12 V, 150 mA non-isolated buck converter using the VIPER06XS

AN4260
Application note
12 V, 150 mA non-isolated buck converter using the VIPER06XS,
from the VIPer™ plus family
Introduction
This document describes the STEVAL-ISA115V1, a 12 V, 0.13 A power supply set in buck
topology with the VIPer06XS, a new offline high voltage converter by STMicroelectronics,
specifically developed for non-isolated SMPS.
The features of the device are:
• 800 V avalanche rugged power section
• PWM operation at 30 kHz with frequency jittering for lower EMI
• Limiting current with adjustable set point
• On-board soft-start
• Safe auto-restart after a fault condition and low standby power consumption
The available protection includes: thermal shutdown with hysteresis, delayed overload
protection and open loop failure protection. All protection is auto-restart mode.
Figure 1. STEVAL- ISA115V1 product evaluation board
GIPG2305140928LM
December 2014
DocID024275 Rev 4
1/29
www.st.com
Contents
AN4260
Contents
1
Adapter features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4
Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5
Testing the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6
7
5.1
Typical waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2
Line/load regulation and output voltage ripple . . . . . . . . . . . . . . . . . . . . . . 9
5.3
Burst mode and output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.4
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5
Light load performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Functional check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1
Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2
Overload protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.3
Feedback loop failure protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Feedback loop calculation guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1
Transfer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.2
Compensation procedure for a DCM buck . . . . . . . . . . . . . . . . . . . . . . . . 19
8
Thermal measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9
EMI measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Appendix A Test equipment and measurement of efficiency and light load
performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
A.1
10
Measuring input power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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AN4260
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.
Figure 34.
Figure 35.
Figure 36.
Figure 37.
Figure 38.
Figure 39.
Figure 40.
STEVAL- ISA115V1 product evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Layout (top). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Layout (bottom). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Waveforms at VIN = 115 VAC, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Waveforms at VIN = 230VAC, full load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Waveforms at VIN = 80 VAC, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Waveforms at VIN = 265 VAC, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Line regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Output voltage ripple at 115 VAC, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Output voltage ripple 230 VAC, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Output voltage ripple at 115 VAC, no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Output voltage ripple at 230 VAC, no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Active mode efficiency vs. VIN and comparison with CoC4 and DOE . . . . . . . . . . . . . . . . 11
PIN vs. VIN @ no load and light load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Efficiency @ PIN = 1 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
PIN @ POUT = 0.25 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Startup at VIN = 115 VAC, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Startup at VIN = 115 VAC, full load, zoom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Startup at VIN = 230 VAC, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Startup at VIN = 230 VAC, full load, zoom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Output short-circuit applied: OLP tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Output short-circuit maintained: OLP steady-state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Output short-circuit maintained: OLP steady-state (zoom) . . . . . . . . . . . . . . . . . . . . . . . . . 15
Output short-circuit removal and converter restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Feedback loop failure protection: tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Feedback loop failure protection: steady-state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Feedback loop failure protection: steady-state zoom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Feedback loop failure protection: converter restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Control loop block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Thermal measurement @ VIN = 80 VAC, full load (130 mA) Rbl = 8.2 kohm . . . . . . . . . . . 20
Thermal measurement @ VIN = 115 VAC, full load (130 mA) Rbl = 8.2 kohm . . . . . . . . . . 21
Thermal measurement @ VIN = 230 VAC, full load (130 mA) Rbl = 8.2 kohm . . . . . . . . . . 21
Thermal measurement @ VIN = 265 VAC, full load (130 mA) Rbl = 8.2 kohm . . . . . . . . . . 22
Average measurement at full load, 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Average measurement at full load, 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Connections of the UUT to the wattmeter for power measurements . . . . . . . . . . . . . . . . . 24
Switch in position 1 - setting for standby measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Switch in position 2 - setting for efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . 25
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Adapter features
1
AN4260
Adapter features
The electrical specifications of the evaluation board are listed in Table 1.
Table 1. Electrical specifications
Parameter
Symbol
Value
VIN
[90 VAC; 265 VAC]
Output voltage
VOUT
12 V
Max. output current
IOUT
0.15 A
Precision of output regulation
ΔVOUT_LF
±5%
High frequency output voltage ripple
ΔVOUT_HF
50 mV
Max. ambient operating temperature
TAMB
60 °C
Input voltage range
2
Circuit description
The converter schematic is given in Figure 2. The input section includes a resistor R1 for
inrush current limiting, a diode D1 and a Pi filter (C1, L1, C2) for rectification and EMC
suppression.
The FB pin is the inverting input of the internal transconductance error amplifier, internally
referenced to 3.3 V. This allows the output voltage value to be set in a simple way through
the R4-R5 voltage divider between the output terminal and the FB pin, according to the
following equation:
Equation 1
R5
V O UT = 3.3V ⋅  1 + --------
R4
where R4 has been split into R4a and R4b; and R5 into R5a and R5b so to allow a better
tuning of the output voltage value.
The compensation network is connected between the COMP pin (which is the output of the
error amplifier) and the GND pin and is made up of Cc, R3 and C7.
The bleeder resistor Rbl provides about 1 mA minimum load, in order to avoid overvoltage
when the output load is disconnected. Its value is a trade-off between output voltage
increase and power consumption rise in no load.
At power-up the DRAIN pin supplies the internal HV startup current generator which
charges the C3 capacitor up to VDDon (13 V typical). At this point, the power MOSFET starts
switching, the generator is turned off and the IC is powered by the energy stored in C3,
waiting for VOUT reaches its steady-state value. After that, the IC is supplied from the output
through the diode Daux. This allows the system to reach very low values of standby
consumption because, keeping the VDD voltage always above the VDDCSon threshold,
prevents the HV startup generator from being turned on.
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AN4260
Circuit description
Figure 2. Application schematic
Daux
R5b
AC IN
R1
D1
R5a
L1
DRAIN DRAIN DRAIN DRAIN DRAIN
IC
C1
C2
VIPer06XS
GND
VDD
LIM
FB
C8
COMP
D3
R4a
CFB
C3
CF
R4b
R6
R3
CC
C7
L2
Vout
D4
C9
GND
Rbl
GND
AM16629v1
DocID024275 Rev 4
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Bill of material
3
AN4260
Bill of material
Table 2. Bill of material
Name
Value
Description
C1
4.7 µF, 400 V
Electrolytic capacitor
Saxon
C2
4.7 µF, 400 V
Electrolytic capacitor
Saxon
C3
2.2 µF, 25 V
Ceramic capacitor
SMD: 0805
CFB
n.c
Ceramic capacitor
SMD: 0805
Cf
100 nF, 50 V
Ceramic capacitor
SMD: 0805
CC
n.c
Ceramic capacitor
SMD: 0805
C7
22 nF, 25 V
Ceramic capacitor
SMD: 0805
Murata
C8
150 nF, 50 V
Ceramic capacitor
SMD: 0805
Murata
C9
100 µF, 25 V
Electrolytic capacitor
D1
1N4007
High voltage rectifier
DO-41
Fairchild
D3
STTH1L06
High voltage ultra fast rectifier
SMB (SOD87)
ST
D4
STTH1L06
High voltage ultra fast rectifier
SMB (SOD87)
ST
Daux
1N4148
100 V, 0.15 A fast switch diode
SOD-123
Zetex
IC
VIPer06XS
High voltage converter
SSO-10
ST
L1
1 mH
Input filter inductor
SMD
Epcos
L2
RFB0810-152
1.5 mH power inductor
R1
22 ohm, 1%
1 W resistor
SMD 2010
Panasonic
R3
1.2 kohm, 1%
1/4 W resistor
SMD: 0805
Panasonic
R4a
12 kohm, 1%
1/4 W resistor
SMD: 0805
Panasonic
R4b
0 ohm
1/4 W resistor
SMD: 0805
R5a
0 ohm
1/4 W resistor
SMD: 0805
R5b
33 kohm, 1%
1/4 W resistor
SMD: 0805
R6
not mounted
1/4 W resistor
SMD: 0805
Rbl
10 kohm, 1%
1/4 W resistor
SMD: 0805
6/29
DocID024275 Rev 4
Footprint
Manufacturer
Murata
Murata
Rubycon, ZL series
Coilcraft
Panasonic
Panasonic
AN4260
4
Layout
Layout
Figure 3. Layout (top)
AM16630v1
Figure 4. Layout (bottom)
AM16631v1
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Testing the board
AN4260
5
Testing the board
5.1
Typical waveforms
GND voltage and the current across the inductor L2 (I_L2) in full load condition are shown
for the two nominal input voltages in Figure 5 and Figure 6, and for minimum and maximum
input voltage in Figure 7 and Figure 8 respectively.
Figure 5. Waveforms at VIN = 115 VAC, full load
Figure 6. Waveforms at VIN = 230VAC, full load
I_L2
I_L2
AM16633v1
AM16632v1
Figure 7. Waveforms at VIN = 80 VAC, full load
Figure 8. Waveforms at VIN = 265 VAC, full load
I_L2
I_L2
AM16334v1
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AM16635v1
AN4260
Testing the board
5.2
Line/load regulation and output voltage ripple
The output voltage of the board has been measured in different lines and load conditions.
The results are shown in Figure 9 and Figure 10.
Figure 9. Line regulation
Figure 10. Load regulation
13
13
12.8
25%
50%
12.4
75%
12.2
12.6
90
12.4
115
VOUT [V]
VOUT [V]
12.8
0
12.6
100%
230
12.2
12
265
12
11.8
80
105
130
155 180
VIN[V AC ]
205
230
11.8
255
0
AM16636v1
0.05
IOUT [A] 0.1
0.15
AM16637v1
The output voltage ripple in full load condition is shown in Figure 11 at VIN = 115 VAC and in
Figure 12 at VIN = 230 VAC.
Figure 11. Output voltage ripple at 115 VAC, full
load
Figure 12. Output voltage ripple 230 VAC, full
load
AM16638v1
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AM16639v1
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Testing the board
5.3
AN4260
Burst mode and output voltage ripple
When the converter is lightly loaded, the COMP pin voltage decreases. As it reaches the
shutdown threshold, VCOMPL (1.1 V, typical), the switching is disabled and no more energy
is transferred to the secondary side. So, the output voltage decreases and the regulation
loop makes the COMP pin voltage increase again. As it rises 40 mV above the VCOMPL
threshold, the normal switching operation is resumed. This results in a controlled on/off
operation (referred to as "burst mode”) as long as the output power is so low that it requires
a turn-on time lower than the minimum turn-on time of the VIPER06XS. This mode of
operation keeps the frequency-related losses low when the load is very light or
disconnected, making it easier to comply with energy saving regulations.
The figures below show the output voltage ripple when the converter is no/lightly loaded and
supplied with 115 VAC and with 230 VAC respectively.
Figure 13. Output voltage ripple at 115 VAC, no
load
Figure 14. Output voltage ripple at 230 VAC, no
load
AM16640v1
5.4
AM16641v1
Efficiency
The active mode efficiency is defined as the average of the efficiencies measured at 25%,
50%, 75% and 100% of maximum load, at nominal input voltage (VIN = 115 VAC and VIN =
230 VAC).
External power supplies (the power supplies which are contained in a separate housing
from the end-use devices they are powering) need to comply with the Code of Conduct,
version 4 "Active Mode Efficiency" criterion, which states an active mode efficiency higher
than 65.9% for a power throughput of 1.8 W.
Another standard to be applied to external power supplies in the coming years is the DOE
(department of energy) recommendation, whose active mode efficiency requirement for the
same power throughput is 70.9%.
The presented evaluation board is compliant with both standards, as per Figure 15, where
the average efficiencies of the board at 115 VAC (79.2%) and at 230 VAC (76.4%) are plotted
with dotted lines, together with the above limits. In the same figure the efficiency at 25%,
50%, 75% and 100% of load for both input voltages is also shown.
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Testing the board
Figure 15. Active mode efficiency vs. VIN and comparison with CoC4 and DOE
81
79
77
eff [%]
75
73
DOE limit
71
69
67
CoC4 limit
115
230
av @ 115Vac
av @ 230Vac
65
0.2
0.4
0.6
0.8
Iout [% I OUT ]
1
AM16642v1
5.5
Light load performance
The input power of the converter has been measured in no load condition for different input
voltages and results are reported in Table 3.
Table 3. No load input power
VIN [VAC]
PIN [mW]
90
32
115
34
150
37
180
39
230
42
265
48
In version 4 of the Code of Conduct, the power consumption of the power supply when it is
no loaded is also considered. The criteria to be compliant with are reported in Table 4:
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Testing the board
AN4260
Table 4. Energy consumption criteria for no load
Nameplate output power
Maximum power in no load for AC-DC EPS
0 to ≤ 50 W
< 0.3 W
> 50 W < 250 W
< 0.5 W
The power consumption of the presented board is about six times lower than the limit fixed
by version 4 of the Code of Conduct. Even though the performance seems to be
disproportionally better than requirements, it is worth noting that often AC-DC adapter or
battery charger manufacturers have very strict requirements about no load consumption
and if the converter is used as an auxiliary power supply, the line filter is often the main line
filter of the entire power supply that increases greatly the standby consumption.
Even though version 4 of the Code of Conduct does not have other requirements regarding
light load performance, in order to give a more complete overview, the consumption of the
evaluation board in two other light load cases (POUT = 25 mW and POUT = 50 mW) has also
been measured. The results versus line voltage are plotted in Figure 16, together with the
no load measurements reported in Table 3.
Figure 16. PIN vs. VIN @ no load and light load
200
0
25mW
150
PIN [mW]
50mW
100
50
0
80
105
130
155
180
VIN [V AC]
205
230
255
AM16643v1
Several criteria can be adopted to measure the performance of a converter. One criterion is
to measure the output power (or the efficiency) when the input power is equal to 1 Watt. This
measurement is shown in Figure 17 for different input voltage values.
12/29
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Testing the board
Figure 17. Efficiency @ PIN = 1 W
90
85
80
eff [%]
75
70
65
60
55
50
80
110
140
170
200
230
260
VIN [V AC]
AM16644v1
Another requirement for light load performance (EuP lot 6) is that the input power should be
less than 500 mW when the converter is loaded 250 mW. The evaluation board satisfies this
requirement, as shown in Figure 18.
Figure 18. PIN @ POUT = 0.25 W
0.5
PIN [W]
0.45
0.4
0.35
0.3
0.25
80
110
140
170
VIN [V AC]
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200
230
260
AM16665v1
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Functional check
AN4260
6
Functional check
6.1
Startup
The start-up phase at maximum load is shown in Figure 19 and Figure 21 at both nominal
input voltages (115 VAC and 230 VAC).
Figure 19. Startup at VIN = 115 VAC, full load
Figure 20. Startup at VIN = 115 VAC, full load,
zoom
I_L2
AM16646v1
AM16645v1
Figure 21. Startup at VIN = 230 VAC, full load
Figure 22. Startup at VIN = 230 VAC, full load,
zoom
I_L2
AM16647v1
6.2
AM16648v1
Overload protection
In case of overload or short-circuit (see Figure 23), the drain current reaches the IDLIM value
(or the one set by the user through the RLIM resistor). In every cycle where this condition is
met, a counter is incremented; if it is maintained continuously for the time tOVL (50 msec
typical, internally set) the overload protection is tripped, the power section is turned off and
the converter is disabled for a tRESTART time (1 sec typical). After this time has elapsed, the
IC resumes switching and, if the short is still present, the protection occurs indefinitely in the
same way (Figure 24). This ensures restart attempts of the converter with low repetition
rate, so that it works safely with extremely low power throughput and avoids the IC
overheating in case of repeated overload events.
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Functional check
After the short removal, IC resumes working normally. If the short is removed during tSS or
tOVL, before the protection tripping, the counter is decremented on a cycle-by-cycle basis
down to zero and the protection is not tripped.
If the short-circuit is removed during tRESTART, IC must wait for the tRESTART period to elapse
before switching is resumed Figure 26.
Figure 23. Output short-circuit applied: OLP
tripping
Figure 24. Output short-circuit maintained: OLP
steady-state
Output is shorted here
Normal
operation
tRESTART
tOVL
I_L2
tRESTART
I_L2
AM16650v1
AM16649v1
Figure 25. Output short-circuit maintained: OLP
steady-state (zoom)
Figure 26. Output short-circuit removal and
converter restart
Output short is
removed here
tOVL
tRESTART
Normal
operation
tOVL
I_L2
I_L2
AM16651v1
6.3
AM16652v1
Feedback loop failure protection
This protection is available any time IC is externally biased. As the loop is broken (R4
shorted or R5 open), the output voltage VOUT increases and the VIPER06XS runs at its
maximum current limitation. VDD pin voltage increases as well, because it is linked to the
VOUT voltage through the Daux diode.
If the VDD voltage reaches the VDD clamp threshold (23.5 V min.) in less than 50 msec the
IC is shut down by open loop failure protection (see Figure 27 and Figure 28), otherwise by
OLP, as described in the previous section. The breaking of the loop has been simulated by
shorting the low-side resistor of the output voltage divider, R4 = R4a1+R4b. The same
behavior can be induced opening the high-side resistor, R5 = R5a+R5b.
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Functional check
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The protection acts in auto-restart mode with tRESTART = 1 sec (Figure 28). As the fault is
removed, normal operation is restored after the last tRESTART interval has been completed
(Figure 30).
Figure 27. Feedback loop failure protection:
tripping
Figure 28. Feedback loop failure protection:
steady-state
Fault is
applied here
tRESTARTtRESTART
tRESTART
I_L2
I_L2
AM16654v1
AM16653v1
Figure 29. Feedback loop failure protection:
steady-state zoom
Figure 30. Feedback loop failure protection:
converter restart
Fault is
removed here
tRESTART
I_L2
I_L2
< tOVL
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Feedback loop calculation guidelines
7
Feedback loop calculation guidelines
7.1
Transfer function
The set PWM modulator + power stage is indicated with G1(f), while C(f) is the "controller",
the network in charge to assure the stability of the system.
Figure 31. Control loop block diagram
∆ V OUT
∆δ
∆ V OUT
∆δ
G1(f)
∆δ
C(f)
1/HCOMP
∆ V COMP
∆ V COMP
∆ V OUT
AM16666v1
The mathematical expression of the power plant G1(f) in DCM is the following:
Equation 2
j⋅f
1 + ------ΔV O UT
fz
G1 ( f ) = --------------- = G10 ⋅ ----------------Δ∂
j⋅f
1 + ------fp
where fz is the zero due to the ESR of the output capacitor:
Equation 3
1
fz = --------------------------------------------2 ⋅ π ⋅ C O UT ⋅ ESR
and fp is the pole due to the output load
Equation 4
1 + β ⋅ R O UT
fp = --------------------------------------------------------------------------------------------------------------2 ⋅ π ⋅ C O UT ⋅ ( ESR + R O UT + ESR ⋅ β ⋅ RO UT )
with:
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Feedback loop calculation guidelines
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Equation 5
V IN + Vγ
Ipk
⋅ -------α = ----------------------------( VOUT + Vγ ) 2
Equation 6
V IN + Vγ
Ipk
β = ------------------------------⋅ -------- ⋅ ∂
( V OU T + Vγ ) 2 2
Equation 7
Ipk
( V OU T + Vγ ) ⋅ ( V IN + Vγ ) ⋅ -------- ⋅ R O UT
α ⋅ R O UT
2
G10 = ----------------------------- = ----------------------------------------------------------------------------------------------1 + β ⋅ R OU T
Ipk
2
( V O UT + Vγ ) ⋅ ( V IN + Vγ ) ⋅ --------∂ ⋅ R O UT
2
In the above formulas, COUT and ESR are the capacitance and the equivalent series
resistance of the output capacitor respectively, Vy is the forward drop of the free-wheeling
diode, ROUT = VOUT/IOUT is the output load, Ipk is the drain peak current at full load and ∂ =
Ton*fsw is the duty cycle.
If just an RC series between COMP and GND is chosen as a compensation network, as
shown in Figure 2 (in fact Cc and CFB are not mounted), the mathematical expression of
the compensator C(f) is:
Equation 8
j ⋅ f
 1 + ------
fzc
C0
C ( s ) = --------------- ⋅ ----------------------HC OMP j ⋅ 2 ⋅ π ⋅ f
where:
Equation 9
L ⋅ fsw
– Gm
R4
C 0 = ------------------------- ⋅  ---------------- ⋅ ---------------------V IN – V O UT  C7  R4 + R5
and:
Equation 10
1
fzc = -----------------------------------2 ⋅ π ⋅ R3 ⋅ C7
they are chosen in order to ensure the stability of the overall system.
The values of HCOMP = δVCOMP/δICOMP and of Gm (error amplifier transconductance) are
specified in the VIPER06 datasheet.
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7.2
Feedback loop calculation guidelines
Compensation procedure for a DCM buck
The first step is to choose the pole and zero of the compensator and the crossing frequency.
In this case C(f) has only a zero (fzc) and a pole at the origin, thus a possible setting is:
•
fzc = k*fp
•
fcross = fcross_sel_≤ fsw /10
where k is chosen arbitrarily. A starting point could be k = 5
After selecting fcross_sel, G1(fcross_sel) can be calculated from Equation 2 and, since by
definition it is ⏐C(fcross_sel)*G1(fcross_sel)⏐= 1, C0 can be calculated as follows:
Equation 11
H C OMP
j ⋅ 2 ⋅ π ⋅ fcross_sel
C 0 = ----------------------------------------------------- ⋅ -------------------------------------------G1 ( fcross_sel )
j ⋅ fcross_sel
1 + ---------------------------------fzc
At this point the Bode diagram of G1(f)*C(f) can be plotted, in order to check the phase
margin for the stability.
If the margin is not high enough, another choice should be made for k and fcross_sel, and
the procedure is repeated.
When the stability is ensured, the next step is to find the values of the schematic
components, which can be calculated as follows:
from Equation 9
Equation 12
L ⋅ fsw
R4
– Gm
C7 = ------------------------- ⋅  ---------------- ⋅ ---------------------V IN – V OU T  C0  R4 + R5
and from Equation 10
Equation 13
1
R3 = -----------------------------------2 ⋅ π ⋅ fzc ⋅ C7
Quantities found in Equation 12 and Equation 13 are suggested values. Commercial values
are chosen, let us call them C7_act, R7_act, resulting into fzc_act.
Equation 14
1
fzc_act = ----------------------------------------------------------2 ⋅ π ⋅ R3_act ⋅ C7_act
C0 value is also recalculated from Equation 9
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Thermal measurements
AN4260
Equation 15
L ⋅ fsw
– Gm
R4_act
C0 _act = ------------------------- ⋅  ------------------- ⋅ -----------------------------------------------------V IN – VO UT  C7_act R4_act + R5 ( 4 )_act
and the compensator becomes:
Equation 16
f
 1 + -------------------
fzc_act
C 0_act 
C_act(f) = --------------- ⋅ ----------------------------------H CO MP
j⋅2⋅π⋅f
At this point the Bode diagram of G1(f)*C_act(f) should be plotted, and check if the phase
margin for the stability is maintained.
8
Thermal measurements
A thermal analysis of the evaluation board in full load condition at TAMB = 25 °C has been
performed using an IR camera. The results are shown in the following figures.
Figure 32. Thermal measurement @ VIN = 80 VAC, full load (130 mA) Rbl = 8.2 kohm
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Thermal measurements
Figure 33. Thermal measurement @ VIN = 115 VAC, full load (130 mA) Rbl = 8.2 kohm
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Figure 34. Thermal measurement @ VIN = 230 VAC, full load (130 mA) Rbl = 8.2 kohm
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Thermal measurements
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Figure 35. Thermal measurement @ VIN = 265 VAC, full load (130 mA) Rbl = 8.2 kohm
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9
EMI measurements
EMI measurements
A pre-compliant test of the EN55022 (Class B) European normative has been performed
using an EMC analyzer and an LISN. The average EMC measurements at 115 VAC/full load
and 230 VAC/full load have been performed and the results are shown in Figure 36 and
Figure 37.
Figure 36. Average measurement at full load, 115 VAC
AM16660v1
Figure 37. Average measurement at full load, 230 VAC
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Test equipment and measurement of efficiency and light load performance
Appendix A
AN4260
Test equipment and measurement of
efficiency and light load performance
The converter input power has been measured using a wattmeter. The wattmeter measures
simultaneously the converter input current (using its internal ammeter) and voltage (using its
internal voltmeter). The wattmeter is a digital instrument so it samples the current and
voltage and converts them to digital forms. The digital samples are then multiplied giving the
instantaneous measured power. The sampling frequency is in the range of 20 kHz (or higher
depending on the instrument used). The display provides the average measured power,
averaging the instantaneous measured power in a short period of time (1 sec typ.).
Figure 38 shows how the wattmeter is connected to the UUT (unit under test) and to the AC
source and the wattmeter internal block diagram.
Figure 38. Connections of the UUT to the wattmeter for power measurements
Switch
1
WATT METER
U.U.T
(Unit Under test)
Voltmeter
AC
SOURCE
+
V
Multiplier
2
A
Ammeter
INPUT
OUTPUT
AVG
X
DISPLAY
AM16662v1
An electronic load has been connected to the output of the power converter (UUT), allowing
the converter load current to be set and measured, while the output voltage has been
measured by a voltmeter. The output power is the product between load current and output
voltage. The ratio between the output power, calculated as previously stated, and the input
power, measured by the wattmeter, is the efficiency of converter, which has been measured
in different input/output conditions.
A.1
Measuring input power
With reference to Figure 38, the UUT input current causes a voltage drop across the internal
shunt resistance of ammeter (the ammeter is not ideal so it has an internal resistance higher
than zero) and across the cables connecting the wattmeter to the UUT.
If the switch of Figure 38 is in position 1 (see also the simplified scheme of Figure 39), this
voltage drop causes an input measured voltage higher than the input voltage at the UUT
input that, of course, affects the measured power. The voltage drop is generally negligible if
the UUT input current is low (for example when we are measuring the input power of UUT in
light load condition).
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Test equipment and measurement of efficiency and light load performance
Figure 39. Switch in position 1 - setting for standby measurements
Wattmeter
Ammeter
AC
SOURCE
~
A
+
U.U.T.
AC
INPUT
V
-
UUT
Voltmeter
AM16663v1
In case of high UUT input current (i.e. for measurements in heavy load conditions), the
voltage drop can be relevant compared to the UUT real input voltage. If this is the case, the
switch in Figure 38 can be changed to position 2 (see simplified scheme of Figure 40) where
the UUT input voltage is measured directly at the UUT input terminal and the input current
does not affect the measured input voltage.
Figure 40. Switch in position 2 - setting for efficiency measurements
Wattmeter
Ammeter
A
AC
SOURCE
+
~
V
-
U.U.T.
AC
INPUT
UUT
Voltmeter
AM16664v1
On the other hand, the position of Figure 40 may introduce a relevant error during light load
measurements, when the UUT input current is low and the leakage current inside the
voltmeter itself (which is not an ideal instrument and doesn't have infinite input resistance) is
not negligible. This is the reason why it is recommended the setting of Figure 39 to be used
for light load measurements and Figure 40 for heavy load measurements.
If it is not clear which measurement scheme has the lesser effect on the result, try with both
and register the lower input power value.
As noted in IEC 62301, instantaneous measurements are appropriate when power readings
are stable. The UUT is operated at 100% of nameplate output current for at least 30 minutes
(warm-up period) immediately prior to conducting efficiency measurements.
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Test equipment and measurement of efficiency and light load performance
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After this warm-up period, the AC input power is monitored for a period of 5 minutes to
assess the stability of the UUT. If the power level does not drift by more than 5% from the
maximum value observed, the UUT can be considered stable and measurements can be
recorded at the end of the 5-minute period. If AC input power is not stable over a 5-minute
period, the average power or accumulated energy is measured overtime for both AC input
and DC output.
Some wattmeter models allow the integration of the measured input power in a time range
and then measure the energy absorbed by the UUT during the integration time. The
average input power is calculated dividing by the integration time itself.
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10
References
References
[1] Code of Conduct on energy efficiency of external power supplies, version 4.
[2] VIPER06 datasheet.
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Revision history
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Revision history
Table 5. Document revision history
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Date
Revision
Changes
30-May-2013
1
Initial release.
25-Jul-2013
2
Updated: Figure 5, Figure 6, Figure 7, Figure 8, Figure 19,
Figure 21, Figure 23, Figure 24, Figure 25, Figure 26, Figure 27,
Figure 28, Figure 29 and Figure 30.
23-May-2014
3
Changed the title in cover page.
Updated Table 2.
15-Dec-2014
4
Updated Equation 8, Equation 11 and Equation 16.
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