One important thing to know about your flyback converter is whether it will mainly operate in continuous current mode (CCM) or discontinuous current mode (DCM.) For the highest power output per cubic inch, lowest noise, and lower peak currents, CCM is the choice (and most commercial converters are CCM.)
The advantage of DCM is that their is no right-half plane zero in the gain vs. frequency response which is difficult to compensate for. In the CCM condition there is a right-half zero so the bandwidth of the converter must be limited so that the zero does not cause instability.
The use of current limiting in modern converters helps to maintain stability and simplifies design of the feedback compensation network. We will discuss the feedback compensation and design of the feedback network in a later post.
Most practical flyback converters will operate in both CCM and DCM (but not at the same time) if conditions are allowed to put the converter into one mode or the other. Some converters are designed to operate right on the boundary between CCM and DCM.
You can find the operating point of your converter under operating conditions by calculation of the value of the minimum input voltage to maintain DCM which is also the maximum input to maintain CCM operation. This point depends on primary inductance, operating frequency, power required at the input, and the reflected voltage on the primary from the secondary, as well as other factors which we will ignore in this calculation:
Vdccm = ( ( 1 / ( 2 * Lp * Fs * Pin )) - ( 1 / VRO) )^-1
As a practical example, suppose that
VRO = 350 volts
Lp = 0.007 H
Fs = 100000HZ
Pin = 5.8 watts
Then
Vdccm = 126 volts approximately.
So 126 volts is the minimum input voltage to maintain DCM operation. A lower input voltage will force this converter design into CCM operation.
Or conversely, if the input voltage is higher than 126 volts, you cannot maintain CCm operation and you will be operating in DCM mode.
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