AC-DC FLYBACK CALCULATIONS - PART V

One of the most important things to determine on the AC-DC flyback converter are the RMS secondary winding currents. The RMS current must be filtered by a capacitor which is usually an aluminum electrolytic capacitor or some type of large capacitance high ripple current capable capacitor. One of the main failure modes of poorly designed flyback converters is an inadequate current rating for the filter capacitors. If the output capacitor is over-stressed with ripple current beyond its ratings it will run hot, dry out if it is a wet electrolytic, and otherwise fail casuing converter failure.

We can calculate the RMS current for each secondary winding according to a simple formula as follows:

Isecrms = Idsrms * (( 1 - Dmax ) / Dmax )^0.5 * VRO * KL / ( Vo1 + Vf1 )

where we have previously calculated the rms switch current Idsrms = 0.234 A, VRO = 349 volts, and we have specified Vo1 = 14 volts, Vf1 = 1 volts, and the load occupying factor KL has been set to 0.8 or 80% of the total load on the Vo1 output.

The answer is then that Isecrms = 1.8 amps. This is not a trival amount of ripple current to deal with and it will require a large electroylitic capacitor or several capacitors in parallel to handle the current. Note that it is difficult to parallel capaciotrs to handle the total load because one capacitor will more than likely try to handle most of the load and overheat while the other ones are not handling their share.

Another thing to be concerned about in flyback converters is the rating of the diode that feeds current to the filter capacitor from the secondary winding (see Fig. 1 -click on the figure for a larger view.) The current rating of the diode must be able to handle the RMS current we just calculated above and it must have a high enough voltage rating to handle the reverse spikes which are present on every switch cycle of the converter. We can calculate the reverse voltage on the diode from the following formula:

Vdr = Vo1 + Vdcmax * ( Vo1 + Vf1 ) / VRO

where Vo1 is the output DC voltage, 14V, Vdcmax is the max input voltage, 1000V,
Vf1 is the forward diode voltage, 1 volt, and VRO is the voltage reflected from secondary to primary on each switch cycle, 349 volts.

Vdr = 56.9 volts

is the peak reverse voltage on the diode.

It is customary to comput the VRRM (the voltage rating reverse maximum value) as

VRRM = 1.3 * Vdr = 74 volts

The VRRM value we computed is beyond the normal max for schottky diodes of 45 volts but there are some hybrid schottkys that will rate higher. The advantage of schottky diodes is that they have a lower forward voltage drop and therefor dissipate less heat and power for a given current load. If the output voltage had been 5 volts instead of 14, we could then have used a 35V or 45V schottky diode as VRRM would have calculated to 29 volts.