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Questions related to minimum Dc input and diode trr in LNK3207.

Posted by: kaustav chakraverty on Mon, 09/06/2021

Hello all,

We are designing a DC-DC converter using LNK3207 with the following design parameters:

Vin: Either 48V(40Vmin and 58V max) or 72V (63V min and 80V max) battery pack 

Vout: 25V

Max current output: 0.5A

 

Query 1:

The min. voltage given in the corresponding datasheet is 70V at full load or greater. In the PI simulator we can get a rectified output less than 70V, even 50V. This defies the statement of the datasheet that the minimum DC input voltage for full load current to be applied is 70V. Can you please provide the reason behind the following discrepency.

Query 2:

How does  trr of a diode play a crucial  role  in buck converter operation? Hence when the datasheet recommends to use BYV26A as the diode for its less recovery time of <35ns,is it fine to use a diode of ES1A - ES1G with trr=25ns instead. 

Comments

Submitted by PI-Salt on Mon, 09/13/2021

Hi kaustav chakraverty,

Thank you for showing interest in Power Integrations and its products.

Answer to Query 1:

- Could you provide the PIXLS spreadsheet from using the PI simulator?

- For the meantime, you could refer to the app note for LNK3207 (LinkSwitch-TN2 family) here: https://www.power.com/design-support/application-notes/an-70-linkswitch-tn2-design-guide

Answer to Query 2:

trr plays crucial role in switching losses.

- According to the app note and datasheet, "For MDCM operation at tAMB ≤ 70 °C, select an ultrafast diode with tRR ≤ 75 ns. At tAMB > 70 °C, tRR ≤ 35 ns. For CCM operation, select an ultrafast diode with tRR ≤ 35 ns." Make sure to account for the 25% design margin (see app note for ref.)