Downsizing of switching power supplies

Frequent Issues

I would like to replace a film capacitor with a ceramic capacitor. But how do I select . . . ?

The demand for downsizing power supplies is becoming intense along with the general downsizing of electronic equipment.
Company B, which manufactures switching power supplies for office equipment, has also been considering replacing a large film capacitor with a ceramic capacitor.

Example Case: Company B -
Challenge: Can this film capacitor be made smaller?

Company B is a supplier of high-quality customized LLC resonant power supplies, taking advantage of the designs for low loss and low noise switching power supplies.
Until now, the market has accepted the price (to some extent) and the unit size due to its high quality. However, Company B has been sensing impatience from their customers, because equipment manufacturers have hinted on occasion that they may change to a new power supply manufacturer that has been growing, taking advantage of their low cost.

Company B has also been promoting the downsizing of power supply units by increasing the switching frequency to a higher frequency; however, this is mostly based on improvements in the performance of power supply IC and switching elements, and is not a point of distinction from the competitors.
The design team of the next major model under development was also concerned about being able to compete in the business. Meanwhile, a new engineer who was creating CAD of the prototype muttered, "Can this film capacitor be made smaller?"

This capacitor model has not been revised for many generations. In fact, the voltage and capacity of the film capacitor are too large; if this capacitor can be replaced with a smaller one, mounting with a higher density can be accomplished, reducing board size.
The design team started the selection of an alternative capacitor.

It is difficult to select a ceramic capacitor, because the ripple current is not specified.

The capacitors applicable for replacement are a film capacitor used as a capacitor for input filters (C4) of the PFC (power factor correction circuit), and a capacitor for resonance (C9). A ceramic capacitor was selected as a candidate. There are past records of using a disk type ceramic capacitor for the Y capacitor (C2, C3) of the AC input line, and the snubber capacitor (C8) of the low capacity area.

However, the selection of a ceramic capacitor was much more difficult than expected. A large ripple current with a high frequency flows into the capacitor for the PFC input filter, and resonance capacitor. However, since the ripple current is generally not specified in the specifications of a ceramic capacitor like a film capacitor, it is difficult to select a capacitor from the catalog specifications.

power supply circuit,murata,capacitor

As a condition, "it can be mounted by flow soldering." Also a concern of "noise" and "cracks" in the chip type

Since a large capacitance is required for the applicable capacitor, there is not enough capacitance in the disk type ceramic capacitors used in the past. The selection was naturally narrowed down to a multilayer type ceramic capacitor.

There are concerns of problems particular to chip multilayer ceramic capacitors, such as noise caused by ripple voltage, or cracks caused by deflection of the board.
Furthermore, since the size of a chip type product with a rated voltage of more than several hundred Vdc and a large capacitance is too large, it cannot be mounted by flow soldering.
Since the soldering process cannot be changed, the condition is limited to leaded products that can be mounted by flow soldering, the same as a film capacitor.
The replacement of the capacitor with a ceramic capacitor cannot be achieved without solving all of these issues.

Therefore, Company B approached Murata, asking if a large film capacitor being used for an LLC resonant power supply could be replaced with a ceramic capacitor, and we held discussions to solve the issue.
Fortunately the capacitor could be replaced with a lead type ceramic capacitor.

Points of Issue

  • POINT1
    To replace a film capacitor with a ceramic capacitor
  • POINT2
    How to select a ceramic capacitor that can handle a large ripple current flow
  • POINT3
    A ceramic capacitor with a large capacity that can be mounted by flow soldering
  • POINT4
    Preventing noise caused by the ripple load, and cracks by deflection of the board
The Solution

To examples of replacing a large film capacitor with a ceramic capacitor