Implementing the NCP1200 in a 10 W AC/DC Wall Adapter

AND8038
Implementing the
NCP1200 in a 10 W AC/DC
Wall Adapter
Prepared by: Christophe Basso
ON Semiconductor
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APPLICATION NOTE
INTRODUCTION
rugged 10 W adapter. This adapter is designed to operate
from a universal mains (90–260 VAC) while providing a
good standby power at no load.
The NCP1200 implements a standard current mode
architecture where the switch–off time is dictated by the
peak primary current setpoint. By combining fixed
frequency and skip cycle operation in a single integrated
circuit, ON Semiconductor NCP1200 represents an
excellent solution where cost and ease of implementation
are premium: low–cost AC/DC adapters, auxiliary
supplies, etc. Furthermore, the device does not require any
auxiliary winding to operate and thus offers a real
breakthrough alternative to UC384X based supplies. This
application note details how to build an efficient and
+
C2
47 F
400 V
1 Adj HV 8
2 FB
7
3 CS VCC 6
B1
SMD
Universal Input
The Electrical Schematic
Driving an external MOSFET, the NCP1200P60, only
requires a sense element and a Vcc capacitor. Working
together with an internal high–voltage current source, this
Vcc capacitor provides the NCP1200 with an average DC
level of 11 V typically while it also controls the short–circuit
time out. All these parameters are detailed in the application
note AND8023 available to download at www.onsemi.com.
The electrical schematic appears in Figure 1:
C8
10 nF
400 V
R7
22 k
2W
1:01
D2
MBR360T3
Lp
1.8 mH
+
T1
D3
MUR160
L2
22 H
+ C6b
470 F
16 V
12 V @ 0.85 A
+
C7 100 F/16 V
Ground
C6a
470 F
16 V
R3
560
R5
3.9 k
M1
MTP2N60E
4 Gnd Drv 5
C1
R2
1 Meg
R1
10
C4
100 nF
100 nF
X2
L1
2 x 27 mH CM
Schaffner
RN1140–08/2
+
C3
22 F
16 V
R4
1.8
1W
C5
2.2 nF
Y Type
IC2
TL431 R6
1k
C9
1 nF
Figure 1. A 10 W AC/DC adapter built with the NCP1200
 Semiconductor Components Industries, LLC, 2001
April, 2001 – Rev. 3
1
Publication Order Number:
AND8038/D
AND8038
protection activates again. If the short–circuit has gone, the
IC resumes its operation and delivers its normal level. To
check the correct value of the calculated Vcc capacitor, you
need to monitor both output voltage and Vcc level on an
oscilloscope. A shot as proposed by Figure 2 confirms the
validity of a 22 µF choice. We can see that the internal error
flag goes high first but as soon as Vout reaches its target
level, the flag goes back to zero, confirming the normal
controller behavior at the UVLOLow checkpoint. This
experiment should be carried in the worse case conditions,
e.g. low mains and maximum output load.
As stated in AND8023, the Vcc capacitor needs to be
evaluated taking into account the startup sequence (actually
seen as a transient short–circuit by the controller). An
internal error flag is raised within the NCP1200 when an
output overload occurs. If this error flag is still asserted
when the Vcc capacitor reaches UVLOLow (around 10 V
typical), then the IC goes into the latch–off phase: the output
drive is locked and the internal consumption falls down to
350 µA typical. When another Vcc breakpoint is reached
(around 6.0 V), then the internal current source turns on
again and the IC tries to restart. If the error is still present, the
Error flag
Ok, flag = 0
Figure 2. The startup sequence shows a Vout establishment before UVLOLow is reached
Feedback Loop
In this application, a precise output voltage is obtained
through the use of a TL431. Since we target a 12 V output,
you calculate the upper and lower voltage sense elements by
applying the following formula:
Vout (R6) = 1.0 kΩ. This network ensures a bridge current flow
of 2.0 mA which is good for the noise immunity. As any
closed loop systems, a compensation network needs to be
tailored to stabilize the loop. In this aspect, the NCP1200
average SPICE model will save you a tremendous amount
of time. The simulation template appears in Figure 3 on the
following page, showing how to wire the NCP1200 average
model with INTUSOFT’s IsSpice4.
Rupper
1 · Vref
Rlower
Depending on the TL431 type, Vref can be 2.5 V or 1.25 V.
With a 2.5 V reference, Rupper (R5) = 3.9 kΩ and Rlower
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AND8038
Iout
NCP1200
Averaged
IN
CTRL
120
LoL
1 kH
+
CoL
1 kF
0
Vin
out1 L1
22 µF
4
Rs out2
10 m
12.2
+
6
D1
MBR140P
X1
NCP1200_Av
FS = 66 k
L = 1.8 m
RI = 1.5
12
Vin
126
2
FB GND
2.38
1
127
OUT
X1
XFMR
RATIO = 0.1
R4
100 m
R5
100 m
12.2
12.2
15
14
+
C1
470 µF
Vstim
AC = 1
out1
R17
300 m
12.2
7
9
C5
470 µF
Rload
14
C2
10 µF
out2
R15
560
2.38
Vout
11.8
11
5
Cf
100 nF
11.1
10
Rupp
3.9 k
2.50
X3
TL431
13
Rlow
1k
Figure 3. The average model of the NCP1200 when used in AC analysis
the AC stimuli to allow Bode plot generation. Figure 4
portrays the simulated results with a 100 nF feedback
capacitor, while Figure 5 offers the true measurement
curves.
The loop is kept opened in AC thanks to LoL which
exhibits a fairly high value. However, during its bias point
calculation, SPICE opens all capacitors and shorts all
inductors. Therefore, LoL closes the loop in DC but blocks
Mag (dB)
Phase
Gain
BW = 600 Hz
0
Y = 20 dB/div
10
100
Y = 45°/div
1k
10 k
Phase (deg)
80.00
180.00
60.00
135.00
40.00
90.00
20.00
45.00
0.00
0.00
–20.00
–45.00
–40.00
–90.00
–60.00
–135.00
–80.00
–180.00
100 k
10
100
1k
10 k
100 k
Figure 5. confirmed by a network analyzer
measurement
Figure 4. Bode plot obtained using SPICE
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AND8038
switching cycles in standby operation. By default, skip cycle
takes place at 1/3rd of the maximum peak current: 200 mA
in our case. Because skip cycle frequency will naturally
enter into the audible range, it is important that the skip
current value does not engender noise. Fortunately, if that
would be the case, you could still wire a resistor bridge on
pin 4 to fix a DC point different than the default one (1.4 V).
As a result, you can force skip operation to happen at less
than 1/3rd of the maximum peak current. However, keep in
mind that the highest peak currents in skip mode offer the
best standby power. This is because of the switching cycles
population within the bursts: less cycles mean less switching
losses and better efficiency at no load.
A quick method to assess the RMS current in the
MOSFET consists in simulating the whole AC adapter with
SPICE. This has already been presented in AND8029 and
the schematic will not be reproduced here. The simulated
results are given below through Figure 6 and Figure 7 while
the supply is delivering 10 W:
As you can see, curves are in good agreement, despite the
small DC gain error which predicts a slightly lower
bandwidth in the case of SPICE. In both cases, the phase and
gain margins confirm the good stability of the design, but
also the validity of the SPICE model (based on Ben–Gurion
University GSIM approach). The NCP1200 FB pin being a
high impedance path, a 1.0 nF placed between this pin and
ground will prevent any noise picking during operation.
Transient Results
Using the NCP1200 design aid spreadsheet lead us to a
transformer offering the following specs: Lprim = 1.8 mH,
Np:Ns = 1:0.1, RM8 or E25 core. For ease of
implementation, this transformer will be available from
Coilcraft, as referenced in the bill of material. The maximum
peak current has been fixed to 600 mA. This value
essentially defines the air gap requirement in the transformer
but also the final potential transformer mechanical noise
generated in standby. As explained, the NCP1200 skips
1.030 M
1.040 M
1.050 M
1.060 M
1.070 M
Figure 6. Transient results obtained with IsSpice4
Figure 7. Compared to true measurements
power in free–air conditions (without a heatsink) of:
Worse case conditions (low mains, maximum output
current) gives an RMS drain current of 230 mA. Associated
with a 6.5 Ω Rds(ON) @ Tj = 100°C, the conduction losses
grow up to 340 mW. Using a TO220 package for the
MOSFET, offers the ability to dissipate a given amount of
Pmax Tj Tamb
1.3 W. Further switching losses
Rj a
measurements confirm the ability to use this MOSFET
without any heatsink up to an ambient of 80°C.
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4
AND8038
dBµV
90
EN_V_QP
80
60
40
20
0
–20
0.15
1.0
10.0
30.0
MHz
Figure 8. The final composite QP plot carried over one line while
the other is loaded (230 VAC, Pout = 10 W)
Final Performance
We have carried some power tests on the 10 W adapters
and the below numbers will confirm the pertinence of
choosing ON Semiconductor’s NCP1200 for your next
designs:
Conducting EMI Filtering
The 10 W NCP1200 demo board is equipped with a front
stage filter who lets you pass the CISPR22 EMI tests in both
quasi–peak and average detector methods. The method we
used for calculating the filter is described in AND8032
“Conducted EMI Filter Design for the NCP1200’’. The front
stage is made of a single common mode (CM) choke whose
wiring method gives enough leakage inductance for
differential mode (DM) filtering. Figure 8 plots the final
CM+DM noise component confirming the test passing.
VinDC
Pout(W)
Pin(W)
(%)
126
0
0.245
–
126
10.5
12.6
83.3
356
0
0.462
356
10.5
13.17
79.7
The standby power can be further reduced by
implementing one of the method proposed in AND8023
either through an additional diode or an auxiliary winding.
Thanks to its inherent protection circuitry, NCP1200
protects the power supply in presence of a permanent output
short circuit. When shorted, the average output current was
less than 500 mA.
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AND8038
C6a
470 µF/16 V, vertical
C6b
470 µF/16 V, vertical
C7
100 µF/16 V, vertical
C8
10 nF/400 V
D1
MUR160, ON Semiconductor
D2
MBRS360T3, ON Semiconductor
B1 Bridge 1 A/600 V, mini DIP
Transformer available from Coilcraft U.S, ref. : Y8848–A
Mains connector: Schurter GSF1.1202.31 with fuse
10 W Demoboard, Bill of Material
R1
10 Ω, 1 W through holes
R2a R2b 2 times 560 kΩ SMD in series
R3
560 Ω SMD
R4
1.8 Ω, 1W SMD or 1.8 Ω 1 W through holes
R5
3.9 kΩ SMD
R6
1 kΩ, SMD
R7
22 kΩ, 2 W through holes
L1
Schaffner RN114–08/2
L2
22 µH, 1 A
M1
MTP2N60E, TO–220 through holes,
ON Semiconductor
IC1
SFH615A–2, SMD (optocoupler)
IC2
TL431BC (TO–92), ON Semiconductor
IC3
NCP1200P60, DIP8, ON Semiconductor
C1
100 nF X2/ 250 VAC
C2
47 µF/400 V, snap–in vertical
C3
22 µF/16 V, vertical
C4
100 nF, SMD
C5
1.5 nF Y1 type only
Other Available Documents Related to NCP1200:
AND8023/D, “Implementing the NCP1200 in Low–Cost
AC/DC Adapters”
AND8029/D, “Ramp Compensation for the NCP1200”
AND8032/D, “Conducted EMI Filter Design for the
NCP1200”
PSpice, IsSpice4 and Micro–Cap Averaged and Transient
models available in ready–to–use templates at
www.onsemi.com
NCP1200 Design aid spreadsheet with EBNCP1200/D
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AND8038
Printed Circuit Board Details
Figure 9. Component Side, Silk Screen, Scale 1
Figure 10. Solder Side, Silk Screen, Scale 1
Figure 11. Copper Traces, Scale 1
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