AN1064

Application Note 1064
Design and Application Notes for AP3765 System Solution
Prepared by Sun Jun Jie
System Engineering Dept.
charger criteria with AP3765 system solution.
1. Introduction
The AP3765 uses Pulse Frequency Modulation (PFM)
method to realize Discontinuous Conduction Mode
(DCM) operation for flyback power supplies. The
principle of PFM is different from that of Pulse
Width Modulation (PWM), so the design of
transformer is also different.
A typical AP3765 application circuit is shown in
Figure 1.
2. Operation Description
2.1 Constant Primary Peak Current
The primary current ip(t) is sensed by a current sense
resistor RS as shown in Figure 1. The current rises up
linearly at a rate of:
The AP3765 can provide accurate constant voltage,
constant current (CV/CC) regulation by using
Primary Side Regulation (PSR).
dip ( t ) vg( t )
=
dt
LM
The AP3765 can also achieve ultra-low standby
power due to its PFM operation and an innovative
ultra-low startup current technique. Less than 30mW
standby power can be obtained to meet five-star
(1)
D2
T1
VO+
+
C1
R1
+
C2
R5
D1
V
D3
R2
VCC
OUT
R3
CS
Ns
C4 +
VO-
N FB
V
Q1
AP3765
R4
C3
NP
R6
FB
GND
R7
RS
Figure 1. Typical Schematic of AP3765 Solution
Jun. 2011
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
1
Application Note 1064
The output voltage is different from the secondary
voltage in a diode forward drop voltage that depends
on the current. If the secondary voltage is always
detected at a fixed secondary current, the difference
between the output voltage and the secondary voltage
will be a fixed Vd. For AP3765, the voltage detection
point is at 3.2µs of the D2 on-time, which means the
the secondary voltage is detected at a fixed secondary
current. The CV loop control function of AP3765
then generates a D2 off time to regulate the output
voltage.
Figure 2. Primary Current Waveforms
As illustrated in Figure 2, when the current IP(t) rises
up to IPK, the switch Q1 turns off. The constant peak
current is given by:
Ipk =
VCS
RS
(2)
2.3 Constant Current Operation
The energy stored in the magnetizing inductance LM
each cycle is therefore:
1
Eg = ⋅ L M ⋅ Ipk 2
2
(3)
So, the power transferring from input to output is
given by:
1
P = ⋅ L M ⋅ Ipk 2 ⋅ f SW
2
Figure 4. Secondary Current Waveform
In CC operation, the CC loop control function of
AP3765 will keep a fixed proportion between D2
on-time tONS and D2 off-time tOFFS by discharging or
charging the built-in capacitance connected. The
fixed proportion is
(4)
Where the fSW is the switching frequency. When the
peak current IPK is constant, the output power
depends on the switching frequency fSW.
t ONS 4
=
t OFFS 3
2.2 Constant Voltage Operation
The AP3765 captures the auxiliary winding feedback
voltage at FB pin and operates in constant voltage
(CV) mode to regulate the output voltage. Assuming
the secondary winding is master, the auxiliary
winding is slave during the D2 on-time and the
auxiliary voltage is given by:
VAUX =
N AUX (Vo + Vd )
NS
(6)
The relationship between the output constant current
and secondary peak current Ipks is given by:
t ONS
1
Iout = × Ipks ×
2
t ONS + t OFFS
(5)
(7)
At the instant of D2 turn-on, the primary current
transfers to the secondary at an amplitude of:
where the Vd is the diode forward drop voltage. NAUX
is the turns of auxiliary winding, and NS is the turns
of secondary winding.
Ipks =
Np
× Ipk
Ns
(8)
Thus the output constant-current is given by:
Iout =
t ONS
1 Np
2 Np
×
× Ipk ×
= ×
× Ipk
2 Ns
t ONS + t OFFS 7 Ns
(9)
2.4 Leading Edge Blanking
When the power switch is turned on, a turn-on spike
will occur on the sense-resistor. To avoid false
termination of the switching pulse, a 750ns leading
Figure 3. Auxiliary Voltage Waveform
Jun. 2011
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
2
Application Note 1064
The tradeoff between the low standby power and the
output overshoot at no load should be considered
during the selection of the dummy resistor R8. In
order to achieve less than 30mW standby power
while having an acceptable output voltage rise at no
load, 5.1K to 10k is recommended for R8. The power
consumed in the startup resistors (R1+R2) also
becomes considerable in no load or light load
conditions. So 10MΩ to 13MΩ resistance is
recommended for the sum of R1 and R2 considering
the target of less than 30mW standby power and less
than 3s turn-on delay time. And the bias capacitor C2
is recommended as 1µF to 1.5µF accordingly.
edge blanking is built in. During this blanking period,
the current sense comparator is disabled and the gate
driver can not be switched off.
2.5 CCM Protection
The AP3765 is designed to operate in Discontinuous
Conduction Mode (DCM) in both CV and CC modes.
To avoid operating in Continuous Conduction Mode
(CCM), the AP3765 detects the falling edge of the FB
input voltage on each cycle. If a 0.075V falling edge
of FB is not detected at the end of tONS, the AP3765
will stop switching.
2.6 OVP & OCkP
The AP3765 includes over-voltage protection (OVP)
and open circuit protection (OCkP) circuitries as
shown in Figure 5. If the voltage at FB pin exceeds
8V, 100% above the normal detection voltage, the
AP3765 will enter OVP mode. However, if the
AP3765 doesn’t monitor 0.075V rising edge of FB
input at the end of tONP or high voltage (>0.075V)
after tSAMPLE, it will enter OCkP mode. When AP3765
enters OVP or OCkP mode, it will sends out a fault
detection pulse every 18ms until the fault has been
removed.
3.2. Transformer Design
Figure 1 describes a flyback converter controlled by
AP3765 with a 3-winding transformer---Primary
winding (NP), Secondary winding (NS) and Auxiliary
winding (NA) for bias power and output voltage
detecting. The AP3765 senses the auxiliary winding
feedback voltage at FB pin and obtains power supply
at VCC pin. In Figure 6, 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 6 is as the
following:
Vdri---a simplified driving signal of primary transistor
IP---the primary side current
IS ---the secondary side current
VS---the secondary side voltage
tsw---the switching period of frequency
fsw---the switching frequency
tonp---the time of primary side “ON”
tons----the time of secondary side “ON”
toff---the discontinuous time
Ipk---peak current of primary side
Ipks---peak current of secondary side
Vds---the sum of Vo and the forward voltage drop of
rectification diode
OVP
8V
S
pro
Q
COMP
R
OCkP
0.075V
COMP
Timer_18ms
Figure 5. OVP & OCkP Function Block
3. Design Guidelines
3.1 Low Standby Power Design
tonp
Vdri
tsw
Ipk
Ip
Ipks
Is
tons
Vds
Vs
toff
Figure 6. Operation Waveforms
Jun. 2011
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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Application Note 1064
4. Design Steps:
L p I 2pk
(15)
Step 1. Select a Reasonable Ipk for the Flyback
Converter with AP3765
t SW =
1-1. Calculate the Maximum Turn Ratio of XFMR
tSW, tONP and tONS in (10) are replaced with (15), (12)
and (13),
The maximum turn ratio of transformer should be
designed first, which is to ensure that the system
should work in DCM in all working conditions,
especially at the minimum input voltage and full
load.
L p I 2pk
Lp
Vindc
(17)
Lp
(18)
Ls =
n ps
2
Where nPS is the turn ratio of primary winding to the
secondary winding.
(11)
With (16), (17) and (18), then,
I pk
2Pin
When Vindc is the minimum value, the maximum tONP
can be obtained. So,
Lp
(12)
t ONP _ MAX = I pk
Vindc _ min
≥
1
1
+
VS n ps Vin
Pin =
VO I O
η
(20)
Where η is the system efficiency.
(13)
At maximum load, the system will work in the
boundary between CV and CC stages. IO can be given
by,
In (13), Ls is the inductance of secondary winding.
1 t ONS
×
I pks
2 t SW
VS = VO + Vd , Vd is the forward voltage drop of the
secondary diode.
IO =
For (13), in CV regulation, the Vs is a constant
voltage, so tONS is a constant value with different
input voltage.
Then, Ipks can be defined,
In the flyback converter, when the primary transistor
turns ON, the energy is stored in the magnetizing
inductance Lp. So the power stored in Lp is given by,
In the design of AP3765,
1
Pin = L p I 2pk f SW
2
(21)
(22)
I pks = kI O
2t SW
= 3.5 (In CC mode, the proportion of tONS
t ONS
(23)
and tOFFS is 4:3)
k=
(14)
So it can be obtained,
Then,
Jun. 2011
(19)
Because,
For the secondary side current,
t ONS = I pks
(16)
I pks = n ps × I pk
(10)
Where Lp is the inductance of primary winding.
Vindc is the rectified DC voltage of input.
LS
VS
Lp
Ls
+ I pk
Vs
Vindc_min
Because the peak current and inductance of primary
side and secondary side have the following
relationship,
For the primary side current,
t ONP = I pk
≥ I pks
2Pin
If the system can meet equation (10) at minimum
input voltage and full load, it can work in DCM in all
working conditions.
t SW ≥ t ONP + t ONS
2Pin
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
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Application Note 1064
n ps ≤ Vindc_min (
k×η 1
−
)
2VO VS
Then, LP can be got by,
(24)
Therefore, the maximum turn ratio of primary and
secondary side N can be obtained.
k×η
1
N ≤ Vindc_min (
)
−
2VO VO + Vd
2P
I f η
LP =
(29)
O
2
PK SW
Here, to achieve good overall system performance,
the optimum switching frequency fSW is
recommended to be 50kHz to 60kHz under full Load.
(25)
2-2. Re-calculate the Turn Ratio of Primary and
Secondary Sides---nPS
Because above calculations are all based on ideal
conditions without considering precision of system, k
is given an experiential value 3.85 to replace the real
value 3.5.
From formula (26), the turn ratio of primary and
secondary side n can be re-calculated.
1-2. Calculate the Peak Current of Primary Side
and Current Sensed Resistor
n ps =
k ⋅ IO
(k = 3.85)
I pk
(30)
Ipk can be calculated by the output current.
I pk =
I pks
n ps
k × IO
=
n ps
2-3. Calculate the Turns of Primary, Secondary
and Auxiliary Sides
(26)
First, the reasonable core-type and ∆B should be
selected. Then, the turns of 3-winding transformer
can be obtained respectively.
Here, k=3.85
nPS is the calculated value of nMAX.
The turns of primary winding,
In AP3765, 0.5V is an internal reference voltage. If
the sensed voltage VCS reaches 0.5V, the power
transistor will be shut down and tONP will be ended.
R CS =
0.5V
I pk
Np =
(31)
(27)
The turns of secondary winding,
So RCS can be obtained from equation (27) and
selected with a real value from the standard resistor
series. After RCS is selected, Ipk should be modified
based on the selected RCS.
NS =
NA =
Step 2. Design Transformer
The primary side inductance LP is relative with the
stored energy. LP should be big enough to store
enough energy, so that Po_Max can be obtained from
this system.
N S VA
VS
(33)
Step 3. Select Diode and Primary Transistor
3-1. Select Diodes of Secondary and Auxiliary
Sides
Maximum reverse voltage of secondary side
From formula (20), the output power can be given by,
Jun. 2011
(32)
Here, VA can be set a typical value of 20V.
Vs is equal to Vo+Vd.
Ae can be got automatically after core-type is
selected.
2-1. Calculation of the Primary Side Inductance
---LP
1
L p I 2pk f SW η (η: system efficiency)
2
NP
n PS
The turns of auxiliary winding,
From now on, Ipk and RCS have been designed.
PO =
L P I PK 10 4 (L :mH,I :mA,Ae:mm2, ∆B :GS)
p
pk
Ae × ∆B
(28)
Vdr = VO +
Rev. 1. 1
Vindc_max N S
NP
(34)
BCD Semiconductor Manufacturing Limited
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Application Note 1064
Maximum reverse voltage of auxiliary side,
Vdar = VA +
Vindc_max N A
I pk =
I pks
N
=
k × IO
N
(35)
NP
I pk_max = 325mA
In (34) and (35), the maximum DC input voltage
should be used.
Sensed current resistor,
3-2. Select the Primary Side Transistor
R CS =
Vdc_max = Vdc_spike + Vindc_max +
VS N P
NS
(36)
0.5V
I pk
(40)
(41)
(42)
R CS ≈ 1.538Ω (Set: R CS = 1.54Ω )
(43)
Re-calculate peak current of primary side,
Be careful that the value of Vdc_spike will be varied
with different snubber circuit.
I pk_max = 325mA
(44)
.
5. Design Example
Step 2. Design Transformer
Specification:
Input voltage: 85VAC-265VAC
Output voltage: VO=5V
Output current: IO=0.7A
System Efficiency: 75%
Switching frequency: fSW=60kHz
Forward voltage of secondary diode: Vd=0.4V
Forward voltage of auxiliary diode: Vda=1V
Feedback voltage of auxiliary winding: Va=20V
Core_type: EE16 (Ae=19.2mm2)
∆B : ∆B =2450GS
Vdc_spike=100V (with snubber circuit)
Output cable: 28AWG, 1.5m long, 0.214Ω/m
Secondary diode turns on duty cycle: D ons = 4/7
Feedback resistor: R6=36.5k, R7=9.1k
2-1. Calculation of the Inductance of Primary
Side---Lp
(46)
k ⋅ IO
(k ≈ 3.85)
I pk
N = 8 .3
(48)
The turns of primary winding,
Np =
(37)
L P I PK 10 4
Ae × ∆B
Vindc_min = Vinac_min × 2 − 40 ( when IO=0.7A, Set: Vindc
(38)
N MAX = 8.3
(39)
(49)
(50)
NP =102 N
drops about 40V )
(47)
2-3. Calculate the Turns of Primary, Secondary
and Auxiliary Sides
1-1. Calculate the Max. Turn Ratio of XFMR
The turns of secondary winding,
NS =
NP
N
N S = 12T
1-2. Calculate the Peak Current of Primary Side
and Current Sensed Resistor
Jun. 2011
L p = 1.47mH
N=
Step 1. A Reasonable Ipk of Flyback with AP3765
Should be Designed
k×η
1
−
)(k ≈ 3.85)
2VO VO + Vd
(45)
O
2
PK SW
2-2. Re-calculate the Turn Ratio of Primary and
Secondary Side---N
Design Steps:
N MAX = Vindc_min (
2P
I f η
LP =
Rev. 1. 1
(51)
(52)
BCD Semiconductor Manufacturing Limited
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Application Note 1064
The turns of auxiliary winding,
N V
NA = S A
VS
(53)
N A = 44T
(54)
Maximum reverse voltage of auxiliary side
Vindc_max N A
Vdar = VA +
NP
Vdar = 181.8V
Vdc_max = Vdc_spike + Vindc_max +
(56)
Vdr = 49.1V
(59)
(60)
Step 4. Select Reasonable Feedback Resistor R6
and R7
R 7 (Set:R7=9.1kΩ,R6=36.5kΩ)
VFB = 4V = Va ×
R6 + R7
(55)
NP
VS N P
NS
Vdc_max = 520.9V
3-1. Select Diodes of Secondary and Auxiliary
Sides
Maximum reverse voltage of secondary side
Vindc_max N S
(58)
3-2. Select Primary Side Transistor
Step 3. Select Diode and Primary Transistor
Vdr = VO +
(57)
(61)
Design Results Summary:
1. Calculate the Maximum Peak Current of Primary Side and RCS
IPK=
325
mA
Peak current of primary side
RCS=
1.54
Ω
LP=
1.47
mH(±8%)
N=
8.3
NP=
102
T
Turns of primary side
NS=
12
T
Turns of secondary side
NA=
44
T
Turns of auxiliary side
Current sensed resistor
2. Design Transformer
Inductance of primary side
Turn ratio of primary and secondary
3. Select Diode and Primary Transistor
Vdr=
49.1
V
Maximum reverse voltage of secondary diode
Vdar=
181.8
V
Maximum reverse voltage of auxiliary diode
Vdc_max=
520.9
V
Voltage stress of primary transistor
4. Select Feedback Resistor
R6=
36.5
kΩ
Feedback resistor
R7=
9.1
kΩ
Feedback resistor
Jun. 2011
Rev. 1. 1
BCD Semiconductor Manufacturing Limited
7
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