Choosing Output Capacitor Value for Boost Converter at 1MHz When Datasheet Is 100kHz

The discussion addresses selecting the output capacitor value (Cout) for a boost converter operating at 1 MHz switching frequency, while capacitor datasheets typically provide capacitance values measured at 100 kHz. The initial question concerns whether to choose a capacitance ten times greater than the calculated Cout based on the formula Cout = (Iomax * Dmax) / (Fs * ΔVo). Responses clarify that capacitance itself does not inherently decrease with frequency, but the effective capacitance measured can appear lower at higher frequencies due to the capacitor's equivalent series inductance and resistance (ESR). Electrolytic capacitors exhibit significant capacitance variation with frequency, as dielectric properties and ESR impact performance. It is recommended to use low ESR capacitors and consider paralleling multiple capacitors to reduce ESR and improve performance at 1 MHz. Instead of increasing capacitance by a factor of ten, a factor of about 2 to 5 times larger capacitance is suggested to compensate for frequency effects, with parallel capacitor configurations preferred. Datasheet capacitance-frequency characteristics, such as those in the KEMET T495 series, show capacitance dropping from approximately 200 µF at 100 kHz to 15–20 µF at 1 MHz, supporting the recommendation to increase capacitance moderately rather than tenfold. Lowering switching frequency to around 100 kHz is also suggested if feasible to avoid these issues.
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