The first objective in selecting input capacitors is to reduce the ripple voltage amplitude seen at the input of the module. This reduces the rms ripple current to a level which can be handled by bulk capacitors. Ceramic capacitors placed right at the input of the regulator reduce ripple voltage amplitude.
Input capacitors provide a short bypass path for ripple current and stabilize bus voltage during a transient event. In recent years, the advancements in power-MOSFET technology have dramatically increased switching frequency and gate driving speeds of switch-mode power supplies.
Ceramic capacitors have low ESR and they can reduce the input voltage peak-to-peak ripple, which, in turn, reduces the input ripple current for the input bulk capacitors to handle. Figure 3. Input Capacitor RMS Current Calculation Table 3. Output Capacitor Criteria
Based on the input voltage, the input current RMS current, and the input voltage peak-to-peak ripple you can choose the capacitor looking at the capacitor datasheets. It is recommended to use a combination of Aluminum Electrolytic (AlEl) and ceramic capacitors.
Until the regulator can increase the load current to the new value, the deficit must come from the output capacitors. Capacitors all have some parasitic series resistance (ESR). Any current flowing in the capacitor must also flow through the ESR. This causes a voltage drop due to the I×ESR product.
The low power factor is not an issue because the capacitive power supply power rating is not high enough for a power factor correction (PFC) to be required. The Standard IEC 61000-3-2 requires PFC for power supplies only with a power output of more than 25 W. 04. CONSTRUCTION OF A CAPACITIVE POWER SUPPLY