This article based on Knowles Precision Devices blog elaborates on importance of safety capacitors and its benefits especially in power electronic applications.
Safety capacitors are designed to mitigate the effects of transient voltages and interference in electrical and electronic circuits, especially high-voltage applications, ensuring their safe operation.
Even everyday devices need safety capacitors: modems and other telecoms equipment, AC-DC power supplies, power distribution switchgear, and electric vehicles (EVs) and other automotive applications.
The Role of the Safety Capacitor
In a circuit, safety capacitors used on AC lines filter high-frequency electromagnetic interference (EMI), or undesirable electromagnetic emissions or disturbances generated either by electronic devices or natural sources in the environment that can interfere with the proper functioning of other nearby devices or systems. This is known as EMI filtering, with the filter providing a safe path for noise while maintaining electrical isolation and safety.
When placed in combination with a series of inductors or resistors, safety capacitors form low-pass filters that attenuate the high-frequency signals (EMI) and allow the lower-frequency power signals to pass. This is the classic EMI filter setup—attenuating unwanted noise above a cutoff frequency. As more electronic devices enter the market, EMI filtering becomes even more crucial as there is an increased likelihood that EMI could cause a device to malfunction, crash, or fail.
In power electronics specifically, EMI filters take the form of power line filters that protect the line from upstream noise. While this list may differ depending on system requirements, the core components of a power line filter include:
- A common-mode filter circuit with two or more line-to-chassis capacitors and a common-mode inductor
- A differential mode (DM) filter circuit with at least one pair of series inductors and one line-to-line capacitor
- Compensation networks to adjust the filter’s quality (Q) factor and adapt the output impedance as needed
- Transient voltage suppression device(s) to defend against surges
Figure 1. shows an EMI filter structure that could be useful for single-phase AC applications or for DC power inputs.
Where Safety Capacitors are Used
To ensure safe operations and compliance with electrical safety standards, you’ll find safety capacitors in nearly every electronic device developed today – from industrial motor drives and HVAC systems to consumer appliances to medical devices, telecommunications equipment, and automotive electronics. For many of these applications, safety capacitors are crucial components that ensure user safety and reliable performance.
Classes of Safety Capacitors
In general, there are two classes of capacitors, Class-X and Class-Y, that are both used to minimize EMI in different applications.
- X Capacitors: Class-X capacitors, also known as “across-the-line capacitors,” are used between the wires carrying the incoming AC current. These offer line-to-line protection, which means that if there is a failure, a short may occur, but there is no risk of shock. An X capacitor failure usually causes a fuse or circuit breaker to open.
- Y Capacitors: Class-Y capacitors, also known as “line-to-ground capacitors” or “line bypass capacitors,” offer line-to-ground protection, which generally means that if a failure with the ground occurs, there is a risk for shock. However, Class-Y safety capacitors must meet rigorous specifications, minimizing the chance of electric shock.
- X/Y Capacitors: Some safety capacitors handily combine classes, for example, X1/Y2. This simply means that the capacitor can be used as an X1 capacitor in an across-the-line application or as a Y2 capacitor in the line-to-ground portion of the circuit.
Safety Capacitor Subclasses
As mentioned, both X and Y capacitors are further broken into the following subclasses according to their pulse voltage withstand capability and rated voltage (more details in Table 1):
- X Capacitor Subclasses:
- X1: For higher surge levels (≥2.5 kV and ≤4 kV)
- X2: Most common in consumer electronics (≤2.5 kV)
- X3: For ≤1.2 kV, less common
- X Capacitor Subclasses:
- Y1: For higher insulation and impulse voltage (>500 VAC, up to 8 kV)
- Y2: Common for 250 VAC operation
- Y3/Y4: Lower ratings, used in less critical applications
*2x Y2 or Y4 rated may bridge double or reinforced insulation when used in series
By understanding the differences between the different capacitor classes and subclasses, you can optimize product development while ensuring user safety, product compliance, and reliable circuit performance.
Choosing a Safety Capacitor – Ceramic or Film?
Two common types that can fit the role of safety capacitors are multilayer ceramic capacitors (MLCCs) and plastic film capacitors. Each has its benefits depending on the specific application. Some characteristics to consider when choosing between capacitors include the following:
- Temperature range
- Size constraints
- Voltage rating
- Capacitance value
- Certification standards (like X and Y ratings for line-to-line and line-to-ground applications)
- Cost
Film Capacitors
Film capacitors can withstand a wide temperature range and often have higher capacitance value. Film capacitors also offer self-healing, which enables the device to recover from a dielectric breakdown with only a small reduction in capacitance. However, film capacitors are large in size relative to their capacitance. They are also often more expensive and used in voltage/capacity ranges that cannot be covered by ceramic capacitors.
Multilayer Ceramic Capacitors
MLCCs can achieve higher capacitance values in a smaller volume than film capacitors. They are usually better for low-power applications. For example, MLCCs are well-suited for space-sensitive applications like antenna coupling. They are also available with special terminations to accommodate automotive technology with applications like onboard chargers, inverters, DC/DC converters, and other equipment in EVs.
And while film capacitors have an impressive temperature range, MLCCs can also withstand high temperatures, high voltage, and high power. Additionally, Knowles Precision Devices MLCCs are available with a FlexiCap™ termination. FlexiCap™ is a proprietary flexible epoxy polymer termination material that is applied to the device under the usual nickel barrier finish to accommodate a greater degree of board bending than conventional capacitors, reducing the risk of mechanical cracking.