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UCC28950: how does magnetizing current provides part of compensating ramp?

Nitish Agrawal
Expert 1595 points
Part Number: UCC28950

Hello,

Equation 169 of SLUSA16D states that "magnetizing current provides part of compensating ramp" and is given by Mmag = Vinmax * Rcs/(Lmag * CTrat).

We are trying to understand how this is happening.  Can you help?

Regards,

Nitish Agrawal

  • 0
    TI__Guru* 85806 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 85806 points

    Hello,

    The following equations will help describe how the magnetizing current adds slope compensation.

    The voltage across the transformer primary (Vprimary)  will develop a change in current (di) based on  the magnetizing inductance (Lm) and the time the voltage is applied across the primary of the transformer.

    Vprimary = Lm*di/dt

    di = Vrprimary*dt/Lm

    di can also be calculated based on duty cycle (D) and switching frequency (fsw).

    di = Vrprimay*(D)/(fsw*Lm)

    This di cause by the magnetizing current will develop a change in current across the current sense transformers through the turns ratio and will result in dV across the current sense resistor (Rs).  This dV will act as slope compensation that is caused by the transformers magnetizing current.

    dV = di*Np/Ns*Rs 

    Regards,

  • 0 in reply to Mike O'
    Expert 1595 points

    Hi Mike,

    Thank you for your response.  The total slope of signal at CS pin is the sum of the slopes caused by the transformer magnetizing current and the secondary side inductor current (each scaled by appropriate factors).
    My understanding is that to determine the required slope compensation, we should only be considering the slope contribution from the secondary side (output) inductor, which is:
    m_Lout = (Vout/Lout)*Rcs/a1/CTrat.

    Then, shouldn't the minimum required compensation just be 0.5*m_Lout = 0.5(Vout/Lout)*Rcs/a1/CTrat ?

    I am not following why m_mag is subtracted out to obtain m_sum in eqn. 170 of SLUSA16D.
    What I am asking is that shouldn't m_sum just equal 0.5(Vout/Lout)*Rcs/a1/CTrat ?

    Regards,
    Nitish

  • +1 in reply to Nitish Agrawal
    TI__Guru* 85806 points

    Hello,

    The following link will bring you to an application note that discusses how to design a phase shift full bridge and there is a section on how to setup slope compensation for the device.

    https://www.ti.com/lit/pdf/slua560

    You are correct that the slope compensation you add should be 50% of the output inductor ripple reflected across the transformer.  Application note slua560 should give you the correct guidance on how to program the internal slope compensation when using the UCC28950.

    Regards,