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Original question:

48V/12V 60A reference design

PMP7282: Design challanges

Furkan Fehmi Sefiloglu
Expert 3118 points
Part Number: PMP7282
Other Parts Discussed in Thread: LM5116

Hi Team, 

This is customer request and related to topic on:https://e2e.ti.com/support/power-management/f/196/p/943795/3496463#3496463

"

We made our own design by taking PMP7282 reference design as an example. But we are having trouble with the signal we receive regarding the SW pin.

Problem: We reduced our switching frequency to 50K. Because the controls get very hot. When we read the SW pin, the resulting signal is attached. What needs to be done to see a proper PWM?

"

Thanks in advance

Furkan Sefiloglu

  • 0
    TI__Intellectual 1235 points

    Hi Furkan,

    There is nothing wrong with your switching waveform. The short pulse and ringing is due to discontinuous conduction-mode (DCM) operation at light load. When the load current is less than 1/2 the inductor ripple current, the triangular inductor ripple current goes to zero, causing the synchronous rectifier to shut off. Ringing occurs due to circulating current in the inductor and MOSFET switch capacitance. Only when you increase the load above 1/2 of the inductor ripple current will continuous conduction-mode (CCM) operation occur. 

    Reducing the switching frequency or inductor value will cause DCM to occur at higher load current, since there is more ripple current. Increasing the switching frequency or inductor value will cause DCM to occur at lower load current. Ween you increase the load above the DCM boundary, CCM operation with square wave switching occurs and no ringing.

    DCM operation always occurs at light load with diode rectified switching regulators. Some synchronous controllers have a MODE pin to select forced CCM at light load or DCM operation. The LM5116 has a DEMB pin where the DCM threshold can be adjusted. Please read the notes in the data sheet section 7.4.1 Soft-Start and Diode Emulation. It is possible for the controller to default to diode emulation mode under some conditions.

    Best Regards,

    Bob Sheehan

    Power Design Services