The role of ceramic capacitors in switching power adapters

Ceramic capacitors are widely used in switch-mode power adapters due to their long lifespan, high operating frequency, and compact size. Also known as ceramic dielectric capacitors, they use ceramic as their dielectric material. Ceramic capacitors come in various structural forms, but their fundamental principle remains the same. The thickness, surface area, smoothness, and flatness of the ceramic are carefully controlled based on its physicochemical properties, and they are then precision-manufactured using modern techniques. There are many types of ceramics, classified by composition, such as titanium-based ceramics, thermal ceramics, and barium titanate ceramics. Titanium-based ceramics have a high dielectric constant and a negative temperature coefficient. Thermal ceramics have a lower dielectric constant and a smaller negative temperature coefficient, offering better capacitor stability. Barium titanate ceramics have the highest dielectric constant but poor stability due to a large temperature coefficient, making them unsuitable for high-end electronic products. In switch-mode power adapters, ceramic capacitors are often used to suppress common-mode noise, typically connected between the circuit and ground, such as in the case of Y capacitors.

• Environmental Resilience: Ceramic capacitors are highly resistant to acids, alkalis, and salts, allowing them to operate reliably even in corrosive environments without significant aging.
• High Dielectric Constant: The dielectric constant of ceramic materials ranges from tens to hundreds of picofarads, and in some cases, even thousands. This enables the production of compact capacitors, and multilayer designs can significantly increase capacitance.
• Simple Structure and Cost-Effectiveness: The manufacturing process is straightforward, with abundant raw materials, making these capacitors affordable.
• High Insulation and Voltage Resistance: Ceramic capacitors exhibit strong insulation properties and high insulation resistance, capable of withstanding voltages up to 2 kV.
• Excellent High-Frequency Performance: The ceramic dielectric material cannot be coiled, resulting in capacitors with minimal inductance. This makes them ideal for high-frequency applications in aerospace, telecommunications, and switch-mode power adapters.
• Thermal Stability: They can operate stably at high temperatures, up to 500–600°C.
• Wide Temperature Coefficient Range: Ceramic capacitors can be designed with varying temperature coefficients to suit different applications.
• Low Loss Tangent: The loss tangent of ceramic capacitors is minimally affected by frequency, but they are mechanically fragile and prone to cracking.

In general, capacitors block DC signals while allowing AC signals to pass, playing a crucial role in noise suppression, high-frequency electromagnetic interference filtering, and stabilizing electrical performance. In switch-mode power adapters, ceramic capacitors serve the following functions:

• Smoothing Ripple Current: The output of the power conversion circuit in switch-mode adapters is pulsating DC with significant ripple. While electrolytic capacitors are traditionally used for smoothing, the trend toward higher frequencies and miniaturization has led to the adoption of multilayer ceramic capacitors. These capacitors, treated with nickel and carbon film at high temperatures, offer improved capacitance, voltage resistance, and reduced leakage current. Their low impedance compared to aluminum electrolytic capacitors makes them highly effective in smoothing ripple currents, with minimal self-heating. Unlike electrolytic capacitors, whose capacitance decreases over time due to electrolyte drying, ceramic capacitors maintain their capacitance over time.
• Surge Voltage Absorption: During operation, electronic circuits can experience surge voltages due to environmental factors, load changes, or sudden circuit transitions. These high-energy surges can damage power transistors. To protect them, ceramic capacitors are often connected in series with resistors and placed across the primary winding of transformers to form snubber circuits. These capacitors typically have a capacitance below 470 pF and a voltage rating above 630 V.
• Noise Bypassing: To prevent noise from entering the load or low-frequency electromagnetic waves from entering the power supply, ceramic capacitors are used as bypass capacitors. These capacitors, connected in the output circuit, effectively suppress normal-mode noise and act as low-pass filters. Their capacitance typically ranges from 220 to 3300 pF, with voltage ratings between 500 V and 1.2 kV, depending on the circuit requirements.
• Noise Filtering: The input circuit of switch-mode adapters often includes an AC line filter to block external noise and internal noise emissions. Ceramic capacitors are commonly used for this purpose. Capacitors connected to the power line, known as X capacitors, are designed to suppress lower-frequency noise and have higher capacitance values, typically rated at 275 V.