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UC3845: Application

Sonu Jayachandran
Prodigy 140 points
Part Number: UC3845
Other Parts Discussed in Thread: TL431

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?

  • 0
    TI__Mastermind 31580 points
    Hi Sonu,

    In answer to your questions

    1. Customers use both configurations. The simpler solution for isolated designs is to use a TL431 on the secondary side which shunts current through the diode of the opto-coupler. On the primary side the FB is connected to ground which in the absence of any pull down on the COMP pin will drive COMP high. The opto-transistor collector is connected to comp and its emitter to ground and the TL431 provides regulation through the opto in this way. If the output voltage exceeds the setpoint the TL431 shunts more current through the opto diode and the opto transistor pulls the COMP pin to a lower voltage reducing the PWM duty cycle.

    2. In a forward converter the core must be reset before the next cycle. If a reset winding is used and the turns ratio between the reset winding and primary winding is 1 then the duty cycle is limited to 50%. It is also possible to design resonant reset forward converters where the duty can be greater than 50% and the transformer does not need a reset winding. Please take a look through for TI's library of technical documents on power supply design at www.ti.com/.../login.shtml

    3. In a forward converter the power stage will transition into CCM once the average load current exceeds half the peak to peak ripple current in the output inductor. If you design the output inductor to have 30% ripple for a 10A full load current then the peak to peak current will be 3Appk. Once the load current rises above 1.5A the design will be in CCM.

    4. There may be something in the TI library or the "Fundamentals of Power Electronics " by Robert Ericsson is a good reference book

    5. Again the TI library has papers on control loop theory that may be useful

    6. The peak current in the primary switch is the magnetizing current and the reflected output current. The power stage designer tool is useful to do the first pass calculations for power stage design and it also include control loop design. www.ti.com/.../powerstage-designer

    Regards

    Peter