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Question on transformer specifications in flyback converters for IGBT driving

Neil Yu
Prodigy 70 points

I have some questions in reading TI's help document(TIDU411), which is used for isolated IGBT gate-drive flyback power supply design. Specifications for power supply are

• Continuous output power of 2 W to drive each IGBT
• Support up to a 6-A peak current, with an output voltage ripple of less than 200 mV

However, I find that spcifications for the transformer are

• Four isolated outputs: Vout1 = 25 V at 150 mA, Vout2 = 25 V at 150 mA, Vout3 = 25 V at 150 mA, and Vout4 = 25 V at 450 mA.

It seems like that the power of transformer is 3.75W for each IGBT, totally different from that of the power supply!

How could I define my specifications for the transformer used for isolated IGBT gate-drive flyback power supply?

  • 0
    TI__Expert 7395 points
    Neil,
    Thanks for your interest in TI here. I'll have an engineer support you here soon.

    Regards
    John
  • 0
    TI__Genius 12951 points

    Hello Neil,

    Sorry for the delay in getting back to you.

    Yes, the transformers are designed for higher power than required.

    Typically, IGBT gate drive power supplies are specified for average current (one PWM cycle) which would be typically in the range of few 10’s to 100’s of mA depending upon the IGBT.

    The currents (typically 2.5A/5A) specified in gate driver ICs (for driving IGBTs) are peak currents which lasts for about 100-500nsec. This is achieved using bulk capacitors.

    For the document TIDU411, please note that 25V is further divided to 16V and -8.2V as bipolar supply for the IGBT.

    If you can help with exact requirements (like application, IGBT part number, required output power), we will be able to support further.

    Thank you.

    Sanjay R. Pithadia

  • 0 in reply to Sanjay Pithadia
    Prodigy 70 points

    Dear Sanjay,

    Thank you for your reply.

    Is there any rule I can obey to specify the transformer's power?

    Peak currents which are needed at the time for IGBTs'   turning-on or turning-off are drawn from the bulk capacitors. This causes the transient reponse of the primary side's current, which will  force the transformer for more power. Is this the only thing I need to care about in specifying the transformer or how should I cope with this procedure?

    In my design, I use a flyback topology for 4 outputs of 24V, which is further divided to 16V and -8V too. The main circuit is single phase T-type three-level inverter with four IGBTs for each bridge, whose output power is 3kW. IGBT's part number is FGH40T65SHD, coming from Faichild.

    Thank you!

    Neil

  • 0 in reply to Neil Yu
    TI__Genius 12951 points
    Hi Neil,

    Sorry for the delay as I am seeing this post today itself. (Somehow I did not get any email when you replied here).

    The average power taken from the transformer - is mainly required to be specified along with efficiency and margin for the transformer design.
    We have designed the transformer for slightly higher power so as to take care of variety of IGBT gate-drivers.

    Here is a link for a very good document from TI which explains the design of transformer.
    www.ti.com/.../slup127.pdf

    Hope this helps, thanks.

    Regards,
    Sanjay R. Pithadia.