DN05131/D
Oct.. 2019, Rev. 0 www.onsemi.com 1
Design Note – DN05131/D
5-V/300-A Two-Switch Forward Ac-Dc
Converter
Description
This design note provides elementary information
about a two-switch forward converter built with the
NCL30125 operated in current-mode control. This
controller offers many features to build an energy-
efficient converter with all the needed protections like
cycle-by-cycle current limit with a 500-mV sense
voltage, over temperature protection with a dedicated
NTC pin and brown-out feature. In addition to the low
side MSOFET drive, the controller integrates also an
high-side section to drive the floating N-channel power
MOSFET. Dedicated pins are available to adjust the
switching frequency (RT pin – pin 6) or the soft-start
duration (SS pin – pin 7). Finally, a high-voltage
current source with Dynamic-Self Supply (DSS) is
embedded to quickly start the power supply and
maintain the V
cc
voltage in light load or standby.
The primary-side section drives a transformer whose
primary inductance is 3.8 mH. The energy
accumulated in this inductance is sending back to the
bulk capacitor thanks to the freewheel components.
One of the classical freewheel diode has been replaced
by a MOSFET M
9
driven by the controller in order to
refresh the bootstrap capacitor. The current is sensed
via a 87-mΩ resistance. The switching frequency of
100 kHz and the soft-start duration are set by two
individual components (R
23
for f
SW
and C
22
for SS).
The power stage is made of two switching N-channel
transistors M
1
and M
8
. These two transistors are
switched in same time and seen the input voltage as
maximum. In the secondary side, three MOSFETs are
self-driven by the transformer but require regulation
transistors Q
9
and Q
10
to limit the maximum V
GS
these
elements can accept. The three forward MOSFETs M
2
,
M
3
and M
4
are driven by the NCP4306 synchronous
rectification controller. The regulation is ensured by a
TL431 wired in a type-2 configuration. Loop gain
measurements show a 74° phase margin at a 60-A
output obtained with a 230-V rms source.
Key Features
• Integrated High-side driver
• Adjustable switching frequency up to 300 kHz
• Peak current-mode control
• Skip mode to maximize performance in light load
conditions
• Internal 600-V start-up source operated in
dynamic self-supply during start-up or skip mode.
• Brown-out (BO) detection
• Adjustable soft-start duration
• 15-ms timer-based short-circuit protection with
auto-recovery or latched operation
• Auto-recovery or Latched OVP on V
cc
• Latched OVP/OTP input for improved robustness
• +0.9 A / -1.2 A peak source/sink drive capability
Devices Applications
Input
voltage
Output power
Topology Board Size
NCL30125 Lighting 176-265 V rms 300 W
Two-Switch
Forward
148 x 152 x 53
mm
Output spec. Turn on time Efficiency
Operating
temperature
Cooling
Standby
power
5 V/60 A < 200 ms
above 89 %
@ full load
0 – 40
o
C
Open Frame
in Still Air
Does not apply
DN05131/D
Oct.. 2019, Rev. 0 www.onsemi.com 2
Demonstration Board Schematic Diagram
NCL30125 5 V/300 W Power Converter
R19
2.5k
R35
43k
R24
100
TP9
DRV_FW
1
C11
1800uF
C10
1800uF
D18
4.3V
R6
4.7
D4
D1N4148
D5
MRA4007
C13
1800uF
C22
22n
C16
1nF / Y 1
D14
NC
L
C28
47u
50V
Q3
MMBTA06LT1G
R28
0.1
U1
NCL30125
HV
1
FAULT
3
BO
4
FB
5
RT
6
SS
7
FW
8
CS
9
VCC
10
GND
11
DRV_LO
12
DRV_HI
14
HB
15
BOOT
16
D17
18V
L1
2.2uH
1 2
TP17
Vcc_sec
1
5Vout
ST1
GND_Pri
1
2
R30
2.5k
R8
47k
L2
2.2mH
1 2
R15
590
D3
MUR160
TP13
BO
1
TP19
Aux
1
VBulk
M6
NTMFS5C410
TP8
HB
1
R32
1k
C27
100n
C7
470nF/X2
D7
NC
C19
1.5n
C20
33n
C17
4.7n
D6
MRA4007
R23
100k
Aux1
TP18
Vcc
1
6.3 V
TP4
HV
1
R33
24k
TP5
Drain
1
R9
2.2k
M8
FDPF20N50
C21
1n
R31
47k
C26
100n
C30
1n
Q1
FMMT624
0
0
FB
TP11
FB
1
M5
NTMFS5C410
TP12
DRV_HI
1
N
D10
D1N4148
IC1
Diode_bridge
R18
10
Q2
MMBTA06LT1G
TP2
Vout
1
TP10
Va
1
R21
20k
C29
47u
50V
GND
TP6
GND_Sec
1
C5
150u
400V
D8
18V
C1
100p
6.3 V
R7
10k
TP15
CS
1
R34
12k
R25
NTCLE100E3
L3
820uH
1 2
D12
12V
D1
D1N4148
R29
10
6.3 V
T1
0
1
2
3
4
5
C3
1nF/Y 2
M3
NTMFS5C423
0
TP3
DRV_direct
1
C4
150u
400V
IC3
opto
C14
10n
C15
100n
R27
0.68
R13
10R
R26
1k
D13
D1N4148
D9
MUR160
R3
10
C25
390p
M2
NTMFS5C423
0
D11
BAV23CL
C2
1nF/Y 2
C12
1800uF
R14
3M
Vbulk
Earth
R4
47k
C8
NC
CM1
5mH
1
2 3
4
R11
3M
R1
470
C18
1n
D15
BAV23CL
M1
FDPF20N50
IC4
NCP431
R10
2.2k
GND
0
D2
15V
R12
10
5Vout
C24
22u
50V
R17
NC
C6
470nF/X2
R2
3R/5A
R20
4.7
M9
CS1N60
6.3 V
TP1
Bulk
1
IC5
NCP4306
VCC
1
MIN_TOFF
2
MIN_TON
3
LLD
4
TRIG
5
CS
6
GND
7
DRV
8
D16
BAT54
TP14
DRV_LO
1
M4
NTMFS5C423
6.3 V
R16
200k
TP16
FW
1
R22
47k
C9
1800uF
TP7
Boot
1
C23
22u
50V
M7
NTMFS5C410
FB