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UCC28950: Gate Driver Circuit Design

abhishek moghe
Intellectual 290 points
Part Number: UCC28950

In isolated gate driver where pulse transformer is used, it is recommended to use capacitor in series with pulse transformer to avoid the Saturation of the same. 

What should be the type of capacitor that is to be used for this purpose. in my previous experience i have seen 1uF 50V Box type capacitor ? can we use ceramic capacitor instead of Box type capacitor where space is constraint. in my application i have limited board space and i can not afford to use a box type capacitor due to its size. Can i use Multi layer Chip Capacitor of same value (1uf 50V) ?  ( C26 in below circuit. which is in series with pulse transformer )

 

  • 0
    TI__Mastermind 25570 points

    Hello Abhishek

    The short answer is yes you can use a ceramic capacitor in this application.

    The main thing to watch out for is that the temperature coefficient of capacitance and the voltage coefficient of capacitance of the capacitor you choose are acceptable. Normally this means using a capacitor with a fairly high voltage rating, 50V is a good initial value and a reasonably temperature stable dielectric and X7R is a reasonable choice here. Please note that these suggestions may not be the best if your design has to survive extremes of temperature or needs an automotive qualification but in general they would be a good starting point.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Intellectual 290 points
    Hi Colin,
    Thank you for your reply.
    product is class II Power supply.
    operating temperature range : 0 to 55 deg cel.
    another doubt i am having in ceramic capacitor is are they able to handle peak currents that will flow through Pulse Transformer during MOSFET Turn ON Period. i.e. when internal Cgs is charging. Ceramic caps may get derate because of peak currents fetched at 200KHz through them. (100KHz is switching frequency of DC DC Converter). So is ceramic capacitor really preferred ?
  • 0 in reply to abhishek moghe
    TI__Mastermind 25570 points
    Hello Abhishek

    You are correct in observing that the peak currents in the pulse transformer are high. 1A peak certainly isn't unusual and I have seen peak currents of up to 10A in at least one design that I worked on. However the duty cycle of the current is very small so if you calculate the RMS current you will find that it too is very low. The current rating of a ceramic capacitor is usually set by self heating considerations and this self heating is proportional to the RMS current not the peak current. You could, and probably should, monitor the capacitor's temperature rise during operation and make sure that it is not excessive. The Murata site at www.murata.com/.../0001 discuss this but I'm certain that the general conclusions apply to other manufacturers product as well.

    While we are on the topic of gate drive currents - please make sure that the resistors in the gate driver circuit can handle the peak currents. We have seen problems where 0603 sized parts fail slowly over a period of months because they cannot handle the peak current. Over time their resistance increases and eventually the MOSFET is not being driven correctly and fails. Normally we use 1206 sized parts in our EVMs for this reason.

    If you need any more detailed information on the behaviour of ceramic caps then I would have to ask you to contact one of the manufacturers of these parts.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Intellectual 290 points

    Hi Colin,

    Yes i agree the resistor sizing. i have selected 1206 package for Gate Resistor. 

    Thank you for your reply.  i will definitely look for more details on the same.