Semiconductor Components Industries, LLC, 2004
June, 2004 − Rev. 1
1 Publication Order Number:
AND8127/D
AND8127/D
Implementing NCP1207 in
QR 24 W AC−DC Converter
with Synchronous Rectifier
Prepared by: Petr Lidak
ON Semiconductor
Introduction
The NCP1207 is a controller dedicated for driving the
current−mode free running quasi−resonant Flyback offline
converter.
This converter is designed for consumer products like
notebooks, offline battery chargers, consumer electronics
(DVD players, set−top boxes, TVs), etc.
The growing interest for EMI pollution reduction,
efficiency improvement, and maximum safety has been
taken into account while designing the NCP1207.
By implementing the NCP1207 one can build a power
supply that can meet all those requirements. This can be
achieved with help of the following NCP1207 main
features:
• Current−Mode Control: Cycle−by−cycle primary
current observation is helping to prevent any significant
primary overcurrent which would cause transformer’s
core saturation and consequent serious power supply
failure.
• Critical Mode Quasi−resonant Operation: Prevents the
converter operation in Continuous Conduction Mode in
any input and output condition. It is provided by the
zero crossing detection of the auxiliary winding’s
voltage.
• By addition of the reasonable delay the switch turn−on
instant can be shifted to the minimum (valley) of drain
voltage. This improves EMI noise and efficiency.
• Dynamic Self−Supply: Ensures IC proper operation in
applications where the output voltage varies during
operation like battery chargers. The DSS also supplies
the IC when the overvoltage event is being latched and
converter operation is stopped.
• Overvoltage Protection: By sampling the plateau
voltage on the auxiliary winding, the NCP1207 enters
into latched fault condition whenever the overvoltage is
detected. The controller stays fully latched until the
V
CC
decreases below 4.0 V, e.g. when the user unplugs
the power supply from the mains outlet and re−plugs it.
The OVP threshold can be adjusted externally.
• Over−Load Protection: by continuously monitoring the
feedback loop activity, NCP1207 enters hiccup
operation as soon as the power supply is overloaded. As
soon as overload condition disappears, the NCP
resumes operation.
The 24 W AC−DC Adaptor Board Specification
The adaptor has following maximum and performance
ratings.
Output Power 24 W
Output Voltage 12 VDC
Output Current 2.0 A
Minimum Input Voltage 180 VAC
Maximum Input Voltage 240 VAC
Maximum Switching Frequency 70 kHz
The schematic diagram of the adaptor can be seen in
Figure 1.
Transformer Design
The bulk capacitor voltage than can be calculated:
(eq. 1)
V
bulk−min
V
AC−min
2
180 2
255 VDC
(eq. 2)
V
bulk−max
V
AC−max
2
240 2
339 VDC
The requested output power is 24 W.
Assuming 87% efficiency the input power is equal to:
(eq. 3)
P
IN
P
OUT
24
0.87
27.6 W
The average value of input current at minimum input
voltage is:
(eq. 4)
I
IN−AVG
P
IN
V
bulk−min
27.6
255
108 mA
APPLICATION NOTE
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AND8127/D
http://onsemi.com
2
Taking into account the absence of a clamping network the
suitable reflected primary winding voltage for 800 V rated
MOSFET switch is:
(eq. 5)
V
flbk
800 V V
bulk−max
V
spike
800 339 330 131 V
Using calculated Flyback voltage the maximum duty
cycle can be calculated:
(eq. 6)
max
V
flbk
V
flbk
V
bulk−min
131
131 255
0.339 0.34
The following equation determines peak primary current:
(eq. 7)
I
ppk
2 I
IN−AVG
max
2 108 10
−3
0.34
635 mA
The maximum switching frequency at minimum input
voltage is 70 kHz. Taking into account Quasi−Resonant
(QR) and valley switching operation of the NCP1207 the
QR time interval from the instant of the total core
demagnetization to the valley of switch’s drain voltage
needs to be taken into account when calculating the switch
max. ON−time interval.
Using QR time of 2s appropriate for 70 kHz switching
frequency the ON−time can be calculated as follows:
(eq. 8)
t
ON
1
f
sw
t
QR
max
1
70 10
3
2 10
−6
0.34 4.177 s 4.18 s
The EF25 core for transformer was selected. It has
cross−section area A
e
= 52.5 mm
2
. The N67 ferrite material
allows to use maximum operating flux density
B
max
= 0.25 T.
The number of turns for the primary winding is:
(eq. 9)
n
p
V
bulk−min
t
ON
B
max
A
e
255 4.18 10
−6
0.25 52.5 10
−6
80 turns
The primary inductance can be calculated as follows:
(eq. 10)
L
p
V
bulk−min
I
ppk
t
ON
255
0.635
4.18 10
−6
1.68 mH
The A
L
factor of the transformer’s core can be calculated
as follows:
(eq. 11)
A
L
L
p
(n
p
)
2
1.68 10
−3
(80)
2
263 nH
C2
47 F/
400 V
470
Q3
BC238
C5
1 nF
R10
1 k
PMEC
L1
21
34
+
R2
100
R3
39
D3
1N4148
C11
100 nF
C4
47 pF
C1
+
R13
18 k
R4
1 k
ISO1
PC817
1N4148
C6
1nF
Q1
STP4NB80
Q4
BC308
IC2
TL431BILP
T2
12
43
R6
R7
100
R9
1 k
R8
470
IC1
NCP1207
1
2
3
4
5
6
8
Demag
FB
CS
GND Out
Vcc
Vi
+
+
4k7
R12
R5
1.5
L2
15 uH
+
Q2
BC238
Q5
IRF2807
C12
1 nF
D2
1N4148
J2
1
2
J1
1
2
R11
33 k
T1
6
5
2
1
10
3
4
9
7
C7
4.7 nF
1 nF / Y1
F1
T1A
R1
39 k
−+
DB1
B250
3
1
4
2
Figure 1. Schematic Diagram of the QR 24 W AC−DC Converter with NCP1207 and Synchronous Rectifier
C13
C8
470 F/
25 V
C9
470 F/
25 V
C10
100 F/
35 V
C3
10 F/25 V
100 nF/
D1
X2