Selecting an inductor answers a basic question: How much current ripple can the design tolerate? A larger inductance reduces ripple and noise, but it also increases size and cost. This article introduces a general-purpose equation, calculates the required inductance for both input conditions, and selects standard values. The equations do not depend on a particular IC.
What Does the Inductor Do in a Boost Converter?
An inductor can be compared to a reservoir:
- When the switch is on, the source “fills” the inductor with energy.
- When the switch is off, the inductor “releases” that energy to the output.
The larger the inductance, the more slowly the current changes, which means lower ripple. The measured inductor-current waveform below illustrates this behavior:
Figure 1: Inductor current is a triangular waveform with ripple rather than flat DC. This is also the actual input-current waveform of a boost converter (image sourced from Analog Devices).
General Inductor-Selection Equation
Inductance is determined by four quantities: minimum input voltage, maximum duty cycle, allowable ripple current, and switching frequency:
Where:
- V_{IN(MIN)} is the minimum input voltage.
- D_{MAX} is the maximum duty cycle at V_{IN(MIN)}.
- \Delta I_{SW} is the allowable peak-to-peak switch, or inductor, ripple current.
- f_{SW} is the switching frequency.
Design tip: \Delta I_{SW} is the ripple tolerance chosen by the designer. A smaller ripple requires a larger inductor; a smaller inductor requires accepting more ripple. There is no free lunch.
Design Goal: Stay in CCM at Light Load
To reduce output noise, we want the converter to operate in continuous conduction mode (CCM) whenever possible. In CCM, the inductor current does not fall to zero during a switching cycle. A smaller ripple current makes CCM easier to maintain because the current valley is less likely to reach zero.
For this example, set the ripple current \Delta I_{SW} to 0.3\,\text{A}, approximately 80% of the peak current calculated for the 12 V input. This is a relatively conservative choice that helps maintain CCM at light load.
Calculate the Inductance for Both Input Voltages
12 V Input
Substituting V_{IN1}=12\,\text{V}, D_{MAX1}=0.647, \Delta I_{SW}=0.3\,\text{A}, and f_{SW}=2\,\text{MHz}:
28 V Input
Similarly, for V_{IN2}=28\,\text{V} and D_{MAX2}=0.176:
Account for Tolerance: Round Up to a Standard Value
Real inductors commonly have a tolerance of \pm20\%. If a nominal 12 \mu\text{H} inductor is selected for the calculated value of 12.94 \mu\text{H}, a negative 20% tolerance would reduce the actual inductance to approximately 9.6 \mu\text{H}, noticeably increasing ripple current. The safer approach is to round up to the next standard value:
| Input | Calculated value | Selected standard value |
|---|---|---|
| 12\,\text{V} | 12.94\,\mu\text{H} | 15\,\mu\text{H} |
| 28\,\text{V} | 8.24\,\mu\text{H} | 10\,\mu\text{H} |
Even with a 20% negative tolerance, these choices still provide useful margin for maintaining CCM.
A Practical Reminder: Higher Input Voltage Makes Discontinuous Current More Likely
Here is a counterintuitive point: A higher input voltage can make it easier for the converter to leave CCM. At a 28 V input, the duty cycle is small and the inductor charging interval is short. Even with the 10 \mu\text{H} inductor selected above, the current can briefly approach 0\,\text{A}, resulting in DCM:
Figure 2: Inductor current with a 28 V input and a 10 \mu\text{H} inductor. The current becomes discontinuous, with alternating large and small spikes (image sourced from Analog Devices).
This discontinuous behavior results from cycle-by-cycle current changes and switch-node ringing, making the noise more difficult to predict. The design must therefore consider both the minimum input voltage, which determines the inductance, and the maximum input voltage, which tests whether CCM can be maintained.
Summary
- Select 15 \mu\text{H} for the 12 V input and 10 \mu\text{H} for the 28 V input.
- These values limit ripple current to approximately 0.3\,\text{A} peak-to-peak and help maintain CCM at light load.
- After selecting the inductor, the next step is to select the IC and define the light-load operating mode.
Key Takeaways
- Inductance is determined by V_{IN}, duty cycle D, target ripple \Delta I_{SW}, and switching frequency f_{SW}. The general equation is independent of the specific IC.
- To support CCM at light load, keep the ripple current relatively small; this example uses 0.3\,\text{A} peak-to-peak.
- Because inductors may have \pm20\% tolerance, round up to a standard value.
- Higher input voltage makes it more difficult to maintain CCM, so both minimum and maximum input voltage must be considered.
Source: This article references the Analog Devices technical article Boost Converter Design for Sensitive Loads at Light Load.
Related Products
- LT8336 | 40 V, 2.5 A low-IQ synchronous boost converter
- LT8330 | 60 V, 1 A low-IQ boost/SEPIC converter
Additional References
More Content
- Boost Converter Design: Part 1 — The Light-Load Challenge: Choosing Between Efficiency and Noise
- Boost Converter Design: Part 2 — Design Goals and Input Current / Duty Cycle Calculations
- Boost Converter Design: Part 3 — Selecting the Inductor: Larger or Smaller?
- Boost Converter Design: Part 4 — IC Selection and Light-Load Operating Modes
- Boost Converter Design: Part 5 — The Final Gate: An Output LC Filter
- Boost Converter Design: Part 6 — Reviewing the Seven-Step Design Process