Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Power Electronics Tips: How to select input capacitors for a buck converter

by Maniing Xie

Electromagnetic interference (EMI) can create serious issues for manufacturers in an industrial environment. A buck converter generates a pulsating ripple current with high di/dt at the input. Without input capacitors, ripple current is supplied by the upper power source. Printed circuit board (PCB) resistance and inductance causes high-voltage ripple that disrupts electronic devices. The circulating ripple current results in increased conducted and radiated EMI. 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. Therefore, reducing the input-voltage ripple of a buck converter has become more challenging. This article uses a buck converter as an example to demonstrate how to select capacitors to achieve optimal performance. Figure 1 shows the basic circuit of a buck converter. The converter input current (iIN_D) consists of an alternating ripple current (ΔiIN_D) and DC current (IIN_DC).
switch mode charger block diagram TI.jpg

Design parameters:

Output voltage, VO = 1.2 V
Maximum load current, IO = 6 A
Estimated efficiency at maximum load, h = 87%
Switching frequency, fSW = 600 kHz
DC input bus voltage = 12 V with 5% tolerance
Worst-case maximum input voltage, VIN_max = 16 V
Bus converter control bandwidth = 6 kHz
Transient load step, IStep = 3 A
Worst-case board temperature = 75°C

Design requirements:

Allowed input peak-to-peak ripple voltage, ΔVIN_PP ≤ 0.24 V
Allowed input transient undershoot or overshoot, ΔVIN_Tran ≤ 0.36 V

The capacitor voltage rating should meet reliability and safety requirements. For this example, all input capacitors are rated at 25 V or above. The following discussion focuses on meeting electrical and thermal requirements, optimizing performance, and lowering size and cost.

Select key ceramic capacitors to bypass input ripple current

Among the different types of capacitors, the multilayer ceramic capacitor (MLCC) is particularly good regarding allowable ripple current. A starting point....
Read the full article on Power Electronics Tips here


There are no comments to display.

Part and Inventory Search

Blog entry information

Read time
2 min read
Last update

More entries in Uncategorized

More entries from EastbayA

Share this entry