Designing for AC circuits requires thinking differently about MLCC choices

Wilmer Companioni

Tuesday, March 26th, 2019

The Rise of AC

At the beginning of the 20th Century, Edison and Tesla, known bitter rivals, battled for years to see whose power delivery technology would thrive in the Electrical Age. Edison, the self-taught, self-made titan of industry was already revered for his invention of the lightbulb; one that would ultimately go on to be the light to the darkest parts of the Northeast United States. Tesla was the foreign, brilliant scientist with an extensive education and an ability to understand some of the most esoteric topics of electromagnetics. Tesla previously worked for Edison in Menlo Park. The two had a difference of opinion and work ethic, resulting in their working relationship ultimately sour to a bitter rivalry. Their differences were perhaps no greater reflected than in their opinions on how to distribute electricity to the masses. Tesla favored alternating current and Edison was a proponent of direct current. Both methods had their advantages and disadvantages, but ultimately, DC won over AC primarily because it was quite easy to transmit, and to step down, the high voltages generated by the source of power to low voltages usable by households using the technology of the time. Whether DC or AC, any electrical system makes use of capacitors. MLCCs were developed well after Edison won The War of the Currents, but they can still be used to serve the needs of today’s AC systems.

The Many Sides of Impedance

The one of the main parameters that define how circuits operate is impedance. Impedance is the property that determines how a component behaves when an alternating current is applied to that component. Impedance is a complex quantity; not complex in the sense that it is difficult, but complex in the sense that it is a quantity composed of various items.


Z = Impedance
R = Resistance
XC = Capacitive reactance
XL = Inductive reactance

Two of those items are resistance and reactance. Resistance is simply a component’s ability to prevent the flow of direct current. Reactance is a similar quantity for alternating current. Because almost no system is purely AC or purely DC, it is important to think of the total impedance, or rather the combination of resistance and reactance.

The story doesn’t stop there. Reactance is something that is given rise to by the inductive and capacitive effects of a component. So, in short, the reactance, and thereby the overall impedance, of a component is highly dependent of the value of its capacitance and of its inductance. For capacitors, the primary contributor to impedance is, not surprisingly, the capacitance. The total power dissipated by a device when AC is applied to it, is called apparent power. The portion of that apparent power that is dissipated by resistive effects is called real power and the portion of power that is dissipated by reactance is called reactive power.

MLCCs in AC Circuits

For some time MLCCs have been used in AC circuits primarily as safety devices. X/Y safety capacitors have been around since the rise of AC. Their purpose is to filter out unwanted noise in a system. The reason they carry the moniker of “safety” is that in the unlikely event of a capacitor failure the system will fail in a “safe” manner, meaning they won’t cause harm. The role of capacitors in AC circuits now goes beyond just safety-rated EMI capacitors. They are used in various functions, such as AC filtering, power factor correction circuits, and AC to DC conversion. Safety-rated MLCCs do not make good options for these circuit applications. KEMET’s new AC-rated MLCCs have 50 times more capacitance than their safety-rated counterparts. The AC-rated MLCCs have the low ESR, low ESL properties of typical MLCCs and they are suitable for 250 VAC continuous. All the typical ceramic options such as Flexible Terminations are available in AC-rated MLCCs.

Look no more, KEMET has it!

KEMET’s new 250 VAC rated MLCCs are ideal for non-safety critical applications such as: AC-DC converters, AC filtering, power factor correction, and power supply. This series offers up to double the capacitance versus Safety rated MLCCs and a waterfall with industry leading CV values. Check out our extensive catalog of electronic components including AC-rated MLCCs on our industry-leading search engine, Component Edge.

 

If our new AC-rated MLCC sparks your fancy, here’s a little more about the benefits:

  • Continuous AC voltage rating 250V, 50/60Hz
  • Base metal electrode (BME) dielectric system
  • −55°C to +125°C operating temperature range
  • Low ESR and ESL
  • Lead (Pb)-free, RoHS, and REACH Compliant
  • Temperature stable dielectric
  • Non-polar device, minimizing installation concerns
  • 100% pure matte tin-plated termination finish allowing for excellent solderability
  • Flexible termination option available upon request