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

3 Power Considerations

Flyback converters are often used in common household power adapters, meaning many of these converters must follow national and global power consumption standards. The most popular standard designers must follow is the DoE Level VI standard, which specifies maximum standby power consumption based on the normal output power.

Table 3-1 Standby Energy Requirements
Output PowerStandby Power
US DoE Level VI (≤ 49 W)< 100 mW
US DoE Level VI (50W to 249 W)< 210 mW
US DoE Level VI (> 249 W)< 500 mW

Minimizing the power dissipation of the shunt reference in the control loop is critical for meeting global power consumption standards. The power dissipation of the control loop can be calculated as the voltage being applied to this control system multiplied by the current flowing through. The voltage applied to the control system is the output voltage of the flyback converter, which is set to a desired value; therefore, minimizing the current through the control network is the key to reducing power dissipation.

GUID-20230801-SS0I-MBG2-PV9M-DBHFXHMKTQJJ-low.svgFigure 3-1 Current Through an SSR Feedback Network

Figure 3-1 shows that the total current that flows through the control loop is the sum of the Iq current through the resistor divider, the feedback current through the optocoupler diode, IFB(secondary), and the biasing current, Ibias.

Equation 4. Pdissipation=Vout×(IFB(secondary)+Ibias+Iq)

Equation 4 shows the formula for calculating the power dissipation of a feedback network with SSR (for the second biasing topology). Choosing an optocoupler with a higher CTR (current transfer ratio) allows the secondary side feedback current, IFB(secondary), to be reduced via a larger optocoupler resistance. Increasing the resistances on the resistor ladder while maintaining the same ratio can reduce Iq; however, this can come at the expense of increasing the output voltage reliance on Iref, which can vary widely under different conditions. The most common design choice for reducing power dissipation in the feedback network is to reduce the biasing current, Ibias. This biasing current can only be reduced to IKA(min) without negatively impacting the performance of the flyback converter. In applications requiring reduced power consumption, selecting a shunt reference with a lower minimum IKA (min) is the best practice.

Table 3-2 Device Critical Spec Comparison
DeviceBandgap ReferenceNominal Iref CurrentMinimum Cathode Current
TL431Vref = 2.495 VIref = 2 µAIKA(min) = 1 mA
TLVH432Vref = 1.24 VIref = 0.1 µAIKA(min) = 100 µA
ATL431Vref = 2.5 VIref = 30 nAIKA(min) = 35 µA
TL431LIVref = 2.495 VIref = 0.2 µAIKA(min) = 1 mA
ATL431LIVref = 2.5 VIref = 0.2 µAIKA(min) = 80 µA

Table 4-4 shows some of the key specs when selecting a shunt reference, where a lower IKA(min) allows for reduced power dissipation, and a lower nominal Iref allows for improved accuracy reliability at the flyback output. These specs make the ATL431 an excellent design for minimizing power dissipation to meet global regulations.