FLYBACK CALCULATIONS - PART VI

In this part we will calculate the RMS current ripple in the main output filter capacitor.

First we need to calculate the main output secondary current using the following data:

Idrms = 0.2 A
Dmax = 0.85
VRO = 350 volts
KL = 1.0
VF1 = 1.0 volt
Vo1 = 14 volts

where Idrms is the rms transistor switch current, Dmax is the maximum allowable duty cycle (usually limited by the main PWM control IC), VRO is the secondary voltage reflected to the primary side, Vo1 is the nominal output voltage, VF1 is the forward voltage of the output diode, and KL is the load occupying factor for the main output (1.0 if there is only one output winding.)

The equation for the secondary RMS current is

Isecrms = Idsrms * (((1 - Dmax) / Dmax )^0.5) * VRO * KL / (Vo1 + VF1)

Isecrms = 1.96 A

The output current Io is specified as 0.3 amps, so now we can calculate the output capacitor ripple current. This current is usually inconveniently large for flyback converters and cheap converters are sometimes found to have an output filter capacitor that is under-rated for ripple current. The risk is that the capacitor will overheat, dry-out too fast and the converter will fail to operate as intended. The ripple current is calculated as follows:


Icaprms = ( Isecrms^2 - Io^2 )^0.5

Icaprms = 2.28 A

Note that a continuous current mode (CCM) converter will have a lower secondary RMS current for a given output current load.

So we will need to have a capacitor that is rated at 2.3 amps or better for this design. It is better to use one large capacitor instead of smaller capacitors in parallel to meet the current requirement as parallel capacitors will not necessarily share ripple current equally and one or more of them could overheat. It is usually good though to put small capacitors in parallel such as ceramic chip caps for filtering high frequency noise.

Assume that we have the additional data as follows:


Co = 330UFD

Rc = 0.07 ohm

Idspeak = 0.3 A

Fs = 100KHZ

where Co is the output filter capacitance total, Rc is the equivalent series resistance of the capacitance, Idspeak is the peak transistor switch current, and Fs is the converter switching frequency.

Now we can calculate the theoretical output ripple or noise voltage using the equation

dVo1 = Io * Dmax / (Co * Fs ) + Idspeak * VRO * Rc * KL / (Vo1 + VF1 )

dVo1 = 0.498 volts

On an output voltage of 14 volts, 0.498 volts is about 3.6%. This value is probably OK for a battery charger converter, but probably too high if the voltage is to be used to power some electronic circuit. For a lower output ripple voltage, it would be necessary to increase capacitance value, reduce the equivalent series resistance of the capacitor, or both. Notice that the above calculation ignores the effects of the capacitor series inductance and the inductance of the circuit wires or circuit board traces. A low inductance layout is very important to keep noise low.