NCP1129: 12 W, Off-line Buck Regulator

DN05053/D
Design Note – DN05053/D
NCP1129, 12 Watt, Off-line Buck Regulator
Device
Application
Input Voltage
Output Power
Topology
I/O Isolation
NCP1129
MBRS3200
Smart Meters
Electric Meters,
White Goods
85 to 265 Vac
12W at 12Vout
Off-Line 65 kHz
Buck
Non-isolated
Output Specification
Output Voltage
Output Ripple
Typical Current
Max Current
Min Current
12 to 28 Vdc depending on selected Z1 zener value
Less than 1%
250 mA to 1 amp
1 amp maximum (12 Vout)
zero
PFC (Yes/No)
Efficiency
Inrush Limiting / Fuse
Operating Temp. Range
Cooling Method /
Supply Orientation
Signal Level Control
No, Pout < 25 watts
Dependent on Freewheel diode selection
Fused input
0 to +50°C (dependent on U1 heatsinking)
Convection
None
Circuit Description
This design note describes a simple, low power (15 W or
less), constant voltage buck power supply intended for
powering electronics for white goods, electrical meters,
and industrial equipment where isolation from the AC
mains is not required. The efficiency limitations of the offline buck converter are also discussed with emphasis on
switching losses caused by the freewheel diode recovery
characteristics. The output voltage can be set from 12 to
approximately 28 volts (or higher) by using the
appropriate zener diode for Z1 and selection of trim
resistor R4. The new ON Semiconductor NCP1129 copackaged controller/Mosfet in a DIP 8 package is utilized
as the buck switching element. Output reguation is
accomplished by utilizing a simple zener/optocoupler
sensing scheme. The optocoupler is necessary because
the control logic is at a switching node common to the
freewheel diode (D3) cathode. This was done along with
half-wave input line rectification to allow a common
connection from the input line neutral to the negative
output terminal.
The main problem associated with off-line buck converters
is the efficiency associated with the combination of low
duty cycle (Vout less than 50V) and the power Mosfet and
freewheel diode turn-on switching loss. ON Semiconductor
application note AND8318 discusses one solution for both
of these issues by using a tapped buck inductor. This
solution, however, typically requires a non-standard
inductor which may not be an “off-the-shelf” component.
October 2013, Rev. 0
At initial Mosfet turn-on, the freewheel diode recovery
characteristics causes a significant leading edge spike of
current to flow through both devices that can easily exceed
3 to 5 times the normal peak load current of the devices.
This is particularly acute at high input line (230 Vac) if a
fast recovery type of diode is used where the reverse
recovery characteristics are slower with increased diode
PRV voltage rating. Conventional high voltage Schottky
diodes (typically several in series) and silicon carbide (SiC)
diodes will greatly reduce the turn-on switching loss but
there will still be a leading edge current spike associated
with the devices’ junction capacitance. The table and
waveforms shown below compare the representative
efficiencies and switching characteristics when using the
three diode types. The SiC diode definitely exhibited the
best performance, but the present day cost of this part may
be prohibitive for the intended application, and a pair of (or
3) series standard Schottky rectifiers are probably the best
compromise. Tests have shown that the use of a typical
ultra-fast recovery rectifier for D3 may prohibit continuous
output currents much greater than about 500 mA due to
excessive heating of the Mosfet and/or the freewheel diode
itself. Obviously good pc board layout with liberal copper
clad for heatsinking will certainly help the thermal issues.
Protection features in the NCP1129 include peak
overcurrent limiting which can be set by proper selection of
current sense resistor R7, and the Vcc pin will latch the
chip off if the Vcc exceeds 27 volts, allowing for
overvoltage detection. Over-temperature protection at
150C (internal) is also included.
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DN05053/D
Schematic
Line
F1
1A
AC input
85 - 265Vac
L1, 820 uH
0.5 A
D1
MRA4007
C2
C1
100nF
"x"
R1
1M
0.5W
100nF
"x"
D2
MMSD4148A
1
NCP1129-65
5 U1
C3
330uH, 1.5A
1.5 ohms
6
4
1
3
2
8
47 uF,
400Vdc
L2
R7A
R5
C7
R7B
2.4K
C8
D3B
0.1uF, 50V
D4
R6
+
C5
4.7uF
50V
1nF
C4
U2
3
1
2
1000uF 0.1uF
16V
50V
Z2
_
1N5352B
(15V, 5W)
MURA160 100 ohms R2
4
Output
12V, 1A
C9
MBRS
3200
x2
100pF
C6
Neutral
+
D3A
33
Z1
MMSZ5241B
R4
Vadj.
R3
680
Notes:
1. Vout set by Z1 (Vout = Vz + 1V approx).
2. L1 is Wurth 7447728215; L2 is Wurth 7447709331.
3. Thick lines indicate recommended ground plane area.
4. U1 should use clad heatsinking around pins 5 and 6.
5. D3A/B should have large pad areas for heatsinking.
6. Z2 is optional output OVP zener.
7. Crossed schematic lines are not connected.
8. U2 is NEC PS2561L-1 optocoupler or similar (CTR > 0.5)
9. R4 will adjust Vout upwards only.
10. Red ground plane area should be minimized to lower
capacitance (switched node).
11. Clad pour recommended around U1 pins 5 and 6 for
heatsinking.
Off-Line Buck Converter Using NCP1129
With Optocoupler Voltage Sensing (Rev 1A)
1
© 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]
October 2013, Rev. 0
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DN05053/D
Mosfet Source Voltage (yellow) and Current (blue)
12V, 1A output, 120 Vac input with ultra-fast recovery freewheel diode.
Expanded drain current waveform showing current spike magnitude.
October 2013, Rev. 0
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DN05053/D
Waveforms with 1.5A, 600V SiC freewheel diode.
Waveforms with pair of 3A, 200V MBRS3200 Schottky freewheel diodes in series.
October 2013, Rev. 0
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DN05053/D
Freewheel Diode Efficiency Comparisons
Freewheel Device
Efficiency (12V/1A output; 120Vac input)
3A/600V Ultra-fast
76% (unable to run at 230Vac without eventual thermal shutdown)
1.5A/600V SiC diode
83% (operation at full load at 230Vac without thermal issues)
3A/200V Schottky x 2
(MBRS3200 in series)
81.5% (operation at full load at 230Vac with acceptable thermals)
Summary
The NCP1129 co-packaged controller/Mosfet in a DIP 8 package can be used to design a simple, yet
effective off-line buck converter for applications that don’t require input to output isolation. Care must
be taken, however, in the selection of the freewheel diode due to its reverse recovery characteristics
and the overall impact on conversion efficiency. Utilizing the buck controller at the high-side switching
node in conjunction with optocoupler feedback provides the most effective, low noise technique for
sensing and regulating the output voltage.
References:
ON Semiconductor Application Notes: AND8318, AND8328
ON Semiconductor Design Notes: DN05014, DN06011, DN06052
ON Semiconductor NCP1129/1126 Data Sheet
October 2013, Rev. 0
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