Formula Used
Input power: Pin = Pout / efficiency
Boost duty cycle: D = 1 - Vin / Vout
Average input current: Iin = Pin / Vin
CRM peak inductor current: Ipk = 2 × Iin
Required inductor: L = Vin × D / fs × Ipk
Equivalent form: L = Vin² × (Vout - Vin) / (2 × Pin × fs × Vout)
Switch on time: Ton = L × Ipk / Vin
Switch off time: Toff = L × Ipk / (Vout - Vin)
Stored energy: E = 0.5 × L × Ipk²
Approximate turns: N = L × Ipk / (Bmax × Ae)
How to Use This Calculator
- Enter the lowest and highest expected input voltage.
- Enter the output voltage and output power target.
- Add the estimated efficiency for input power correction.
- Set the CRM frequency for the selected design point.
- Enter tolerance and current margin for practical part selection.
- Add core area and flux density to estimate turns.
- Press calculate and review the result above the form.
- Download the CSV or PDF result for your records.
Example Data Table
| Case |
Vin Design |
Vout |
Pout |
Efficiency |
Frequency |
Typical Use |
| Low voltage module |
12 V |
24 V |
60 W |
91% |
120 kHz |
Battery boost stage |
| Industrial rail |
18 V |
48 V |
120 W |
92% |
100 kHz |
Control supply |
| High output bus |
36 V |
96 V |
250 W |
94% |
80 kHz |
Power front end |
Understanding CRM Boost Inductor Design
A boost converter in critical conduction mode works at the border between continuous and discontinuous inductor current. The inductor current starts near zero, rises during switch on time, then falls back to zero through the diode or rectifier. This simple pattern makes peak current, timing, and energy easy to inspect.
Why The Inductor Matters
The inductor sets the peak current for a selected input voltage, output power, and switching frequency. A smaller inductance raises peak current and core stress. A larger inductance lowers peak current, but it may reduce the operating frequency or require a bigger magnetic part. The best value is usually a balance between efficiency, size, temperature, and cost.
Critical Mode Behavior
In CRM operation, the switching period changes with line voltage and load. The design frequency normally applies at one selected operating point, often the lowest input voltage and full load. When input voltage changes, the same inductor creates a different frequency. That is why this calculator also estimates high line behavior.
Practical Design Checks
Peak current is not the only concern. RMS inductor current affects copper loss. Switch RMS current affects transistor heating. Diode or rectifier RMS current affects conduction loss. Stored energy shows how much magnetic energy moves each cycle. Turns and air gap estimates help the designer judge whether a selected core is realistic.
Using The Results
Use the calculated inductance as a starting point. Then choose the nearest standard value. Add tolerance and current margin. Check saturation current above the worst peak current. Confirm winding temperature, core loss, and clearance. If the frequency becomes too high at light load or high line, review the control method and clamp limits.
Design Limits
This tool uses ideal CRM boost equations with efficiency correction. Real designs also include diode drop, switch voltage loss, winding resistance, parasitic capacitance, and controller delay. These effects can change the final part value. Always verify the result with a bench test, thermal run, and trusted simulation before releasing a power design. Record every assumption beside the result. Input tolerance, load range, thermal target, and selected core data should stay visible. Clear records make later debugging faster and safer for teams during prototype changes and design reviews.
FAQs
What is CRM in a boost converter?
CRM means critical conduction mode. The inductor current returns to zero at the end of each switching cycle. The next cycle begins near that zero-current point.
Why is peak current double the average input current?
In ideal CRM, inductor current forms a triangle from zero to peak and back to zero. The average value is half the peak value, so peak current is about twice average input current.
Which input voltage should I use?
Use the worst design point. For many boost converters, this is the minimum input voltage at full load. It usually creates higher current stress.
Why does high line frequency change?
CRM converters do not always run at one fixed frequency. With the same inductor, different input voltages and loads change the on time, off time, and total period.
Does this calculator include diode and switch losses?
It uses efficiency to correct input power. It does not separately model diode drop, switch resistance, winding loss, or controller delay. Use simulation for final checks.
How should I use the tolerance result?
The nominal inductance with tolerance helps keep the minimum possible inductance near the required value. Choose a practical standard value and verify current rating.
What does the turns estimate mean?
It estimates turns from inductance, peak current, flux density, and core area. It is a first pass value. Core shape and gap details still matter.
Can I use this for final production design?
Use it as an engineering starting point. Final production work should include magnetic design, thermal testing, safety review, layout checks, and prototype measurement.