SN6505B: 15V/3W Transformer Options

Hi WB,

Thank you for posting to E2E!

The Pulse transformer part options you shared, PH9185.013NL and PH9385.013NL, are expected to be compatible with SN6505B. They have a slightly lower turns ratio than the Wuerth part, 750316818, so PH9385.027NL might be a more similar option. What is the necessary load current for the isolated loads?

Problems aren't expected when using a buck converter over an LDO to generate a power supply, but do ensure the buck converter outputs are stable and the SN6505 has a 0.1uF capacitor placed closely to its Vcc pin in layout.

Regarding point 3), we do not have a part that can accept >5V inputs at this time. Please accept my Request to Connect so I may follow up on this. Would the topology below be helpful instead? A regulator would still be required for 15V -> 5V to power the SN6505, but efficiency could be higher since the higher voltage is connected to the transformer directly:



Please let me know your thoughts on the above! The Pulse transformers we discussed should be able to handle 2.5W - 3W of isolated power transfer.


Respectfully,
Manuel Chavez

Hello Manuel,

Thank you for your feedback. 

The peak load is around 3W. The average is around 2W.

I had used similar design in the early revision of the same project. Here is the circuit I had:

I used SN6505A along with 750315240 (150KHz TR). A WE FAE indicated that 15V could cause the transformer to saturate. But, PH9185.011NL + SN6505B  could work because it has 66 V-us comparing to 23V-us on 750315240. https://www.mouser.com/datasheet/2/336/P708-1526895.pdf 

PH9185 is preferred for the reinforced insulation.  PH9385 has Basic insulation but of an interest for a different project. Is there a minimum requirement for the transformer Pri inductance?  PH9185.011NL has 750uH.

As for efficiency, I am in favor of the topology with the minimum no load current and better efficiency. I believe that the no load current was around 10mA but might be mistaken since it's been a while. Please advise if using FETs to buffer D1 and D2 outputs is a better option. Any idea what would be the no load current and the efficiency at 50% load for this topology? Any other recommendations before modifying the circuit? 

Thank you!

Hi WB,

You're welcome! A 15V input paired with SN6505A would likely lead to transformer 750315240 saturating, however this transformer could be used with SN6505B (faster switching frequency) and a 15V input if that is an option for you.

There is not a minimum or maximum requirement for primary-side inductance for transformers in SN650x systems: the functional requirements are covered by V-t product and turns ratio parameters. PH9185.011NL should be acceptable for reinforced isolation using either SN6505 device with a 15V power supply on the input-side.

System efficiency depends on which transformer is selected along with which external components and in which topology they're configured. For a transformer with 750uH primary inductance, "no load" current should be ~10mA in the typical configuration and increase for transformers with lower inductance and DCR than PH9185.011NL. Efficiency at across loads will also be transformer-specific, and, if external FETs are used, also dependent on their Rds-on value.

My apologies, but we do not have relevant measurements with these parts to share at this time. Please let me know if there is additional helpful information I may provide.


Respectfully,
Manuel Chavez

Thanks Manuel... Any suggestions for the FETs?