Preventing Capacitor Arcing with HVArc Guard MLCCs

Advantages of HVArc Guard capacitors in lighting applications

MLCCs used in ballast circuits can withstand high voltages in excess of 1000VDC in air. A block diagram of a typical lighting ballast circuit is shown in Figure 1. Capacitors are susceptible to surface flashover and internal breakdown. In either case, circuit instability caused by surface flashover can cause failures and damage surrounding components, even if the capacitors can temporarily maintain normal functionality. The unique design of the HVArc Guard capacitors prevents flashovers while allowing for a smaller ballast housing.

Figure 1 Typical electronic ballast circuit

Until now, MLCCs used in high-voltage lighting ballasts have come in large case sizes such as 1210, 1808 and 1812. The new HVArc Guard high-voltage MLCC capacitors can replace these standard high-voltage capacitors in ballast circuits, allowing engineers to design more compact circuits and reduce device costs.

Standard high voltage MLCC capacitors and HVArc Guard capacitors used in lighting ballasts. For example, a common 630V MLCC capacitor with a case size of 1206 (0.126 inches x 0.063 inches) can be replaced with an HVArc Guard MLCC capacitor with a case size of 0805 (0.079 inches x 0.049 inches). Using HVArc Guard capacitors can save 50% or more of board space.

The HVArc Guard surface mount MLCC capacitors feature small radial dimensions and high breakdown voltage, making them ideal for use in the high voltage inverter portion of compact electronic fluorescent lamp ballasts. In fact, the HVArc Guard capacitors have a breakdown voltage that is twice that of standard high voltage capacitors. Additional information on the voltage characteristics of this capacitor is also provided below.

Eliminate expensive outer encapsulation with HVArc Guard capacitors

Until now, design engineers have needed to take various costly measures, such as outer encapsulation, to prevent discharge flashover in high-voltage applications. Although encapsulation increases manufacturing and design costs, it is often used to meet the requirements of electrical safety standards. In some applications, people still use old-fashioned coated and leaded through-hole capacitors to avoid the use of outer encapsulated devices.

Vishay HVArc Guard adopts a special protection structure inside, which does not need to be encapsulated to prevent surface arcing. HVArc Guard can well replace the old through-hole capacitors with coatings and leads, and can greatly save manufacturing costs by eliminating the high-cost manual insertion process.

HVArc Guard capacitors provide improved voltage breakdown capability.

Text in the picture: Average voltage breakdown in Air (VDC): Average breakdown voltage in air (VDC)

HVArc Guard capacitors offer improved breakdown voltage performance compared to standard high voltage capacitors. The bar graph above compares the average breakdown voltage in air for standard high voltage 1812 size capacitors versus HVArc Guard capacitors. Because HVArc Guard capacitors prevent surface flashover, their breakdown voltage in air is twice as high as conventional standard high voltage capacitors.

Using HVArc Guard capacitors in lighting control circuits

In lighting control circuits, energy is provided by the rectified AC mains circuit, as shown in the block diagram below. A pre-converter is also used in many ballast circuits, and the power factor of the converter is close to 1. The output voltage of the pre-converter is regulated and very accurate. The filament of a fluorescent lamp needs to be preheated and then a very high starting/arcing voltage is used to light the bulb. When the arc is generated and the bulb is conducting, the basic equivalent circuit looks like an inductor in series with a parallel resistor and capacitor.

Figure 2 Lighting control circuit

The input voltage comes from the AC mains with a sine wave. To shape the input current into a waveform close to the line voltage, the converter generates a boost inductor current, just like the rectified input voltage. The regulated voltage from the diode rectifier bridge is sent to the ballast part of the circuit. When the ballast part starts working, the current pump composed of the capacitor buffer (Csnubber) and the diode will limit the rise and fall time of the rectifier bridge output. The buffer is also used to reduce EMI.

Many lighting ballasts use two power MOSFET switches, as shown in Figure 2. The MOSFETs alternately drive and conduct the transformer windings.

This basic circuit has been used for many years as a standard lighting ballast circuit, but it has several disadvantages. The circuit has no self-starting function, no dimming function, and requires a large transformer to work properly.

Advances in driver ICs have led to significant improvements in lighting ballasts. These circuits can drive either low-side or high-side MOSFETs, depending on the logic level/ground reference input, without the need for a drive transformer. The example in Figure 3 shows the various capacitors required for a ballast circuit.

Figure 3 Actual fluorescent lamp ballast circuit