NCP1251: 20 W Flyback with Primary Side Regulation (PSR)

DN05033/D
Design Note – DN05033/D
A 12V, Off-line Power Supply With Primary Side Feedback
Device
NCP1251B
NDD04N60
Application
Input Voltage
Output Power
Topology
I/O Isolation
White Goods, Small
Instruments, EMeters, Industrial
Equipment
90 – 267 Vac
20 Watts
DCM Flyback
2.5 kV primary to
secondary
Other Specification
Output Voltage
Ripple
Nominal Current
Max Current
Min Current
Output 1
Output 2
Output 3
Output 4
12 Vdc +/- 5%
< 2%
1.8 Amps
2 Amps
zero
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
PFC (Yes/No)
Nominal Efficiency
Inrush Limiting / Fuse
Operating Temp. Range
Cooling Method /
Supply Orientation
Signal Level Control
No
>80%
Yes
0 to 50C
Convection
NA
No
Others Input EMI filter
output voltage. The load regulation over the
usable load range and the nominal output
voltage can be further optimized by adjusting the
values of R5 and R11, respectively.
Circuit Description
This Design Note describes a very simple, low
cost, primary side controlled, off-line flyback
power supply using ON Semiconductor’s
NCP1251B current mode controller (TSOP6
package) and an NDD04N60 D-Pak Mosfet.
The design dispenses with the conventional
TL431/optocoupler feedback scheme and uses
primary side voltage sensing on the NCP1251’s
Vcc rail which is derived from an aux winding on
the transformer. This scheme provides tight line
regulation and a load regulation of +/- 5% which
is more than adequate for most applications. The
acceptable load regulation is achieved by tight
coupling (bifilar wound) between the main 12V
secondary and the 12V aux winding in the
transformer. The voltage sensing circuit is
comprised of Z2 and Q2. The zener voltage of
Z2 plus the VBE drop of Q2 sets the nominal
June 2012, Rev. 0
Over-current limiting is provided by sensing the
peak current in the MOSFET Q1 via R8. Once
the 800 mV threshold level on U1’s pin 4 is
exceeded, the circuit will go a “hiccup” mode
until the over-current condition is removed. An
optional output OVP clamp is implemented via
Z1 to limit the maximum no load output voltage.
Key Features
 Primary side voltage sensing – no optocoupler
 Input EMI filter for conducted EMI compliance
 Schottky output rectifier for high efficiency
 Current mode control with adjustable output
current limit setpoint
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DN05033/D
Schematic1
L1A
F1
C1
C2
0.1uF
"X2"
0.1uF
L1B "X2"
R1
4.7 ohms
3W
10
D1-D4
MRA4007
X4
820uH
2A
250Vac
AC Input
T1
C4
R14
47uF
400V
R2
R3
360K
360K 360K
R9
1K C5
1nF
2
1N4937
MBRS360
R6
10
5
R7
R8
1 ohm
1/2W
10K
R4
3
D5
Q1
D6
C6B
D8
U1
5
1
4
J2
C9A C9B
R13
3K
14V
1W
+
Z1 C10
0.1uF
50V
12 Vout
_
C12
1K
6
2
D9
6
R10
NCP1251B
10nF
EE20/10/6
9
NDD03N60Z
C3
820uH
C8
4.7nF 1
1kV
43K
1W
MMSD4148
J1
22uF
25V
R5
D7
24
MMSD
4148
MMSD
4148
C6A
4.7uF
50V
R11
10
Q2
1nF "Y1"
3
8
C7
220pF
Z2
MMBT
2222A
MMSZ5241B (11V)
R12
680
NOTES:
1. Crossed lines on schematic are NOT connected.
2. Z1/R13 are optional OV clamp/pre-load for zero load protection.
3. R1 is for inrush limiting - use carbon comp or wire wound.
4. L1A/L1B are Wurth 7447728215 components (820 uH, 500mA).
5. Output caps (C9A/B) are radial lead, low impedance types (UCC LXV series or similar).
6. R5 should be adjusted for optimum load regulation depending aux winding leakage
inductance (typically 15 to 68 ohms).
7. Z2 sets nominal Vout.
8. R11 is for Vcc/Voutput trimming.
9. R8 sets max output current.
10. See drawing for T1 details.
20 Watt NCP1251 Power Supply
with Primary Side Sense (Rev 7A)
1
© 2012 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]
June 2012, Rev. 0
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DN05033/D
MAGNETICS DESIGN DATA SHEET
Project / Customer: 24 watt, 12 vout NCP1251 Flyback with primary side sensing
Part Description: 24 watt flyback transformer, 12Vout (Wurth part # 750313284)
Schematic ID: T1
Core Type: EE20/10/6 ferrite core; 3C90 material or similar
Core Gap: Gap for 190 +/- 200uH across Primary A (pins 1 - 10)
Inductance: 750 uH total (+/- 5%) measured from pin 1 to pin 9 with pins 2 and 10 connected
Bobbin Type: 10 pin horizontal mount for EE20/10/6
Windings (in order):
Winding # / type
Turns / Material / Gauge / Insulation Data
Primary A (1 - 10)
30T of #28HN over 1 layer (25 TPL). Insulate for
1 kV to next winding. Self leads to pins.
12V Secondary/Vcc (5 - 6, 3 - 8)
6 turns trifilar of #26 triple insulated wire over one.
layer. The outer winding on schematic is Vcc (3 - 8).
The 12V secondary (5 - 6) uses two wires.
Self leads to pins per drawing.
Primary B (2 - 9)
Same as Primary A. Insulate with tape and selfleads to pins.
Hipot: 2.5 kV from primaries & Vcc to secondary for 1 minute.
Lead Breakout / Pinout
Schematic
10
3
(Top View of Bobbin)
1
5
10 9 8 7 6
9
12V
sec
Pri A
Pri B
2
6
Trifilar
Wound
0.15" pin
separation
(8 places)
0.60" pin rows
8
1 2 3 4 5
June 2012, Rev. 0
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DN05033/D
2
Output Load Regulation
Efficiency Plot
References:
NCP1251 Data Sheet: http://www.onsemi.com/pub_link/Collateral/NCP1251-D.PDF
ON Semiconductor NCP1251 Design Notes: DN05012, DN05017, DN05028, DN05029
Flyback Transformer Design Aid:
http://www.onsemi.com/PowerSolutions/supportDoc.do?type=tools&rpn=NCP1075
June 2012, Rev. 0
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