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Original question:

MC33063A: Help Calculating CT

MC33063A: MC3306 Getting Hot

Michael M1
Prodigy 140 points
Part Number: MC33063A
Other Parts Discussed in Thread: TPS61089

Hello,

I posted earlier but accidentally included the wrong screen shot. Please see my schematic attached. I have this boost converter working fine and pumping out the correct output voltage (11v). The input is 3.5v. However, when I add any sort of load of around .8-1 amps reported on my powewr supply, the IC gets really hot, the voltage sags to about 8v and then shuts down. I have calculated the values for 1.5A maximum. What could be the issue here? I am using a frequency of 100khz, hence the 100 pf timing cap. I have also tried 1nf as reccomended previously. The output on my scope looks clean and nice.

What is causing the MC3306 IC to overheat? Is my math incorrect? Thanks for your help

  • 0
    Prodigy 140 points

    Can someone reply please? Bumb

  • 0 in reply to Michael M1
    TI__Mastermind 21565 points

    Hi Michael:

    So sorry for keeping you waiting.

    May you share the waveform with us? Better including the current of inductance. What's the saturation current of the inductance?

    And the issue just happened when load is higher than 0.81A, but it's fine with lighter load?

  • 0 in reply to Minqiu Xie
    Prodigy 140 points

    Hi,


    Thanks for getting back to me. I have tried a few different inductor sizes, all with similar result. I can pull less current and it does not heat up as fast and the voltage drops less. I have tried 1uh, 3uh, 10uh and 20uh, based on different numbers, none of which have solved the problem. The inductor stays cool but the IC gets really hot to touch.

    Please see scope readings attached.  Our load pulls 0.5A straight from the PSU, when attached to the boost, it pulls between 0.8-1.3A. Something in the circut is causing a large inefficiency. I am not sure why this is happening. I had another engineer take a look and they are also not sure. This is my last hope! Thanks for your help.

    Input: 3-4.2V

    Output: 11V

    Max current: 1.5A

    F: 100khz

    Inductor Current Saturation: 2.2A

    no load:

    with load:

    Minqiu Xie said:

    Hi Michael:

    So sorry for keeping you waiting.

    May you share the waveform with us? Better including the current of inductance. What's the saturation current of the inductance?

    And the issue just happened when load is higher than 0.81A, but it's fine with lighter load?

  • 0 in reply to Michael M1
    TI__Mastermind 21565 points

    Hi Michael:

    I highly suspect the inductance is saturation.

    For a boost converter, the Iin=Vout*Io/Vin/Efficiency. Take Vin=4V, Vout=11V, Io=0.8A , Eff=90% for example. Then Iin=2.44A. Consider the current ripple and inrush, it could be much higher. It's exceed the inductance spec.

    It's better to use a current probe to double check it. And try the inductance with large current capability. 

    Actually, we recommend the choose the external parameters follow datasheet recommendation.  

  • 0 in reply to Minqiu Xie
    TI__Mastermind 21565 points

    Hi Michael:

    I just thought of it's exceed the current capability of MC33063, as it only support 1.5A current. It's the peak current of switch like peak current of the inductance, not output current. 

    I recommend you to see TPS61089, which has enough current capability and much smaller solution size. 

  • 0 in reply to Minqiu Xie
    TI__Mastermind 21565 points

    Hi Michael:

    As long time no hear from you, I assume you have solved the issue. If not, just reply below. Thank you!