NCP1014: Low Power, Off-Line, Constant Voltage Power Supply

DN06066/D
Low Power, Off‐Line,
Constant Voltage Power
Supply
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DESIGN NOTE
Table 1. DEVICE DETAILS
Device
Application
Input Voltage
Output Power
Topology
I/O Isolation
NCP1014
Smart Meters,
Electric Meters
85 to 265 Vac
Up to 10 W
Off-Line Buck
Non-Isolated
Characteristic
Output Specification
Output Voltage
10 to 28 Vdc Depending on Selected Z1 Zener Value
Ripple
Dependent on L2 Inductance and C6 ESR, Typically 1% Max
Nominal Current
50 to 325 mA Typical
Max Current
350 mA Maximum
Min Current
Zero
PFC (Yes/No)
No, (POUT < 25 W)
Minimum Efficiency
75% (Dependent on VOUT and IOUT Combination)
Inrush Limiting/Fuse
Inrush Resistor (R1)
Operating Temp. Range
0 to +60°C (Dependent on U1 Heatsinking)
Cooling Method/Supply Orientation
Convection
Signal Level Control
None
Circuit Description
breakdown level and the B-E drop on Q2, then the feedback
pin of U1 (pin 2) is pulled down to achieve regulation. It
should be noted that this type of level shifted voltage sensing
is not compatible with the tapped inductor type of buck.
Due to the additive magnetizing component of L2, the true
load current should be limited to about 350 mA maximum
to avoid over-current limiting in U1. An inductance smaller
than approximately 2.5 mH for L2 will probably lower the
effective usable output current, so a minimum inductance
value of 2.5 mH is recommended to keep the choke’s
magnetizing current low. Commercially available chokes
should be rated for an output current of 0.5 A to assure best
efficiency. For lower currents and/or output voltages, some
compromises can be made on the maximum inductance
value; however, lower values could also impact the output
ripple if output capacitor C6 has high ESR.
The output power will also be thermally limited, so for
power levels above 5 W, the ground tab on U1 (SOT−223
package) should be soldered to a sufficient area of copper pc
board clad to assure that the internal over-temperature
circuit is not activated.
This design note describes a simple, low power (10 W or
less), constant voltage power supply intended for powering
electronics for white goods, electrical meters, industrial
equipment or lighting applications where isolation from the
AC mains is not required. The output voltage can be set from
10 to approximately 28 V by using the appropriate zener
diode for Z1. Output voltage sensing is accomplished by
utilizing a level shifting (“bootstrap”) circuit comprised of
D7, C8, Z1, Q2 and the associated resistors. In applications
where the output negative terminal must be common with
the input bulk capacitor negative, voltage sensing of the
output directly is not possible. This is because the ground
node of the controller is switched between the peak input dc
voltage and the common line. In order to sense the voltage
without an optocoupler in the feedback path, the level
shifting “bootstrap” circuit acts as a sample and hold
network which stores a sample of the output voltage on C8
(through D7) when the MOSFET in U1 is in the off state.
During the on-time, U1’s ground reference is shifted up to
the input voltage level of choke L2, and sense diode D7 is
back biased. When the voltage on C8 exceeds Z1’s
© Semiconductor Components Industries, LLC, 2015
December, 2015 − Rev. 1
1
Publication Order Number:
DN06066/D
C1
4.7 nF
“x”
C2
+
10 mH,
400 Vdc × 2
C3
L1
1 mH, 0.6 A
D1−D4
MRA4007 × 4
+
4
Figure 1. Circuit Schematic
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2
R7
1 kW
R5
100 kW
+
C8
4.7 mF
35 V
MURA160
D7
+
R4
10 kW
C6
330 mF
35 V
C7
0.1 mF
50 V
−
Output
25 V,
325 mA
+
Attenuation
Constant Voltage Output Regulation with Minimal
Components and No Optocoupler for Feedback
100 W
Z1
MMSZ24T1
D6
MURA160
L2
2.5 mH, 0.5 A
• Input Filter (Pi-Network) for Conducted EMI
Q2
MMBT2222
R6
NCP1014
100 kHz
(SOT−223)
3
2
1
Key Features
C9
Notes:
1 nF
1. VOUT set by Z1 (VOUT = VZ +1 V)
2. L1 is Coilcraft MSS1260−105KL or similar.
3. Thick lines indicate recommended ground plane area.
4. U1 should be heatsunk via ground tab to copper clad area.
5. Crossed schematic lines are not connected.
F1
0.5 A
AC Input
85−265 Vac
R1
5.1 W, 2 W
•
C4
22 mF +
25 V
U1
DN06066/D
• NCP1014 Monolithic Current Mode Controller with
• Circuit Easily Configured for Different Output Voltages
• Inherent Short Circuit and Over-temperature Protection
Internal 700 V MOSFET for Maximum Simplicity
SCHEMATIC
DN06066/D
Output Choke Design
References
L2 can be constructed by winding 200 turns of #28 magnet
wire on the bobbin of an EF−16 (E16/8/5) ferrite core with
a cross sectional area (Ae) of 0.2 square centimeters (or
similar ferrite core and bobbin), and gaping the core to
achieve an inductance of 2.5 mH when measured across the
winding. Another, but less efficient option would be to use
two “off-the-shelf” Coilcraft RFB1010−122L inductors in
series.
[1] ON Semiconductor Design Note DN06037/D: Low
Power, Off-Line Buck, CVCC Power Supply
[2] Data Sheet NCP1014/D
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DN06066/D