SLVAFD4 February   2024 TPS54308 , TPS54320 , TPS54350 , TPS54620 , TPS54622 , TPS54821 , TPS54824 , TPS62933F

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Components Selection for Secondary Stage Filter
  6. 3Experimental Validation
  7. 4How to Estimate Inductance of Ferrite Bead for Ripple Reduction
  8. 5Summary
  9. 6References
  10. 7Appendix

3 Experimental Validation

The experimental validation is completed with PCM converter TPS62933F. Operating condition is: Vin=24V, Vout=1.2V, fsw=500kHz, Iout=0A. Required output voltage ripple amplitude is smaller than 800uV pk-pk.

As comparison, an application case without second stage filter is given at first. With typical recommended components value in data sheet, L=2.2 uH and Co=94uF are selected for this operating condition. 2 x 47uF capacitors are used for the 94uF Co.

The output voltage ripple test waveform is shown as Figure 3-1, the output voltage ripple pk-pk amplitude is about 5.7mV and it is much larger than the requirement.

GUID-20230616-SS0I-MJCX-R1ZW-SKCCXC74FS2J-low.svg Figure 3-1 Output Voltage Ripple Without Second Stage Filter

Then the proposed components selection method of second stage filter is verified. With same 2 x 47uF as the total capacitance for both first stage and second stage filters, we set the Co=1x47uF and C2=1 x 47uF. Substituting all parameters into Equation 7, L2≥14.9nH could be got as the restriction for 800uV output voltage ripple.

Select L2=20nH and Figure 3-2 shows the output voltage ripple with second stage filter. The output voltage ripple amplitude is 640uV pk-pk and could satisfy the 800uV requirement.

GUID-20230616-SS0I-CCHZ-GKZN-T2XJZ9SZCF7W-low.svg Figure 3-2 Output Voltage Ripple with Second Stage Filter

The effectiveness of proposed design method is verified by the example. More experimental results are shown in Section 7.