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I've a question regarding the inductor selection in a boost converter.

In this TI App note under Inductor Selection on page 3, it is asking to assume the inductor ripple current as a percentage of OUTPUT CURRENT.

This is a general circuit of a boost converter below,

enter image description here

In the above circuit, we can see the inductor is connected to the input voltage. So, should the inductor ripple current be calculated as a percentage of input current? Below, I have attached a snap shot from the TI App note.

enter image description here

In the datasheet of the A80604 LED driver, on page 36, step 4c, they have calculated the inductor ripple as a percentage of the input current.

Below, I have attached the snapshot from the A80604 LED Driver Datasheet.

enter image description here

So, which one should be followed? Inductor ripple current based on input current or output current?

Similarly, for the boost converter forward diode (shown in the above image,) the forward current (need to calculate for the diode power dissipation) would be the output current of the boost converter only, right? It cannot be the input current, right?

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  • \$\begingroup\$ I don't see those words on page 3 - embed the exact words so they can be searched or embed a section of the text. As an alternative to the TI thing, this might help you experiment with values to get a better feel. \$\endgroup\$
    – Andy aka
    Apr 1, 2021 at 8:23
  • \$\begingroup\$ I have attached the snapshots from both the application note as well as the datasheet for easy reference, Please do check \$\endgroup\$
    – Newbie
    Apr 1, 2021 at 8:34
  • \$\begingroup\$ A boost converter running in CCM will have a low ripple current but, trying to make the p-p ripple current about 30% of the DC output current is a bit random. Boost converters can operate in DCM and the inductor ripple current will be higher than the DC current on the output so maybe the TI document isn't correct or it's targeting a very thin application area in CCM? \$\endgroup\$
    – Andy aka
    Apr 1, 2021 at 8:45
  • \$\begingroup\$ Calculating the inductor value whether it is for a buck or a boost converter remains an iterative process. This is because the inductor not only affects the ripple current (input current for a boost and output current for a buck) but it also conditions the converter's reaction time and the crossover frequency you want (among other things like rms currents). This is especially true with a boost or buck-boost converter considering the right-half-plane zero present in the transfer function: the larger the \$L\$ the smaller the ripple but the slower the converter. \$\endgroup\$ Apr 1, 2021 at 8:47
  • \$\begingroup\$ So, for inductor ripple, it should be calculated as a percentage of Input current, right? \$\endgroup\$
    – Newbie
    Apr 1, 2021 at 8:47

2 Answers 2

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The inductor value when we talk about a switching cell affects numerous parameters in the converter. Among those that I think of, we have:

  1. the input current ripple: in a boost converter, the input current is that of the inductor. We say it is a smooth non-pulsating current while the output current is highly pulsating. The higher the inductor, and the smaller the input ac ripple will be. The lower the ac ripple, the best iron losses in the core as the BH minor loop dynamics is reduced (unlike in discontinuous conduction mode where it is the highest).

  2. a small input ripple obviously helps for designing the front-end filter and the burden on the filtering capacitor will be less with a boost than with a buck converter for instance (highly-pulsating input current, like a buck-boost).

  3. unfortunately, if shooting for a low input ripple is a possible choice, having a large inductor clearly hampers the converter's reaction time. An inductor naturally opposes current variation so do not expect a fast converter if you have a large inductor, pushing operations in deep continuous conduction mode (CCM). The boost converter being an indirect-energy-transfer type of converter, the delay in the conversion process is modeled as a right-half-plane zero (RHPZ). And you can't reasonably select a crossover frequency higher than 30% of the lowest RHPZ position.

  4. finally, a low ac ripple implies smaller circulating rms current and less stress on the filtering capacitors. This is true for the input caps in a boost structure and for a buck output capacitor. In both cases, the inductor smooths the circulating input (boost) and output (buck) currents. All the ohmic paths benefit from this lower ripple such as the power MOSFET for instance.

As a preliminary conclusion, designing the inductor goes through an iterative process building on the above remarks. But as you have to start from somewhere, I have given a design formula in my book on switching power supplies based on the following inductor waveforms:

enter image description here

The formula is here:

\$L=\frac{\eta V_{in}^2D}{\delta I_rF_{sw}P_{out}}\$

So if you start with a ripple percentage (it peaks at \$\frac{V_{in}}{2}\$), think about the points I highlighted. When I started looking at designs 35 years ago, people were suggesting 10% of the output current for the design of the inductor in a buck for instance. Considering the progress in core materials, the ripple selection nowadays is more between 30-40% of the output current: the inductor is smaller, it has less dc losses, the ac core losses are less of a problem. Good luck with your design!

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  • \$\begingroup\$ Thank you for the answer. Based on the duty cycle calculated and the input voltages, i have arrived at 3 inductor values. Min, typ and Max. From 39uH to 90uH. What would be the right inductor value for my application with 9V to 16V input. Output current 0.3A with 30V boost output. \$\endgroup\$
    – Newbie
    Apr 1, 2021 at 9:46
  • \$\begingroup\$ You usually go for the worst-case operation which is the lowest input and the highest output current. Before ordering components, run a quick simulation to check the value you have selected is ok. Check my 60+ SIMPLIS templates which work on the free demo, there are boost converter examples. \$\endgroup\$ Apr 1, 2021 at 10:29
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Both of your examples do the same thing: They estimate the input current from the required output current, then calculate the inductor ripple from that and from there go on to calculate the value of the required inductor.

You are designing a circuit to meet certain output criteria: Output voltage and maximum output current. You have to work backwards from those values to design your circuit.

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  • \$\begingroup\$ So, for inductor calculation, the inductor ripple should be first calculated as a percentage of Input current, right? \$\endgroup\$
    – Newbie
    Apr 1, 2021 at 8:48
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    \$\begingroup\$ The inductor ripple current is a percentage of the input current, correct. The input current itself is unknown - you must estimate it from the output current. \$\endgroup\$
    – JRE
    Apr 1, 2021 at 8:50

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