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Texas Instruments Minimize Standby Consumption for UCC287XX Family Application notes
Application Report
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Minimize Standby Consumption for UCC287xx Family
Sonal Singh
ABSTRACT
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Contents
Introduction ..................................................................................................................
Where Does Standby Power Go? ........................................................................................
Relationship Between Standby Power and Design Parameters .......................................................
Design Example..............................................................................................................
UCC287xx Progression Towards Standby ...............................................................................
Conclusion ....................................................................................................................
References ...................................................................................................................
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List of Figures
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Typical ADDC Flyback Application ........................................................................................ 2
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Control Law Profile of the UCC28730
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....................................................................................
Block Diagram of TIDA-01560 .............................................................................................
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List of Tables
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UCC287xx Standby Current Feature...................................................................................... 3
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Feature Description of UCC287xx Family ................................................................................ 3
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
Any appliance that is constantly plugged in such as washing machines, microwaves, and coffee makers,
requires either a mechanical disconnect switch or dissipates power in standby condition. In end
applications, the microcontroller is surrounded by peripheral circuits, like display or other similar units, to
make user interface easier. These circuits act like a resistive circuit and consume some energy during
standby. The power consumption of this standby load can range from a couple to few hundreds of mW,
depending on the application. It is becoming a growing trend to keep these standby losses to zero power
levels in next-generation devices. IEC 62301:2011: clause 4.5 specifies the standby power less than 5
mW as “Zero Standby”. Any appliance that adheres to this can achieve a zero power label.
International Electrotechnical Commission (IEC) is a standard international organization that publishes
international standards for various electrical and electronic equipment performances like power
generation, transmission, and distribution to home appliances. IEC 62301 “Household Electrical
Appliances – Measurement of Standby Power” standardizes measurement methods of standby power in
various appliances and electronic equipment. IEC 62301:2011 specifies measurement of electrical power
in low power modes (network mode), also called standby.
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Where Does Standby Power Go?
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Where Does Standby Power Go?
Figure 1. Typical ADDC Flyback Application
The standby power dissipation can be broken down into the following areas as highlighted in Figure 1:
2.1
X-cap Discharge
The filter capacitor known as “X-cap” (Cx in Figure 1) is connected between the live and neutral in order to
help filter out the differential mode noise from the power supply. According to the IEC 62368 standard
para 5.5.2.3, the circuit connected to mains supply should have a means of discharge, resulting in a time
constant not exceeding two seconds. This calculates the time constant of the discharge period based on
the X-cap capacitance and resistive values:
5X u &X
where
•
•
•
Cx is the capacitance of the X-cap
Rx is the discharging resistance
Pdis is the power dissipation across the X-cap
(1)
You can assume that some power would be constantly dissipated by this discharging resistor. The power
dissipation can be calculated as:
VAC _ rms _ max 2
Pdis
RX
(2)
This Pdis is very significant during the no-load condition, especially at high line. Practically, lower discharge
resistance is associated with a higher X-cap value, so there is a trade-off between the standby power and
EMI performance.
The UCC287xx family operates in discontinuous conduction mode with valley switching in order to
minimize the switching losses of the MOSFET and improve the efficiency. These controllers are designed
to detect the resonant ring during the dead time, and the MOSFET is turned on at the valley which is the
falling edge of this resonant ring, allowing for a higher frequency operation and a smaller input EMI filter.
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Where Does Standby Power Go?
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2.2
IC Bias Power
The UCC287xx family enables the controller to operate with a very low IC bias current during standby
(Iwait) that enables low standby power. The controller current consumption is device specific. For
example, the UCC28730 controller has a very low bias current of ~50 µA during standby conditions, which
varies for different members of this family. The control law feature enables the UCC28730 controller to
vary the switching frequency at very light to no-load condition down to 28 Hz. This is called a wait-state,
enabling the power consumption of the controller to be <1 mW.
Table 1 shows the IC bias current (Iwait) comparison of various UCC287xx members.
Table 1. UCC287xx Standby Current Feature
IC BIAS CURRENT AT STANDBY (µA)
UCC287xx CONTROLLER
Typ
Max
UCC2870X
85
110
UCC2871X
95
120
UCC2872X
95
150
UCC28730
52
75
UCC28740
95
125
UCC28742
80
115
If standby is a primary concern, then choosing a controller with a lower bias current spec adds a major
limit to the IC power consumption during standby.
2.3
Startup Resistor Loss
The start-up resistor is required to provide for the initial charging current across the VDD capacitor to
supply for an active source of start-up charging current. The Vdd capacitor is required to provide the initial
charge until the auxiliary bias gets high enough to maintain regulation. The value of the start-up resistor
depends on the UVLO start threshold and the bias current of the controller. The power dissipation across
this resistor can be calculated as:
VAC _ rms _ max 2
Pstrup
R strup
where
•
•
Pstrup is the power loss across the startup resistors
Rstrup is the resistances required to provide sufficient current to charge the Vdd cap
(3)
To eliminate this loss, some members of the UCC287xx family have an integrated high voltage start-up
circuit built in as referred to in Table 2. The HV pin connects directly to the bulk capacitor to provide startup current to the VDD capacitor. The typical start-up current is approximately 250 µA, which provides fast
charging of the VDD capacitor. Once the controller is past the start-up state and is running, no current is
drawn from the bias cap and the internal FET is disabled, thus enabling the application to save the power
loss across it. Choosing a controller with integrated high voltage start-up can help limit this power
dissipation. Figure 2 has a tabular comparison of various UCC287xx family members against the high
voltage start-up feature.
Table 2. Feature Description of UCC287xx Family
UCC287xx CONTROLLER
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INTEGRATED HV START-UP
UCC28700
No
UCC2871X
Yes
UCC28720
Yes
UCC28722
No
UCC28730
Yes
UCC28740
Yes
UCC28742
No
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Where Does Standby Power Go?
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According to the IEC 62368 Para 5.5.2.3: Safety Regulations against Capacitor discharge, for any power
supply where a capacitor voltage is or becomes accessible upon disconnection from connector, the circuit
should be provided with a means of discharge within 2 sec, complying with ES1 limits. The start-up
resistor is required to ensure enough current to charge the VDD capacitor at low-line condition in order to
achieve a certain start-up time (<2 s). Typically, the Rstrup value ranges from 13 MΩ to 20 MΩ.
2.4
Bulk Cap Leakage
The bulk capacitor dissipation is considered in regards to the leakage current: Ileak, which is dependent on
input voltage, temperature, and operation time. The leakage current becomes practically insignificant after
one to five minutes of applying the rectified voltage.
In accordance to the EN130300, the leakage current measured after five minutes should be less than:
I leak
0.3 u VAC _ rms _ max u C bulk
0.7
4 $
(4)
The leakage power dissipation can be calculated as the energy dissipated over time:
Pleakage VAC _ rms _ max u I leak
(5)
It is recommended to choose a capacitor with low leakage current rating in order to minimize the standby
power loss.
2.5
Pre-load Resistor Loss
In order to stabilize the power supply during no-load condition, it is required to have a certain load that
equalizes the power consumption between the auxiliary winding and secondary winding. A lower bias
standby current 52 µA of the UCC287xx controller helps reduce the pre-load resistor value. The power
dissipated in the pre-load resistor can be estimated as:
Vout 2
Ppre load
R PL
where
•
•
RPL is the pre load resistor value
Ppre-load is the power loss across the pre load resistors
(6)
In order to maintain stable operation, the output load current should be greater than the auxiliary load
current or the IC bias current.
2.6
Control Law
The UCC287xx family has implemented a control law function in order to improve the efficiency and
standby power, where the IC switches between the frequency modulation and amplitude modulation,
depending on the output load. This enables the controller to vary the operating frequency based on the
line and load conditions. This flexibility helps the device to achieve better light load efficiency and low
standby power.
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Figure 2. Control Law Profile of the UCC28730
As the output load (x-axis) goes towards zero, the switching frequency (y-axis) can be seen moving from
83.3 kHz to 32 Hz. Along with this, the primary peak current also reduces by a 3-to-1 ratio, also referred to
as the “Kam” ratio in the data sheet.
The maximum and minimum input power can be specified by Equation 7 and Equation 8:
1
Pin _ max
L pri u I pk _ max 2 u f SW _ max
2
(7)
and
Pin _ min
1
L pri u I pk _ min 2 u f SW _ min
2
where
•
•
•
•
Pin_max/min is the power consumption of the power stage
Lpri is the primary inductance
Ipk_max/min is the primary peak current
fsw_max/min is the switching frequency of the controller
(8)
Equation 7 and Equation 8 estimate the standby power consumption by looking at the ratio of the min to
max input power for the UCC28730 controller.
Note that this calculation assumes an ideal operating condition and the losses calculated are just across
the control law profile of the UCC28730 controller.
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Relationship Between Standby Power and Design Parameters
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Relationship Between Standby Power and Design Parameters
While all these features that were previously described can help limit the standby power losses, they are
heavily dependent on the choice of controller. In order to resolve this challenge, every data sheet has a
standby power number on the first page which is only applicable for a specific operating condition.
The application design specification plays a very critical role in determining the standby power. Equation 9
can be referred to for calculating the estimated standby power:
V out u I out u f min
Pstdby
n sb u K am 2 u f max
where
•
•
•
•
Kam is the ratio of maximum to minimum primary current peak amplitude, 2.99 V/V typical
nsb is the estimated internal pre-load power efficiency of the flyback converter when its output power is zero.
An estimation 50%–70% can be used at beginning of a design.
fmin is the actual minimum switching frequency of the converter, it can be estimated at three to four times fsw
(min)
fmax is the full load maximum switching frequency of the converter, referred from the electrical characteristics
(9)
This is assuming the load current during standby condition is greater than the auxiliary or the controller
bias current for stable operation.
The parameter nsb represents any associated losses top either the internal consumption of the controller or
the power dissipated across the pre-load on the secondary side of the transformer or the bias
consumption of the Optocoupler, and so forth.
nsb is the transformer no-load conversion efficiency. This factor is dependent on the bias power
consumption of the flyback controller and the pre-load resistance for that design. Typically, the transformer
no-load conversion efficiency lies within the range of 0.5 to 0.7. The UCC28730 allows extremely low bias
current (52 µA typ), improving the ratio of the internal bias power to the converted power.
The pre-load resistance is dependent on the current consumption of the controller IC during standby
condition. The higher the bias current the bigger resistance is required to balance out the currents
between the transformer windings.
The UCC287xx family devices with primary side regulation (for example, the UCC28730) eliminates the
need of an opto coupler which further improves the converter efficiency.
Considering all these advantages from choosing the UCC28730, you can assume a higher factor of 0.6 to
0.7 for an initial estimate.
The UCC287xx family allows the converter to minimize the standby losses by minimizing the switching
frequency at no load conditions and lower bias power, therefore, smaller secondary to aux turns ratio.
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Design Example
The TIDA-01560 is an example where the UCC28730 can achieve less than 4 mW standby power at high
line. This reference design is an example of how the UCC28730 in conjugation with the UCC24650 can
provide ultra-low standby power without sacrificing start-up time or output transient response. The
UCC28730 uses the following in its control algorithm to maximize efficiency over the entire operating
range:
• Frequency modulation
• Peak primary current modulation
• Valley switching
• Valley skipping
This design is a 15 W bias supply with two isolated outputs (12 V/1.125 A and 3.3 V/0.3 A). The average
standby power for this design is:
Pout u f min
Pstdby
n sb u K am 2 u f max
(10)
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Pstdby _ theo
15 W u 4 u 32 Hz
0.7 u 2.99 2 u 83.3 kHz
3.68 mW
(11)
NOTE: Equation 11 is an estimation of the standby losses and calculates the minimum loss this
design can achieve without any additional circuitry. Practically, the losses across the X-cap
and bulk caps are dependent on line condition and you might see some deviation from the
theoretical value at high line.
The practical results in the following table can be seen aligning with the theoretical values.
Vin_ac
STANDBY POWER (mW)
115 V
3.1
220 V
4.2
Figure 3 shows the block diagram of the TIDA-01560 reference design. The main parts of this reference
design are the isolated-flyback power supply controller (UCC28730), voltage monitor (UCC24650) as a
wake-up device, and next-generation, low-dropout regulators (TLV74333).
Figure 3. Block Diagram of TIDA-01560
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UCC287xx Progression Towards Standby
All the members of the UCC287xx family are different in terms of factors like bias power consumption,
switching frequency, integrated start-up, and so forth. Hence, the standby performance differs from each
other. The following table shows the progression of achievable standby power for the various controllers of
the UCC287xx family. The standby measurement comparisons are for a universal input 5 Vout/2 A flyback
power supply design.
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Conclusion
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INPUT VOLTAGE
115 Vac
Regulation
PSR
SSR
6
TI controller
230 Vac
Standby Power (mW)
Design Out: 5 V/2 A
UCC28730
2.7
3.2
UCC28730EVM-552
UCC28710
14
16
PMP9202
UCC28700
24
35
PMP4351
UCC28704
45
63
PMP11600
UCC28742
30
40
PMP40487
UCC28740
57
64
PMP9204
Conclusion
1. Choose a suitable X-cap to optimize the standby consumption losses and balance the EMI
performance.
2. Try to optimize the controller losses by choosing a UCC287xx member with lower standby bias current,
thereby limiting pre load resistor losses.
3. Try to eliminate the losses on the start-up resistor by choosing a UCC287xx member with integrated
HV start-up.
4. Appropriate bulk capacitors with relatively low leakage current specification should be used. Beware of
the increased cost.
Following these suggestions help limit the standby power consumption losses by either reducing the loss
on each component or eliminating the need of it.
NOTE: The transformer design single-handedly has a significant impact on the power stage efficiency and
EMI performance of the controller. The Flyback Transformer Design Considerations for Efficiency and EMI
Seminar is a detailed description of the core and switching losses of the transformer, the snubber clamp
levels, and how to improve the overall transformer performance.
7
References
•
•
•
•
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Texas Instruments, UCC28730-Q1 Zero-Power Standby PSR Flyback Controller for Automotive Data
Sheet (SLUSCR9)
Texas Instruments Training, Introduction to EMI in Power Supply Designs: Sources, Measurements
and Mitigation Methods
Commission guidelines: Eco Design Requirements For Off Mode, Standby and Networked Standby
Texas Instruments, TIDA-01560 Reference Design
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