SN6501: SN6501 - Driving 2 Transformers in Parallel

Awesome Manuel, thanks for that! Dave

Hi Manuel,

At first I was thinking there might be a problem with core magnetization when driving two transformers, but after thinking about it I've convinced myself there isn't an issue.  The purpose of my post is to get confirmation that I am thinking about it correctly.

I would like to mention that there is a paragraph in the SN6501 datasheet that is missing in the SN6505 datasheet, and it seems like it is very good information.  It is the paragraph at the end of section 8.3.2.  It is the one that starts out with "Fortunately, due to the positive temperature coefficient of a MOSFET’s on-resistance".  Is there a reason this paragraph does not appear in the SN6505 datasheet?  Doesn't it apply to the SN6505 too?

Is it true that the core magnetization issue is solely because of small inaccuracies in the timing of the SN650x circuitry, such that one transistor is on longer than the other?  If the SN650x didn't have these small timing differences, then the whole section on core magnetization could be omitted?  If the answer is yes, then I am understanding it correctly.  Otherwise I have more thinking to do because other factors affect core magnetization.

I copied a schematic from the data sheet and added a 2nd transformer to it.  This allowed me to visualize the circuit better and conclude that core magnetization does not apply.  Basically the SN650x sees one inductor, which is really two in parallel?  Here is the schematic:

RDS increases as the transistor heats up, which causes VDS to increase, which causes VP to decrease because VIN is a constant.  I don't see how two transformers in parallel would affect this.

Thanks for the help!

Regards,

Greg

I had one other thought about this.  Per the datasheet, Magnetic Flux Density is approximately equal to V_P x t_on.  Assuming t_on is exactly the same for both transistors, what if a transformer wasn’t constructed perfectly? If the center tap was misplaced, V_P would not be the same for both transistors because more current would flow in one leg than the other due to a slightly different resistance. Would this cause a shift in the magnetization? And if so, how would this differ from an error in t_on?  The datasheet for the Wurth transformer says the error spec on the turns ratio is a max of 2%.  I'm not sure if that would apply to the positioning of the center tap or not.

Regards,

Greg

Hi Greg,

Thank you for your posts! The paragraph you mentioned end of section 7.3.2 in the SN6501 datasheet could apply to SN6505 devices as well.

The concern for imbalances in core magnetization does come from small, potential inaccuracies in the switching device (SN650x) and, as mentioned in your second post, slight inaccuracies in the transformer's windings between the center tap and outer pins. In both cases, the FETs in D1 and D2 will heat according to the current flow through each pin, helping keep the V-t balance stable. There are cases where a core could saturate due to large external magnetic fields, for which this self-regulation may be insufficient. In most datasheets, the transformer turns ratio tolerance refers to the mismatch between the turns ratio from total primary turns to total secondary turns.

The schematic with two windings in parallel does show how the SN650x will "see" the two-transformer system, and with two (or more) transformers in parallel, the same self-correcting effect applies for each pair of windings. Part of the reason why this is permissible is a benefit of push-pull transformer drivers: they are compatible with a wide range of transformers and inductances.

Please let me know if the information above is helpful!


Respectfully,
Manuel Chavez

Thank you very much Manuel!  Your answers have been very helpful.  Thank you for confirming that there isn't a core magnetization issue.

I've tried to be respectful to Dave's original post and keep my posts in line with his topic so that the discussion is helpful and relevant.  I have one more question which I believe is related to the topic of this thread. You had said "There are cases where a core could saturate due to large external magnetic fields, for which this self-regulation may be insufficient". Would you please comment on the PCB parts placement when using two transformers? My concern is the coupling between the transformers.  Is there a recommended distance to space them apart? Is there a preferred orientation of one with respect to the other? Any EMI issues related to placement?  Here are two examples to illustrate what I’m thinking (I didn't bother to place the SN6501 in them):

Aren’t toroidal inductors pretty good at containing fields?  Maybe it isn't a big issue?

Thank you again for your superb support!

Regards,

Greg

Hi Greg,

You're welcome! We are happy to help :) for future reference, additional questions or follow up questions to a thread can be linked in a new thread by using the yellow "+ Ask a related question" button in the upper right corner of this window.

There isn't a recommendation for placement of the two transformers since this isn't a typical configuration, and customers have not reported issues with transformers interfering with each other in multi-transformer configurations. The transformers will contain most of the magnetic fields in their core, since it behaves like a shunt for best magnetic coupling, so placement of the transformers could be selected as the one which facilitates your PCB layout the most, or to keep the layout symmetrical, a layout resembling the top one you shared could help keep radiated emissions low:




Best,
Manuel Chavez

HI Manuel,

Thank you for answering that question.  So far all of your answers have been great news!  I think that layout orientation is the most compact and easy to lay out.

I see there is a way for a user to see if a thread was started as a related question to another.  There is a link at the top of the page that says "Original question:" and the link to it.  That is very helpful.  But is there a way to see related threads when starting from a parent thread?  I realize threads can get out of hand and stray off topic, so it is best to keep them short.  But without a way to easily see the related questions, it is likely that pertinent information about the topic could be missed without an exhaustive search.  Now I am straying off topic!  You don't need to answer this question unless you can easily point me to where this feature is.  Otherwise I will ask this question in the Site Support forum.

Thank you for all the help on this topic.  I feel very comfortable about using multiple transformers, and the SN6505B is probably the better device when doing this.

Regards,

Greg

Thanks for the follow up questions Greg, and thanks Manuel. I hadn't gotten as far as thinking about transformer placement so this is a great help, cheers. Dave

I'm glad it was helpful for you too Dave :-).  I just thought of one more question that I'd like to ask and then I'll move to related threads.

Manuel, is the SN6501 able to self-regulate better than the SN6505?  I'm thinking that SN6501's R_DS is greater than the SN6505, which would allow the MOSFETs to heat up more causing a greater voltage drop.  Or is the SN6505's R_DS enough, and it self-regulates just as well?  Sometimes more isn't better, and if the SN6501 has the power capability, it might be a better fit.

Regards,

Greg

Hi David,

You're welcome!

Greg,

Please do ask the question about seeing related threads when looking at a parent thread on the Site Support forum, since this is currently not a feature of E2E.

Although the Rds-on values of SN6505 devices is lower than SN6501, the same principle applies and we do expect SN6505 to operate just as well if not better than SN6501 in conditions where core imbalances exist. This is due to the built-in safety features of SN6505 (current-limiting, thermal shutdown) and faster switching frequency of SN6505B (reduces how much of available BH curve is used, allowing for greater margin before saturation). The additional benefit of lower Rds-on is higher system efficiency, especially at higher load currents.

If low emissions is the top priority in a design, SN6505A should yield circuits with the lowest emissions with the tradeoff a higher V-t product requirement due to the lower switching frequency.

Thanks to you both!
Manuel Chavez

Thank you professor Manuel!  In case you're interested, here is the link to the thread I created in the Site Support forum.  Feel free to jump in and support the cause!  Hopefully they'll say that they are implementing it already.

https://e2e.ti.com/support/site-support/f/1024/t/981310

I hope they will be able to add this feature because it would be really helpful to crawl around the site.

Thank you for the info on the SN6505 having more features and potentially performing better than the SN6501 where core imbalances exist.  One of the questions I alluded to earlier is about safety features, which I will save for a new thread.

All the best to the both of you too.

Greg

Hi Greg,

You're very welcome! Thanks for linking the new thread you posted in the Site Support forum -- I'm looking forward to the addition of this feature.

Please feel free to continue posting on E2E using the red and yellow buttons in the top right corner of this window meanwhile.


Have a great day,
Manuel Chavez