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UC3825: Using method of UC3825

Part Number: UC3825

Hi

I got questions from my customer about UC3825.
1.
 This device has two drive output.
 Could I use this device only one drive output?
 If this method is ok, are there the consideration?
2.
 Could I use t the two drive output as the common?
Best regards,
Yokota
  • Hi Takahiko-san,

    Thank you for your interest in TI's UC3825. I have asked one of our applications engineers to respond to your post. You should see a response soon.

    Peter

  • In reply to Peter Meaney:

    Hello Takahito-san

    You may use only one of the two outputs from the UC3825. The switching frequency of the single output will be half that of the oscillator (set by RT and CT). The maximum duty cycle of the output used will be half that of the two outputs or 42.5%

    The two outputs from the UC3825 are complementary - you cannot connect them in parallel.

    Can you send me a block diagram showing how the customer plans to use this controller please ? you can send it to me directly at colingillmor@ti.com if you can't post it here.

    Please let me know if you need any further information.
    Regards
    Colin
  • In reply to Colin Gillmor:

    Hi Colin-san
    I'm sorry for my late reply.
    My customer think they use UC3825 by multi topology such as pushpull, fullbridge and forward(single/double).
    Could you tell me the method if there are the method using it in parallel?
    Best regards,
    Yokota
  • In reply to takahiko yokota:

    Hello Yokota-san

    You can parallel multiple power stages using the UC3825 - or indeed any controller operating in current mode control (but not Voltage mode control) by connecting their error amplifiers as voltage followers and then connecting their inputs to the output of an error amplifier. This could be an external error amplifier or an error amplifier in one of the power stages. I don't have a diagram of this arrangement specifically for hte UC3825 but the general idea is sketched out in the attached file.

    Please let me know if you have any further questions - especially if it is not clear how this method applies to the UC3825. I'm sorry for the short reply but I'm travelling all week on business with little free time.

    Regards

    Colin

  • In reply to Colin Gillmor:

    Hi-Colin-san

     
    Thank you for the reply.
    The attached document is the customer's consideration circuit.
    I think they can not use UC3825 with that design.
    Can I use the circuits of ① or ②?
     
     
    Best regards,
    Yokota
  • In reply to takahiko yokota:

    Hi-Colin-san
     
    I'm sorry to be pressing, Any update this issue?
     
    Best regards,
    Yokota
  • In reply to takahiko yokota:

    Hello Yokata-san

    Thanks for reminding me of this - I had 'lost' the task and was unable to find it.

    Anyhow, the customer's option won't work, the two outputs of the IC will 'fight' each other - one pulling up while the other pulls down.

    The option with the 'OR' gate is the correct concept but it won't work because the OR gate won't have enough output current to drive the MOSFET properly. (your diagram had a small 'typo' and showed the two OR gate inputs shorted together

    The option with the two diodes is the best - and lowest cost - solution. The Vf of the diodes will subtract from the output voltage of the IC but if you use Schottky devices that won't be a problem. Do make sure that the diodes can carry the pulse currents delivered by the IC into the MOSFET.

    Regards

    Colin

  • In reply to Colin Gillmor:

    Hi Colin-san

    Thank you for the reply.
    Let me ask you the further question.
    If I use UC3825 as the single output, should I open the other output terminal?


    Best regards,
    Yokota
  • In reply to takahiko yokota:

    Hello Yokota-san

    If the output OUTA or OUTB is not used then you should leave it open-circuit.

    I have not used the UC3825 in this manner and this is not the way it was intended to work - I think it should work ok but it would be best to check before spending too much time on it.

    Regards
    Colin
  • In reply to Colin Gillmor:

    Hi


    I’m sorry for replying to you so late.
    My customer conisder designing attached circuit.
    Could they operate UC3825 over 50% duty cycle using attached circuit?

    Best regards,
    Yokota