NIKO-SEM
APPLICATION NOTE
AN-1002
Designing a Forward Power Supply Using The N3858x Series
By Steven Hsu
INTRODUCTION
The N3858x series of synchronous rectification controller are designed for secondary side to
control a forward rectification MOSFET and a fly-wheeling rectification MOSFET with the low
Rds(on) characteristic, instead of the forward rectifier diode and fly-wheeling diode. Base on
the power MOSFET technology, a few mΩ Rds(on) of power MOSFETs are very prevalent ,we
can use the feature of low Rds(on) ,easily replace the forward rectifier diode and fly-wheeling
diode of conventional forward power supply and obtain lower power loss .
The N3858x implements a constant dead time control (patent pending), however, the AC input
voltage is changing, the dead time will keep to be constant .
BLOCK DIAGRAM
VD
VCC
VIN
VCC
1%
COMP. 1
regulator
+
VCC
COMP. 3
GATE1
-
driver1
+5V
+
VREF
-
COMP. 4
+5V
-
UVLO
+
7V
VCC
VCC
COMP. 2
AND
+
RCT
driver2
GATE2
-
2.5V
front edge
GND
trigger
Fig 1 N3858x Function Block Diagram
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PIN FUNCTIONS
GATE 2 ( Pin 1 ) : This pin is the driver output pin to drive the gate of the fly-wheeling
synchronous rectification MOSFET. It’s capable of source and sink 1A
current ,directly connected to the Gate of the fly-wheeling synchronous
rectification MOSFET or series with a small resistance of resistor .
VCC ( Pin 2 ) : This pin is for supply voltage of the control IC.
RCD( Pin 3 ) : This pin connected RT and CT to generate a constant saw-tooth waveform for
obtaining a proper dead time control, RT connected to VREF pin and CT
connected to GND.
VIN( Pin 4 ) : Input pin (+) of the Comparator 3., connected R1and R2 to filter the sinusoidal
voltage waveform during the DCM condition.
VD ( Pin 5) : Input pin (-) of comparator 1, inhibit function detect pin, connected to the Drain
pin of the fly-wheeling synchronous rectification MOSFET, if VD> 0V, the GATE2
output will be disable to turn off the synchronous rectification MOSFET Q2
when the current through it tends to reverse ,allowing discontinuous mode
condition and providing protection to the device from eventual sinking current
from the output tank.
VREF (Pin 6) : 5V reference voltage output, it provides the charging current for CT through RT.
GND ( Pin 7 ) : This is the GND pin of the ship, connected to the Source pin of the fly-wheeling
synchronous rectification MOSFET.
GATE 1 ( Pin 8 ) : This pin is the output driver pin to drive the gate of the forward rectification
MOSFET, It’s capable of source and sink 1A current ,directly connected to
the Gate of forward rectification MOSFET or series with a small resistance of
resistor .
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TYPICAL APPLICATION
See the Fig 2 ,it‘s showing the typical application with N3858x .The TVS1 is a transient voltage
suppresor , it’s optional to clamp the spike voltage for using the lower voltage rating MOSFET ,
And R3,C2 ,R4,C3 are the snubber components to be needed for damping spike voltage .
If a strong spike voltage appears on the VD pin ,then it is needed to connect a 10pf capacitor
for filtering the spike voltage , prevent a wrong trigger to the output drivers.
L1
B+
2
R5
T1
C5
3
1
2
7
3
CT
PWM
Vo
4
9
8
D3
12V
4
Q3
N3858
GATE2 GATE1
VCC
RCT
C4
VREF
VD
VIN
7
R7
TVS1
D2
0~200R
R3
6
Q2
5
P6KE68A
OPTION
C1
C2
RT
R2
R6
GND
8
10P
R1
ENABLE(LOW)
Q4
2N3904
OPTION
OPTION
Q1
D1
R4
C3
FEEDBACK
Fig 2
Typical Application With N3858x
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OPERATION
•Synchronization
The N3858x synchronization is obtained directly from the secondary side using the voltage
across the fly-wheeling synchronous rectification MOSFET as the information for the switching
transitions. Refer to FIG.3 , a squire wave dectector filters the eventual sinusoidal waveform
caused by discontinuous mode operation to prevent a wrong driving of the forward rectification
MOSFET by a wrong synchronization .
square wave dectector
input (VD pin)
VSIN
VDP
square wave dectector
output (comp.3)
on time of forward rectification MOSFET
FIG. 3
VSIN R2
R1+R2
< 5V
DCM waveform
VDP R2
R1+R2
；
> 5V
A proper setting R1 and R2 to make sure VSIN R2/(R1+R2)< 5V , VDP R2 /(R1+R2)> 5V to get
a good driving of the forward rectification MOSFET .
•How to Set The Constant Dead Time
The RCD pin is connected a RT and CT to adjust a optimal dead time to prevent the flywheeling synchonous rectification MOSFET and the forward rectification MOSFET working
on-state condition when during fly-wheeling period is ended.If RT and CT fixed,however AC
input or duty cycle is changing ,then the dead time will be fixed to be constant.
Before connect the GATE 1 and GATE 2 outputs of the N3858x to each MOSFET. First,short
gate and source of each MOSFET ,then check the dead time is OK or not ,adust RT or CT for
tuning a proper dead time, then connect the GATE 1 and GATE 2 outputs to each MOSFET .
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1.44
f(osc) ≈
RT CT
Dead time(max) =Ts x Total Tol.%+ td(off) of S.R. MOSFET , TS = 1/fS
Dead time(min) = td(off) of S.R. MOSFET , Total Tol.%=(Rt +Ct )Tol.%
• Enable/Disable Function
The VIN pin is able to provide the enable/disable function to control the outputs of GATE1 and
GATE2 , if connect VIN to GND , the outputs of GATE1 and GATE2 stopped to output .
See the Fig 4 ,it’s showing disable function in the DCM application with N3858x , The voltage
detected from the photo-coupler of the feedback circuit , if the output is in the light load (DCM)
condition , this detected voltage is lower than middle load , then connected this dectected
voltage to a comparator , the comparator output pulled VIN pin to the low level when output is
in the light load (DCM) condition . By the way ,if the comparator connect a PNP transistor to
control the VCC , it also can disable the outputs of GATE1 and GATE2 of the N3858x .
•Inhibit Function
The inhibit function is detected the voltage of the VD pin , the VD pin is connected to the drain
of the fly-wheeling synchronous MOSFET , if VD > 0V ,the GATE2 output will be disable to turn
off the fly-wheeling synchronous rectification MOSFET Q2 ,when the current through it tends to
reverse, allowing discontinuous mode condition and providing protection to the device from
eventual sinking current from the output tank.
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L1
B+
2
R5
D3
9
8
C5
3
T1
4
Vo
N3858
1
GATE2 GATE1
2
7
VCC
3
CT
PWM
12V
GND
RCT
4
VREF
VIN
VD
8
7
R15
TVS1
D2
0~200
R3
6
C2
Q2
5
P6KE68A
OPTION
C1
Q3
R2
C4
RT
R1
10P
OPTION
R6
Q1
D1
R4
VCC
8
R9
R7
220K
C6
4700P
U2
1
4
3
2
R8
1M
1
2 -
C7
330P
R10
COMP
R13
Vo
R14
R11
PC817
C8
U3
TL431
Fig 4
3 +
4
D4
C3
R12
Disable Function in The DCM Application With N3858x
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High Side MOSFET Application With N3858x
See the Fig 5 , the N3858x is able to be used in high side MOSFET application .
C3
R4
D1
L1
12V
Q1
B+
T2
1
R6
3
5
D3
C5
6
TVS1
N3858
8
1
2
7
TRANSFORMER
PWM
R7
9
Q3
3
CT
4
GATE2 GATE1
VCC
GND
RCT
VREF
VIN
R2
8
7
6
P6KE68A
OPTION
D2
0~200
R3
Q2
C1
C2
5
RT
R1
C4
R5
VD
R8
Q4 ENABLE(LOW)
10P
OPTION
2N3904
OPTION
FEEDBACK
Fig 5
High Side MOSFET Application With N3858x
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