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Texas Instruments Designing with the ATL431LI Application notes
Application Report
SNVA850 – November 2018
Designing with the ATL431LI in Flyback Converters
Marcoo Zamora, Xiaodong Cai, Lakshmikant B R
ABSTRACT
Power adapters are ubiquitous given the increase in the number of electronic devices. This is especially
common in household appliances and portable electronics that each require a separate AC/DC power
supply adapter. With the increase in power adapters, the standby power in households has increased
dramatically to over 300kWh/yr. Currently there are several government organizations, such as the US
Department of Energy (DoE) and the EU Code of Conduct (CoC), that have implemented power
requirement standards to lower the power consumption of power supplies. This has forced power supply
manufacturers to change their designs to meet these new regulation standards for their respective
markets, but it is not uncommon to see universal power adapters that need to meet all the global
requirements. One common device in all flyback converters is the industry standard TL431LI precision
shunt regulator. The TL431LI is used in the feedback network to regulate the output voltage and it also
drives the power consumption of the feedback network. The need for low standby power has caused
designers to look for improvements in the TL431LI feedback network. One simple solution to this is to
replace the TL431LI with an ATL431LI, as this new device can operate at a lower IKA(min) which translates
to a lower system standby power. This application note will go over the flyback converter topology with
optocoupler feedback and will explain how the ATL431LI can help reduce the system standby power. In
addition to this, other possible design considerations, such as optocouplers and stability, will be discussed.
5
6
Contents
Shunt Regulators in Flyback Converters .................................................................................
Power Savings with the ATL431LI.........................................................................................
Flyback Power Breakdown Application Example ........................................................................
Feedback with ATL431LI Design Considerations .......................................................................
4.1
Optocoupler Considerations for Level VI and European CoC Power Supply Designs ....................
4.2
ATL431LI Bandwidth ...............................................................................................
4.3
ATL431LI Stability ..................................................................................................
Conclusion ....................................................................................................................
References ...................................................................................................................
1
Schematic Diagram for a Flyback Converter............................................................................. 2
2
Current Paths in the Secondary Control Loop ........................................................................... 3
3
Gain and Bandwidth for ATL431LI ........................................................................................ 5
4
Stability for TL431
5
Stability for ATL431LI
1
2
3
4
2
2
3
4
4
5
5
6
6
List of Figures
...........................................................................................................
.......................................................................................................
5
5
List of Tables
1
Example Specifications of a Flyback Power Adapter ................................................................... 3
2
Standby Energy Standard Requirements................................................................................. 3
3
Example TL431LI Based Flyback Converter Standby Power .......................................................... 4
4
ATL431LI Based Flyback Converter Standby Power ................................................................... 4
5
Typical '817' Optocoupler Suffix Decoder ................................................................................ 5
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1
Shunt Regulators in Flyback Converters
1
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Shunt Regulators in Flyback Converters
The flyback converter is the most popular switched mode power supply topology, as the inductor is split to
form the transformer and it offers isolation. This design is very common in all power adapters that need to
convert high voltage AC to an isolated DC for power electronic devices at high efficiency.
VOUT
VIN AC
VDD
VPC
VDD
UCC28740
PWM Controller
VS
VSC
HV
UCC24636
SR Controller
DRV
DRV
FB
TBLK
CS
GND
TL431LI
Copyright © 2018, Texas Instruments Incorporated
Figure 1. Schematic Diagram for a Flyback Converter
Figure 1 shows the simplified basic flyback converter that uses a shunt regulator such as TL431LI or
ATL431LI. The primary purpose of the A/TL431LI is to help in the regulation of the control loop due to its
internal error amplifier and internal voltage reference. By using resistors R1 and R2, the A/TL431LI device
sets the output voltage of the loop through negative feedback. The A/TL431LI acts as a voltage to current
controller to drive the optocoupler. It is important to design a control loop that is capable of driving the
feedback pin of the PWM controller, such as UCC28740, over its complete dynamic operating range.
2
Power Savings with the ATL431LI
The TL431LI is the most commonly used shunt regulator for flyback converters due its low cost and high
accuracy. The TL431LI does require at least 1mA of cathode current (Ika) for proper operation but
generally the TL431 is biased at a higher cathode current than 1mA due to an improvement in stability.
This is a design challenge because the optocoupler in the feedback loop does not require as much current
as the TL431LI to operate to its full range, thus generating a significant amount of wasted power.
The ATL431LI aims to lower the power required by the feedback loop by replacing the TL431LI. The
ATL431LI has a much lower IKA(min) requirement of 80uA compared to the TL431LI that allows for a lower
biasing point of Ika for lower power consumption.
2
Designing with the ATL431LI in Flyback Converters
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Flyback Power Breakdown Application Example
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3
Flyback Power Breakdown Application Example
VOUT
Rs
Iq
Iq
IKA
R1
IREF
TL431LI /
ATL431LI
R2
Copyright © 2018, Texas Instruments Incorporated
Figure 2. Current Paths in the Secondary Control Loop
Figure 2 shows the output voltage regulation by either the ATL431LI or TL431LI and the optocoupler,
which offers feedback to the isolated primary side. In this simplified flyback converter, the IKA bias current
is limited by the value of Rs which is used to bias the optocoupler and A/TL431LI. When the system is on
standby, the IKA and current going through the feedback resistors are not reduced and heavily count
towards the standby system Iq.
Table 1. Example Specifications of a Flyback Power Adapter
System Specifications
Output Voltage
20V
Output Current
2.25A
Total Output Power
45W
Table 2. Standby Energy Standard Requirements
Output Power
Standby Power
US DoE Level VI (≤ 49 W)
< 100 mW
US DoE Level VI (50 W to 249 W)
< 210 mW
US DoE Level VI (> 249 W)
< 500 mW
EU CoC Tier 2 (≤ 49 W)
< 75 mW
EU CoC Tier 2 (> 50 W)
< 150 mW
Table 1 shows example system requirements of a system under a no load condition based on Table 2. It
is important to be aware of the current standby power requirements in countries such as USA and in
Europe. The USA requires that all power adapters must meet the DoE Level VI standby power
requirements and currently in Europe the CoC tier 1 are in the process of being updated to the CoC tier 2.
Because most manufactures want to ensure universally-compliant power adapters, this example will focus
on the highest standard, the CoC tier 2.
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Feedback with ATL431LI Design Considerations
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Table 3. Example TL431LI Based Flyback Converter Standby Power
Specifications
UCC28740
TL431LI
MOSFET
UCC24636 SR
Other Components
Power (mW)
15
40
15
2.2
30
Total Power (mw)
102.2
Let’s take an example of a flyback converter using TL431LI. The systems standby power breakdown is
shown in Table 3. This example assumes a system such as the one shown in Figure 1 and Figure 2,
where there is a PWM controller, such as the low power UCC28740 and UCC24636, to control the high
speed switching. Other power loss components consist of leakage current from passives that are used in
the flyback converter. In this example, the TL431LI is biased above 1 mA to allow for the maximum gain
and a boost in phase margin for better performance. During standby, the bias current is also larger
because during standby the voltage across the TL431LI decreases which increases the current through
the optocoupler. In such a system the standby power of the loop cannot meet tier 2 or level VI and there is
no margin for leaky passive component power losses.
Table 4. ATL431LI Based Flyback Converter Standby Power
Specifications
UCC28740
ATL431LI
MOSFET
UCC24636 SR
Other Components
Power (mW)
15
4
15
2.2
30
Total Power (mw)
66.2
In Table 4, the ATL431LI was used to replace the TL431LI and Rs was increased to allow for a lower Ika
while keeping the other system devices constant. In Table 3, the standby power for the ATL431LI is 4 mW
which is a drastic improvement over the 40 mW used in the TL431LI. Further improvements in standby
power can be done by choosing a lower Idd optocoupler with higher current transfer ratio (CTR) or better
passive components to allow for a larger margin for leaks. In this example, the ATL431LI-based system
can meet both Level VI and CoC tier 2 as shown in the total power row in Table 4.
4
Feedback with ATL431LI Design Considerations
When designing a flyback converter with the ATL431LI, the are external components and devices such as
the optocoupler and compensation network will affect the stability and loop gain of the system. In the case
of the optocoupler, the CTR and response time of the optocoupler must be changed to improve the
frequency response and standby current with the ATL431LI. Another design choise is the stability and
bandwidth of the ATL431LI as flyback converters typically require a compensation network to stabilize the
shunt regulator. The ATL431LI has changes and improvements in the bandwidth and stability compared to
other TL431-like devices that a designer can use for optimization. Due to this, the compensation network
might require modification. Compensation network design is beyond the scope of this document, for more
information on compensation networks check out SLUA671.
4.1
Optocoupler Considerations for Level VI and European CoC Power Supply Designs
With the ATL431LI's reduced Ika(min) of 80uA it is also important to ensure the optocoupler will have
sufficient CTR without sacrificing the optocoupler speed. This typically makes ATL431LI-like devices be
biased with an additional current margin for a higher CTR but which increases leakage current. One way
to minimize the leakage current is to choose the appropriate optocoupler. One such example of
optocoupler selection would be optocouplers with “817” in their part number, which corresponds to pin-topin compatible optocouplers. Table 5 shows an example of 817 devices with different CTR ranges
denoted by a single letter suffix. In TL431LI designs a '817' optocoupler with the suffix "A" or "B" can be
suitable for its IKA but for the lower ATL431LI IKA a 'C' or D' can allow for lower leakage. For Class VI and
European CoC power supply designs, one must select an optocoupler with high CTR while satisfying the
feedback voltage requirement for the PWM controller in the flyback system. For further details on how to
select and properly bias an optocoupler in a flyback system, please refer to this article for more
information.
4
Designing with the ATL431LI in Flyback Converters
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Table 5. Typical '817' Optocoupler Suffix Decoder
4.2
Part No. Suffix
CTR Min
CTR Max
A
80%
160%
B
130%
260%
C
200%
400%
D
300%
600%
None
80%
600%
ATL431LI Bandwidth
75
200
60
160
45
120
30
80
15
Phase - q
AV - Small-Signal Voltage Amplification - dB
The ATL431LI offers lower Iq, greater stability, and still offers a high unity gain bandwidth of 2MHz. The
high unity gain bandwidth allows for greater flexibility when designing flyback converter compensation
networks to achieve higher efficiency. This is because the typical bandwidth of a flyback converter is kept
around 10kHz, which is usually much less than the bandwidth of the ATL431LI. Lower bandwidth shunt
regulators can restrict certain designs. This is because optocouplers normally have a low frequency pole
due to parasitic capacitance at 2-5kHz, which limits the mid-band gain of the system and cuts off high
frequencies. Having a flat and high gain bandwidth from the ATL431LI, as shown in Figure 3, allows
designers to achieve higher system cutoff frequencies that allow for a larger mid-band gain. A larger midband gain will help the dynamic response of the system and allow higher frequency power supplies.
40
AV
Phase
0
100
1k
10k
100k
f - Frequency - Hz
1M
0
10M
D000
Figure 3. Gain and Bandwidth for ATL431LI
4.3
ATL431LI Stability
In a closed loop configuration such as in Figure 2, the ATL431LI and industry standard TL431 form a
feedback loop between its cathode and reference pin. One design challenge is that this feedback loop is
susceptible to instability depending on the output load capacitance and sink current. In Figure 4 the region
of instability occurs due to the shift of the dominant pole from the addition of an external capacitor load.
This is because there are certain bias conditions that the dominant pole changes from being an internal
pole to an output pole. In the ATL431LI, the instability region was reduced compared to the TL431 which
allows for a greater selection of load capacitor as shown in Figure 5.
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5
Conclusion
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90
I KA − Cathode Current − mA
80
A VKA
B V KA
C VKA
D VKA
15
= Vref
=5V
= 10 V
= 15 Vf
B
70
A VKA = Vref
B VKA = 5 V
C VKA = 10 V
13
TA = 25°C
60
C
Stable
Stable
50
A
40
A
30
D
20
IKA - Cathode Current - mA
100
Stable Region
11
9
7
5
3
B
10
0
0.001
0.01
0.1
10
1
1
0.001
0.01
0.1
1
CL - Load Capacitance - µF
CL − Load Capacitance − µF
Figure 4. Stability for TL431
5
10
ATL4
Figure 5. Stability for ATL431LI
Conclusion
The ATL431LI is a lower quiescent current device compared to the TL431LI due to the ATL431LI's lower
Ika(min) of 80μA. This is a benefit for systems due to the TL431LI having a quiescent current of 1mA, which
can be larger than other more expensive components. Systems where the power budget is critical, such
as 20V power adapters that need to meet power standards like the DoE Level VI or European CoC, the
ATL431LI’s Ika(min) of 80uA can offer a substantial power savings. In the simplified example provided by this
application note, the ATL431LI is able to lower the Iq of the system by 40 mW. Additionally the ATL431LI
has a large bandwidth that makes it versatile for optocoupler compensation network designs. The stability
of the ATL431LI was also optimized over the industry standard TL431, which allows for the usage of more
common capacitor values at lower bias currents.
6
References
•
•
•
•
•
•
•
•
6
Designing with the Improved TL431LI SNOAA00
ATL43xLIx High Bandwidth Low-Iq Programmable Shunt Regulator SLVSDU6
TL43xLIx Programmable Shunt Regulator with Optimized Reference Current SLVSDQ6
Compensation Design With TL431 for UCC28600 SLUA671
“Make sure your optocoupler is properly biased” Brian King, 10/25/2017
https://www.edn.com/electronics-blogs/power-tips-/4459005/Make-sure-your-optocoupler-is-properlybiased
Low-cost flyback solutions for 10-mW standby power SLYT557
MOSFET power losses and how they affect power-supply efficiency SLY664
Improving Power Supply Efficiency – The Global Perspective
http://www.ti.com/download/trng/docs/seminar/Topic1BM.pdf
Designing with the ATL431LI in Flyback Converters
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