Capacitor Recommendations for the PTH12020 Series of Power Modules

Input Capacitor

Output Capacitors (Optional)

Ceramic Capacitor

Tantalum Capacitors

Capacitor Table

PTH12020W/L

The recommended input capacitance is determined by the 560

µF minimum capacitance and 800 mArms

minimum ripple current rating.

Ripple current, less than 100 m

Ω equivalent series resistance (ESR), and temperature are major considerations

when selecting input capacitors. Unlike polymer-tantalum capacitors, regular tantalum capacitors are not

recommended for the input bus. These capacitors require a recommended minimum voltage rating of

2×(maximum DC voltage + AC ripple). This is standard practice to ensure reliability. There were no tantalum

capacitors, with sufficient voltage rating, found to meet this requirement. When the operating temperature is

below 0°C, the ESR of aluminum electrolytic capacitors increases. For these applications Os-Con,

polymer-tantalum, and polymer-aluminum types should be considered.

Adding a 10-

µF ceramic capacitor to the input will reduce the ripple current reflected into the input source.

For applications with load transients, the regulator response will benefit from external output capacitance. The

recommended output capacitance of 330

µF will allow the module to meet its transient response specification

(see product data sheet). For most applications, a high quality computer-grade aluminum electrolytic capacitor is

adequate. These capacitors provide decoupling over the frequency range, 2 kHz to 150 kHz, and are suitable

when ambient temperatures are above 0°C. For operation below 0°C, tantalum, ceramic, or Os-Con type

capacitors are recommended. When using one or more non-ceramic capacitors, the calculated equivalent ESR

should be no lower than 4 m

Ω (7 mΩ using the manufacturer's maximum ESR for a single capacitor). A list of

preferred low-ESR type capacitors are identified in Table 4.

Above 150 kHz the performance of aluminum electrolytic capacitors is less effective. Multilayer ceramic

capacitors have very low ESR and a resonant frequency higher than the bandwidth of the regulator. They can be

used to reduce the reflected ripple current at the input as well as improve the transient response of the output.

When used on the output their combined ESR is not critical as long as the total value of ceramic capacitance

does not exceed 300

µF. Also, to prevent the formation of local resonances, do not place more than five identical

ceramic capacitors in parallel with values of 10

µF or greater.

Tantalum type capacitors can only be used on the output bus, and are recommended for applications where the

ambient operating temperature can be less than 0°C. The AVX TPS, Sprague 593D/594/595 and Kemet

T495/T510 capacitor series are suggested over many other tantalum types due to their higher rated surge, power

dissipation, and ripple current capability. As a caution many general purpose tantalum capacitors have

considerably higher ESR, reduced power dissipation and lower ripple current capability. These capacitors are

also less reliable as they have reduced power dissipation and surge current ratings. Tantalum capacitors that

have no stated ESR or surge current rating are not recommended for power applications.

When specifying Os-con and polymer tantalum capacitors for the output, the minimum ESR limit will be

encountered well before the maximum capacitance value is reached.

Table 4 identifies the characteristics of capacitors from a number of vendors with acceptable ESR and ripple

current (rms) ratings. The recommended number of capacitors required at both the input and output buses is

identified for each capacitor type.

This is not an extensive capacitor list. Capacitors from other vendors are available with comparable

specifications. Those listed are for guidance. The RMS ripple current rating and ESR (at 100 kHz) are critical

parameters necessary to insure both optimum regulator performance and long capacitor life.

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