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UCC28950 Example Schematic

Bill Drumheller
Prodigy 135 points
Other Parts Discussed in Thread: UCC28950

Hello,

Page 61 of the UCC28950 data sheet (SLUSA16C) shows a schematic of a daughter card.  There are two resistors shown (R5 and R26) that are not populated that I am uncertain of their purpose.  For R5, it would appear that this would serve as a voltage divider for the COMP pin.  Why would you need this?  R26 would seem to allow the ADEL and ADELEF pins to be tied to the CS pin.  Again, would this ever be beneficial?  I am assuming that if you did populate R26 that both R21 and R23 would then be removed from the circuit.  Is that correct?  Or would R23 remain in the circuit therefore tying the DCM voltage divider to CS (again not sure why this would be done).

Thanks,

Bill

  • 0
    TI__Mastermind 25570 points
    Hello Bill

    R5 was probably put in place by the designer of the EVM as a 'placeholder' part to allow some experimentation with the compensation during EVM development. You won't need it in an actual production design.

    Connecting the ADEL and ADELEF pins to the CS signal allows the designer to adapt the dead time between high side and low side (ADEL) and between primary and SR switching (ADELEF) as a function of the load current. Normally a potential divider (R16/R17 and R19/R20) is used to deliver a proportion of the CS signal to the ADEL and ADELEF pins. This allows longer dead times at light loads and shorter dead times at heavy loads. Whether this is beneficial in a given design depends on whether maintaining ZVS on the primary, reducing body diode conduction time in the SRs and primary switches is important. In general, ZVS is really useful in reducing switching losses at high input voltages (PFC Boost stage outputs at 385V typ), less so at 48V telecom inputs. Keeping body diode conduction times as short as possible is more important at lower input voltages where the currents are higher.

    The ADEL / ADELEF functions of the UCC28950 are described in the datasheet on P1 et seq. Of course if you don't want to use ADEL or ADELEF, these pins may be tied directly to GND.

    If you use ADEL and ADELEF, then yes, R26 gets populated (0 Ohm) and R23 gets removed.

    The DCM resistor potential divider (R24/R25) sets the CS current at which the UCC28950 turns the SRs off (diode rectification). It is not connected to the CS pin at all - either directly or through a resistor - effectively, R21 is ALWAYS needed. Do take care to set the DCM level such that the SRs are disabled while the output inductor current is still continuous. DCM is described on P26 of the datasheet - do observe the conditions regarding DCM threshold level in the last paragraph on P27.

    Again, the EVM and DS were drawn up as general purpose schematics - allowing a user to experiment - hence the many non populated resistors.

    Please let me know if you have any further questions.
    Regards
    Colin