Electrical – Guarantee Continuous conduction mode dc/dc buck converter

dc/dc converter

In a buck converter the continuous conduction mode it depends on following equation:

$$ L_{min} \geq \frac{(1-D)R}{2f_s} $$

For a fixed load and inductance value, the operation mode will depend on the switching frequency.

For following specifications:

$$ V_i = 30V $$
$$ R = 6\Omega $$
$$ L = 43 \mu H$$

I made the following plot:

enter image description here

Where yellow line is the output current, and the curves represents the minimum current required to operate in CCM at a given frequency. According to that, any Fs over 100 kHz will guarantee CCM in all range for that specific load and for any bigger one.

To find the minimum switching frequency to operate in CCM search iteratively the frequency where the line and the curve is tangent ang got: Fs = 70859Hz.

enter image description here

Is this a proper way to approach this problem or there is a better/easier alternative?

Best Answer

Here are a few observations: -

  • Most buck converters are voltage regulators so, if the load is fixed (3.5 ohms) the graphs plotting load current against duty cycle take you down a non-constant output voltage situation - this seems to restrict your ideas to very few applications (CC applications).
  • Having the load resistor fixed in value also restricts the application even more so I'm even less inclined to see the benefits of this approach.
  • A lot of modern buck regulators are now becoming synchronous and the excessive ripple seen in these types of converter (what would be previously due to DCM) is a thing of the past IMHO.

Maybe I've missed some intrinsic benefit about your process/approach?

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