TDK: Guide for replacing of film capacitors with MLCC

source: TDK news

TDK published a technical note guide for replacing of film capacitors with C0G MLCC.

Vol.1 Features of High voltage MLCCs with C0G Characteristics and Replacement Solutions Overview

A wide variety of capacitors, each with their own special characteristics, are used in electronic devices. Generally speaking, the capacitance and withstand voltage (rated voltage) of capacitors are in a trade-off relationship which is difficult to balance. In MLCC of the same size, when increasing the withstand voltage, the capacitance tends to decrease.
Film capacitors possess a good balance of high withstand voltage and capacitance. Since they also possess outstanding frequency characteristics and temperature characteristics, they are widely used in automotive electronics, industrial equipment, home appliances, etc.

However, in recent years, there have been remarkable increases in withstand voltage and capacitance in MLCCs (multilayer ceramic chip capacitors) for temperature compensation (class 1). In particular, even in fields where film capacitors have traditionally been used, resonance circuits for example, replacement with MLCC is now possible. TDK has developed high voltage MLCCs with C0G characteristics. Through C0G characteristics, these MLCCs achieve withstand voltage of 1000V at the broadest capacitance range (1nF to 33nF) in the industry.

In this guide, we explain the numerous benefits of replacement while comparing the features of high voltage C0G MLCCs with those of film capacitors.

Characteristics of main capacitors
MLCCs are divided into two major categories according to the type of ceramic materials used for their dielectric, namely class 1 (temperature compensating) and class 2 (high dielectric constant).
Class 2 MLCCs have a large capacitance. However, they also have a disadvantage in terms of a large capacitance change caused by temperature. On the other hand, while class 1 MLCCs do not offer as high a capacitance as class 2, they display a smaller capacitance change caused by temperature. They also possess outstanding frequency characteristics and are used in circuits which require high precision.

Figure 1 shows the corresponding regions for rated voltage-capacitance in main capacitors: aluminum electrolytic capacitors, film capacitors, and MLCCs (class 1 and class 2).

Figure 1: Corresponding regions of rated voltage-capacitance for different capacitors

In terms of capacitance, class 2 MLCCs achieve a capacitance of more than 100μF, as offered by aluminum electrolytic capacitors. Furthermore, even in the past, class 1 MLCC voltage-capacitance overlapped a portion of film capacitor regions. However, the withstand voltage and capacitance have increased in recent years, and the overlapping regions are increasing rapidly.
Table 1 summarizes a comparison of the characteristics of aluminum electrolytic capacitors, film capacitors, and MLCCs.

Table 1: Comparison of characteristics in main capacitors

	Aluminum electrolytic capacitor	Film capacitor	MLCC (class 1)	MLCC (class 2)
Large capacitance	◎	○	△	○
Withstand voltage	○	◎	○	○
Temperature characteristics	△	◎	◎	△
Frequency characteristics	△	◎	◎	◎
DC bias characteristics	◎	◎	◎	△
Ripple resistance	△	◎	◎	◎
Moisture resistance	○	○	◎	◎
Lifespan/reliability	△	◎	◎	◎
Compact size	△	△	◎	◎

◎: Outstanding　○: Good　△: Relatively poor

The advantage of aluminum electrolytic capacitors is their large capacitance. In terms of other characteristics, film capacitors and MLCCs are superior. Unlike class 1 MLCCs, it is difficult to achieve compact size for film capacitors. The table also shows how it is difficult to increase the capacitance and withstand voltage of class 1 MLCCs.

The capacitance value of class 2 MLCCs changes greatly with changes in temperature. In comparison, class 1 MLCCs exhibit a nearly linear change. The straight line slope in relation to temperature is called the “temperature coefficient.” It is expressed in units of [ppm/°C].
In JIS and EIA standards, the temperature coefficient value and the related tolerance are categorized into classes. The strictest EIA standards for C0G MLCCs (class 1) require a temperature coefficient of 0 ppm/°C and a tolerance of ±30 ppm/°C at a temperature range of -55 to +125°C. Figure 2 shows temperature characteristics (changes in capacitance due to temperature change) for film capacitors and MLCCs.

Figure 2: Comparison of temperature characteristics (changes in capacitance due to temperature change) in C0G MLCCs and various capacitors

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