Mpq2484ugf-aec1-p_inrush_current

Lighting Solutions
1

Dear MPS Technical Support,

We are currently using the MPQ2484UGF-AEC1-P for an LED lighting application in PWM dimming mode. Our operating conditions are as follows:

  • PWM Frequency: 40 Hz

  • ON Time: 1 ms

  • Input Voltage: 12 V

  • Load: 10 LEDs in series

  • Output Voltage: ~60 V

  • Output Current: 600 mA

During the first PWM pulse, we observe a peak input current of approximately 9 A, which causes the driver to enter fault mode. During subsequent PWM pulses, the input current settles to approximately 4 A.

Schematics:

image
image1004×613 104 KB

Input Current:

To reduce this transient current, we performed several trial-and-error modifications by reducing the output capacitance and increasing the compensation capacitor. Our observations are summarized below.

Trial 1

Design Changes:

  • Unmounted C25 (47 µF), resulting in a total output capacitance of 22 µF.

  • Increased the compensation capacitor C40 to 2 µF.

Observations:

  • Initial pulse input current reduced from 9 A to 4.5 A.

  • A delay 50uS observed to settle the output current to 600mA

  • The steady-state input current increased to approximately 4.7 A.

  • Blue - PWM RED- Output Capacitor Voltage Green- Input Current

  • image
    image946×503 117 KB

To mitigate Observations 2 and 3 from Trial 1, we performed another set of modifications as described below.

Trial 2

Design Changes:

  • Unmounted C21, C22, and C24, resulting in a total output capacitance of 49 µF.

  • Increased the compensation capacitor C40 to 6 µF.

Observations:

Initial pulse input current reduced from 9 A to 5.2 A.

  • The delay between the PWM signal going HIGH and the output current reaching 600 mA reduced significantly to approximately 680 ns.

  • The steady-state input current decreased to approximately 4 A.

  • However, the output current ripple increased compared to Trial 1.

  • BLUE- PWM; YELLOW- OUTPUT CURRENT

    image
    image958×572 144 KB

We would appreciate your guidance on the following:

  1. Based on the above results, which of the two compensation approaches is likely to provide better loop stability?

  2. Could you please explain the recommended method for selecting the RC compensation network (COMP pin components) for the MPQ2484UGF-AEC1-P? We could not find any design guidelines or compensation calculation method in the datasheet.

We look forward to your recommendations.

Attached Waveforms and Schematic for your understanding

Thank you for your support.

Best regards,