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LTC4278

4278fc

APPLICATIONS INFORMATION

Capacitor Selection

In a flyback converter, the input and output current flows

in pulses, placing severe demands on the input and output

filter capacitors. The input and output filter capacitors are

selected based on RMS current ratings and ripple voltage.

Select an input capacitor with a ripple current rating

greater than:

IRMS(PRI) =

VIN(MIN)

1– DCMAX

DCMAX

Continuing the example:

IRMS(PRI) =

29.5W

41V

1– 49.4%

49.4%

= 0.728A

Keep input capacitor series resistance (ESR) and

inductance (ESL) small, as they affect electromagnetic

interferencesuppression.Insomeinstances,highESRcan

alsoproducestabilityproblemsbecauseflybackconverters

exhibit a negative input resistance characteristic. Refer to

Application Note 19 for more information.

The output capacitor is sized to handle the ripple current

and to ensure acceptable output voltage ripple. The output

capacitor should have an RMS current rating greater than:

IRMS(SEC) =IOUT

DCMAX

1– DCMAX

Continuing the example:

IRMS(SEC) = 5.3A

49.4%

1– 49.4%

= 5.24A

This is calculated for each output in a multiple winding

application.

ESRandESLalongwithbulkcapacitancedirectlyaffectthe

output voltage ripple. The waveforms for a typical flyback

converter are illustrated in Figure 17.

The maximum acceptable ripple voltage (expressed as a

percentage of the output voltage) is used to establish a

starting point for the capacitor values. For the purpose of

simplicity, we will choose 2% for the maximum output

OUTPUT VOLTAGE

RIPPLE WAVEFORM

SECONDARY

CURRENT

PRIMARY

CURRENT

IPRI

∆VCOUT

4278 F17

RINGING

DUE TO ESL

IPRI

∆VESR

Figure 17. Typical Flyback Converter Waveforms

ripple, divided equally between the ESR step and the

charging/discharging

ΔV. This percentage ripple changes,

depending on the requirements of the application. You can

modify the following equations.

For a 1% contribution to the total ripple voltage, the ESR

of the output capacitor is determined by:

ESRCOUT ≤1%•

VOUT • 1–DCMAX

(

)

IOUT

The other 1% is due to the bulk C component, so use:

COUT ≥

IOUT

1% • VOUT • fOSC

In many applications, the output capacitor is created from

multiple capacitors to achieve desired voltage ripple,

reliability and cost goals. For example, a low ESR ceramic

capacitor can minimize the ESR step, while an electrolytic

capacitor satisfies the required bulk C.

Continuing our example, the output capacitor needs:

ESRCOUT ≤1%•

5V • 1– 49.4%

(

)

5.3A

= 4mΩ

COUT ≥

5.3A

1% • 5 • 200kHz

= 600µF

These electrical characteristics require paralleling several

low ESR capacitors possibly of mixed type.

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