high current capabilities - even in a TO220 package. On-state resistance in small fractions of Ohms. Gate current ridiculously small.

    Kick the tires on some N-channel MOSFETS. Try looking around International Rectifier's site in the IRF7xx series as a starting point.
    IRF730; N-Channel 400v, Rds(on) 0.75 Ohm, Id 5.5A, TO-220 MOSFET
    IRF3415 has a max Vdss of 150, can do 43A, Rds(on)=0.042 Ohms. Those are some mighty impressive numbers for a TO220 case.
  7. beenthere

    beenthere Retired Moderator

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    Why didn't I think of that? Also, consider IGBT's (insulated gate bipolar transistors). They don't switch as fast as FET's, but handle more power.
  8. recca02

    recca02 Thread Starter Senior Member

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    thanks MR Sgtwookie and MR beenthere,
    yeah most inverters now use IGBT's and pwm is possible i'll try that too. i'lll post back if i get it working somehow with the the goof up that i made :D
  9. SgtWookie

    SgtWookie Expert

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    200V is about the threshold for IGBT's; below that use MOSFET/HEXFETs.

    Power MOSFETs are a snap to parallel, because of the positive temperature coefficient. If one MOSFET is carrying more current, it heats up, which raises it's on-state resistance, which causes it to carry less current. Just about everything else has a negative temp coefficient, which makes it much harder to parallel them efficiently; without careful planning, you'll wind up with runaway thermal conditions or lots of wasted energy.
  10. recca02

    recca02 Thread Starter Senior Member

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    actually i m studying applications of VFD for power plant auxiliaries so they must handle large power and voltages. right now i m studying about IGBT's .Will try to work out how MOSFETs compare to them for such applications ,thanks.
    BTW i need one clarification about the carrier frequency for IGBT's : does the o/p voltage
    for an inverter decrease with increase in carrier frequency-(will post back more info if required).


    since i would like to simulate the simple circuit using scr (for satisfaction sake) .
    here is a quote from wiki(on thyristors) please comment (i need your helps since i have no experience in power electronics-but will gain enough soon)
    so if i keep the pulse off after triggering for some time the SCR will turn off?
    if so i think all i need to do is some rethinking about the firing pulses.
    thanks for all the help thus far.
  11. SgtWookie

    SgtWookie Expert

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    The on-state resistance is one thing - the power dissipation will certainly generate some heat.

    What REALLY generates heat is the power dissipation during transition from state to state. The more time the device spends in transition, the more waste heat you'll have to dissipate. Switching a slow device at high frequency = bad news; you'll spend porportionately more and more time in transition, thus decreasing available output current while increasing waste heat. You need to find the "happy medium" or the optimum frequency to switch to maintain proper output while minimizing time spent in transition. That will be device-dependent.

    Once the SCR is conducting, it's going to stay that way until the current through the device drops behold threshold. The gate just serves to trigger the device to an ON state, providing there is sufficient potential across it.

    If your load is inductive, you might do something like Beenthere suggested in reply #4, or possibly use a UJT through an inductor to "spike" the SCR off.
  12. recca02

    recca02 Thread Starter Senior Member

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    so no change at gate i/p pulse wud turn the SCR off?

    where do i give that spike?(forgive my ignorance)
    right now all i want to do is turn the scr's at the desired instances so that the bridge starts behaving.
  13. SgtWookie

    SgtWookie Expert

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    Well, if you managed to "spike" it between the SCR's output and the inductive load, you might be able to momentarily cause the potential across the SCR to fall below the conductance threshold.

    I haven't worked this out; it's just a SWAG. ;)
  14. recca02

    recca02 Thread Starter Senior Member

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    ok i get what u are saying will try.thanks.
    whenever i see an inverter bridge they only show a simple scr one so i thought it wont be
    such a big problem,
    i'll post the corrections if i find what i was missing(hopefully not my head)
    for now i stick to IGBT's.
  15. techroomt

    techroomt Senior Member

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    many of the scr circuits i see use diacs to control the scr, then the gate can be reduced to zero for commutation.
  16. recca02

    recca02 Thread Starter Senior Member

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    thanks,
    now i get it, will check for a circuit using diac.
  17. sahil4ever1

    sahil4ever1 New Member

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    to turn off the scr, reduce the anode current to zero or below 'threshold level' and reverse the voltage across its terminals. this process is termed as commutation.
    DIAC's are uncontrolled devices and and they cant be used in VFD as they require variable frequency control.
    SCR's are used in power systems as their power handling capacity is higher than MOSFETS, IGBT, UJT, etc. well most.

    SCR can never be turned OFF using a gate signal.

    look into commutation circuits.
  18. Kermit2

    Kermit2 Well-Known Member

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  19. BillB3857

    BillB3857 Senior Member

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    For those curious about capacitor commutation , here is an example. This is from an early to mid 1970's vintage control made by General Electric. This is a simplified drawing of only one phase of a 3 phase drive. The Commutation SCRs are of much lower capacity than the inverter SCRs since they only have to handle a short pulse of current. In this application, the main SCRs are of the "hockey puck" design.

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