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# SN6505B-Q1: How to design turns ratio

Part Number: SN6505B-Q1

Hi team,

From the datasheet, we can see that Vo = n*Vin*D. If the design requirements are Vin=5V, Vout=12V, so how to choose turns ratio and duty cycle? because i am not famliar with push-pull, so can you please help me learn the device more?

thanks and best regards,

Sophia

• Hi Sophia,

Minimum turns ratio for a transformer used with the SN6501 or any SN6505 part can be calculated as:

This is essentially N = Vout / Vin + compensation for transformer, diode, and LDO inefficiencies. Rds-max and Id max are given in the SN650x datasheet with Vf (or diode drop voltage) being found in the Schottky diode datasheet, Vdo-max (max dropout voltage) being found in the LDO datasheet if one is used, and Vin and Vo being determined by the user. Please note the calculation yields a minimum turns ratio, so a transformer with slightly higher N than calculated should be selected.

Turns ratio is defined as the number of turns on the secondary (output) side of the transformer divided by the number of turns on the primary (input) side. This can be presented as a ratio such as N = 1:3 for 5V:12V operation. The center tap for Vcc or GND connections should made on the exact center turn of each side to prevent imbalancing, and duty cycle is set by the SN650x at 50%; this is necessary for proper operation.

Are you in the process of selecting a transformer? Please let me know how else I may help with a selection!

Thank you for posting to E2E,
Manuel Chavez