SELECTION CRITERIA FOR HIGH FREQUENCY CAPACITORS

In passive components, there are high-frequency versions of components with operational specifications reaching up to very high frequencies. Capacitor selection for high applications depends not just on capacitance, but also its frequency response. It behaves inductively rather than capacitively beyond resonance frequency. Capacitor are specially designed to retain their ideal behaviour up to very high frequencies.

Important parameters of high frequency capacitors

1. Capacitance range: May vary from sub-pF to nano farads
2. Voltage rating: May range from as low as 10 Volts to tens of kilo volts
3. Operating frequency and Self Resonant Frequency (SRF): Self-resonant frequency (SRF) of capacitor should be above the highest significant operating or harmonic frequency. This may be checked from capacitor data sheet.
4. ESR and dissipation factor: Must be low to maintain high quality factor, lower heating and power loss, especially where ripple current is high.
5. ESL and package size: Internal structure and physical leads result in inductance. Mounting and lead inductance can be significantly large at high frequencies.
6. dV/dt rating: This is crucial for RF capacitors as it directly affects the current handling capability and overall performance of capacitor in high-frequency applications. It is the ability of capacitor to withstand rapid voltage changes safely without damage or failure. High dV/dt can have a significant impact on circuit function and reliability.
7. Linearity behaviour limits: At high frequencies, capacitors exhibit parasitic ESL and ESR, causing non‑ideal behaviour.
8. Package and mounting: Smaller MLCC packages usually provide lower ESL and higher SRF.

At GHz frequencies, capacitor and its PCB footprint form integral electromagnetic structure. For example, 10 GHz, a 1 mm length is a substantial fraction of a wavelength, and mounting structure cannot be treated as an ideal wire.

Frequency response of capacitors

ESL and ESR make the impedance of capacitor complex, and impedance changes with frequency. As frequency increases, capacitive reactance decreases, while ESL increases, and impedance gradually reduces. At one point (resonance frequency), capacitive reactance becomes equal to inductive reactance, and the capacitor becomes a resistive element. Impedance is lowest at this point. Beyond this point, the capacitor acts as an inductive element. A high frequency circuit design must avoid frequencies close to resonance frequency.

The frequency curve may not be truly followed observed once the components are placed on a real PCB. Parasitic effects modify impedance of a circuit, thereby modifying the input impedance and this needs to be verified in actual situations with proper circuit analysis.

dV/dt characteristics of RF capacitor

Capacitor current is generated by the change in capacitor terminal voltage. This current is proportional to the capacitance and the rate of change of capacitor terminal voltage over time. The larger the dv/dt, the larger the capacitor current.

Theoretically, capacitors can withstand unlimited current surges. In high-frequency applications like snubbers, IGBT, rectifier or bypass circuits, despite low ripple voltage, they may carry high currents.

Smaller physical sizes of capacitors (like metallized or MLCC capacitors), electrode conducting areas are small, and current carrying capacity of electrodes is limited. Therefore, practical capacitors have current limitations of maximum current (RMS and peak current), decided by dV/dt rating. This data is available in manufacturer’s data sheet.

Selection criteria for RF capacitor

1. Application needs
   • RF coupling, matching, and resonant circuits.
   • High-frequency bypass or decoupling.
   • Frequency range
   • Switching-converter snubber or ripple suppression.
   • High-voltage pulse or resonant power circuit.
2. Desired capacitance and voltage
3. SRF (Self-resonant frequency) and impedance
   • Resonance frequency should much above operating frequency.
   • Multiple capacitors with different values and SRFs may be used where needed.

General choices for capacitor dielectric

• 1–30 MHz bypassing: 100 pF to 10 nF C0G/NP0 MLCC, supplemented by a suitable X7R capacitor for lower-frequency energy.
• RF signal coupling: C0G/NP0 (low insertion loss and adequate voltage rating).
• MHz resonant converter: PP film, ceramic, or a dedicated resonant capacitor.
• High-voltage RF or induction heating: Ceramic, vacuum, or water-cooled capacitor specifically rated for RF current and frequency.
• Microwave frequencies: RF chip capacitors with good S-parameters or impedance data- PCBs and transmission-line geometry.

Power Ratings of High frequency capacitors

High-frequency capacitors may handle power from a few watts to tens of kVA in industrial or high-power RF applications.

1. Continuous RMS current limits can range from under 1 A for small SMD capacitors to over 10 A for high-voltage capacitors.
2. Temperature derating must be considered beyond normal operating range- may require derating voltage as much as by 50% to maintain reliability.

Capacitors for GHz frequencies

For GHz-frequency circuits, RF/microwave ceramic capacitors, usually in very small SMT packages. Correct choice depends more on the manufacturer’s specifications, insertion loss, Q, and mounting data. RF capacitors are available for applications from several GHz to above 40 GHz, or higher.

Recommended capacitor types

GHz application	Suitable capacitor	Typical use
1–3 GHz	C0G/NP0 MLCC, RF ceramic	Coupling, matching, bias bypass
3–10 GHz	RF multilayer ceramic or thin-film capacitor	Filters, matching networks, microwave amplifiers
10–20 GHz	Thin-film, silicon, or dedicated microwave capacitor	Radar, microwave, high-speed RF
Above 20 GHz	Thin-film, broadband, or distributed PCB structure	mmWave matching and broadband bypassing
High-Q resonator	Air, mica, RF ceramic, or thin-film capacitor	Oscillators, filters, resonant networks
Wideband bypass	Broadband capacitor or multiple RF capacitors	Bias-line and power-supply bypassing

C0G/NP0 is preferred for stable RF signal paths because it has low dielectric loss and relatively low voltage and temperature dependence. X7R may be acceptable for noncritical supply bypassing, but its capacitance changes with DC bias and temperature.

Some typical applications

• RF multilayer ceramic capacitors: Suitable for approximately 500 MHz to 10 GHz, with low ESR and high Q.
• Dedicated microwave ceramic capacitors: These cover low-to-mid GHz range with capacitance values around 0.1–100 pF.
• Thin-film RF capacitors: Operate up about 20 GHz, and  have close tolerance and high Q.
• Broadband capacitors: Good performance up to 16–18 GHz range, and some capacitors extend to approximately 40 GHz.
• Ultra-broadband ceramic capacitors: Operational range up to tens of GHz, and may extend tp 110 GHz.