STMicroelectronics AN2929 In consumer applications such as lcd or plasma tv Datasheet

AN2929
Application note
Wide range [90 V - 265 V] input, 5 V - 12 W output VIPER27LN
demonstration board with improved standby performance
Introduction
In consumer applications such as LCD or plasma TVs, some models of DVD recorders, settop boxes with hard disk, as well as desktop computers, the power supply often includes two
modules: the main power supply that provides most of the power and is off when the
application is in standby mode and the auxiliary power supply that provides energy for
specific peripherals such as USB ports, remote receivers, and modems.
The auxiliary power supply is also on when the application is in standby mode and it is often
required that its input power be as low as possible. The demonstration board presented in
this application note meets the specification of a wide range of auxiliary power supplies for
these applications and is optimized for very low standby consumption, helping to meet the
most stringent energy-saving requirements.
Figure 1.
April 2011
VIPER27LN demonstration board
Doc ID 15333 Rev 2
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www.st.com
Contents
AN2929
Contents
1
2
Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1
Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2
Schematic and bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3
Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Testing the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1
Typical board waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2
Precision of the regulation and output voltage ripple . . . . . . . . . . . . . . . . 10
2.3
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4
Light-load performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5
Overload protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.6
Secondary winding short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . 22
2.7
Output overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.8
Brownout protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.9
EMI measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
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AN2929
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
VIPER27LN demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Transformer size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Transformer size and pin diagram [inches (mm)] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Drain current and voltage at full load 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Drain current and voltage at full load 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Drain current and voltage at full load 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Drain current and voltage at full load 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Output voltage ripple 115 VINAC full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Output voltage ripple 230 VINAC full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Output voltage ripple 115 VINAC at no load (burst mode) . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Output voltage ripple 230 VINAC 50 mA at no load (burst mode) . . . . . . . . . . . . . . . . . . . . 12
Efficiency vs. VIN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Efficiency vs. load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Active mode efficiency vs. VIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Input voltage averaged efficiency vs. load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
ENERGY STAR® efficiency criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Input power vs. input voltage for different loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Average switching frequency vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Output power vs. input voltage with input power of 1 W . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Output short-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Operation with output shorted. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2nd OCP tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Operating with secondary winding shorted - restart mode . . . . . . . . . . . . . . . . . . . . . . . . . 23
OVP circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
OVP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Jumper J7 setting, brownout disabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Jumper J7 setting, brownout enabled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Brownout protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Input AC voltage steps from 90 VAC to 75 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Input voltage steps from 90 VAC to 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
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List of tables
AN2929
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
Table 17.
Table 18.
4/32
Electrical specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Transformer characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Output voltage and VDD line-load regulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Output voltage ripple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Burst mode related output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Active mode efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Line voltage averaged efficiency vs. load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Energy efficiency criteria for standard models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Energy efficiency criteria for low-voltage models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
No-load input power (no brownout) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Energy consumption criteria for no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Low-load performance POUT = 25 mW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Low-load performance POUT = 50 mW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Output power when the input power is 1 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Input and output load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
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AN2929
Board description
1
Board description
1.1
Electrical specifications
The electrical specifications of the VIPER27LN demonstration board are listed in Table 1.
Table 1.
Electrical specifications
Symbol
Parameter
VIN
VOUT
Output voltage
IOUT
Max output current
Unit
[90VRMS; 265VRMS]
V
5
V
2.4
A
ΔVOUT_LF
Precision of output regulation
ΔVOUT_HF
High-frequency output voltage ripple
50
mV
TA
Max ambient operating temperature
60
°C
±5%
Schematic and bill of material
The schematic of the board is shown in Figure 2. Table 2 gives the list of components (bill of
material).
Figure 2.
Schematic
4
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3403
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4
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-EG 2
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8 # !0
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2
K
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8
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2
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5
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1.2
Input voltage range
Value
N&
2
2
K K
*
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!-V
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Board description
Table 2.
AN2929
Bill of material
Reference
Part
Description
Manufacturer
BR1
DF06M
600 V 1 A diodes bridge
Fairchild \ General
Semiconductor
C1, C13
22 nF X2 capacitor
Evox Rifa
C3
450 V 33 µF electrolytic capacitor
C4
35 V 22 µF electrolytic capacitor
C5
Not mounted
Not mounted
C6
25 V ceramic capacitor
C7
25 V ceramic capacitor
C8
Y1 2.2 nF capacitor
C9, C14
16 V ZL 1000 µF 10X20
1000 µF 16 V electrolytic capacitor
Rubycon
C10
16 V 100 µF YK
100 µF 16 V YK rubycon
Rubycon
C11, C12
10 nF
25 V ceramic capacitor
D1
BAT46
Diode
D2
1N4148
Diode
D3
STTH1L06
Diode
STMicroelectronics
D4
STPS745
Diode
STMicroelectronics
D5
1.5KE250
Transil
STMicroelectronics
D6
BZX79-C18
Zener diode
NXP
F1
TR5 250 V 1 A
Fuse
L1
6/32
STMicroelectronics
3.3uH
NTC1
B57236S160M
16 Ω NTC
EPCOS
OPTO1
PC817
Opto coupler
Sharp
R1
3.3 Ω axial resistor
R16, R17
1600 kΩ 1% axial resistor
R3
56 kΩ 1% axial resistor
R6
12 kΩ axial resistor
R8
120 kΩ 1% axial resistor
R9
39 kΩ 1% resistor
R10
270 kΩ axial resistor
R14
220 kΩ 1% axial resistor
R12
27 kΩ axial resistor
R13
1 kΩ axial resistor
R19
120 Ω axial resistor
R20
Heatsink
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Board description
Table 2.
1.3
Bill of material (continued)
Reference
Part
Description
Manufacturer
T1
750871111
Switch-mode power transformer
Würth Elektronik
T2
BU15-4530R4BL
Common-mode choke
Coilcraft
U1
VIPER27LN
Offline high-voltage converters
STMicroelectronics
VR1
TS431AIZ-AP
Voltage reference
STMicroelectronics
Transformer
The transformer characteristics are listed in the table below.
Table 3.
Transformer characteristics
Properties
Test condition
Value
Manufacturer
Würth Elektronik
Part number
750871111
Primary inductance
Measured at 10 kHz 0.1 V
1.7 mH ± 10%
Leakage inductance
Measured at 100 kHz 0.1 V
60 μH
Primary-to-secondary turn
ratio (6 - 4)/(7, 9 – 12, 14)
Measured at 10 kHz 0.1 V
13.5 ± 3%
Primary-to-auxiliary turn
ratio (6 - 4)/(3 - 1)
Measured at 10k Hz 0.1 V
5.19 ± 3%
Insulation
Primary to secondary
4 kV
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Board description
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The figures below show the size and pin distances (inches and [mm]) of the transformer.
Figure 3.
Transformer size
(a) Top view
Figure 4.
(b) Bottom view
Transformer size and pin diagram [inches (mm)]
(a) Pins distances
8/32
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(b) Electrical diagram
AN2929
Testing the board
2
Testing the board
2.1
Typical board waveforms
Drain voltage and current waveforms were captured for the two nominal input voltages and
for the minimum and the maximum voltage of the converter’s input operating range.
Figure 5 and Figure 6 show the drain current and the drain voltage waveforms at the
nominal input voltages of 115 VAC and 230 VAC when the load is the maximum (2.4 A).
Figure 7 and Figure 8 show the same waveforms for the same load condition, but for the
minimum (90 VAC) and the maximum (265 VAC) input voltages.
The converter is designed to operate in continuous conduction mode (in full-load condition)
at low line. CCM (continuous conduction mode) allows reducing the root mean square
current values at the primary side in the power switch inside the VIPER27LN and at the
secondary side in the output diode (D4) and in the output capacitors (C9 and C14).
Reducing RMS currents means reducing the power dissipation (mainly in the VIPER27LN)
and the stress on these components.
Figure 5.
Drain current and voltage at
full load 115 VAC
Figure 6.
Drain current and voltage at
full load 230 VAC
Figure 7.
Drain current and voltage at
full load 90 VAC
Figure 8.
Drain current and voltage at
full load 265 VAC
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Testing the board
2.2
AN2929
Precision of the regulation and output voltage ripple
The output voltage of the board was measured in different line and load conditions (see
Table 4). The output voltage is practically not affected by the line condition. The VDD voltage
was also measured to verify that it is inside the operating range of the device.
Table 4.
Output voltage and VDD line-load regulation
No load
Half load
Full load
VINAC (V)
VOUT (V)
VDD (v)
VOUT (V)
VDD (V)
VOUT (V)
VDD (V)
90
5.05
9.79
5.05
18.9
5.04
19.8
115
5.05
9.71
5.05
18.9
5.04
19.7
230
5.05
9.37
5.05
18.8
5.04
19.6
265
5.05
9.22
5.05
18.8
5.04
19.6
In a dual-output flyback converter, when just one output is regulated, the unregulated output
does not rigorously respect the turn ratio. The unregulated output voltage value depends not
only on the turn ratio but also, approximately, from the output current ratio (output current of
the regulated output divided by the output current of the unregulated output).
As confirmed from the results in Table 4, the VDD voltage (unregulated auxiliary output)
increases as the load on the regulated output increases. In order to avoid that the VDD
voltage exceeds its operating range, an external clamp was used (D6, R19). See schematic
in Figure 2.
The ripple at the switching frequency superimposed at the output voltage was also
measured and the results are given in Table 5. The board is provided with an LC filter for
cleaner output voltage. The high-frequency voltage ripple across capacitor C9 (VOUT_FLY),
which is the output capacitor of the flyback converter before the LC filter, was also measured
to verify the effectiveness of the LC filter and to provide complete results.
Table 5.
Output voltage ripple
Half load
Full load
VINAC (VRMS)
10/32
VOUT (mV)
VOUT_FLY (mV)
VOUT (mV)
VOUT_FLY (mV)
90
25
275
40
172
115
26
275
37
201
230
26
273
36
194
265
25
272
36
195
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Testing the board
The waveforms of the two voltages (VOUT and VOUT_FLY) are shown in the figures below.
Figure 9.
Output voltage ripple 115 VINAC full load
Ch2: VOUT_FLY
Ch3: VOUT
Ch4: VDRAIN
Figure 10. Output voltage ripple 230 VINAC full load
Ch2: VOUT_FLY
Ch3: VOUT
Ch4: VDRAIN
When the device is working in burst mode, a lower frequency ripple is present. In this mode
of operation the converter does not supply continuous power to its output. It alternates
periods when the power MOSFET is kept off and no power is processed by the converter,
and periods when the power MOSFET is switching and power flows towards the converter
output. Even if no load is present at the output of the converter, during no switching periods,
the output capacitors are discharged by their leakage currents and by the currents needed
to supply the circuitry of the feedback loop present at the secondary side. During the
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Testing the board
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switching period the output capacitance is recharged. The figures below show the output
voltage and the feedback voltage when the converter is not loaded. In Figure 11 the
converter is supplied with 115 VAC and with 230 VAC in Figure 12.
Figure 11. Output voltage ripple 115 VINAC at no load (burst mode)
CH1: VOUT
CH2: VOUT_FLY
CH4: IDRAIN
Figure 12. Output voltage ripple 230 VINAC 50 mA at no load (burst mode)
CH1: VOUT
CH2: VOUT_FLY
CH4: IDRAIN
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Testing the board
Table 6 shows the measured value of the burst mode related ripple measured in different
operating conditions. The measured ripple in burst mode operation is very low and always
below 25 mV.
Table 6.
2.3
Burst mode related output voltage ripple
VIN
No load (mV)
25 mA load (mV)
50 mA load (mV)
90
5.1
13.5
14.5
115
5.2
14.5
15.5
230
5.4
15.4
16.9
265
5.7
15.8
18
Efficiency
The efficiency of the converter was measured in different load and line voltage conditions.
According to the ENERGY STAR® average active mode testing efficiency method, the
measurements were done with full load and with 75%, 50%, and 25% of the full load for
different input voltages, seeTable 7.
Table 7.
Efficiency
Efficiency (%)
VINAC
Full load
75% load
50% load
25% load
(2.4 A)
(1.8 A)
(1.2 A)
(0.6 A)
90
72.46
75.34
78.503
79.841
115
74.96
76.95
79.634
80.395
230
77.70
79.04
80.690
79.990
265
77.80
78.97
80.264
78.287
(VRMS)
The results were plotted in the following figures for better visibility. Figure 13 shows the
efficiency versus VIN for the four different load values and Figure 14 shows the efficiency
versus load for different input voltages.
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Testing the board
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Figure 13. Efficiency vs. VIN
Efficiency
85.00
80.00
100%
75%
75.00
50%
25%
70.00
65.00
50
100
150
200
V INAC (V RMS )
250
300
AM02177v1
Figure 14. Efficiency vs. load
Efficiency
85.00
80.00
90V A C
115V A C
230V A C
75.00
265V A C
70.00
65.00
2.000
5.000
8.000
Pout (W )
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11.000
14.000
AM02178v1
AN2929
Testing the board
The active mode efficiency is defined as the average of the efficiencies measured at 25%,
50% and 75% of maximum load and the maximum load itself. Table 8 gives the active mode
efficiencies calculated from the Table 7 measured values. For clarity the values from Table 8
are plotted in Figure 15. Figure 16 shows the averaged (average was done considering the
efficiency at different input voltage) value of the efficiency versus load.
Table 8.
Active mode efficiencies
VINAC (VRMS)
Efficiency (%)
90
76.54
115
77.98
230
79.35
265
78.83
Figure 15. Active mode efficiency vs. VIN
85.00
80.00
75.00
70.00
65.00
50
100
150
200
250
V INAC (V RMS )
Table 9.
300
AM02179v1
Line voltage averaged efficiency vs. load
Load (% of full load)
Efficiency (%)
100
79.39
75
79.85
50
78.67
25
75.56
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Testing the board
AN2929
Figure 16. Input voltage averaged efficiency vs. load
Efficiency
85.00
80.00
75.00
70.00
65.00
2
5
8
P OUT (W )
11
14
AM02180v1
STAR®
program requirement for single voltage external
In the version 2.0 of the ENERGY
AC-DC power supplies (see Section 4: References), the power supplies are divided in two
categories: low-voltage power supplies and standard power supplies with respect to the
nameplate output voltage and current. An external power supply, in order to be considered a
low-voltage power supply, needs to have a nameplate output voltage lower than 6 V and a
nameplate output current greater than or equal to 550 mA.
The tables below give the EPA energy efficiency criteria for AC-DC power supplies in active
mode for standard models and for low-voltage models, respectively.
Table 10.
Energy efficiency criteria for standard models
Nameplate output power
Table 11.
(Pno)
Minimum average efficiency in active mode
(expressed as a decimal)
0 W < Pno ≤ 1 W
= 0.48 * Pno + 0.140
1 W < Pno ≤ 49 W
= [0.0626 * In (Pno)] + 0.622
Pno > 49 W
= 0.870
Energy efficiency criteria for low-voltage models
Nameplate output power
16/32
(Pno)
Minimum average efficiency in active mode
(expressed as a decimal)
0 W < Pno ≤ 1 W
= 0.497 * Pno + 0.067
1 W < Pno ≤ 49 W
= [0.075 * In (Pno)] + 0.561
Pno > 49 W
= 0.860
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Testing the board
Figure 17. ENERGY STAR® efficiency criteria
7
Ș
%FFICIENCY
Ș
ST 0NO
LV 0NO
0NO
Figure 17 shows the efficiency criteria where the red line is the criteria for the standard
model and the blue line is the criteria for the low-voltage model. The PNO axe is in
logarithmic scale.
The power supply presented belongs to the low-voltage power supply category and, in order
to be compliant with ENERGY STAR® requirements, needs to have efficiency higher than
74.7%. For all the considered input voltages, the efficiency results (see Table 8) are higher
than the recommended value.
2.4
Light-load performance
The input power of the converter was measured in no load condition, with brownout
protection disabled (see Section 2.8) for different input voltages and the results are given in
Table 12.
Table 12.
No-load input power (no brownout)
VINAC (VRMS)
Pin (mW)
fSW_AVG (kHz)
90
15
0.590
115
16
0.558
230
24
0.434
265
29
0.401
In version 2.0 of the ENERGY STAR® program the power consumption of the power supply
when it is not loaded is also considered. The criteria for compliance are given in the table
below.
Table 13.
Energy consumption criteria for no load
Nameplate output power (Pno)
Maximum power in no load for AC-DC EPS
0 W < Pno ≤ 50 W
< 0.3 W
50 W < Pno < 250 W
< 0.5 W
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Testing the board
AN2929
The performance of the VIPER27LN board is much better than required. The power
consumption is about twelve times lower than the ENERGY STAR® limit. Even if the results
seem to be disproportionally better than the requirements, it is worth mentioning that often
AC-DC adapter or battery charger manufacturers have very strict requirements concerning
no-load consumption and if the converter is used as an auxiliary power supply, the line filter
of the entire power supply is much bigger and considerably increases the standby
consumption.
Even if the ENERGY STAR® program does not have other requirements regarding light-load
performance, in order to give complete information we have provided the input power and
efficiency of the demonstration board in two other low-load cases also. Table 14 and
Table 15 show the performances when the output load is 25 mW and 50 mW respectively.
Table 14.
Low-load performance POUT = 25 mW
VIN_AC
POUT (mW)
PIN (mW)
Eff. (%)
PIN-POUT
(mW)
fSW_AVG (kHz)
90
25.760
48
53.22
22.640
2.017
115
26.760
49
54.95
21.940
1.886
230
25.760
61
41.95
35.640
1.461
265
25.760
67
38.74
40.740
1.355
Table 15.
Low-load performance POUT = 50 mW
VIN_AC
POUT (mW)
PIN (mW)
Eff. (%)
PIN-POUT (mW)
fSW_AVG (kHz)
90
51.510
80
64.39
28.490
3.407
115
51.510
82
62.82
30.490
3.177
230
51.510
98
52.56
46.490
2.435
265
51.510
104
49.53
52.490
2.284
The input power and the average switching frequency versus the input voltage for different
load conditions are shown in the following figures.
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Testing the board
Figure 18. Input power vs. input voltage for different loads
3,1 :
3287
P:
P:
P:
9 ,1$&
!-V
Figure 19. Average switching frequency vs. input voltage
FSW_AVG (kHZ)
4
3
POUT
25 mW
2
0 mW
50mW
1
0
80
120
160
200
240
280
V INAC
AM02173v1
Depending on the equipment supplied, we can have several criteria to measure the standby
or light-load performance of a converter. One criterion is the measure of the output power
when the input power is equal to one watt. Table 16 gives the output power needed to have
1 W of input power in different line conditions and Figure 20 illustrates this.
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Testing the board
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Table 16.
Output power when the input power is 1 W
VIN
PIN
POUT
Efficiency
Pin-Pout
(VRMS)
(mW)
(mW)
(%)
(mW)
90
1000
767.1
76.71
232.853
115
1000
762.1
76.21
237.910
230
1000
752.0
75.20
248.024
265
1000
726.2
72.62
273.815
Figure 20. Output power vs. input voltage with input power of 1 W
POUT (mW)
770.0
760.0
750.0
740.0
730.0
720.0
80
120
160
200
240
280
VINAC (VRMS)
AM02174v1
If brownout protection is required, for the same load condition the input power increases.
The brownout external circuit (R16, R17 and R18 see schematic in Figure 2) is connected in
parallel with a bulk capacitor (C3 in the schematic) and some power is dissipated on it
according to the voltage across the bulk capacitor. The following formula gives the additional
power dissipation in the brownout circuit:
Equation 1
PBR _ LOSS =
2
VBULK
_ RMS
(R16 + R17 + R18)
In light-load condition the voltage across bulk capacitor can be considered constant and
equal to the peak of the AC input voltage. Considering the worst case of maximum input
voltage, the dissipation in the brownout circuit is:
Equation 2
PBR _ LOSS =
20/32
2
VACMAX
_ PK
(R16 + R17 + R18)
=
Doc ID 15333 Rev 2
( 2 ⋅ 265 ⋅ V)2
≅ 43mW
3222 ⋅ kΩ
AN2929
2.5
Testing the board
Overload protection
The VIPER27LN has several protections, one of which prevents converter damage in case
of overload or output short-circuit. If the load power demand increases, the output voltage
decreases and the feedback loop reacts by increasing the voltage on the feedback pin. The
increase of the feedback pin voltage leads to the PWM current set point increase which
increases the power delivered to the output until this power equals the load power. If the
load power demand exceeds the converter’s power capability (which can be adjusted using
RLIM), the voltage on feedback pin continuously rises but the power delivered no longer
increases. When the feedback pin voltage exceeds VFB_lin (3.3 V typ.), VIPER27LN logic
assumes that it is a warning for an overload event. Before shutting down the system, the
device waits for a period of time set by the capacitor present on the feedback pin. In fact if
the voltage on the feedback pin exceeds VFB_lin, the internal pull-up is disconnected and the
pin starts sourcing a 3 A current that charges the capacitor connected to it. As the voltage
on the feedback pin reaches the VFB_olp threshold (4.8 V typ.), VIPER27LN stops switching
and is not allowed to switch again until the VDD voltage goes below VDD_RESTART (4.5 V typ.)
and rises again up to VDD_ON (14 V typ.).
The following waveform shows the behavior of the converter when the output is shorted.
Figure 21. Output short-circuit
Ch1: V OUT
Ch2: V FB
Ch4: IDR AIN
VIN =115VAC
Fu ll Load
Before the short
If the short-circuit is not removed, the system starts to work in auto-restart mode. The
behavior, when a short-circuit is permanently applied on the output, is a short period of time
where the MOSFET is switching and the converter tries to deliver to the output as much
power as it can, and a longer period where the device is not switching and no power is
processed.
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Testing the board
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The duty cycle of power delivery is very low (around 1.55%) so the average power
throughput is also very low (see the figure below).
Figure 22. Operation with output shorted
Ch1: V DD
Ch2: V FB
Ch4: IDR AIN
VIN =115VAC
2.6
Secondary winding short-circuit protection
The VIPER27LN is provided with an adjustable first level of primary overcurrent limitation
that switches off the power MOSFET if this level is exceeded. This limitation acts cycle by
cycle and its main purpose is to limit the maximum deliverable output power. A second level
of primary overcurrent protection is also present and in this case it is not adjustable, it is
fixed to 1 A (typical value). If the drain current exceeds this 2nd OCP (second overcurrent
protection) threshold, the device enters a warning state. The next cycle the MOSFET is
switched on, and if again the second level of overcurrent protection is exceeded, the device
assumes that a secondary winding short-circuit or a hard saturation of the transformer is
happening and stops the entire operation. In order to re-enable the operation, the VDD
voltage has to be recycled which means that VDD has to go down to VDD_RESTART, then rise
up to VDD_ON. When the VIPER27LN is switched on again (VDD equals VDD_ON), the
MOSFET can start to switch again. If the cause of the 2nd overcurrent protection activation
is not removed, the device goes in auto-restart mode.
This protection was tested on the present board. The secondary winding of the transformer
was shorted, in different operating conditions. The following figures show the behavior of the
system during these tests.
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Testing the board
Figure 23.
2nd OCP tripping
Ch1 = VOUT
Ch2 = VFB
Ch4 = IDRAIN
Test condition:
Vin = 230VAC
Full load before
short
In Figure 23 when the board was working in full-load condition with an input voltage of
115 VAC, the secondary winding was shorted. The short on the secondary winding leads to
a high drain current. After two switching cycles, the system stops and continuous running
with high currents in the primary as well as in the secondary windings and through the
power section of the VIPER27LN is avoided. Figure 24 shows the situation when a
permanent short-circuit is applied on the secondary windings. Most of the time the power
section of the VIPER27LN is off, eliminating any risk of overheating.
Figure 24. Operating with secondary winding shorted - restart mode
Ch3 = VDD
Ch4 = IDRAIN
Test condition
Vin=230VAC
Secondary winding
shorted
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Testing the board
2.7
AN2929
Output overvoltage protection
An output overvoltage protection is implemented by monitoring the voltage across the
auxiliary winding during the MOSFET off-time, through the D2 diode and the resistor divider
R3 and R14 (see schematic of Figure 2) connected to the CONT pin of the VIPER27LN. If
the voltage on the pin exceeds the VOVP thresholds (3 V typ.), an overvoltage event is
assumed and the power section is no longer allowed to switch on. To re-enable operation,
the VDD voltage has to be recycled. In order to provide high noise immunity and avoid that
spikes erroneously trip the protection, a digital filter was implemented so the CONT pin has
to sense a voltage higher than VOVP for four consecutive cycles before stopping operation.
Figure 25. OVP circuit
Current Limit Comparator
Rov p (R14)
Curr. Lim.
BLOCK
CONT PIN
-
Dov p (D2)
+
To PWM Logic
Auxiliary
w inding
Rlim
OVP DETECTION
(R3)
LOGIC
From SenseFET
To OVP Protection
AM02175v1
The value of the output voltage when the protection has to be tripped can be set by properly
selecting the resistor divider R3 and R14. R3 is selected according to the maximum power
that the converter has to provide to the output, and R14 is selected according to Equation 3:
Equation 3
ROVP_(R14) =
RLIM_(R3) ⎛ NAUX
⎞
⋅ ⎜⎜
⋅ VOUT_ OVP − Vdrop_ Dovp_(D2) − VOVP ⎟⎟
VOVP ⎝ Ns
⎠
The protection was tested by disconnecting the opto-coupler from the feedback pin and
applying a minimum load to the converter. In this way the converter operates in open loop
and delivers the maximum power that it can to the output. The excess of power with respect
to the load charges the output capacitance, increasing the output voltage since the OVP is
tripped and the converter stops working.
In Figure 26 it is possible to see that the output voltage increases and, as it reaches the
value of 5.58 V, the converter stops switching. In the same figure the CONT pin voltage
(Ch1, yellow waveform) is shown. The crest value of the CONT pin voltage tracks the output
voltage.
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Testing the board
Figure 26. OVP
Ch1: VCONT
Ch2: VFB
Ch3: VOUT
Working condition:
input voltage: 115VAC
No load
Feedback pin: disconnected.
The test was performed in different line and load conditions, to check the dependence of the
output voltage value, when the protection is activated, on the converter’s input voltage and
load.
The results are shown in Table 17.
Table 17.
Input and output load
VOUT_OVP (V)
Load
VIN
(VRMS)
No load
25% (0.6 A)
50% (1.2 A)
75% (1.8 A)
100% (2.4 A)
90
5.56
5.50
5.46
5.45
5.45
115
5.56
5.51
5.49
5.48
5.46
230
5.59
5.55
5.52
5.53
5.53
265
5.59
5.54
5.54
5.53
5.53
The variation with load and line condition is very low ([5.45 V; 5.59 V],
ΔVOUT_OVP = 140 mV), less than 3%. Considering a precision of the OVP threshold on the
CONT pin of 10% ([2.7 V; 3.3 V]), using as ROVP and RLIM 1% precision resistance and with
a turn ratio between the secondary and auxiliary windings that has a precision of 5%, it is
possible to fix the output overvoltage 20%, 25% higher than the nameplate output voltage of
the converter, without risk of undesired protection tripping due to the spread in the values of
the components (R3 and R14), of the transformer parameter (turn ratio) and of the
VIPER27LN parameter (VOVP).
It is possible to not implement this protection if it is not necessary, by not mounting diode D2
and resistor R14, thus reducing the number of components.
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Testing the board
2.8
AN2929
Brownout protection
The brownout protection is basically an unlatched device shutdown functionality whose
typical use is to sense flyback converter input undervoltage. The VIPER27LN has a pin (BR,
pin 5) dedicated to this function that must be connected to the DC HV bus. If the protection
is not required, it can be disabled by connecting the pin to ground. In the present converter
the brownout protection is implemented but can be disabled by changing the setting of
jumper J7 (see schematic in Figure 2). The settings of jumper J7 are shown in Figure 27
and Figure 28. The shutdown of the converter is accomplished by means of an internal
comparator internally referenced to 450 mV (typ, VBRth) that disables the PWM if the voltage
applied to the BR pin is below the internal reference, as shown in Figure 29. The PWM
operation is re-enabled as the BR pin voltage is more than 450 mV plus 50 mV of voltage
hysteresis that ensures noise immunity. The brownout comparator is also provided with
current hysteresis. An internal current generator is ON as long as the voltage applied to the
brownout pin is below 450 mV and is OFF if the voltage exceeds 450 mv plus the voltage
hysteresis.
Figure 27. Jumper J7 setting, brownout
disabled
Figure 28. Jumper J7 setting, brownout
enabled
Figure 29. Brownout protection
100mV Typ.
Disable
+
-
VBRhyst
VAC_OK
+
VBRth
IBRhyst
AM02176v1
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Testing the board
The current hysteresis provides an additional degree of freedom. It is possible to set the ON
threshold and the OFF threshold for the flyback input voltage separately by properly
choosing the resistors of the external divider. The following relationships can be established
for the ON (VIN_ON) and OFF (VIN_OFF) thresholds of the input voltage:
Equation 4
⎛ R + RL
VIN _ OFF = VBRth ⋅ ⎜⎜ H
⎝ RL
⎞
⎟⎟
⎠
Equation 5
⎛ R + RL
VIN _ ON = VBRth + VBRhyst ⋅ ⎜⎜ H
⎝ RL
(
)
⎞
⎟⎟ + RH ⋅ IBRhyst
⎠
where IBRhyst = 8.5 µA (typ.) is the current hysteresis, VBRhyst = 50 mV (typ.) is the voltage
hysteresis and VBRth = 450 mV (typ.) is the brownout comparator internal reference.
One purpose of this protection is to stop operation of the converter when the line voltage is
too low, avoiding an excessive root mean square current value flowing inside the main
switch and consequently its overheating. Another purpose is to avoid a false restart of the
converter and then having a monotonic decay to zero of the output voltage when the
converter itself is unplugged from the mains. A typical situation, in most cases for converters
designed for the European range (230 VAC), could be a converter that, when unplugged,
shuts down due to the overload protection (due to the low input voltage the converter is not
able to supply the full power), but the voltage on the bulk capacitor is higher than VDRAIN
RESTART so the device restarts and the output voltage rises again. This situation could be
dangerous for some loads, and in many applications it is better to avoid it.
The following figures show how the brownout protection works in the VIPER27LN board
when used. Figure 30 shows the behavior of the board when the input voltage is changed
from 90 VAC to 75 VAC with a full load applied. The system stops switching and the output
load, no longer supplied, decays monotonically to zero.
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Testing the board
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Figure 30. Input AC voltage steps from 90 VAC to 75 VAC
Ch1: V OU T
Ch2: VBR
Ch3: V BUL K
Ch4: ID RAIN
I OUT : 2.4A
Figure 31. Input voltage steps from 90 VAC to 0
Ch1: V OU T
Ch2: VBR
Ch3: V BUL K
Ch4: ID RAIN
I OUT : 2.4A
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2.9
Testing the board
EMI measurements
A pre-compliance test for EN55022 (Class B) European normative was also performed and
the results are shown in the two figures below.
Figure 32. 115 VAC
Figure 33. 230 VAC
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Conclusion
3
AN2929
Conclusion
The presented flyback converter is suitable for different applications and can be used as an
external adapter or as an auxiliary power supply in consumer products. Special attention
was given to low-load performance and the bench results are good with very low input
power in light-load condition. The efficiency was compared to the requirements of the
ENERGY STAR® program (version 2.0) for external AC/DC adapters with very good results
in that the measured active mode efficiency was always higher than the minimum required.
4
References
[1] ENERGY STAR® program requirements for single voltage external AC-DC adapter
(Version 2.0)
[2] VIPER27 datasheets
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5
Revision history
Revision history
Table 18.
Document revision history
Date
Revision
Changes
07-May-2010
1
Initial release
06-Apr-2011
2
Modified: Table 2: Bill of material
Doc ID 15333 Rev 2
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