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Overcoming the Duty Cycle Limitations of the 2 Switch Forward.

John Bottrill
Mastermind 25130 points

The limitations of the Two Switch Forward Topology are well known and are the result of having to balance the magnetic flux in the transformer. There is a method of overcoming this limitation that is covered in an article published in Power management Design Line on 03/14/05 to which TI holds the patent.

This offers the opportnity to extend the duty cycle of the power switch and hence allows a wider operating duty over the input voltae range. The article can be accessed by the link below. A license is prvided to use the patent as long as a TI part  is the controller.

http://www.powermanagementdesignline.com/howto/showArticle.jhtml;jsessionid=RMDU534TNDEF4QSNDBESKHA?articleID=159402553 

  • Prodigy 70 points

    Hi John,

     

    I did not quite understand the advantage  using the 2 switch forward converter for increase the duty cycle. 

    1. If one wants to work at a higher duty cycle. they would be better off

    a) by using single switch forward converter with a resonant snubber. The Voltage rating needs to be twice the input voltage but the cost won't be as high as using 2 mosfets. In thsi case 3 mosfets. Apart from the mosfet, there is a cost on the board space and the drive circuitry too. Single switch coverter would be advantages because parralleling 2mosfets reduces the conduction loss by half.

    b) The proposed circuit needs 3 mosfets, One would be better off by using half bridge and get duty cycle around 95%. and put the cost of mosfet over the capacitors.

     Maybe i am not looking on a particular appliation where this scheme would be the best. Can  you explain the application with input voltage and output voltage and current ratings.

     

    Thanks

    Jose Thomas

    (www.powerelectronicsconsultant.weebly.com)

     

     

  • in reply to jose thomas16200
    TI__Mastermind 25130 points

    Jose,

    The article that you referred to shows how to overcome the 50% duty cycle limitation of the 2 switch forward without losing most of the advantages of the 2 switch forward such as lower voltage FETs, recovery of the magnetizing inductance energy, removal of the need for primary side lossy snubbers, and reduction on EMI generation.

    Compared to the single switch forward there are probably significant efficiency advantages with a two switch forward but you are restricted to the 50% duty cycle limit. The cost is the drive circuit and component count and board space.

    A three switch forward gives you a 67% max duty cycle with limited additional component cost over a two switch forward as you already have most of the components in the two switch forward design.

    It is a engineering decision and the article just presents the idea. It has enough advantages that it was patented. The license is free to anyone using our parts to control the converter using the topology.

    John

  • in reply to John Bottrill
    Prodigy 70 points

     

    Thanks for your feedback.

    Single switch forward converter with resonant snubber allows the duty cycle to go to 65%. The topology is well documented in APEC 2001 or 2002 publication.  The resonant snubber capacitor is charged with a negative voltage during the turn on off the device which decreases the swing on the drain voltage during turon off of the device.

     

    Jose Thomas

  • in reply to jose thomas16200
    TI__Mastermind 25130 points

    Jose,

    Sorry if I wasn't clear. The duty cycle limitation applies to the 2 switch forward which is what the paper also refered to.

    John

  • in reply to John Bottrill
    Prodigy 30 points

    Is  it possible to increase duty cycle of a 2 switch forward by  inserting  zeners in series with the catch diodes ? . This can increase the reset voltage by the amount

    equal to the breakdown voltage of zeners.therby  alowing the core to reset in lesser time. only the voltage rating of fets shall increase by value of vdc plus 2 zeners

  • in reply to feesah haseef
    TI__Mastermind 25130 points

    Feesah,

    I don't see any reason why that won't work to expand the duty cycle. 

    The efficiency cost will be higher voltage components for the FETs (higher Rdson) and power dissipative zeners. This will also impact the component cost for the supply.

    It is an engineering decision you will have to make.

    Regards,

    John