Boost converter inductor selection - inductor ripple current calculation based on input current or output current?

Question

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,

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.

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.

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?

Answer 1

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:

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!

Answer 2

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.