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UCC2801: Regulation Principles of a Isolated Flyback in Peak Currrent Mode

Oskars Jascenoks
Prodigy 60 points
Part Number: UCC2801
Other Parts Discussed in Thread: TL431, TLV431

Dear TI Community,

I am currently designing an isolated flyback using the UCC2801 Controller IC. I have some general questions on how the regulation of a isolated flyback in peak current mode is operating.

I looked at the Functional Block Diagram (§9.2) and the typical circuit Diagram (§10.2) in the datasheet. I made following  conclusions and wanted to ask if I get it wright. Please correct if I am wrong with something.

1. The exact control of the duty cycle is made by the primary current of the transformer. When the voltage on the CS Pin reaches 1V the OUT pin is switched off until the next cycle begins.  

2. The maximum primary current must must be set so that the output voltage can be reached at the minimal input voltage @ maximum output load.

3. The regulation via the FB Pin gets active only when the Output voltage ist too high. As long the Output voltage is smaller than the nominal, the FB Pin and the error amplifier has no effect on the duty cycle.

4. If the Output voltage gets too high, the TL431 turns on the optocoupler LED and the voltage across the resistor Reg rises also above 2.5V. Due to the inverting configuration of the error amplifier the FB Pin is Held at 2.5V and the Output of the error amplifier( COMP pin) gets lower. Vcomp minus one voltage drop is then devided by 1.65 and fed to the PWM comparator. If this voltage is smaller than 1V, the OUT is switched off (overrides the voltage at PIN CS).

If I get it right, the voltage regulation can only decrease the output voltage by switching the OUT Pin off before the maximal primary current is reached. When the Output voltage falls below the nominal the voltage regulations is switched off again and the current regulation drives the output voltage upwards and so on and so on....

One point I am struggling with is the regulation range of the classical TL(V)431 isolated feedback. I understand it this way:

1. When the reference voltage reaches 2.5 (1.24) V, the TL(V)431 switches the optocoupler LED on. A more or less constant current flows through the LED and is transferred to the primary side where it is again converted to voltage and fed to the FB pin for the voltage regulation. TL(V)431 switches on very sharply (simplified no linear reagion, just on or off). So if I get it right, it says only "Voltage too high" or "volatge to low" and not how much too high or how much  too low. Or am i mixing something up?

One last question: does the TL(V)431 have a hysteresis? The simulation model I am using (TLV431 spice model from TI) is turning on at 1.24V and turning off at 1.19V. Converted back to output voltage scale (5V in my scale) this corresponds to 5V on and 4.8V off. This hyteresis combined with the reaction time of the UCC2801 current and voltage loops I am struggling to get a stable output voltage. At least in simulation.

I know there is a lot of literature on flyback design and isolated feedback loops out there. I wanted to be sure I understand everything right in the times base before I start with frequency compensation etc.

Best Regards,

Oskars

  • 0
    TI__Mastermind 31580 points
    Hi Oskar,

    I think you have a pretty good understanding of the basics of the control loop. If the output voltage is below the regulation value the controller will move to maximum duty cycle in an attempt to bring the loop back into regulation. It depends on the loading condition and the amount of power been transferred whilst the controller is operating in this condition whether or not the current hits the current limit value.

    The TL431 behavior you describe is the open loop behavior where the device stops and starts shunting current as the voltage on its reference input varies about the reference voltage level. For stable control loop behavior a series RC network is place between the cathode and reference pin to limit the rate of change of voltage on the cathode.

    There are some good references in the past articles from the TI power supply design seminars which can be found here.

    www.ti.com/.../login.shtml

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 60 points

    Dear Peter,

    Thank you for the hint with the feedback for the TL431. I used some dummy default values in the simulation. The feedback was not deep enough and the TL431 went into saturation thus breaking the feedback loop. I readjusted the values and now it working fine in the simulation. Time to built first prototype.

    Best Regards,

    Oskars