Part Number:UC3845
I am designing a 300W single switch forward DC-DC converter with reset winding. I am using UC3845 current mode PWM controller. I am confused with the following things:
1. In the datasheet of UCx84x current mode PWM controller it is said in feature description that: The error amplifier output, COMP, is frequently used as a control port for secondary-side regulation by using an external secondary-side adjustable voltage regulator, such as a TL431, to send an error signal across the secondary-to-primary isolation boundary through an optoisolator, in this configuration connect the COMP pin directly to the optoisolator feedback. On the primary side, the inverting input to the UCx48x error amplifier, VFB, should be connected to GROUND.
But in the typical application section it is said: When used in an off-line isolated application, the voltage feedback of the isolated output is accomplished using a secondary-side error amplifier and an adjustable voltage reference, such as the TL431. The error signal crosses the primary to secondary isolation boundary using an optoisolator whose collector is connected to the VREF pin and the emitter is connected to VFB. Kindly clarify this contradiction.
2. Can forward converter be operated for > 50% duty cycle? Is the 50% margin kept for resetting of transformer core?
3. How to find whether the forward converter is in DCM/CCM Mode. Is the calculation of critical inductance same as that of buck topology?
4. I am finding different methods for finding Power stage transfer function CCM current mode forward converter. Is there any standard document?
5. Is the open loop transfer function of the forward converter consists only of Control to Output function? How the Phase Margin and Gain margin of the open loop transfer function used to determine the compensator values?
6. How to find the exact peak current in the switch. Does it consist of only magnetizing current plus secondary reflected inductor current?
