Implementing Constant Current Constant Voltage AC Adapter by NCP1200 and NCP4300A

AND8042/D
Implementing Constant
Current Constant Voltage
AC Adapter by NCP1200
and NCP4300A
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Prepared by: Hector Ng
ON Semiconductor
APPLICATION NOTE
Circuit Description
Circuit and BOM of the AC adapter is shown in Figure 1
and Table 1. This design can accept universal AC input from
90 V to 264 VAC. Bulk capacitors C5 and C6 are split by
inductor L1 and L2 to form the EMI filter as well as to
provide energy storage for the remaining DC to DC
converter circuit. Thanks to dynamic self supply of
NCP1200 (please refer to NCP1200 data sheet), Vcc
capacitor C7 is charged to startup voltage 11.4 V and the
power MOSFET MTD1N60E starts switching. To reduce
power consumption of NCP1200, HV pin (pin 8) is supplied
by half wave rectification through a parallel combination of
diode D6 and resistor R13. A small signal diode 1N4148 is
enough for this function because diode D6 just has to
withstand one diode drop during negative half cycle. R13 is
to equilibrate the voltages on the 1N4148 when both diodes
and high volt current source of NCP1200 are in the off state.
R12 is to set the power level at which NCP1200 goes into
pulse skipping, please refer to below section for more
details. RCD snubber R1, C1 and D3 provides the necessary
snubbing function to prevent drain voltage of MTD1N60E
to exceed 600 V. Choosing suitable value for the sensing
resistor R7 is very important as it limits the primary peak
current during power up. If its value is too low, the system
cannot deliver enough power during full load low AC input.
On the contrary, the transformer may go into saturation and
damages Q1 and NCP1200. Information on how to
determine value of R7 is elaborated in latter paragraph.
Introduction
This paper describes a compact design of constant current
constant voltage (CCCV) AC adapter based on the current
mode PWM controller NCP1200 and the secondary side
feedback IC NCP4300A. By these two ICs from ON
Semiconductor, circuit design is much simplified. These
devices enable users to meet ever increasing demand of
smaller dimension and more sophisticated protection
feature of AC adapter.
On the primary side, NCP1200 is used as the PWM
controller. This current mode controller requires very few
external components and no auxiliary winding is needed to
supply this IC. In addition, NCP1200 can fulfill IEA
recommendation easily because it features a pulse skipping
low power consumption mode.
NCP4300A is a general purpose device which consists of
two operational amplifiers and a high precision voltage
reference. One of the operational amplifiers is capable of rail
to rail operation. NCP4300A is employed to provide voltage
as well as current feedback to NCP1200.
Output of the AC adapter is maintained at 5.2 V from no
load to 600 mA. Further increase in load enters constant
current output portion and output is kept at 600 mA down to
zero volt. This output characteristic assures a basic
protection against battery overcharge which is needed by a
lot of applications, for instance cellular phone AC adapter.
 Semiconductor Components Industries, LLC, 2001
February, 2001 – Rev. 1
1
Publication Order Number:
AND8042/D
D1
MUR120
L1
470 µH
0.2 A
470p 250 V 100 k 1 W
C1
+
C5
4.7 µ
400 V
90–264 VAC
+
DF06S
–
V4
1 U2
4
+
C6
4.7 µ
400 V
2
R1
U1
1
2
3
4
3
8
7
6
5
D3
1N4937
+
C2
10 µ
Q1
MTD1N60E
R10
68 k
3
1
In1–
2
In1+
3
Ground
4
U4
D4
1N4148
D5
1N4148
Figure 1. Circuit Description
VCC
8
Out2
7
In2–
6
In2+
5
NCP4300AD
C9
0.047 µF
+
R5
10 k
1%
U3
2
R12
10 k
R3
10 k
1%
C4
47 µ
Out1
R7
3.3
0.6 W
SFH6156–3
4
1
L2
470 µH
0.2 A
R2
3.3 k
R4
1.5 k
C3
330 µF
C10
1 nF
250 VAC
Y1
D6
1N4148
+
C7
47 µF
+
5.2 V, 600 mA
R6
0.15
R8
2.7 k
1%
R9
470
R11
75 k
1%
C8
0.1 µ
AND8042/D
2
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NCP1200
NCP1200D60
R13
220 k
L3
4.7 µH
1A
D2
1N5819
AND8042/D
Table 1.
Reference
Part
Quantity
Manufacturer
U1
NCP1200D60
1
ON Semiconductor
U2
DF06S
1
General Semi or IR
U3
NCP4300AD
1
ON Semiconductor
U4
SFH6156–3
1
Infineon
Q1
MTD1N60E
1
ON Semiconductor
C1
470 p, 250 V
1
C2, C7
10 F, 25 V
2
C3
330 F, 35 V
1
Panasonic FC Series or Rubycon JXA Series
C4
47 F, 16 V
1
Panasonic FC Series or Rubycon JXA Series
C5, C6
4.7 F, 400 V
2
C8
0.1 F
1
C9
0.047 F
1
R1
100 K, 1.0 W
1
R2
3.3 K
1
R3, R5
10 K, 1%
2
R4
1.5 K
1
R6
0.15 W, 0.1 W SMT
1
R7
3.3 , 0.6 W
1
R8
2.7 K, 1%
1
R9
470 1
R10
68 K
1
R11
75 K, 1%
1
R12
10 K
1
R13
220 K
1
D1
MUR120
1
D4, D5, D6
1N4148
3
D2
1N5819
1
ON Semiconductor
D3
1N4937
1
ON Semiconductor
L1, L2
470 H, 0.2 A
2
L3
4.7 H, 1.0 A
1
T1
Transformer
1
C10
1.0 nF, 250 VAC, Y1 Cap
1
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ON Semiconductor
AND8042/D
OP2 is below ground. Once the output current reaches
600 mA, feedback action is taken over by OP2 and one will
see a drop in output voltage if load is further increase but
output current remains constant. C9, R10 and C8, R9
provide necessary feedback compensation for voltage and
current loop respectively.
The secondary side of the transformer consists of 2
windings, the output winding as well as a higher voltage
winding which is used to supply power to NCP4300A. As
the output may drop to 0 V during constant current
operation, turn ratio of this higher voltage winding must be
able to sustain minimum Vcc as specify by NCP4300A. Or
else, the system will be lost of feedback and the output is not
under control anymore. Figure 2 shows the internal block of
NCP4300A. A 2.6 V, 1.0% tolerance voltage reference is
connected to the non–inverting terminal of OP1. Thus, OP1
gives voltage feedback when its inverting terminal is
connected to the potential divider R3 and R5. Characteristic
of the voltage reference is similar to industry standard
TL431 and a bias current supplied by R2 is needed to
guarantee proper operation. This 2.6 V is also divided down
by R11 and R8 to provide reference for output current
sensing. Voltage developed at the non–inverting terminal of
OP2 is:
Transformer Design
Transformer design involves very tedious calculation. An
Excel spreadsheet has been specially designed for NCP1200
to facilitate user with a quick determination of transformer
parameters. Table 2 and Table 3 display the results of the
spreadsheet after keying in system parameters. Although
recommended transformer primary inductance is 4.6 mH,
3.2 mH is chosen instead. A lower primary inductance
enables us to have a lower flyback voltage added to the drain
of the power MOSFET. This in turn allow us to use a less
heavy snubber which implies less power dissipated on the
snubber. Disadvantage of a lower primary inductance is the
increase in MOSFET conduction loss because of higher
primary peak current. However, output of this AC adapter is
only 3.0 W and typical RDS(on) of MTD1N60E is merely 5.9
. Increment in conduction loss is not significant in this
case.
After the primary inductance is determined, we have to
decide on the ferrite core. It can be seen from the Excel
spreadsheet that E16/8/5 core is big enough for this
transformer. Primary (N1) and secondary (N2) number of
turns needed are 166 and 12 respectively. However, one
more winding N3 is required to supply NCP4300A. It is
critical that voltage output of N3 must be higher than
minimum operating voltage of NCP4300A even when
output has dropped to 0 V. Under this condition, output
winding loop can be represented by Figure 3.
VCC
Out1
Out2
OP1
OP2
-
+
+
In1–
In2–
GND In1+ In2+
Figure 2.
Vcurrent reference 2.7 K2.7K75 K · 2.6 0.09 V
D2
1N5819
Since Out1 and Out2 are wired together by diodes D4 and
D5, feedback current through the opto–coupler U4 is
dominated by whichever op–amp output that has a lower
voltage. Thus feedback is dominated by OP1 until voltage
developed across R6 reaches 0.09 V and this is equivalent to
600 mA passing through R6. Thanks to the rail to rail
capability of OP2 in NCP4300A, current sensing function is
guaranteed although voltage of non–inverting terminal of
L3
47 µH
1A
Short
Circuit
VO(SC)
R6
0.15
Figure 3.
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AND8042/D
Table 2.
NCP1200 DISCONTINUOUS MODE DESIGN WORKSHEET
System Parameters
Vmax
264 V
Maximum AC Input Voltage
User Input Cells
Vmin
90 V
Minimum AC Input Voltage
Results
Fline
50 Hz
Line Frequency
Vmin(DC)
85.73 V
Minimum DC Voltage
Fs(max)
69 KHz
Maximum Switching Frequency
Fs(typ)
60 KHz
Typical Switching Frequency
Fs(min)
51 KHz
Minimum Switching Frequency
Vo
5.2 V
Output Voltage
Selected Device
Io
0.6 A
Maximum Output Current
60 KHz
75%
Efficiency
Vbd
600 V
Power MOSFET Breakdown Voltage
Vd
1V
PI
4.16 W
Input Power
Iin(pk)
0.21 A
Maximum Primary Peak Current
Vo′
85.72 V
Reflected Output Voltage
Vpwr_sw(max)
459.07 V
Maximum Voltage across the Power Switch Circuit (Less Leakage Spike)
Output Diode Voltage Drop
Dmax
0.50
Iin(av)
0.05 A
Maximum Input Average Current
13.83
Turn Ratio Between Primary and Secondary
Ratio N1/N2
Maximum Turn On Duty (Full Load, Low Line)
Recommended Lp
4.650 mH
Recommended Primary Inductance
Lp
3.200 mH
Primary Inductance
RDS(ON)
16 ohm
Maximum RDS(ON) of Power MOSFET
Pdls(pwr_sw)
0.12 W
Maximum Conduction Loss of Power MOSFET
Input Filter Capacitor
Recommended Cin
14 F
Recommended Input Filter Capacitance
Cin
9.4 F
Input Filter Capacitance
Io(pk)
2.40 A
Output Peak Current
Vro
32.20 V
Output Maximum Reverse Voltage
Output Diode Selection
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AND8042/D
Table 2. (continued)
NCP1200 DISCONTINUOUS MODE DESIGN WORKSHEET
Wire Selection
Iin(rms)
0.08 A
Maximum Input RMS Current
Io(rms)
0.98 A
Maximum Output RMS Current
Lay_p
1
Layer of Primary Winding
Lay_s
1
Layer of Secondary Winding
Primary Wire Size
AWG 35
Maximum Wire Size
Secondary Wire Size
AWG 24
AWG 24
RMS Current Density
4.9
(A/mm2)
Core Selection
Flux Density Safety Factor
0.4
Bobbin Usage Factor
0.4
Core Type
Core
Type A
Core
Type B
Core
Type C
Core
Type D
Core
Type E
Core Name
E 16/8/5
EI28–Z
E25/13/7
E 30/15/7
E32/16/9
Ae
20.1
86
52.5
60
83
mm2
Bsat
0.5
0.5
0.5
0.5
0.5
T
Aw
22.3
39.4
61
90
108
mm2
Bobbin Winding Window Area
Abob
8.92
15.76
24.4
36
43.2
mm2
Usable Area of Bobbin for
Winding
Gap Length d
0.22
0.05
0.08
0.07
0.05
mm
N1
166
39
63
56
40
N2
12
3
5
4
3
Ap
0.02
0.02
0.02
0.02
0.02
1
1
1
1
1
Lay_p
Apri
As
Lay_s
Asec
3.98
0.93
1.52
1.33
Area of Single Turn of Primary
Wire
Layer of Primary Winding
Area of Primary Winding
Area of a Single Turn of
Secondary Wire
0.26
0.26
1
1
1
1
1
1.04
Secondary Number of Turns
mm2
mm2
0.26
1.19
Primary Number of Turns
0.96
0.26
0.73
Saturation Magnetic Flux
Density
mm2
0.26
3.12
Effective Area
Layer of Secondary Winding
0.75
mm2
Area of Secondary Winding
mm2
Total Winding Area
Asum
7.09
1.66
2.72
2.38
1.72
Enough Space?
OK
OK
OK
OK
OK
Maximum Peak Current (Sensing Resistor) Setting
DLp
10%
Tolerance of Primary Inductance
Lp(min)
2.880 mH
Lowest Primary Inductance
Lp(max)
3.520 mH
Highest Primary Inductance
Ip(worst)
0.24 A
Worst Case Maximum Primary Peak Current
(Lowest Switching Frequency and Lowest Primary Inductance
Rsense(max)
4.20 ohm
Maximum Allowable Sensing Resistance
Rsense
3.30 ohm
Sensing Resistance
Binit
0.32 T
Magnetic Flux Density During Startup
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AND8042/D
Table 3.
Transformer Specification
Primary Inductance
Lp
3.200 mH
Core Type
=
E 16/8/5
Primary Wire Size
=
AWG 35
Layer of Primary Winding
=
1
Primary Number of Turns
N1
166
Secondary Wire Size
=
AWG 24
Layer of Secondary Winding
=
1
Secondary Number of Turns
Select Core Type
Core Type A
N2
12
Gap Length
d
0.22 mm
Enough Space?
=
OK
Cin
9.4 F
Vro
32.20 V
Rsense
3.30 ohm
Input Filter Capacitor
Input Filter Capacitance
Output Diode
Maximum Reverse Voltage
Sensing Resistor
Sensing Resistance
During flyback cycle, voltage across the output winding
Vo(sc) is:
N3
N1
Vo(sc) = V(D2) + V(L3) + V(R6) + V(PCB trace)
N2
V(D2) = forward voltage drop of 1N5819 ≈ 0.6 V
If resistance of L3 is 0.1 , V(L3) = 0.1 × 0.6 A = 0.06 V
V(R6) = 0.15 × 0.6 A = 0.09 V
If resistance of PCB trace is 0.15 , V(PCB trace) =
0.15 × 0.6 A = 0.09 V
N1 = 166T, AWG # 34, : 0.16 mm
N2 = 12T, AWG # 24, : 0.51 mm
N3 = 40T, AWG # 34, : 0.16 mm
Core = E16/8/5
Magnetic Material = PC40 or N67
Air Gap = 0.22 mm (center limb)
Primary Inductance (Across N1) = 3.2 mH
Therefore Vo(sc) is 0.84 V and volt/turn is 0.84/12 = 0.07.
Minimum operating voltage of NCP4300A is 3.0 V. Its
supply winding voltage has to be 0.6 V higher if we assume
forward drop on MUR120 is 0.6 V. Minimum number of
turns required for this winding is 3.6/0.07 ≈ 52 turns. As can
be seen from the schematic, these 52 turns can be added on
top of the output winding. Therefore 40 turns is enough for
N3. When output is 5.2 V, supply winding voltage of
NCP4300A is approximately 24.5 V. Thanks to its wide
operating voltage, 24.5 V is below maximum operating
voltage of NC4300A (35 V). The final design of the
transformer is shown in Figure 4.
Another important consideration is the value of sensing
resistor R7. Value of R7 control maximum primary peak
current by the following equation.
Figure 4.
For discontinuous mode operation, maximum power that
can be delivered by the system is:
P max 1 LpI2pk(max) f
2
Where Lp is the primary inductance which we already
decided and f is the switching frequency. In other words,
Ipk(max) must be high enough to give full load power and this
implies that R7 cannot be too high. The Excel spreadsheet
has calculated for us that R7 must be lower than 4.2 . 3.3
is chosen to give some headroom during transient
response. Before finalizing on this value, one must make
Ip(max) 1.0 V
R7
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AND8042/D
Vstby/4
sure that transformer does not saturate at power up. During
power up when output voltage is much lower than rated
value, MTD1N60E is switched off not by PWM action. The
power MOSFET is switched off because the primary peak
current has reached its maximum allowable value, Ip(max).
Ip(max) drives the transformer core up the B–H curve of the
magnetic material. B, magnetic flux density must be lower
than the saturation value Bsat. For most magnetic material,
Bsat equals 0.5 T at room temperature. Nevertheless, Bsat
falls as temperature increases and at 120°C, Bsat becomes
0.35 T. Last row in Table 2 shows the magnetic flux density
during startup. The value is 0.32 T, thus 3.3 should give
us a safe startup.
VCS
Figure 6.
Therefore the input power level Pstby that enters standby
mode is given by the following equation.
Pstby 1 Lp
2
0.5 3.2 E 3 Pulse Skipping Mode
NCP1200 has a pulse skipping standby mode feature and
the power level to enter standby mode is adjustable. Figure 5
shows the equivalent circuit of the Adj pin with a 10 K
resistor connecting Adj pin to ground. When the voltage at
FB pin falls below Adj pin, NCP1200 starts to skip cycle.
This voltage Vstby is:
Vstby V4Rstby7 f
40.466
2 60000
3.3
0.12 W
At light load condition, efficiency should be lower than
that of full load. Assume efficiency is 50% when input
power is at 0.12 W, load current Io(stby) at that time is:
Io(stby) 0.12 W 50% 0.01 A
5.2 V
Remember that Vo drops when Io attains 0.6 A. When Vo
drops below certain voltage, NCP1200 will also enters pulse
skipping mode. Once again, assume efficiency is 50% when
input power is at 0.12 W, Vo(stby) at that time is:
10 K29 K
· 5.2 V 0.466 V
10 K29 K 75.5 K
Vo(stby) 0.12 W 50% 0.1 V
0.6 A
NCP1200
75.5 k
+
In summary, NCP1200 starts pulse skipping when Io is
below 0.01 A or Vo is below 0.1 V.
Adj
10 k
5.2 Vdc
Actual Performance
Figure 7 and Table 4 shows the actual performance of the
circuit.
–
29 k
6
OUTPUT VOLTAGE
5
Figure 5.
Since NCP1200 is a current mode device, there is a direct
relationship between voltage at the FB pin and the voltage
developed by the peak current across the sensing resistor, ie.
voltage at CS pin, Vcs. As can be seen from the block
diagram of NCP1200 datasheet, Vcs is compared with one
fourth of FB pin voltage. Therefore at the verge of entering
into pulse skipping mode, we should see a relationship as
shown on Figure 6.
4
3
2
1
0
0
0.2
0.4
0.6
0.8
OUTPUT CURRENT
Figure 7. Vo–Io Characteristic @ 110 VAC Input
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AND8042/D
Table 4.
Test
Conditions
Results
Line Regulation
Vin = 90 to 264 VAC, Io = 0.6 A
= 0.5 mV
Load Regulation
Vin = 110 VAC, Io = 0 to 0.6 A
Vin = 220 VAC, Io = 0 to 0.6 A
= 3.0 mV
= 3.0 mV
Vin = 110 VAC, Io = 0.6 A
Vin = 220 VAC, Io = 0.6 A
40 mVpp
40 mVpp
Vin = 110 VAC, Vo = 5.2 V, Io = 0.6 A
Vin = 220 VAC, Vo = 5.2 V, Io = 0.6 A
68%
61%
Output Ripple
Efficiency
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AND8042/D
Notes
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Notes
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AND8042/D
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