By Takanori Hibino, Product Manager – Capacitors, Murata Electronics Europe
There is constant pressure in the electronics industry to make everything smaller, faster, lighter and sleeker. Nowhere is this trend stronger than in the consumer electronics sector, where the look, feel and user experience that a product offers is often more important than the details of its functionality – case in point, who buys earbuds for their sound quality rather than their convenience or looks?
This pressure to miniaturise electronics is felt particularly keenly by the component makers, who are constrained by the laws of physics as to the scope of features they can cram into a given area or volume. IC vendors may have benefited for decades from the scaling of process nodes, allowing them to keep doubling device density every 18 to 24 months, but for less complex passive components (such as inductors, resistors and capacitors), the options can be more limited. For example, the electrical energy that a capacitor can store is defined by the area of its two plates, the dielectric material chosen to separate them and their distance apart. Much ingenuity has been expended over the years to work within these constraints in order to maximise the charge storage density of capacitors.
There is a particular demand at the moment for incorporating smaller capacitors (and other constituent components) into the power supplies of audio-visual equipment, such as flat-panel TVs. Simultaneously, there is rising demand for low-profile power supplies to go into the 1U-high rack-mounted equipment targeted at the servers and storage systems used in data centres.
All this equipment, be it the latest high-end 4K UHD TV or a commodity data centre server, needs an electrically quiet environment in order to work properly. Uncontrolled noise on the power rails of a TV may make the screen image jump from time to time, which is irritating. In data centre hardware, however, uncontrolled noise may cause malfunctions, unexpected reboots and possibly catastrophic loss of service.
Power supply designers address the noise issue by using Y capacitors to connect commercial power lines to ground and coupling capacitors to connect the primary and secondary sides. These capacitors need to meet elevated safety standards, so they can withstand unexpectedly high voltages and offer good flame-retarding properties. Fortunately for end users, safety standards bodies, such as UL and CQC, have considered the issues that such components face and have developed testing strategies and certification procedures that ensure that properly tested and certified components are safe for use.
If you want to make power supplies smaller, all the components clearly need to get smaller too. This can sometimes be constrained by the form factor of the components most commonly used in their circuitry. For example, radial-lead disc ceramic capacitors have successfully supported the trend to downsize power supplies by increasing their charge storage density. However, the mounted height of radial-lead components is considerable and there are times when this restricts efforts to further reduce equipment dimensions.
In response, Murata has introduced the DK series type EA ceramic capacitor, an IEC 60384-14 X1/Y1 class certified surface-mount component. DK series capacitors offer capacitance values in the range 10 to 1500 pF at Y1 rated voltages up to 300 VAC rms.
Along with its shallow height, the surface-mount component also helps overcome another issue with radial-lead components, which is that their lead wires are usually inserted through a hole in the associated PCB, creating a projection on its underside. The height of this projection may be small in itself, but safety standards for power sources require that a stipulated distance (the insulation distance) is maintained between the ends of the lead wires and other metal components, such as the power supply chassis. This consequently hampers efforts to create lower-profile equipment.
The DK series type EA capacitor enables a Y capacitor to be surface mounted by soldering flat metal terminals onto a disc-type ceramic dielectric (the capacitor element) and then encasing the result in a resin melding. The surface-mount configuration gives the component a lower profile and eliminates lead wires from the underside of the PCB. This takes away the need to consider the insulation distance between the lead wire ends and the chassis. The resulting profile reduction can have a significantly impact on power supply form factors.
The fact that these components use a disc ceramic dielectric element as the storage node means that they can provide the high dielectric strength needed to qualify for the IEC 60384-14 X1/Y1 class and also results in a 2.5mm height from the PCB surface.
The chemical composition and amount of solder that joins the metal terminals to the ceramic element has been chosen so that the capacitor can be used in reflow processes that rely on high temperatures and relatively long soldering times. The surfaces of the metal terminals have also been tin-plated to make them easier to solder.
Murata already offers the safety-certified DE series of radial-lead disc ceramic capacitors. The DE series type KX is certified for the IEC 60384-14 X1/Y1 class and has been widely adopted. The capacitor elements in the company’s EA-type components uses the same disc-shaped ceramic as the DE series, so can offer a high dielectric strength between terminals of 4kV AC for 60 seconds, as required for IEC 60384-14 Y1 class compliance.
As an example of the dielectric strength performance, Figure 2 shows a comparison of the between terminal insulation breakdown voltage for the DK series type EA and the DE series type KX. The minimum value for between-terminal insulation breakdown with the DK series is the same as for the DE series, providing more than enough between-terminal dielectric strength to meet safety certification requirements.
Addressing reliability issues
The composite nature of these components, which use both ceramic and resin materials, means that it is important to ensure that the differing thermal expansion rates of the two materials do not cause reliability issues. If the adhesion between the ceramic and resin is insufficient, it can lead to a drop in dielectric strength and moisture resistance. On the other hand, if the linear expansion coefficients of the two materials differ greatly, the unequal expansion of the two materials during temperature cycles can cause mechanical stress that may lead to cracking.
Figure 3 shows how the insulation resistance changes before and after a moisture resistance load test (done at 60°C, 95% relative humidity, 440V AC, over 1000 hours) for this DK series and the comparable DE series type KX. The DK series has a moisture resistance equal to the DE series, and neither series shows the kind of change in moisture resistance that would be expressed by a marked reduction in insulation resistance after the test. The DK series also eliminates structural concerns about chip cracks from circuit-board flexing and solder cracks due to thermal cycles, as can sometimes happen with large multilayer ceramic capacitors.
Meeting safety requirements
The safety standards for class Y1 capacitors that connect commercial power lines to ground address issues including the dielectric strength between terminals, the distance between electrodes (creepage distance) and combustion resistance. We have already addressed dielectric strength above. For creepage distance control, the capacitor’s body and the shape of its terminals have been designed to achieve at least an 8mm landing space around the mounted component, as required by the safety standard.
The capacitor body is moulded using a material that conforms to UL 94 V-0 –recognised as the most demanding flammability test requirement. The manufacturing process also controls the thickness of the covering resin to maintain the dielectric strength between the terminals and the enclosure, again to meet the safety standard’s requirements. These steps have resulted in the component acquiring the major safety certifications available in Europe, the US and Asia – such as UL, CQC and KTC.
A capacitor appears to be a relatively simple part, but something as basic as its form factor can have serious implications on later design decisions, such as the overall height of a power supply, how closely it can be fitted into a chassis and how much space it needs on the PCB to overcome the effects of any possible component creepage.
Murata’s DK series type EA surface-mount capacitor borrows proven technology from previous lines of axial-lead disc ceramic parts, but packages it in such a way that its mounted height from the PCB surface is much reduced, enabling greater freedom of design. Once adopted, the availability of this component should lead to flatter TVs and thinner servers for all.