NCP1351 Evaluation Board Printer Power Supply

AND8278/D
NCP1351 Evaluation Board
16 V / 32 V – 40 W Printer
Power Supply
Prepared by: Nicolas Cyr
ON Semiconductor
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APPLICATION NOTE
designer can thus easily combine startup time and standby
consumption.
Overload Protection Based on Fault Timer: every
designer knows the pain of building converters where a
precise over current limit must be obtained. When the fault
detection relies on the auxiliary VCC, the pain even
increases. Here, the NCP1351 observes the lack of feedback
current and starts a timer to countdown. At the end of its
charge, the timer either triggers an auto−recovery sequence
(auto−restart, B and D versions) or permanently latches−off
(A and C). On C and D versions the fault timer is started at
an output power corresponding to 60% of the maximum
deliverable power; to allow transient peak power delivery.
Latch Fault Input: a dedicated input lets the designer
externally trigger the latch to build additional protections
such as over−voltage (OVP) or over−temperature (OTP).
The present document describes a printer power supply
operated by the NCP1351, a fixed ton / variable off time
controller. The board can deliver 10 W average on a 16 V
output and 30 W average on a 32 V output with a transient
peak power capability of 80 W. It however exhibits a low
standby power: below 150 mW at no load whatever the input
voltage. Let us first review the benefit of using the
NCP1351:
The NCP1351 at a Glance
Fixed ton, Variable toff Current−mode Control:
implementing a fixed peak current mode control (hence the
more appropriate term “quasi−fixed” ton), the NCP1351
modulates the off time duration according to the output
power demand. In high power conditions, the switching
frequency increases until a maximum is hit. This upper limit
depends on an external capacitor selected by the designer. In
light load conditions, the off time expands and the NCP1351
operates at a lower frequency. As the frequency reduces, the
contribution of all frequency−dependent losses accordingly
goes down (driver current, drain capacitive losses, switching
losses), naturally improving the efficiency at various load
levels.
Peak Current Compression at Light Loads: Reducing
the frequency will certainly force the converter to operate
into the audible region. To prevent the transformer
mechanical resonance, the NCP1351 gradually reduces –
compresses – the peak current setpoint as the load becomes
lighter. When the current reaches 30% of the nominal value,
the compression stops and the off duration keeps expanding
towards low frequencies.
Low Standby−power: the frequency reduction technique
offers an excellent solution for designers looking for low
standby power converters. Also, compared to the skip−cycle
method, the smooth off time expansion does not bring
additional ripple in no−load conditions: the output voltage
remains quiet.
Natural Frequency Dithering: the quasi−fixed ton mode
of operation improves the EMI signature since the switching
frequency varies with the natural bulk ripple voltage.
Extremely Low Start−up Current: built on a
proprietary circuitry, the NCP1351 startup section does not
consume more than 10 mA during the startup sequence. The
© Semiconductor Components Industries, LLC, 2007
March, 2007 − Rev. 0
The Schematic
The design must fulfill the following specifications:
Input voltage: 88 – 265 Vac
Output voltage: 16 V @ 0.625 A and 32 V @ 1 A nominal
(40 W); with transient 80 W peak power capability during
40 ms, and 62 W peak during 400 ms
Over power protection below 100 W for the whole input
voltage range (LPS)
Latched short−circuit protection
Latched Over voltage protection
Latch recovery time below 3 s
Brown−out protection
Start−up time below 3 s
In order to deliver the peak output power, the NCP1351
will increase its switching frequency up to the upper limit set
by the CT capacitor. To not jeopardize the EMI test
compliance, the switching frequency should be kept below
150 kHz. We will choose 100 kHz to have a good margin. As
a result the switching frequency at nominal load will be
around 50 kHz. Since we need to deliver 80 W of transient
peak power while ensuring the power will never be above
100 W, we will use the C version of NCP1351, specially
tailored for this kind of application. When the controller
detects a need for a frequency higher than 60 kHz, implying
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Publication Order Number:
AND8278/D
AND8278/D
an overload condition, it will start to charge the timer
capacitor: if the overload disappears, the timer capacitor
goes back to zero. If the fault remains, the timer capacitor
voltage reaches 5 V and latches off the controller. During the
fault condition, the power supply will anyway deliver the
output power while the switching frequency is below its
maximum value of 100 kHz.
The transformer has been derived using the Excel®
spreadsheet available from the ON Semiconductor website
which also gives transformer parameters. We came up to the
following values:
Lp = 270 mH
Np:Ns = 1:0.2
Np:Naux = 1:0.2
Ipk = 3 A
The transformer has been manufactured by Coilcraft
(www.coilcraft.com). The leakage inductance is kept
around 3% of the primary inductance, leading to a good
efficiency and reduced losses in no−load conditions. The
schematic appears on Figure 1. The converter operates in
DCM at nominal power; and for peak power it goes CCM
with close to 50% duty−cycle at low mains and stays CCM
at high line.
D5
L3
D13
+
+
C13
U1
C12
R5
R24
R6
R15
D7
D4
+
C19
C18
32 V
C20
L2
C15
+
C16
+
C17
GND
16 V
X6
R23
R10
L1
C14
X4x
Fuse
R13
R14
R8 C4
D9
NCP1351C
1
2
3
4
C6
R20
D3
R2
Aux
D10
8
7
6
5
X11
X4
R1
C1
+
C3
C8
C10
R28
R18
D11
+
C7
R30
C23
R11
R31
X10
R19
R21
C21
R22
Figure 1. The Simplified 40 W Printer Board Featuring the NCP1351 Controller
controller is latched (a direct connection to the AC line
would also work). Despite a small value for C3, the VCC still
maintains in no−load conditions thanks to the split
configuration:
Two 330 kW resistors in series with a 60 V zener diode
ensure a clean start−up sequence with the 4.7 mF capacitor
(C3), not from the bulk capacitor as it is usually done; but
from the fully rectified, unfiltered haversine. This
configuration allows for a quick release time after the
HV rail
R5
330 k
+
Cbulk
R6
330 k
1
8
2
7
3
6
4
5
D10
60 V
VCC
+
C3
4.7 m
NCP1351
+
C7
22 m
Aux
Figure 2. The split VCC configuration helps to start−up in a small period of time (C3 to charge alone)
but the addition of a second, larger capacitor (C7), ensures enough VCC in standby.
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AND8278/D
The primary−side feedback current is fixed to roughly
300 mA via R8 and an additional bias is provided for the
TL431. 1 mA at least must flow in the TL431 in worse case
conditions (full load). Failure to respect this will degrade the
power supply output impedance and regulation will suffer.
A 2.7 kW value for R19 has proven to do just well, without
degrading the standby power.
The overvoltage protection uses a 17 V zener diode (D9)
connected to the auxiliary VCC. When the voltage on this rail
exceeds 17 V plus the NCP1351 5 V latch trip point (total is
thus 22 V), the circuit latches−off and immediately pulls the
VCC pin down to 6 V. The reset occurs when the injected
current into the VCC pin falls below a few mA, that is to say
when the power supply is disconnected from the mains
outlet. To speed−up this reset phase, a connection to the fully
rectified haversine resets the system faster (Figure 2).
To satisfy the maximum power limit, we don’t need to add
a true Over Power Protection (OPP) circuit since our
NCP1351C transiently authorizes higher power, but safely
latches off if the overpower lasts too long. To ensure a fault
timer duration of at least 500 ms (to be able to deliver the
62 W power peak during 400 ms), the timer capacitor C10
must be 1.5 mF. This value will be adjusted depending on the
specification, according to the maximum peak power
duration the adapter must sustain.
If anyway a constant overpower protection is needed over
the whole input voltage range, a simple arrangement can be
used: given the negative sensing technique, we can use a
portion of the auxiliary signal during the on time, as it also
swings negative. However, we don’t want this compensation
for short TON durations since standby power can be affected.
For this reason, we can insert a small integrator made of
C9−R26 (see Figure 3). To avoid charging C9 during the
flyback stroke, D14 clamps the positive excursion and offers
a stronger negative voltage during the on time.
Rcomp
R26
470 k
2.2 k
−N.Vin
OPP Adjust
C9
220 p
D14
1N4148
Rcs
1
8
2
7
3
6
4
5
VCC
+
+
Vaux
Rsense
NCP1351
Figure 3. A Simple Arrangement Provides an
Adjustable Overpower Power Compensation
A simple resistor connected between the auxiliary
winding (that swings negative during the ON time) and the
CT capacitor ensure a stable operation in CCM despite the
duty cycle above 50% at very low line, due to the ripple on
the bulk capacitor. The unique features of NCP1351C allow
using a 100 mF bulk capacitor while delivering the transient
peak power and ensuring the output is still regulated during
line drop−outs.
Finally, the clamping network maintains the drain voltage
below 520 V at high−line (375 Vdc) which provides 85%
derating for the 600 V BVdss device.
Measurements
Once assembled, the board has been operated during 15
min at full power to allow some warm−up time. We used a
WT210A from Yokogawa to perform all power related
measurements coupled to an electronic ac source.
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AND8278/D
Efficiency:
VIN (POUT)
120 Vac
230 Vac
40 W
84.4%
85.4%
25 W
85.9%
85.9%
10 W
86.0%
85.1%
5W
85.5%
83.2%
2W
83.4%
79.5%
1W
77.7%
73.3%
0.5 W
70.0%
66.3%
VIN (POUT)
120 Vac
230 Vac
No−load
75 mW
140 mW
No−load Power:
Overpower Protection Level:
The power supply is able to deliver a peak power of 85 W
during 500 ms from 85 Vac to 270 Vac.
It can deliver a constant output power of more than 40 W,
but less than 80 W over the same input voltage range.
Start−up Time:
VIN (POUT = 40 W)
85 Vac
230 Vac
Start−up Duration
2.7 s
0.5 s
In the above tables, we can see the excellent efficiency,
especially at light load conditions thanks to the natural
frequency foldback of the NCP1351.
The no−load standby power stays below 150 mW at high
line, a good performance for a dual output power supply able
to deliver 80 W. Please note that the high−voltage probe
observing the drain was removed and the load totally
disconnected to avoid leakage.
Despite operation in the audible range, we did not notice
any noise problems coming from either the transformer or
the RCD clamp capacitor.
120
100
Vin(max)
Vin(min)
FSW
80
60
CCM Transition
40
20
0
0
20
40
60
80
100
120
140
Pout
Figure 4. Switching Frequency Variations vs. Output Load
Scope Shots
Below are some oscilloscope shots gathered on the demoboard:
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AND8278/D
2.7 s
32 V
VOUT
VDRAIN
Figure 5. Startup Time, VIN = 85 Vac
538 V
VDRAIN
VDRAIN
Figure 6. Maximum Output Power, VIN = 265 Vac
Conclusion
The printer power supply built with the NCP1351 exhibits
an excellent performance on several parameters like the
efficiency and the low−load standby. The transient
switching frequency increase allows to deliver peak power
during a limited time; but if the overpower lasts longer than
the set fault timer, the controller safely latches off.
The limited number of surrounding components around
the controller associated to useful features (timer−based
protection, latch input…) makes the NCP1351 an excellent
choice for cost−sensitive printer adapter designs.
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AND8278/D
Bill of Materials
Designator
Qty
Description
Value
Tol.
Footprint
C1, C4, C15, C18, C21
5
SMD capacitor
100 nF / 50 V
5%
SMD 1206
C3
1
electrolytic capacitor
4.7 mF / 50 V
20%
radial
C5
0
SMD capacitor
−
5%
SMD 1206
C6
1
SMD capacitor
180 pF / 50 V
5%
SMD 1206
C7
1
electrolytic capacitor
47 mF / 50 V
20%
radial
C8
1
SMD capacitor
10 nF / 50 V
5%
SMD 1206
C9
0
SMD capacitor
−
5%
SMD 1206
C10
1
SMD capacitor
1.5 mF
C12
1
Film capacitor
10 nF / 630 V
10%
radial
C13
1
electrolytic capacitor
100 mF / 400 V
20%
radial
C14
1
x2 capacitor
330 nF / 250 Vac
20%
radial
C16
1
electrolytic capacitor
1000 mF / 50 V
20%
radial
C17
1
electrolytic capacitor
100 mF / 50 V
20%
radial
C19
1
electrolytic capacitor
1000 mF / 25 V
20%
radial
C20
1
electrolytic capacitor
100 mF / 25 V
20%
radial
C23
1
y1 capacitor
2.2 nF / 250 Vac
20%
radial
C101
0
SMD capacitor
−
5%
SMD 1206
D1
1
SMD resistor
0 W / 0.25 W
5%
SMD 1206
D2
0
Zener diode
−
5%
SOD−123
D3
1
High−voltage switching diode
BAS20
200 mA / 200 V
−
SOT−23
D4
1
Fast−recovery rectifier
1N4937
1 A / 600 V
−
axial
D5, D7
2
Schottky rectifier
MBR20100CT
20 A / 100 V
−
TO−220
D9
1
Zener diode
17 V / 0.5 W
5%
SOD−123
D10
1
Zener diode
60 V / 0.5 W
5%
SOD−123
D11
1
Switching diode
200 mA / 75 V
−
SOD−123
D12
1
Zener diode
6.2 V / 0.5 W
5%
SOD−123
D13
1
Standard rectifier
1 A / 1000 V
−
axial
SMD 1206
D14
0
Switching diode
−
−
SOD−123
HS1
1
Heatsink
6.2°C / W
−
radial
HS2, HS3
2
TO−220 heatsink
27°C / W
−
−
U1
1
Rectifier bridge
DB105
1A / 600 V
−
DIP−4
U2
1
CMOS IC
NCP1351A
−
−
SOIC−8
X4
1
Optocoupler
SFH615
−
−
DIP−4
X6
1
Common−mode choke
Panasonic ELF−25F108A
2 * 15 mH/ 1 A
−
radial
X10
1
shunt regulator
TL431
2.5 – 36 V
5%
TO−92
X11
1
Power MOSFET N−Channel
3 A / 600 V
−
TO−220
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AND8278/D
Bill of Materials
Designator
Qty
Description
Value
Tol.
Footprint
Q1
0
PNP transistor
−
−
TO−92
T1
1
Transformer Coilcraft
GA0007−AL
−
−
radial
J1
1
connector
230 Vac
F1
1
Fuse
2 A / 250 Vac
L1
1
SMD inductor
Coilcraft
10 mH
L2, L3
1
inductor
4.7 mH / 10 A
−
radial
R1
1
SMD resistor
15 W / 0.25 W
5%
SMD 1206
R2
1
resistor
4.7 MW / 0.33 W
5%
axial
R5, R6
2
SMD resistor
330 kW / 0.25 W
1%
SMD 1206
radial
T
radial
SMD
DO1605T
R7
1
SMD resistor
0 W / 0.25 W
5%
SMD 1206
R8, R19
2
SMD resistor
2.7 kW / 0.25 W
5%
SMD 1206
R9, R12
2
SMD resistor
0 W / 0.25 W
5%
SMD 1206
R10, R11, R18
3
SMD resistor
1 kW / 0.25 W
5%
SMD 1206
R13
1
SMD resistor
3.4 kW / 0.25 W
1%
SMD 1206
R14
1
SMD resistor
0.33 W / 0.5 W
1%
SMD 2010
R15
1
resistor
150 kW / 2 W
5%
axial
R16
0
SMD resistor
−
−
SMD 2010
R17
0
SMD resistor
−
−
SMD 1206
R20
1
SMD resistor
100 kW / 0.25 W
1%
SMD 1206
R21
1
SMD resistor
56 kW / 0.25 W
1%
SMD 1206
R22
1
SMD resistor
10 kW / 0.25 W
1%
SMD 1206
R23, R24
2
SMD resistor
3.3 MW / 0.25 W
5%
SMD 1206
R25
0
SMD resistor
−
1%
SMD 1206
R26
1
SMD resistor
0 W / 0.25 W
1%
SMD 1206
R28
1
SMD resistor
8.2 kW / 0.25 W
1%
SMD 1206
R30
1
SMD resistor
47 kW / 0.25 W
1%
SMD 1206
R31
1
SMD resistor
180 kW / 0.25 W
1%
SMD 1206
RV1
1
NTC
−
−
Radial
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AND8278/D
PCB Layout
Figure 7. Top Side Components
Figure 8. Copper Traces
Figure 9. SMD Components
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AND8278/D
U1
Fuse
DF05M
220 n
0.33
NC
R13
3.3 M R23
3.3 M
C9
R2
4.7 M
OptoBase
X4x
C5
FB
1.5 m 2.7 k R8
C4 0 R9 Ct
R26
0
D12
6V2
NC C101
4 3 2 1
5 6 7 8
NTC
NCP1351A
100 n
Timer
1k
R10
C10
C1
100 n
C8
D9
10 n
R11 1 k D2
17 V
VCC D10
60 V
1N4148
NC
0 R12
R5
330 k
330 k
+
+
NC
Q1
D1
NC
10 n
1N4007
0
R1
15
R17
R6
D11 R7
4.7 m
C7
47 m
R24
R25 OPP
NC
C6
C3
Jitter
CS
180 p
NC
C13
100 m
+
3.4 k
D14
NC
1N4007
D13
ELF−25F108A
R14
Rsense
X6
C14
D4
BAS20
D3
10 m
L1
Aux
0
C12
R15
150 k
D7
R30
Vaux
MBR20100
C15
C18
100 n
47 k
D5
MBR20100
100 n
X11
IRFIB6N60
C16
X4
1000 m
+
C23
1000 m
C19
+
2.2 n
1 k R18
4.7 m
TL431
C17
SFH615A
C21
10 k
R22
100 n
R28 3.24 k
105 k R20
R21 3.65 k
200 k R31
+
X10
C20
+
2.7 k R19
L3
4.7 m
L2
100 m
32 V
16 V
GND
Figure 10. The 40 W Printer Board Featuring the NCP1351 Controller
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100 m
AND8278/D
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to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability
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“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
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AND8278/D