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TPS60403: Ultra-low Power DC/DC converter

Part Number: TPS60403
Other Parts Discussed in Thread: SN6505B, SN6505A, TPS62125, TPS60402, , TIDA-00349

I have been using the SN6505B to drive a Wurth pulse transformer #760039012.  It is very inefficient at low output currents.  The output voltage in my application is 3.0V after rectification.  My application requires a range of output currents from 745 uAmps to 25 mA.  The input voltage is 12V and is stepped down to 3.1V using a TI regulator TPS62125 then the SN6505B transfers the power through the pulse transformer to provide 3.0V after rectification with 2500Vrms isolation.  The power input at 12V to achieve the 745 uAmps at the isolated output of the pulse transformer is 21 mW so the overall efficiency is 10%.  Similarly, with 25 mA output the input power requirement is 113 mW so the overall efficiency for that load is 64%.  These efficiencies are to be expected as per the TI SN6505A document on page 10, which has a graph showing an efficiency of 80% with 30 mA load dropping off exponentially to about 30% with 5mA load. It's difficult to estimate on the graph provided what the efficiencies are for lower values of load current.

I did some WEB searching and found the TI design #TIDU813B which showed a uniquely efficient switching architecture at ultra low power incorporating the TPS60402 device driving a Wurth transformer #750314839 which achieves efficiencies ranging from 70% to 85% in the 500uA to 10 mA range (page 31 in the document TIDU813B).  This is exactly what I would like to achieve in my application except that the Wurth transformer used in the TI design is about 2X  the height of the #760039012 pulse transformer.

My question is this, would it be possible to substitute the TI part TPS60402 which operates at 50 kHz with the TPS60403 which operates at 250 kHz and the Wurth transformer #750314839 with the Wurth pulse transformer #760039012 to achieve similar efficiencies or is a pulse transformer not suited to this particular architecture? 

  • Hello Daniel,

    The design can also be used with TPS60403, however, the efficiency will be most likley less compared to the solution of TIDA-00349. Due to the higher switching frequency, the losses will be more. In addition, the TPS60403 has higher Iq which also affects the efficiency. Chapter 5.4 of the Reference Design Guide describes the background to that.

    Regarding the usage of the transformer, we cannot comment, as no data is available to us.

    The selection of the parts in TIDA-00349 has been made with full focus on efficiency for very light loads as this is needed in 4-20mA loop powered sensor transmitter.



  • There were lower efficiencies because of the higher operating frequency just as you said would be the case.

    I built a prototype of the circuit using a TPS60403DBVR and Wurth Electronics 760390014 pulse transformer and it worked awesomely, it vastly improved the efficiencies at very low currents over my previous push-pull and SN6505B solution.  The efficiencies at 3.0V input were as follows:

    64% at 800uA output, 75% at 3.2 mA output, 75% at 6.1mA output,  and 74% at 12.4mA output.  The output voltage was 3.33V out at 800uA and 2.98V at 12.4mA.

    My application required a minimum of 3.0V output so I boosted the input voltage to 3.1V to obtain 3.10Voutput at 12.9mA with the efficiency at 75%.  I eliminated C12, C15, R10, R13 in Figure 50 by tying the transformer leg directly to ground thus simplifying the circuit. I have a few other ideas for improving the design to try out as well.

    I would like to thank TI very much for this great circuit idea.