# Why did the designer of this flyback circuit choose to set the oscillation frequency as high as possible with the smallest resistor tolerated?

This circuit uses a current mode controller with an oscillation frequency response of:

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:

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.

• 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! Aug 14 '20 at 17:29
• @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? Aug 14 '20 at 17:31
• Doesn't "Off-line" imply an AC mains connection? Aug 14 '20 at 17:35
• 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. Aug 14 '20 at 19:13
• @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 Aug 14 '20 at 23:57