AN1074

Application Note 1074
Application Notes for AP3771 System Solution
Prepared by Zhao Jing Jing
System Engineering Dept.
Regulation (PSR). AP3771 has the special technique to
suppress the audio noise, internal line compensation to
reduce the number of system components, fixed cable
compensation to compensate the voltage drop on different
output cable for achieving good CV regulation.
1. Introduction
The AP3771 uses Pulse Frequency Modulation (PFM)
method to realize Discontinuous Conduction Mode (DCM)
operation for FLYBACK power supplies. The principle of
PFM is different with that of Pulse Width Modulation
(PWM), so the design of transformer is also different.
The AP3771 can achieve low standby power less than
30mW.
The AP3771 can provide accurate constant voltage,
constant current (CV/CC) regulation by using Primary Side
D2
T1
+
C1
RST1
+
R5
C2
D1
C3
Np
+
Ns
D3
RST2
VCC
CS
Q1
GATE
VO +
C4
RDUMMY
VO-
Na
RFB1
AP3771
C5
FB
CPC
GND
RFB2
RLINE
Rcs
Figure 1. Typical Application Circuit of AP3771 for Adapter
Figure 1 is AP3771 typical application circuit, which is a
FLYBACK converter controlled by AP3771 with a
3-winding transformer---Primary winding (Np), Secondary
winding (Ns) and Auxiliary winding (Na). The AP3771
senses the auxiliary winding feedback voltage at FB pin
and obtains power supply at VCC pin.
VSEC---The transient voltage of secondary
VS---the sum of VO and forward voltage of rectification
diode
Ip---The primary side current
Is ---The secondary side current
IPK---Peak current of primary side
IPKS---Peak current of secondary side
tSW---The period of switching frequency
tONP ---The time of primary side “ON”
tONS ---The time of secondary side “ON”
tOFF ---The discontinuous time
tOFFS --- The time of secondary side “Off”
Figure 2 is the typical operation waveforms of PFM
controller. In this figure, a series of relative ideal operation
waveforms are given to illustrate some parameters used in
following design steps. And the nomenclature of the
parameters in Figure 2 is illustrated.
Vdri---A simplified driving signal of primary MOSFET
Mar. 2012
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Application Note 1074
tONP
Vdri
tSW
IPK
IP
IPKS
IS
tONS
VS
VSEC
tOFFS
tOFF
Figure 2. Operation Waveforms
In order to achieve low standby power, AP3771 decreases
the minimum operating voltage. In order to achieve the
lower power loss performance, the startup resistor
RST1+RST2 should be as high as possible on the premise of
meeting turn on delay time requirement. The selection of
dummy load resistor is a tradeoff between standby power
and I-V Curve.
2. Five Aspects for System Design
1.
2.
3.
4.
5.
Low Standby Power Design
Switching Frequency Design
Transformer and Power Devices Design
Feedback Resistors Design
Line Compensation Design
2.1 Low Standby Power Design
2.2 Switching Frequency Design
VH=0.5V
VCS_REF
1.4V
VCPC
fSW
fSW
47.6kHz
20kHz
IO
42%IO
Figure 3. Relationship Between VCPC, fSW and IO at Constant Peak Current Mode
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Application Note 1074
Where, the fSW is the switching frequency. η is the
efficiency of the system. When the peak current IPK is
constant, the output power depends on the switching
frequency fSW. fSW is linearly increased with load
increasing.
When the constant peak current is adopted, the voltage of
CPC pin is increased linearly with load increasing. The
maximum value of VCPC is equal to
tONS
4
⋅ VDD = ⋅ 3.5V = 1.75V
t SW
8
(1)
In AP3771, two-segmented peak current is used to realize
audio noise suppression. The peak current is about 0.5V
when IO>42%IOmax, and the peak current is about 0.5V/1.5
when IO<42%IOmax.
The primary current ip(t), as shown in Figure 2, is sensed
by a current sense resistor RCS. The power transferring
from input to output is given by:
PO =
1
2
⋅ LP ⋅ I pk ⋅ f SW ⋅ η
2
(2)
VLOAD VCPC VH=0.5V VCS_REF VL=0.5V/1.5 0.42хIO_MAX
IO_MAX fSW
55kHz
52kHz
fSW 23.1kHz
20kHz
3.85kHz 0.42хIO_MAX
IO_MAX ISOURCE 2/3×I SOURCE 0.42хIO_MAX
IO_MAX Figure 4. Relationship Between VCPC, fSW and IO at Variable Peak Current Mode
So, the voltage of CPC pin (VCPC) and switching frequency
(fSW) has a leap at about 42% of load. At the leap point, if
the peak current is changed from 0.5V( high IPK) to
0.33V(low IPK), the voltage of CPC pin at low IPK will be
increased to 1.5 times of VCPC at high IPK, and the
switching frequency fSW at low IPK will be increased to
2.25 times of fSW at high IPK. So the range of load working
in the audio frequency is suppressed.
VCS_REF
VH=0.5V
VL=0.5V/1.5
39%IO
42%IO
IO
Figure 5. Hysteresis at Conversion Between Low IPK and High IPK
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Application Note 1074
For the primary side current,
In order to avoid oscillation, a hysteresis is added at the
conversion between low IPK and high IPK. Considering the
relationship between audio noise and flux density of
transformer, deltaB≤2500 gauss is better for audio noise
suppression.
t ONP = I pk ⋅
Lp
(8)
Vindc
Where LP is the inductance of primary winding.
Vindc is the rectified DC voltage of input.
When Vindc is the minimum value, the maximum tONP can
be obtained. So,
The low limitation of maximum switching frequency is
given by audio noise suppression. And the upper limit of
the AP3771 can be up to 120kHz. But this is only the limit
of the IC; the finally designed maximum switching
frequency is determined by the tradeoff between the
efficiency,
mechanical
dimensions
and
thermal
performance.
t ONP_MAX = I pk ⋅
Lp
Vindc _ min
(9)
For the secondary side current,
2.3 Transformer and Power Devices Design
In Constant Current operation of AP3771, the CC loop
control function of AP3771 will keep a fixed proportion
between D1 (in Figure 1) on-time tONS and D1 off-time
tOFFS (in Figure 2) by discharging or charging a capacitor
embedded in the IC. The fixed proportion is
tONS
4
=
tOFFS 4
t ONS = I pks ⋅
VS = VO + Vd , Vd is the forward voltage of secondary diode.
For (10), in CV regulation, the VS is a constant voltage, so
tONS is a constant value with different input voltage.
(3)
t ONS
1
⋅ I pks ⋅
2
t ONS + t OFFS
In FLYBACK converter, when the primary transistor turns
ON, the energy stored in the magnetizing inductance Lp.
So the power transferring from the input to the output is
given by,
(4)
NP
⋅ I pk
NS
tONS
1 NP
1 N
⋅
⋅ I pk ⋅
= ⋅ P ⋅ I pk
2 NS
tONS + tOFFS 4 N S
(12)
Here, Pin ' is input power of transformer, not including all of
the power loss at primary side (Rectifier, RCD snubber,
BJT and so on).
(5)
ηin is definition to the input efficiency of system, which is
about 0.9.
(6)
Then,
t SW =
The maximum turn ratio of XFMR should be designed first,
which is to ensure that the system should work in DCM in
all working conditions, especially at the min. input voltage
and full load.
2
L p ⋅ I pk
(13)
2 ⋅ Pin ⋅ η in
tSW, tONP and tONS in (7) are replaced with (13), (9) and (10),
2
Lp ⋅ I pk
2 ⋅ Pin ⋅ ηin
As we know, if the system can meet equation (7) at
minimum input voltage and full load, it can work in DCM
under all working conditions.
Mar. 2012
1
2
⋅ L p ⋅ I pk
⋅ f SW
2
'
2.3.1 Calculate the Max. turn ratio of XFMR (NMAX)
tSW ≥ tONP + tONS
(11)
Pin =
Thus the output constant-current is given by:
IO =
Pin = Pin ⋅ηin = Vin ⋅ I in ⋅ηin
'
At the instant of D1 turn-on, the primary current transfers
to the secondary at an amplitude of:
I pks =
(10)
In (10), LS is the inductance of secondary winding.
The relationship between the output constant-current and
secondary peak current IPKS is given by:
IO =
LS
VS
≥ I pks ⋅
Lp
Ls
+ I pk ⋅
Vs
Vindc_min
(14)
Because the peak current and inductance of primary side
and secondary side have the following relationship,
I pks = N ⋅ I pk ⋅ η i
(7)
Rev. 1. 0
(15)
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Application Note 1074
Ls =
using 1% tolerance resistors for RCS. After RCS is selected,
IPK should be modified based on the selected RCS.
Lp
N
2
(16)
From formula (23), the turn ratio of primary and secondary
side N can be re-calculated.
Here, N is the turn ratio of primary and secondary sides.
ηi = 0.95 , which is the efficiency of IPK and IPKS.
N=
With (14), (15) and (16), then,
I pk
1
ηi
≥
+
2 ⋅ Pin ⋅ηin VS ⋅ N Vin
(17)
VO ⋅ I O
η
The primary side inductance LP determines the stored
energy. LP should be big enough to store enough energy, so
that PO_Max can be obtained first, from the system version.
(18)
From formula (18), the output power can be given by,
η is the system efficiency from input to output.
PO =
At full load, the system will work in the boundary of CC
regulation. IO can be given by,
IO =
1 t ONS
⋅
⋅ I pks
2 t SW
(19)
LP =
Np =
The following can be obtained,
k ⋅η
ηi
)
−
2 ⋅ VO ⋅ ηin ⋅ ηi VO + Vd
(27)
The turns of primary winding,
(21)
N ≤ N max = Vindc_min ⋅ (
2 ⋅ PO ηin
⋅
I ⋅ f SW η
2
PK
2.3.4 Calculate the turns of primary, secondary and
auxiliary
In the design of AP3771,
2 ⋅ t SW
=4
tONS
LP ⋅ I PK
LP ⋅ I PK
≥
Ae ⋅ ∆B Ae ⋅ B max
As we know,
2.3.2 Calculate the peak current of primary side and
current sensed resistor (IPK & RCS)
NS =
IPK can be calculated by the output current.
And the turns of auxiliary winding,
I pks
N
=
k ⋅ IO
N
NA =
(23)
In AP3771, 0.5V is an internal reference voltage. If the
sensed voltage VCS reaches 0.5V, the power MOSFET will
be shut down and tONP will be ended.
0.5V
I pk
NP
N
N S ⋅ VA
VS
(29)
(30)
Where, VS is equal to VO+Vd. VA=VCC+ Vda, VCC is the set
IC supply voltage and Vda is the voltage drop of the
auxiliary diode.
Here, k=4, η i = 0.9 , which is the efficiency of IPK and IPKS.
RCS =
(28)
First, the reasonable core-type and ∆B should be selected.
Ae can be gotten automatically after core-type is selected.
(22)
Then N is fixed as less than or equal to NMAX.
I pk ⋅η i =
(26)
Then, LP can be gotten by,
(20)
I pks = k ⋅ I O
1
η
⋅ L p ⋅ I 2pk ⋅ f SW ⋅
2
η in
Where fSW was set by the user based on definite
requirement.
Then, IPKS can be defined,
k=
(25)
2.3.3 Calculate the inductance of primary side---LP
Because,
Pin =
k ⋅ IO
(k = 4 )
I pk ⋅ η i
For AP3771, the typical value of UVLO is decreased to
6.5V, so the supply voltage of IC，VCC can be set to a
typical value---13V.
(24)
2.3.5 Check the maximum duty cycle of primary side
So RCS can be obtained from (24) and selected with a real
value from the standard resistor series. We recommend
Mar. 2012
After turn ratio of primary side and secondary side is
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Application Note 1074
ηi
k ⋅η
−
) = 13.96
2 ⋅ VO ⋅ ηin ⋅ η i VO + Vd
designed, the maximum duty cycle of primary side at low
line voltage can be calculated again.
N MAX = Vindc_min ⋅ (
Considering the Volt-second balance between magnetizing
and de-magnetizing, the formula of duty cycle is
Vindc_min = Vinac_min ⋅ 2 − 40 ,
We choose N=11
(V + V ) ⋅ N ⋅ 0.5
D= O d
Vindc
(31)
2) Calculate the peak current of primary side and
current sense resistor (IPK & RCS)
2.3.6 Select diodes of secondary and auxiliary sides
Maximum reverse voltage of secondary side，
Vdr = VS +
Vindc_max ⋅ N S
I pk =
(32)
NP
RCS =
Maximum reverse voltage of auxiliary side,
Vdar = V A +
Vindc_max ⋅ N A
I pks
N ⋅ηi
=
Vcs _ ref
I pk
k ⋅ IO
= 0.638
N ⋅ηi
(36)
(37)
= 0.846
We choose
(33)
NP
R CS = 0.85Ω,
(38)
(39)
In (32) and (33), the maximum DC input voltage should be
used.
I PK = 0.64A,
2.3.7 Select the primary side MOSFET
3) Calculate the inductance of primary side---LP
Vdc_max = Vdc_spike + Vindc_max +
(35)
VS ⋅ N P
NS
(34)
LP =
2 ⋅ PO ηin
⋅
= 1.15mH
I ⋅ f SW η
2
PK
(40)
Be careful that the value of Vdc_spike will be different with
different snubber circuit.
We choose Lp=1.15mH
Design Example 1
(for 12V/1A Adapter Application)
4) Calculate the turns of primary, secondary and
auxiliary sides (NP, NS, NA)
Specification:
Input voltage: 90VAC to 264VAC
Output voltage @ cable: VO_CABLE=12V
Output current: IO=1A
Output voltage @ PCB, VO=12.3V (with 1.8m AWG24
cable)
k=2* TSW/TONS=4
Efficiency: ηi = 0.9
Np =
LP ⋅ I PK
LP ⋅ I PK
≥
= 109.43
Ae ⋅ ∆B Ae ⋅ B max
N p = 110
Ns =
NA =
Other setting by users:
Switching frequency: fSW=60kHz
Forward voltage of secondary diode: Vd=0.4V
Forward voltage of auxiliary diode: Vda=1.1V
VCC voltage: VCC=18V
Core_type: EE19/16 (Ae=22.4mm2), Bmax<3000GS
Vdc_SPIKE =50V (with snubber circuit)
= 10
(42)
N S ⋅V A
= 15
VS
(43)
N
5) Check the maximum duty cycle of primary side
The maximum duty cycle of primary side is calculated as
following,
D=
Design Steps:
(VO + Vd ) ⋅ N ⋅ 0.4
= 0.55
Vindc
(44)
6) Select diodes of secondary and auxiliary sides
Maximum reverse voltage of secondary and auxiliary side，
1) Calculate the maximum turn ratio of XFMR
Mar. 2012
Np
(41)
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Application Note 1074
Vdr = VS +
Vindc_max ⋅ N S
(45)
NP
Vindc_m ax = 265V ⋅ 2
Vdr = VS +
Vindc_max ⋅ N S
NP
Vdar = V A +
NP
= 70V
(48)
7) Select primary side MOSFET
(46)
= 47V
Vindc_max ⋅ N A
Vds_mos = Vdc_spike + Vindc_max +
(47)
VS ⋅ N P
= 564V
NS
(49)
Design Results Summary:
1.Calculate the maximum peak current of primary side and RCS
IPK=
640
mA
Peak current of primary side
RCS=
0.85
Current sensed resistor
Ω
2.Design transformer
LP=
1.15
mH
Inductance of primary side
N=
11
Turn ratio of primary and secondary
NP=
110
T
Turns of primary side
NS=
10
T
Turns of secondary side
N A=
15
T
Turns of auxiliary side
DMAX
0.55
Maximum duty cycle of primary side at VINDC=80V
3. Select diode and primary transistor
Vdr=
47
V
Maximum reverse voltage of secondary diode
Vdar=
70
V
Maximum reverse voltage of auxiliary diode
VdcMax=
564
V
Voltage stress of primary transistor
ηi
k ⋅η
−
) = 14.05
2 ⋅ VO ⋅ ηin ⋅ ηi VO + Vd
Design Example 2
(for 12V/1.5A Adapter Application)
N MAX = Vindc_min ⋅ (
Specification:
Input voltage: 90VAC to 264VAC
Output voltage @ cable: VO_CABLE=12V
Output current: IO=1.5A
Output voltage @ PCB, VO=12.24V (with 1.5m AWG22
cable)
k=2*TSW/TONS=4
Efficiency: η = 0.75 , ηin = 0.9 , ηi = 0.9
Vindc_min = Vinac_min ⋅ 2 − 40 ,
We choose N=10
2) Calculate the peak current of primary side and
current sense resistor (IPK & RCS)
I pk =
Other setting by users:
Switching frequency: fSW=50kHz
Forward voltage of secondary diode: Vd=0.4V
Forward voltage of auxiliary diode: Vda=1.1V
VCC voltage: VCC=14V
Core_type: EE20 (Ae=31mm2), Bmax<3000GS
Vdc_SPIKE=50V (with snubber circuit)
RCS =
I pks
N ⋅ηi
=
Vcs _ ref
I pk
k ⋅ IO
= 0.97
N ⋅ηi
= 0.56
We choose
R CS = 0.56Ω,
(53)
I PK = 0.97A,
(54)
3) Calculate the inductance of primary side---LP
Calculate the maximum turn ratio of XFMR
Mar. 2012
(51)
(52)
Design Steps:
1)
(50)
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Application Note 1074
LP =
2 ⋅ PO ηin
⋅
= 0.89mH
I ⋅ f SW η
(55)
2
PK
D=
(VO + Vd ) ⋅ N ⋅ 0.4
= 0.48
Vindc
We choose Lp=0.9mH.
6) Select diodes of secondary and auxiliary sides
4) Calculate the turns of primary, secondary and
auxiliary sides (NP, NS, NA)
Vdr = VS +
LP ⋅ I PK
LP ⋅ I PK
≥
= 92.9
Ae ⋅ ∆B Ae ⋅ B max
N p = 100
(56)
Vindc_max = 265V ⋅ 2
(57)
Vdr = VS +
(58)
Vdar = V A +
Np =
Ns =
NA =
Np
N
= 10
N S ⋅VA
= 12
VS
5) Check the maximum duty cycle of primary side
Vindc_max ⋅ N S
(60)
NP
Vindc_max ⋅ N S
NP
Vindc_max ⋅ N A
NP
(59)
(61)
= 50V
= 60V
(62)
(63)
7) Select primary side MOSFET
The maximum duty cycle of primary side is calculated as
following,
Vds_mos = Vdc_spike + Vindc_max +
VS ⋅ N P
= 550V
NS
(64)
Design Results Summary:
1.Calculate the maximum peak current of primary side and RCS
IPK=
970
mA
Peak current of primary side
RCS=
0.56
Current sensed resistor
Ω
2.Design transformer
LP=
0.9
mH
Inductance of primary side
N=
10
Turn ratio of primary and secondary
NP=
100
T
Turns of primary side
NS=
10
T
Turns of secondary side
N A=
12
T
Turns of auxiliary side
DMAX
0.48
Maximum duty cycle of primary side at VINDC=80V
3. Select diode and primary transistor
Vdr=
50
V
Maximum reverse voltage of secondary diode
Vdar=
60
V
Maximum reverse voltage of auxiliary diode
VdcMax=
550
V
Voltage stress of primary transistor
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Application Note 1074
2.4 Feedback Resistors Design
Figure 6. Feedback Resistors Circuit
From above Figure 6,
(R + RFB 2 ) N S
Vo = VFB ⋅ FB1
⋅
− VD
RFB 2
NA
2.5 Line Compensation Design
The internal line compensation function in AP3771 is
shown in Figure 7. S1 is closed when the primary switch is
“ON”. The line voltage can be detected from the FB pin.
The detected voltage internally compensates the peak
current. So the line compensation is determined by RLINE.
In different application, the value of RLINE is different.
(65)
Through adjusting RFB1 and R FB2, a suitable output voltage
can be achieved. The recommended values of RFB1 and R
FB2 are within 5kΩ to 50kΩ.
Figure 7. Line Compensation Circuit
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Application Note 1074
tSW
FB
tONP
t
VN
tOFF
Figure 8. Waveform of FB Pin
So, RLINE can be adjusted to achieve excellent line
regulation of output current.
The negative voltage VN of FB pin (in Figure 8) is linear to
line voltage. The AP3771 samples VN to realize the line
compensation.
N
RFB 2
VN =
⋅ a ⋅ Vindc
RFB1 + RFB 2 N p
3. Summary
In order to get good performance of AP3771, it’s important
to design transformer, line compensation and feedback
resistance correctly. This application only gives a
preliminary design guideline about these aspects and
considers ideal conditions, so some parameters need to be
adjusted slightly on the basis of the calculated results.
(66)
The compensated voltage of line compensation (VCS_LINE)
can be calculated by the following formula,
1
⋅ VN
670k
N
RFB 2
1
= Rline ⋅ 0.8 ⋅
⋅
⋅ a ⋅ Vindc
670k RFB1 + RFB 2 N p
Vcs _ line = Rline ⋅ K ⋅
Mar. 2012
(67)
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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