NIKO-SEM
APPLICATION NOTE
AN-1001
Designing a Flyback Power Supply Using The N3856x Series
By Steven Hsu
INTRODUCTION
The main advantages of the flyback power supply are cost-effective , simplicity and ease of
reach green mode power consumption requirment without any extra stand-by power circuit.
The N3856x series of synchronous rectification controller are designed for secondary side to
control a low Rds(on) power MOSFET, instead of the turn-on duty of the output rectifier 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 output rectifier diode of
conventional flyback power supply and obtain lower power loss .
The N3856x builded a adjustable slope current dectecter for detecting the current vis MOSFET
in the discontinuous mode operating. It also builded a comparator for comparing the saw-tooth
waveform of RT/CT pin with internal 1/3 VCC voltage to generate a constant pulse width in the
continuous mode operating.
BLOCK DIAGRAM
VCC
GND
10K
PGND
Aout
D
COMP.
OSC
BUFFER
-
RT/CT
DRIVER
GATE
+
1/3 VCC
20mV
ADJ.
SLOPE
CURRENT DETECTER
DRAIN
BIAS
Fig 1 N3856x Function Block Diagram
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PIN FUNCTIONS
GATE ( Pin 1 ) : This pin is driver output of N3856x, It’s capable of source and sink 1A
current ,directly connected to the Gate of synchronous rectification MOSFET
or series with a small resistance of resistor .
PGND ( Pin 2 ) : This is the Power GND for the output totem pole driver of the ship .
GND ( Pin 3 ) : This is the signal GND for the control logic signals of the ship.
BIAS (Pin 4 ) : The pin connected a resistor to VCC ,In the discontinuous mode operating , if
adjust the resistance will cause a different slope voltage between the Gate and
Source of MOSFET , the down slope Vgs waveform is determined turn-off time
when the voltage reach the threshold voltage level of the MOSFET.
DRAIN ( Pin 5 ) : The pin is directly connected to the Drain pin of the synchronous rectification
MOSFET for detecting the drop voltage between Source and Drain .
Aout ( Pin 6 ) : This pin is internal comparator output to control “pin1” output duty , it also
provided the primary control function by a pulse transformer in the continuous
mode operating .It could connect a resistor to VCC pin to modify the rise time
of turn-on , keep this pin is floating if doesn’t use .
RT/CT (Pin 7 ) : This pin connected a resistor and capacitor to generate a saw-tooth waveform
Comparing with internal reference voltage(1/3 VCC) .When the saw-tooth
waveform reach the voltage level will cause output driver turning low voltage
and MOSFET is off. The charging current will discharge via internal circuit.
VCC (Pin 8 ) : This pin is for supply voltage for the ship, it is also a synchronization pin for the
control logic ,so it can’t connect any filtering capacitor for normal operating .
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TRANSFORMER DESIGN
One of the most important factors in design of a flyback power supply is the design of the
transformer .Usualy ,we recommended the Maximum duty is 0.5 in conventional flyback power
supply for obtaining a best efficiency , but in the synchronous rectification application ,we
recommended the Maximum duty is 0.3 , and let the synchronous rectfication MOSFET have
more turn-on period ,resulting a lower power loss during energy tranfer to output stage.
For the N3856x controller application ,we need to provide a AUX winding for the supply voltage
and synchronization purpose . The AUX winding is approximately supply a 12V via a fast
recovery diode(1N4148) to VCC pin of the N3856x derectly without any filtering capacitor , the
AUX winding should be placed near by the main output winding for obtaining a best connection
and less leakage inductance, a higher leakage inductance may cause more spike voltage on
VCC pin that could increase turn-on delay time for the N3856x.
OPERATION
DISCONTINUE MODE OPERATING
In the discontinue mode operating , when the primary power MOSFET is turn-off ,then the
transformer begin to release the storing energy to secondary output , the down slope current
flows through source and drain of the MOSFET , formed a drop voltage comparing with internal
reference voltage 20mV by the adjustable slope current detecter ,when the drop voltage is over
20mV, the driver output high level to turn on the MOSFET , if the drop voltage down to
20mV ,will generate a down slope voltage waveform for the driver output ,if the voltage reach
to the threshold level of power MOSFET, therefore power MOSFET is turn-off, then wait next
operating period ,the MOSFET is turn-on again . There are two detecting pins on the
adjustable slope current detecter ,one is pin 5 ,another is pin 3, the pin 5 is directly connected
to the drain of MOSFET , the pin 3 is connected to the source of MOSFET . For the diffent
Rds(on) MOSFET ,we designed a adjustable slope function for tunning a best turn-off point in
the discontinuous mode operating ,see the Fig 2 , adjust the resistance of the bias resistor ,let
Vgs voltage down to the min. voltage level (near to 0V) before the resonant waveform
beginning (T2) resulting by Land C ,If the resistance of bias resistor is smaller ,the slope will be
faster .
HOW TO ADJUST THE BIAS RESISTOR ( R1)
In the discontinue mode operating ,set the output load heavier ,then adjust the bias resirtor
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until the Vgs slope down to 0V before he resonant waveform beginning (T2) resulting by Land
C , and checking no load codition waveform ,if switching action is not normal , reducing the
resistance until waveform is normal .
T1
T2
T3
WAVEFORM OF
SECONDARY
WINDING
SECONDARY
WINDING
CURRENT ; VSD
20mV reference
DELAY TIME
THE SLOPE OF VGS IS ADJUSTED BY “BIAS” PIN .
VGS
GATE – GND
WAVEFORM
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Fig 2 Control waveform for the discontinue mode
CONTINUE MODE OPERATING
In the continue mode operating , the output control dead time is used by a comparator ,
RT/CT(pin 7) connected a resistor (RT) to VCC ,and connect a capacitor (CT) to GND , the pin
7 is charging current via RT to CT and forming a saw-tooth waveform voltage, the voltage is
reaching 1/3VCC , then the comparator will become low level ,therefore the driver output
changed to low and turn off the MOSFET . See the Fig 3 , the dead time appears on the end of
T1 period .
HOW TO ADJUST DEAD TIME
First, adjust output load from light load to high load slowly ,check the waveform of GATE –GND
When the output load into the boundary of the discontinue mode and continue mode ,checking
the dead time of the waveform of GATE –GND, it appears or not , if dead time not appear ,
then reducing resistance of RT or the capacitance of CT , if dead time appear , check dead
time is OK or not ,adjust it to the best point .
T2=0
T1
T3
duty cylce
dead time
delay time
GATE - GND
WAVEFORM
1/3VCC
VCT
PIN 7
VGS
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Fig 3 Control waveform for the continue mode
See the Fig 4 , showing Delay Time and Dead Time on 120W power supply application ,
Fig 4 Delay Time and Dead Time measurement on 120W,19V/6.3A power application
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POWER SUPPLY DESIGN CRITERIA REQUIRED
D1
B+
1N4148
T1
1
R10
G
Q2
IRFB4710
5
6
+
C5
D3
4
8
ZD1
+
C6
S
D
S
D
D2
SB3100
Vo
+
Q1
G IRFB4710
C3
18V
U1
N3856V
1
PWM
CONTROLLER
Q3
C2
470P
R1
2
3
4
GATE
VCC
PGND RT/CT
GND
AOUT
BIAS
DRAIN
8
7
6
RT
CT
5
16K
R9
R7
U3
4
3
1
R4
R6
2
PC817
U2
C4
TL431
Pin 5 of the U1 is directly connected to "drain" pin
of the Q1 , and this trace can't be connected to
other components .
Pin 2,3 of the U1are directly connected to
"source" pin of the Q1 , and this trace can't be
connect other components .
R5
R8
1)
2)
3)
4)
5)
6)
7)
Fig 5 Typical Flyback power supply using the N3856x Series
Recommended operating frequency for the best efficiency --- fs = 50K~60KHz
Maximum duty cycle --- Dmax ≈ 0.3
Recommended low Rds(on) power MOSFET for the synchronous rectification --- less than
16mΩ .
Recommended AUX winding voltage is approximately 12V.
Adjusted bias resistor R1 for tuning a turn-off point in the discontinue mode operating.
Adjusted RT or CT for tuning a dead time in the continue mode operating ,RT is required for
1% precision resistor and CT is reuired for 1~2%NPO or other low temperature drift characteristic capacitor .
Schottky diode D2 is required(about1/3 output current rating) for reducing conduction loss
during delay time and dead time because the body diode Vf of the MOSFET> schottky
diode Vf , if in the 2 pcs of MOSFETs parallel application , the body diode Vf of the parralel
MOSFETs will be decreased, we may remove the schottky diode D2 .
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Synchronous signal from primary side application
See the Fig 5 , showing synchronous signal from primary side application in the continue mode
operating ,the RT/CT pin (pin 7) is shorted to PGND (pin 2) . the synchronous signal from
primary side must be ahead of the gate waveform of Q2 , let Q1 is turn-off before Q2 turn-on .
D1
B+
T1
R10
1
1N4148
5
SB3100
D2
6
+
C5
D4
4
S
8
D
G
+
C4
1
Q2
VREF
C6
0.1U
SYNC X'FRM
T2A
*
Q3
2SC945
+
2
PWM
CONTROLER
3
C2
CT
4
470P
R1
R9
C3
Q1
U1
GATE
VCC
PGND RT/CT
GND
AOUT
BIAS
DRAIN
8
7
6
5
Q5
2SC945
R3
1K
N3856V
T2B
16K
Q4
2SA733
D3
1N4148
*
R7
U3
PC817
4
3
1
R4
R6
2
U2
R8
Fig 5
TL431
C3
R5
Synchronous signal from primary side , using the N3856x Series
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See the Fig 6 , showing the another synchronous signal from primary side application .
D1
B+
T1
R11
1
1N4148
5
SB3100
D2
6
Vo
+
C5
D4
4
S
8
D
C4
C4 470P
PWM
CONTROLER
R9
2K
D5
C6
102p
SYNC X'FRM
T2A
*
Q3
Q2
1
2
2SC945
3
C2
4
470P
R1
R10
R7
C3
Q1
G
+
+
U1
GATE
VCC
PGND RT/CT
GND
AOUT
BIAS
DRAIN
8
7
6
5
Q5
2SC945
R2
1K
N3856V
T2B
16K
470
D3 *
1N4148
R5
U3
PC817
4
3
1
R3
R6
2
U2
R8
Fig 6
C1
TL431
R4
Synchronous signal from primary side , using the N3856x Series
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See the Fig 7 , we can connect synchronous rectification MOSFET to the GND , and it is
implemented for multiple output application .
B+
1
T1
D1
5
R1
D2
2
4
1N4148
6
8
Vo1
+
C1
U1
3
+
C2
7
9
1
10
3
2
4
C4
Q1
Q2
IRFB4710
PWM
CONTROLER
470P
N3856V
GATE
VCC
PGND RT/CT
GND
AOUT
BIAS
DRAIN
R3
8
7
6
R2
47K
+ C3
5
C5
470P
16K
D3
3A/100V
R4
Vo2
Q3
IRFB4710
D4
+ C6
3A/100V
R5
U2
4
PC817
3
1
R6
R7
2
U3
R8
Fig 7
TL431
R2 or C5 is adjusted for the maximum duty cycle of synchronou
rectifier MOSFET
C7
R9
Multiple output and the MOSFET on the low side application
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DEMO BOARD DESCRIPTION
The Fig .8 presents a demo board for the N3856x , This board replaces the output rectifier
diode with synchronous rectification MOSFET in flyback power supply and includes
all the components needs by the N3856x to operate, R3 and C1 can be adjusted for diffferent
switching frequency and turn-on duty demand
GND
GATE
DRAIN
VCC
D1
1N4148
1
2
C2
470P
3
4
U1
GATE
VCC
PGND RT/CT
GND
AOUT
BIAS
DRAIN
N3856V
8
7
C1
R3
470P
51K
6
5
R1
16K
Fig 8
Demo board layout----componects
Fig 9 Demo board layout----component side
Fig 10 Demo board layout--- soldering side
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