# What are the main characteristics of high reactive power RF capacitors?

This question is related to this one.

I have to design a high quality factor RF parallel resonant circuit, to be placed in parallel to the output of a medium to high power RF generator. Precisely, the design data are the following ones

• $$\Q_F\approx 10\$$, where $$\Q_F\$$ is the resonant circuit quality factor,
• $$\f_\max=(1.0\div 4.0)\, \mathrm{MHz}\$$: the resonant frequency is fixed but its value should be varied if needed,
• $$\P_o\ge 100\mathrm{W}\iff Q_c\approx 1\mathrm{kVAR}\$$ where $$\P_o\$$ is the output power of the generator and $$\Q_c\$$ is the reactive power flowing in the circuit
• The load will not be directly placed in parallel to the resonant circuit, but it will be coupled by the use of a multi winding transformer where a secondary winding inductance will be part of the resonant circuit, so I have some freedom in choosing the voltage and current of the resonant circuit. From the schematics point of view, the situation is this one (the multi winding transformer is shown as two separated transformers just because Circuit Lab does not offer a symbol for such devices).

simulate this circuit – Schematic created using CircuitLab

I have some experience in the design and construction of low to medium power resonant circuits, say up to $$\60\mathrm{W}\$$: in those circuit I used nice low loss, low $$\TC\$$ ceramic RF capacitors, and all worked fine (apart from a couple of high Q SMD capacitors floating on a pool of melted tin in a quick and dirt prototype, and an ETD coil former literally burned by the winding it carried). However, a look at the size and structure of common high power RF capacitors makes me think that there's more than low dielectric loss: and this motivates my questions

1. Are low dielectric losses and low connection resistance the only key parameters (jointly with the maximum working voltage) in the choice of RF power capacitors? Or are there other second order effects arising and influencing capacitor performance at high power levels (for example dielectric hysteresis)?
2. If only contact an dielectric resistances are the only phenomena to be accounted for power dissipation, would it be possible in principle to construct $$\C_r\$$ as a plane capacitor made by two copper layers on a board of appropriate material (for example the standard FR4) connected to the circuit by short and large copper strips?
• Comments are not for extended discussion; this conversation has been moved to chat. Any conclusions reached should be edited back into the question and/or any answer(s). – Dave Tweed May 7 at 20:13

# What is Capacitor Dissipation Factor (DF)?

$$\\mathrm{DF}\$$ is dielectric ratio of power lost to power stored and is the inverse of Quality factor ($$\Q\$$). Power dissipation results in self-heating and due to common thermal resistance of dielectrics.

Capacitors can be specified in many ways.

• $$\\mathrm{DF}\$$ usually for 120Hz to 100kHz
• $$\\tan\delta\$$ for PCB dielectric, RF caps etc..
• $$\\mathrm{ESR}\$$ @ $$\f\$$ (for low $$\\mathrm{ESR}\$$ types)
• $$\Q\$$ for RF types
• $$\\mathrm{VAR}\$$ (VA reactive) for grid or motor power factor correction.
• Ripple Current (Amps) for e-caps and plastic film caps

DF also can be computed/converted many ways.

$$\mathrm{DF} = \tan{{\dfrac{1}{Q}}=\tan \delta ={\frac {\mathrm{ESR}}{\left|X_{c}\right|}}}=\mathrm{ESR}\cdot \omega C$$ where $$\\omega=2\pi f\$$.
Also $$\mathrm{DF}={\text{Power Factor (PF) if }}\mathrm{DF}< 0.01 = 1\%$$

# DF examples

, 1% or more Ceramic general purpose high $$\\mathrm{D}\kappa\$$
, 0.1% NP0/C0G Ceramic low density
, 0.1% to 1% typ. Aluminium Electrolytic
, 0.05 %@ 1kHz Polypropylene (PP) e.g.

With millions of variations in cap specs and costs, this is just a small sample.

1. Ceramic caps with high $$\\mathrm{D}\kappa\$$ change with bias voltage , therefore have some hysteresis but low $$\\mathrm{D}\kappa\$$ C0G/NP0 types do not (negligible but perhaps some).
2. PCB epoxy has reasonably low $$\\mathrm{DF}\$$ but Teflon and ceramic are much lower. They also have large copper heat radiators so can dissipate more heat (1W/sq.in.). There are many grades of PCB materials.