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I am looking for a DC to DC boost converter constant current with very low switching frequency to pass EMC.

  • Vin- 12V
  • Vout- 36V, 100mA.

Any suitable device for my above application?

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  • \$\begingroup\$ "very low switching frequency" of what order of magnitude? \$\endgroup\$ – Paul Ghobril Oct 19 '20 at 5:50
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    \$\begingroup\$ You don't need low switching frequency, you need a good design. \$\endgroup\$ – Gregory Kornblum Oct 19 '20 at 7:26
  • \$\begingroup\$ Why don't you use an amplifier? \$\endgroup\$ – Se1fie Oct 26 '20 at 13:04
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Build a Royer oscillator, the voltage step-up is mostly determined by the turns ratio of the transformer so find a 4:1 trasnformer...

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Consider that:

  • a DCDC converter that switches at a low frequency could actually generate more EM radiation than another DCDC converter that switches at a higher frequency but where measures have been taken to reduce EM emissions.

The speed at which the switching is done is quite important. If you switch fast at a low frequency, you will have lots of harmonics. I once worked on a product where the 283rd harmonic of a DCDC converter switching at 600 kHz was the issue.

  • in your design you have to make sure that the current loops that generate the EM emissions are short and small. The larger they are the more they will emit as they're basically an antenna. Larger antenna: more emissions.

  • consider shielding if you expect EMI issues

  • use a DCDC converter which uses an external clock so that you know at what precise frequency it is switching. You can then use it at a frequency where the clock and its harmonics are no issue.

  • consider a DCDC converter that uses "spread spectrum", the clock isn't constant but varies somewhat and that will spread the EM emissions over a wider frequency range so the power is more "spread out" over frequency which could help in some cases.

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I have successfully used an LM5001 for boost applications in the past. The diagram below, taken from the datasheet is a good starting point. All you need to do is adjust the ratio of R5/R6.

Regarding EMC, the switching frequency is just one factor that influences emissions. If you do this, the size of L1 and C3 typically need to be large. It is often better to use an LC filter at Vin to mitigate current ripple.

enter image description here with an LC input filter.

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    \$\begingroup\$ My answer now contains more information and comments about EMC tradeoffs \$\endgroup\$ – mr_js Oct 26 '20 at 12:50
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    \$\begingroup\$ You can delete your comment now, too... \$\endgroup\$ – TonyM Oct 26 '20 at 13:45

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