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# SN6501: SN6501 transformer

Part Number: SN6501
Other Parts Discussed in Thread: SN6505B

Hi team,

My customer wants to use the SN6501 to design 3.3v input, 6.6v output, then use a LDO to generate 5v, 200ma.

we have found some transformers as the attachments. Could you please help check whether the transformers are fine (smaller inductance as EVM itransformer) and show how to evaluate whether the transformer are fine or not? Thanks.

transformer.zip

• Hi Frank,

Thank you for posting to E2E! Determining whether a transformer can yield desired output voltages depends mostly on its turns ratio, N.

The necessary turns ratio can be calculated using Equation (10) of Section 9.2.2.5 of the SN6501 datasheet, where the desired output voltage is included as Vo-max.

A second parameter to consider is transformer V-t product to ensure a transformer is compatible with the SN6501 switching frequency. From the V-t product perspective, it seems the Pulse transformer and ES835-04196F are compatible with the SN6501 operating at 3.3V. The 6PT3135XCR does not specify V-t product.

However, this system will not be able to supply a 200mA load: at Vcc = 3.3V operation, SN6501 can only transfer ~0.5W of power across the isolation transformer, which is not enough for 6.6V output at 200mA. For this requirement, please encourage our customer to use SN6505B instead, which is a more robust and efficient part that can transfer up to 5W of power across an isolation transformer.

The transformers with turns ratios of 1:2 will output slightly less than 6.6V due to parasitics of the system (Rds_on from SN650x and Vf from diodes). The equation I mentioned above will determine whether the turns ratio of these transformers is sufficient to achieve 6.6V output, but please feel free to share part numbers for the Schottky diodes and LDO, and I can provide further insight.

Manuel Chavez

• hi Manuel,

Thanks for your reply. We can only use the VT as the criteria, correct? My concern is if the transformer primary inductance is too small, the primary peak current will be larger. Whether we need to consider it? Thanks.

• Hi Frank,

This is correct -- V-t product is the main criteria when it comes to switching capabilities. Turns ratio is the main criteria for determining output voltage.

Peak currents on the primary side are mostly affected by the load, so reducing load capacitance is the simplest way to reduce peak currents on the primary side. For SN6501 systems, load capacitance should be <5uF. Inductors may also be used to reduce peak currents as mentioned in the E2E conversation linked below:

Additionally, the SN6505 has soft-start and current protection built in, which ramps output power supplies slowly and protects the device from sudden over-currents. This makes SN6505 systems appropriate for supplies with highly capacitive loads, and the SN6505B is compatible with transformers used for SN6501.

Can our customer use the SN6505B instead of SN6501 in this system?

Thank you,
Manuel Chavez

• Hi Manuel,

I have tried to convince them to use the SN6505B instead of SN6501. Could you please help double check whether the transformer i shared before are fine or not? For the post you shared, it doesn't describe how to consider the inductance very clear. in my opinion, we need to calculate the ΔI pri, then Ipri max= 0.5*ΔI pri, then we can get the inductance influence. Thanks.

• Hi Frank,

Thank you for encouraging our customer to use SN6505B rather than SN6501 in this case. ES835-04196F and PH9085.012 do seem to be compatible with both SN6505B and SN6501.

Which influence from inductance is our customer looking to estimate?

Best,
Manuel Chavez