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enter image description here This circuit uses a current mode controller with an oscillation frequency response of: UC3842 Oscillator Freq vs Time Resistor

In the circuit, the designer chose to use a 10nF cap, with a 680 ohm timing resistor, which according to the datasheet, is the absolute minimum resistance that should be used in the timing circuit, as shown below:

enter image description here

The output frequency of this yields the MOSFET being pulsed at around 147 kHz. I was wondering if there was a reason these components were chosen in this case. I had a hunch it was because of the coupled inductors being used in this case, but I don't know where to begin on finding the best frequency to oscillate that at. And if that was the case, why not use different values to get to that 150kHz, such as a 2.4K resistor and a 5nF capacitor? Any insight is welcome on this.

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    \$\begingroup\$ It worked! The more complex a design, means the more a designer has to juggle, which means shortcuts are taken. Not saying this is the case, but it worked! \$\endgroup\$ Aug 14 '20 at 17:29
  • \$\begingroup\$ @StainlessSteelRat you're saying he probably did this simply because it works? Is there some magic equation for trying to find the best frequency to run the inductor at? \$\endgroup\$
    – James S
    Aug 14 '20 at 17:31
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    \$\begingroup\$ Doesn't "Off-line" imply an AC mains connection? \$\endgroup\$
    – Andy aka
    Aug 14 '20 at 17:35
  • \$\begingroup\$ 0.01\$\mu\$F is more readily available than 1nF. So I'm aledging that it's a combination of what was available and it worked. \$\endgroup\$ Aug 14 '20 at 19:13
  • \$\begingroup\$ @Andyaka, "off-line" can also mean "disconnected from line" ... it is unclear what the datasheet is referring to ... it may be referring to generator produced line power where the voltage and frequency may not be closely controlled \$\endgroup\$
    – jsotola
    Aug 14 '20 at 23:57
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In general for switching power circuits of some fixed power level, a higher operating frequency allows for smaller magnetics and smaller input and output filter components (inductors and capacitors).

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  • \$\begingroup\$ does that mean I should be running this thing as fast as it will possibly go? \$\endgroup\$
    – James S
    Aug 14 '20 at 17:28
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    \$\begingroup\$ @JamesS Not necessarily. Running at higher frequency gives a smaller solution size, but lower efficiency. At some point it's diminishing returns because you might not be able to get the heat out, and parasitics will start to become problematic. I like to run converters with the switching frequency as low as possible while meeting other constraints as it will result in less EMI, higher efficiency, easier layout, probably higher reliability. Switching supply design is a study in tradeoffs, \$\endgroup\$
    – John D
    Aug 14 '20 at 19:12
  • \$\begingroup\$ @JohnD That is one thing that I am worrying about with this design, is EMI testing, a similar circuit I used failed emissions, but I have no clue how to reduce it. So if I understand you correctly, Lower frequency is less efficient, but also reduces EMI? If you have a recommendation for learning more, I would love to know, for now, I think I will attempt to surf TI for some more explainations \$\endgroup\$
    – James S
    Aug 14 '20 at 19:21
  • \$\begingroup\$ @JamesS EMI reduction in switching supplies is a complex subject. Snubbers, layout, (minimizing loop area) switching edge rise/fall time, EMI filtering, shielding, component selection and topology are just a few of the things that can have an influence. It's not really possible to give a comprehensive answer in this forum, but you can find lots of info on the semiconductor vendor sites and other web sources. \$\endgroup\$
    – John D
    Aug 15 '20 at 0:36

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