FLYBACK CALCULTIONS - PART III

This post will calculate the current ripple factor from the peak current step IEDC,
the smaller ramp on top of the current pulse which we will call dl, and the RMS current. Fig. 1 shows the general topology of the converter.

First we calculate IEDC

IEDC = Pin / ( Vdcmin * Dmax )

Assume that Pin is 5.8 watts, Vdcmin is 50 volts and Dmax is 86% or 0.86. Calculating IEDC we get

IEDC = 0.135 Amp

This is the first component or major step of the peak current pulse.

The above calculation is the worst case value at maximum duty cycle, maximum input power required, and the worst case minimum input voltage. Note that you will get higher peak currents as D is reduced and Vdcmin is reduced for a given power input value.

We will also calculate the equivalent RMS current, which we will consider below.

Now we need to calculate the second component or ramp of the current pulse, dl

dl = Vdcmin * Dmax / ( Lm * Fs )

Assume that Lm is 0.007 Henry, and Fs is 100KHZ. Then


dl = 0.058 Amp


We can now calculate the Current Ripple Factor, KRF as

KRF = dl / ( 2 * IEDC ) = 0.215

The point of the above calculations is to determine whether the converter will operate in Discontinuous Current Mode (DCM) or in Continuous Current Mode (CCM). When KRF is less than unity, the converter is operating in CCM at minimum voltage input and maximum duty cycle with the specified maximum power input required.

With the results of the above calculations, we can also calculate the equivalent peak current in the transistor switch as

Idspeak = IEDC + ( dl / 2 ) = 0.164 Amp

Next we calculate the theoretical RMS current for the inductor and transistor from


Idsrms = ( 3 * IEDC^2 + dl^2 * ( Dmax / 12 ) )^0.5 = 0.234 watt


This is the current to use when calculating RMS current in the transistor and switching transformer.