Ist das nicht das altbekannte Thema, dass ein Schaltregler am Eingang einen sehr guten Kondensator haben muss?
Zitat aus dem Datenblatt des LM3405, einem Buck-Regler für LEDs:
INPUT CAPACITOR (C1)
An input capacitor is necessary to ensure that VIN does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage
rating, RMS current rating, and ESL (Equivalent Series Inductance).
The input voltage rating is specifically stated by
the capacitor manufacturer. Make sure to check any recommended
deratings and also verify if there is any significant
change in capacitance at the operating input voltage and the
operating temperature. The input capacitor maximum RMS
input current rating (IRMS-IN) must be greater than:
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle D, is closest to 0.5. The
ESL of an input capacitor is usually determined by the effective
cross sectional area of the current path. A large leaded
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LM3405
capacitor will have high ESL and a 0805 ceramic chip capacitor
will have very low ESL. At the operating frequency of the
LM3405, certain capacitors may have an ESL so large that
the resulting inductive impedance (2ÏfL) will be higher than
that required to provide stable operation. It is strongly recommended
to use ceramic capacitors due to their low ESR and
low ESL. A 10μF multilayer ceramic capacitor (MLCC) is a
good choice for most applications. In cases where large capacitance
is required, use surface mount capacitors such as
Tantalum capacitors and place at least a 1μF ceramic capacitor
close to the VIN pin. For MLCCs it is recommended to use
X7R or X5R dielectrics. Consult capacitor manufacturer
datasheet to see how rated capacitance varies over operating
conditions.
Andernfalls leidet der Wirkungsgrad, der sich dann in einer höheren Stromaufnahme äuÃert.
Jürgen