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SN6501: Step up transformers with 5V on the primary

Expert 1901 points
Part Number: SN6501

I need some help understanding the following from the SN6501 datasheet:

9.2.2.1 SN6501 Drive Capability
The SN6501 transformer driver is designed for low-power push-pull converters with input and output voltages in the range of 3 V to 5.5 V. While converter designs with higher output voltages are possible, care must be taken that higher turns ratios don’t lead to primary currents that exceed the SN6501 specified current limits.

Is there something hidden in that note or is it simply a warning that when you step up the voltage, the max current on the secondary should be proportionately less in order to keep the power the same?

In the Recommended Isolation Transformers table in the datasheet, what is the reason that none of the higher turns ratio transformers are specified for 5V on the primary?  For example, Wurth part number 760390015 is used to step up from 3.3V to 5V.  Why wouldn't it also be specified for stepping 5V up to 9V or so?  Is that note in the datasheet the reason?

Regards,

Greg

  • Hi Greg,

    Thank you for posting to E2E!

    SN6501 (and SN6505) devices are limited in the amount of power they can isolate by the current-sinking capabilities of their D1 and D2 pins, so this note is cautionary.

    As you mentioned, this means although an SN6501 powered by 5V can sink up to 350mA with a 5V output (~1.75W), a 10V output drawing 350mA would require the SN6501 to sink double that amount of current due to conservation of power. Thus, the higher output voltage levels rise, the lower output current is available to keep SN6501 from sinking >350mA through its D1 and D2 pins. If higher amounts of power are needed, SN6505 devices (5W) or our discrete HV solution may be used.

    The transformers shown in our SN6501 data sheet were selected for typical logic-voltage-level applications, but there are many commercially available transformers to choose from since the selection can be optimized for different power levels and output configurations. The same transformers used to step 3.3V to 5V can typically be used to step-up  voltages to higher than 5V so long as V-t product and power ratings are not exceeded.

    Respectfully,

    Manuel Chavez

  • Hi Manual,

    Thank you for answering my question.  So it sounds like there isn't anything hidden in that cautionary statement in the datasheet.  It just seems like it is worded in a way that discourages people from using step-up transformers.  By saying that care must be taken, it makes me nervous that there is something more involved than simply observing that when stepping up the voltage, the available current on the secondary is reduced proportionately.  The way I read that paragraph is that no care must be taken when restricting the voltages to the range of 3.3V to 5V.  But care must be taken when using higher turns ratios.  But we know that isn't the case when stepping up from 3.3V to 5V, because the current on the 5V side needs to be less than the max current on the 3.3V (primary) side.

    I just had a thought about V-t.  Does V-t need to be observed on both the primary and the secondary?  If so, then that would be the concern.  If the turns ratio is 1:2, then V-t on the secondary would need to be twice that of the primary.  Or is V-t only an issue for the driven (primary) side?

    I wouldn't think there would be an issue with the voltage rating of the Wurth transformers.  They have very high isolation ratings, and so voltage increases on the secondary due to turns ratio would be small compared to the isolation rating.

    Regards,

    Greg

  • Hi Greg,

    You're welcome! It is not our intention to make engineers and system designers nervous by our cautions, only aware :) Care should be taken to operate SN6501 within the recommended operating conditions in all input and output voltage conditions. When SN6501 is powered by Vcc = 3.3V, the current available to a 5V output will be I_out = I_in / N, or ~100mA depending on the transformer's turns ratio since SN6501 has a lower current-sinking capability when Vcc = 3.3V (150mA) than when Vcc = 5V (350mA).

    The V-t product equation and specs are meant to be observed for the primary side of the transformer.

    Although transformers have high isolation voltage ratings, which apply from one side of the transformer with respect to the other, their power ratings relate to the amount of power actively isolated by the push-pull power supply. Some transformers are rated for certain input or output currents while others are rated in Watts. When increasing voltage beyond the suggested application (like using a 3.3V to 5V transformer for 5V to 8V), ensure these power limitations are not exceeded.


    Thank you,
    Manuel Chavez

  • Hi Manuel,

    Thank you for pointing out that the device can sink more when VCC = 5V.  It is significantly more capable at 5V.

    I'm glad that the V-t product only applies to the primary side.  Now that I've gotten the background questions out of the way, I would like to ask about whether there are any issues with a specific circuit.  Please assume the following:

    SN6501
    VCC = 5V
    Transformer: Wurth 760390015  1:2.1 turns ratio
    Voltage doubling using the circuit in Figure 47, which is a full-wave bridge without the center tap
    Load current: 20mA

    Ballpark this should produce about 2 x 2.1 x 5 ≈ 20V

    As a contrast, would I be better off using a 1:1.1 transformer and using the circuit of Figure 49 (Quadrupling Vin) instead?

    To complicate it just a little, I would like to use one SN6501 to drive two of these transformer circuits in parallel in order to provide a bipolar supply of +/- 20V.  The positive output of one circuit would connect to the negative of the other to form ground.

    Do you see a problem with any of this?

    Thank you for the help.

    Regards,

    Greg

    »

  • When I made the comment "I wouldn't think there would be an issue with the voltage rating of the Wurth transformers.  They have very high isolation ratings, and so voltage increases on the secondary due to turns ratio would be small compared to the isolation rating", I was envisioning enameled wire wound on top of each other.  If such a large isolation could be realized (2500V), producing a voltage of 20V or so on the secondary winding should not be an issue.  After I said that I was doubting that they would be able to wind the wires on top of one another and achieve such a high voltage isolation.  But I just looked at one of the Wurth transformers and indeed the wires are wound on top of one another.  Granted it is only rated for 1 minute at that voltage, but the enamel is evidently pretty good.  I took a picture with my cell phone, so it isn't the greatest picture.  But it shows how they are constructed.

  • Hi Greg,

    You're welcome! We do not foresee any issues in using the 1:2 transformer in the voltage-doubling configuration or the 1:1.1 transformer in a voltage-quadrupling configuration then connecting an identical circuit's output in series to achieve the bipolar 20V rail. We recommend assembling both options and testing efficiency and other parameters across expected operating temperatures to confirm which is best for your system. The estimated current sourced by SN6501 in these configurations should be ~200mA, which is within its recommended operating limits.

    Thanks for sharing this photo; it is common for SMD transformers to have enameled wires wound on top of each other by design to meet isolation specifications.

    Please let us know if you have follow-up questions.


    Thank you,
    Manuel Chavez

  • Hi Manuel,

    You're welcome about the picture, and as always, thank you for your very helpful answers.  I'll be getting more in the habit of starting new threads instead of making longer ones.  If you think I'm finished with questions about the SN6501, I'm just getting started!  Just kidding, but I do have one or two more questions.

    I'm glad that you think both of those options are viable.  Thank you for the suggestion of breadboarding the circuit and measuring the efficiency and quality.  I'll do that.  I have some time before I do a redesign of the existing implementation which uses charge pumps to boost the voltage.  It would be easier and less expensive to use one of those two options.

    Regards,

    Greg

  • Just in case anyone is wondering exactly which transformer that is, it is the Wurth 760390014 which has a 1:1.3 turns ratio.  The green wires on the right are the primary.

    Greg

  • Hi Greg,

    Great! We appreciate your enthusiasm and are glad that SN650x devices can provide a simpler, more cost effective solution for one of your systems :)

    Please feel free to let us know the results of your measurements -- I'll be looking forward to your future posts that can help others using SN650x devices.


    Best,
    Manuel Chavez