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UCC28C43: ucc28c43

robin gangopadhya18
Mastermind 6296 points
Part Number: UCC28C43
Other Parts Discussed in Thread: TINA-TI,

I have to confess my ignorance in this post upfront!

This topic has to do with slope compensation of a buck converter with 70% duty cycle. 

One can read & I have been doing this for several years until now, but did not have any meaningful rebuttal of the  received truth because what I see does not match with what is stated.

For starters, all baseline  CCM/PCM  is stated to require slope compensation of Mc .

Magically if Mc=M2, down slope of the "inductor current", stability is achieved in 1 cycle.

A lot of ink has been spent on Mc, M2 etc.

But the solution always given to "add a slope compensation" from anywhere BUT the inductor current. In almost all literature, one is advised to just throw in after some  incomprehensible Algebra using Rs, Gi, Ri, Km etc. etc...in the end one adds ramp of the oscillator to the sensed current.

WELL THAT ramp has no connection with the inductor down slope! Nor any connection with what has been derived as "down slope" 

besides, the switch has been already turned off. So what part in the control section has any contribution to make in compensating for Subharmonic oscillation?

Pl see the TINA-TI screen shot: the values of the compensation cannot be derived from SLUA301 not from SLV450. In real circuit, this is a very very slow circuit to respond to transients. 

Does it mean that getting 270V from a 350V is not theoretically possible using a buck converter? 

I suspect, the model  does not give proper values because "slope compensation" --if the SPICE model of UCC28C43 contains the artifact to response to compensation- is not really being added correctly. Even though  we are doing exactly what is written in the cited materials.

So how do we get to a decent cross over freq of this circuit ?

W: we do have an ACS723 after the inductor - but its BW is 80kHz while switching is happening at 200 KHz. We have it so  shut down in case of short circuit.

Can that be used for "Slope comp"?

Any help will be highly appreciated. '

  • 0
    TI__Mastermind 31580 points
    Hi Robin,

    The slope compensation in the circuit shown is added during the on-time of the switch as well as the off-time as you noted.

    The slope compensation signal is taken from the RTCT pin which is a triangle shaped waveform and the amount of slope compensation added to the CS pin signal is controlled by the ratio of the values of R3 and R4. In many power supply application the primary side controller does not have access to the output inductor current. The RTCT signal is used as proxy for the inductor current in this case.

    There should be no issue obtaining 270V from a 350V input for a buck circuit, the duty cycle is 77%, since the duty cycle is over 50%, slope compensation is required to stablise the loop.

    Typical crossover frequencies for the control loops are in the range of 800Hz to 1.5kHz for general purpose industrial power supplies.

    I am not familiar with the ACS723 as it is does not appear to be a TI device. The arrangement in the TINA circuit you shared can provide the slope compensation required once the correct ratio for R3 and R4 has been established.

    It is not clear to me what your application is and what problem you are experiencing with it?

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Mastermind 6296 points
    Thnx Peter. I got a simulation model working that converts 350V to 270V. I got compensation and Slope values through TINA-TI simulation - hundreds of them with different set of values. But I am sure its response to transients is not optimal. Because none of the app notes will yield meaningful values from algebra of CCM/ CMC buck small signal models. I am still looking for a paper or app note that will help "optimize" cross over freq, gm & pm.