NCP1077: 12 Vout, 6 W, Off-line Buck Regulator Using a Tapped Inductor

DN05059/D
Design Note – DN05059/D
NCP1077, 12 Vout, 6 Watt, Off-line Buck
Regulator Using a Tapped Inductor
Device
Application
Input Voltage
Output Power
Topology
I/O Isolation
NCP1077
Smart Meters
Electric Meters,
White Goods
85 to 265 Vac
6W at 12Vout
12W peak
Off-Line 100 kHz
Buck
Non-isolated
Output Specification
Output Voltage
Output Ripple
Typical Current
Max Current
Min Current
3.3 to 28 Vdc depending on selected Z1 zener value
Less than 1%
500 mA continuous
1 amp maximum (several second surge – thermally limited)
zero
PFC (Yes/No)
Efficiency
Inrush Limiting / Fuse
Operating Temp. Range
Cooling Method /
Supply Orientation
Signal Level Control
No, Pout < 25 watts
>75% typical at 120Vac
Fused input
0 to +50°C (dependent on U1 heatsinking)
Convection
None
Circuit Description
This design note describes a simple, low power, constant
voltage output variation of the buck power converter
intended for powering electronics for white goods,
electrical meters, and industrial equipment where
isolation from the AC mains is not required and maximum
efficiency is essential. This buck circuit design has been
modified by tapping the freewheel diode connection to
the inductor to provide several advantages over the
conventional buck circuit. ON Semiconductor application
note AND8318 provides a detailed discussion of the
tapped inductor buck circuit theory which will not be
covered in detail in this design note.
One of the major disadvantages of the conventional buck
circuit configuration is that for off-line applications, the
typical dc input-to-output voltage differential is very high;
resulting is a very short operational duty ratio (D) in the
power MOSFET. Since the buck’s input-to-output voltage
transfer function is defined as Vout = D x Vin, we can see
that for a rectified input of 165 Vdc and an output of 12
Vdc, D will be 12/165 = 0.07 or 7%. Assuming a
switching frequency of 100 kHz (T = 10 us), this results in
a typical on-time of 0.7 us. With this short of a duty ratio,
the conversion efficiency is not very good and this short
of pulse width is approaching the propagation delay time
for some control chips which can affect switching stability
at light load and higher input voltages. In addition, the
December 2013, Rev. 0
maximum dc output load current of the conventional buck
cannot be any greater than the peak current limitation of
the monolithic switcher, and is typically less due to the
magnetizing component of the inductor current.
By tapping the freewheel diode connection to halfway
point on the inductor, two advantages are achieved: 1)
The output current can be effectively boosted nearly
double that possible with the conventional buck
configuration because the power MOSFET duty cycle is
expanded by a factor of 2 without any increase in peak
current; and, 2) The normally high turn-on switching
losses caused by the freewheel diode recovery current in
the conventional buck are reduced due to the leakage
inductance component of the coupled winding in the
tapped choke.
The actual tap point on the inductor can be anywhere,
and, the closer it is to the output end of the inductor, the
greater the current boosting effect and extension of the
effective MOSFET duty ratio. For this design note, a
center tap inductor was chosen because several
commercial vendors provide such a part in a surface
mount configuration that can handle up to one amp peak.
Typical efficiency improvements of 5% or more over a
conventional buck have been achieved with this tapped
configuration and it is particularly effective when low
output voltages of 12V or less are required with highest
efficiency and low standby power.
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DN05059/D
Schematic
F1
1A
L2
N= 1:1
L1, 820uH
D1
AC input
85 - 265Vac
C2
0.1 uF
"X"
C3
D2A
+
C4 1000uF
16V
D2
33 uF,
400Vdc
R1
68
MURS260T3
U1
NCP1077
4
3
C6
10nF
Notes:
4
3 2
1
MRA4007
2
D3
MURA160
D3A
R4
1.5K
+
C1
22uF
25V
Z2
Output
12V, 500mA,
(1A peak)
_
Z1
R2
1
C5
0.1uF
50V
MMSZ5241B
(11V)
4
U2
1
33
R3
680
3
opto
2
1. L1 is Wurth 7447728215
2. L2 is Coilcraft MSD1583-224KE (150 to 220 uH coupled inductor)
3. U2 is Vishay H11A817A or similar opto.
4. U1 is 100 kHz version of NCP1077 in SOT-223 package.
5. R1 is used to trim Vout
6. Crossed lines on schematic are not connected.
7. C4 should be a low-Z electrolytic cap.
8. Vout = Vz1 + 0.9V (approx)
9. For non-tapped Buck configuration move D2 and D3 to "A" (red) positions.
10. Pin 4 of U1 should have heatsink clad pour.
NCP1077 12V, 500 mA Off-line Buck with Tapped Choke (R3)
1
It should be noted that the efficiency of this tapped inductor buck will depend on the selection of the inductor.
Tests have shown that custom made inductors with proper layered winding techniques resulted in the best
efficiency, however, the less expensive, off-the-shelf inductors provided by several vendors (Coilcraft, Wurth,
PalNova) are usually adequate at the lower current levels where the dc resistance of the windings are sufficient for
minimum thermal losses.
The demo/EVAL board associated with this buck converter can be configured in the standard buck configuration
by moving the freewheel diode (D2) and the Vcc bias diode (D3) to the red “A” positions shown in the schematic.
The total buck choke inductance will be the series inductance of the two windings on the inductor and will be equal
to 4 times that of a single section of the winding. If a smaller capacitance value input bulk cap is desired (C3), a
full wave input rectifier should be used instead of the simple, illustrated half-wave rectifier. Depending on the
desired output voltage, resistor R4 should be selected for a Vcc current of about 3 to 5 mA assuming approximately
10V on Vcc pin 1. Zener diode Z2 (in conjunction with input fuse F1) is provided for output OVP protection in the
event the buck switch would fail shorted. The input EMI filter composed of L1 and C2 should be sufficient to meet
Level B for conducted EMI.
For lower output current requirements, the NCP1070, or NCP1075 versions of this monolithic switcher may be
used.
December 2013, Rev. 0
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DN05059/D
Efficiency vs Load
Mosfet Source Voltage – 500 mA Load, 120 Vac Input
December 2013, Rev. 0
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3
DN05059/D
Mosfet Source Voltage – 500 mA Load, 230 Vac Input
Output Ripple – 500 mA Load, 120 Vac Input
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DN05059/D
PC Board Layout Details
Top
Bottom
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DN05059/D
Conducted EMI Profile: Peak (blue) and Average (red)
dBuV
NCP1077 T-Buck
120Vac, 12V @ 500mA
80
70
60
EN 55022; Class B Conducted, Quasi-Peak
50
EN 55022; Class B Conducted, Average
40
Peak
30
20
Average
10
0
-10
-20
1
10
10/16/2013 8:15:19 AM
(Start = 0.15, Stop = 30.00) MHz
BOM
Designator
Qty Description
Value
Tolerance
Footprint
D1
1 Diode - 60 Hz,
1A, 800V
SMA
D2 (or D2A)
1 Ultra-fast rectifier
2A, 600V
SMB
D3 (or D3A)
1 Ultra-fast rectifier
1A, 600V
SMA
Note: For non-tapped Buck configuration, install D2 and D3 in D2A and D3A positions on PCB
D3
1 Diode - UFR
1A, 600V
SMA
Z1
1 Zener diode
11V
SOD-123
Z2
1 Zener diode
15V/5W
Axial lead
U2
1 Optocoupler
CTR >/= 0.5
4-pin SMD
U1
1 Controller - NCP1077
100 kHz
SOT223
C2
C6
C5
C3
C1
C4
1
1
1
1
1
1
"X" cap, box type
Ceramic cap, monolythic
Ceramic cap, monolythic
Electrolytic cap
Electrolytic cap
Electrolytic cap
R4
R2
R3
R1
1
1
1
1
F1
L1
L2
Manufacturer
Manufacturer Part
Number
ON Semi
ON Semi
MRA4007
MURS260T3
ON Semi
ON Semi
ON Semi
Vishay or NEC
ON Semi
MURA160
MMSZ5241B
1N5352B or 1N5929B
SFH6156A-4 or PS2561L-1
NCP1077-100
100nF, X2
10 nF, 50V
100nF, 50V
33uF, 400V
22uF, 50Vdc
1000uF, 16V
10%
10%
10%
10%
10%
LS = 15 mm
1206
1206
LS=7.5mm, D=18mm
LS=2.5mm, D=6.3mm
10x20mm, LS=5mm
Rifa, Wima
AVX, Murata
AVX, Murata
UCC
Panasonic - ECG
UCC, Panasonic
TBD
TBD
TBD
TBD
ECA-1HM220
TBD
Resistor, 1/4W SMD
Resistor, 1/4W SMD
Resistor, 1/4W SMD
Resistor, 1/4W SMD
1.5K
33 ohms
680 ohms
68 ohms
5%
5%
5%
5%
SMD 1206
SMD 1206
SMD 1206
SMD 1206
AVX, Vishay, Dale
AVX, Vishay, Dale
AVX, Vishay, Dale
AVX, Vishay, Dale
TBD
TBD
TBD
TBD
1 Fuse, TR-5 style
1 Inductor (EMI choke)
1 Coupled Output Inductor
1A
820 uH, 500 mA
220uH, 3Apk
TR-5, LS=5mm
LS=5mm, Dia=8.5mm
15mm x 15mm SMD
Minifuse
Wurth Magnetics
Coilcraft
TBD
7447728215
MSD1583-224KE
December 2013, Rev. 0
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DN05059/D
References:
ON Semiconductor Application Notes: AND8318, AND8328
ON Semiconductor Design Notes: DN05014, DN05023, DN06011, DN06052
ON Semiconductor NCP1077 monolithic switcher data sheet.
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© 2013 ON Semiconductor.
Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor
does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to
assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated
with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its
products at any time, without notice.
Design note created by Frank Cathell, e-mail: [email protected]
December 2013, Rev. 0
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