FAQ
TL;DR: At 1 MHz, a tantalum’s effective capacitance can drop from ~200 µF to ~15–20 µF; “couple in parallel” to cut ESR. [Elektroda, Mark Majewski, post #21662128]
Why it matters: This FAQ helps power designers pick stable, low‑ripple output capacitors for 1 MHz boost converters when datasheets show 100 kHz specs.
Quick Facts
- Sizing baseline: Cout = (Io_max × D_max) / (Fs × ΔVo); check your converter’s ripple target first. [Elektroda, Zied Saad, post #21662117]
- At 1 MHz, effective C of some parts can fall by an order of magnitude; verify frequency curves. [KEMET T495 Series Datasheet]
- Use low‑ESR capacitors at 1 MHz to limit ripple and heat in the output stage. [Elektroda, Calinoaia Valentin, post #21662119]
- Parallel capacitors lower ESR and spread ripple current; mix values if needed. [Elektroda, Mark Majewski, post #21662126]
- Losses scale with ESR and frequency: P ≈ U²·ω·C·tanδ; watch thermal rise. [Elektroda, Mark Majewski, post #21662123]
Do I really need 10× more capacitance at 1 MHz than my equation says?
Not automatically. The thread’s guidance is about 5× with parts showing large high‑frequency roll‑off, plus using caps in parallel. Your exact multiplier depends on the device’s frequency‑capacitance curve and ripple goal. Start from the equation, then adjust using the part’s 1 MHz data and thermal limits. “Couple in parallel” to reduce ESR and ripple current. [Elektroda, Mark Majewski, post #21662128]
What equation should I use to size the boost converter output capacitor?
Use Cout = (Io_max × D_max) / (Fs × ΔVo). Pick Io_max, duty cycle at worst case, switching frequency, and allowable output ripple. This gives a baseline at your target ripple. Then check the capacitor’s frequency response and ESR to ensure it meets ripple and thermal limits at 1 MHz. [Elektroda, Zied Saad, post #21662117]
Does electrolytic or tantalum capacitance change with frequency?
Effective capacitance drops at high frequency due to dielectric and parasitics. Example vendor data shows pronounced reduction approaching MHz. Plan with the vendor’s C‑vs‑f curve and verify at 1 MHz. A severe drop can reach ~90%, so derate your target value accordingly. [KEMET T495 Series Datasheet]
How important is ESR at 1 MHz?
Very important. ESR sets ripple and heating under ripple current. Lower ESR reduces ΔV and power loss. Application data discuss ESR rise with frequency and its effect on impedance and temperature. “ESR dominates ripple at high switching speeds.” Choose low‑ESR parts and confirm with impedance curves. [CDE Aluminum Electrolytic Capacitors Application Guide]
What’s a practical way to hit low ripple at 1 MHz?
Combine several low‑ESR capacitors in parallel. This lowers ESR and shares ripple current. Use the baseline Cout from the equation, then split across two or more parts to meet ripple and thermal limits. Mark’s advice: use a couple in parallel for better ESR. [Elektroda, Mark Majewski, post #21662126]
Should I lower the switching frequency instead of oversizing Cout?
If your design allows, moving toward 100 kHz eases capacitor stress and raises effective capacitance. The thread suggests considering a lower Fs when high‑frequency C loss is severe. This trade reduces ESR heating and component count but may increase inductor size. Evaluate efficiency and size impacts first. [Elektroda, Mark Majewski, post #21662128]
What is ESR, and why does tanδ matter?
ESR is the capacitor’s internal resistance that causes ripple voltage and heat. Loss angle (tanδ) relates to ESR and reactance: tanδ = ESR/Xc, with Xc = 1/(ωC). Power loss can be expressed as P = U²·ω·C·tanδ. Higher tanδ or ESR increases temperature rise. [Elektroda, Mark Majewski, post #21662123]
How do I validate capacitor choice for a 1 MHz boost converter?
Three steps: 1) Compute Cout from the ripple equation. 2) Check the vendor’s C‑vs‑f and ESR‑vs‑f at 1 MHz. 3) Parallel enough low‑ESR parts to meet ripple and temperature limits. Measure ripple on hardware to confirm. [Elektroda, Mark Majewski, post #21662126]
Is there a simple rule for parallel capacitors?
Yes. Paralleling halves ESR when you double identical parts, while capacitance sums. This reduces ripple and heating. The thread recommends using a couple in parallel to lower ESR and improve stability at MHz switching frequencies. Keep traces short to limit ESL. [Elektroda, Mark Majewski, post #21662124]
What’s an edge case that can still fail at 1 MHz?
If you select by 100 kHz capacitance only, some parts collapse to ~15–20 µF effective at 1 MHz. Ripple and heating then exceed limits even though nameplate value looked adequate. Always size using MHz data and ESR. [Elektroda, Mark Majewski, post #21662128]
Do I need special capacitor types for 1 MHz?
Use low‑ESR parts explicitly rated for high frequency. The thread advises low‑ESR selection at 1 MHz. Combine technologies if needed, but ensure each part’s datasheet provides impedance at your frequency. Validate with vendor curves and thermal checks. [Elektroda, Calinoaia Valentin, post #21662119]
Where can I see frequency behavior and ESR curves?
Consult vendor application guides and datasheets. They include ESR, impedance, and capacitance versus frequency plots, often up to the self‑resonant region. Review pages that cover ripple current and ESR rise with frequency before finalizing parts. [CDE Aluminum Electrolytic Capacitors Application Guide]
