SLLA618 October   2023 ATL431 , ATL431LI , TL431 , TL431LI , TLVH432

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Designing for SSR With a Shunt Reference
    1. 2.1 Setting the Output Voltage
    2. 2.2 Biasing a Shunt Reference
    3. 2.3 Designing for Transient Response
  6. 3Power Considerations
  7. 4Methodology
    1. 4.1 Shunt Reference Implementation
    2. 4.2 Accuracy Comparison
    3. 4.3 Power Consumption Comparison
    4. 4.4 Transient Response Comparison
  8. 5Results
  9. 6Summary
  10. 7References

2.1 Setting the Output Voltage

The three-terminal shunt references that this paper focuses on shunt current through their cathode pins, and this current increases dramatically as the voltage at the Vref pins exceeds the internal voltage reference. This allows these shunt references to act as error amplifiers. An output voltage can be programmed using a resistor divider to set Vref equal to the internal reference voltage. By doing this, the shunt reference cathode current, IKA, rapidly increases as soon as the output voltage, Vout, exceeds the programmed value or decreases as Vout falls below the programmed voltage. This current being shunted to ground through the shunt reference passes through an optocoupler, acting as a feedback signal. Because the flyback feedback loop requires a continuous signal to be sent through an optocoupler via a current, the shunt reference can be partially on so the current can sink through. Figure 2-2 provides some insight into the workings of a shunt reference by demonstrating that when Vout is at the programmed value, REF is equivalent to the internal Vref, which causes some feedback current to flow through the cathode from the optocoupler's internal LED.

GUID-20230801-SS0I-5TDL-CVGB-PMWQNMXDWR10-low.svgFigure 2-2 Basic Shunt Reference Configuration with Optocoupler
Equation 1. Vout=VREF×1+R1R2+R1×Iref

The formula to set the output voltage is shown in Equation 1, where Vref is the internal voltage reference of the shunt reference, Iref is a small current being sunk through the REF pin, and the resistors R1 and R2 are used as a resistor divider to program the output. These parameters vary from device to device, and designers can use this formula to set the output voltage of their SSR flyback converters.

Equation 1 is used to solve for the programmed output voltage of the flyback converter; however, each parameter in this formula has a tolerance, which is determined by the accuracy grade of that component. Therefore, the accuracy grade of the internal voltage reference of the shunt reference and the accuracy grades of resistors R1 and R2 directly impact the accuracy of the flyback converters' output voltage. While the initial tolerance of Vref can be selected as low as 0.5%, Iref can vary largely with temperature. The effects of Iref on the accuracy can be minimized by selecting a device with a lower nominal Iref value.