AN1062

Application Note 1062
High Voltage Green Mode PWM Controller AP3105
Prepared by Wu Qikun
System Engineering Dept.
1. Introduction
the PCB layout, and methods for reducing the
standby power loss, and finally presents a demo
design of a 12V 1A adaptor.
The AP3105 is a low start-up current, current mode
PWM controller with green-mode power-saving
operation. Different from AP3103, AP3105 PWM
switching frequency at normal operation is fixed at
65kHz dithering with a narrow range. The difference
between AP3103/AP3105 is shown in Table 1. The
dithering of frequency will improve EMI feature.
When the load decreases, the frequency will reduce
and when at a very low load, the IC will enter the
“burst mode” to minimize switching loss. A
minimum 20kHz frequency switching is to avoid the
audible noise as well as reducing the standby loss. A
so-called VCC Maintain Mode is applied under light
load to realize a stable output and to reduce the loss
on the start-up resistor. The standby power of the
system using AP3105 can be reduced to 60mW at
230V input.
AP3103
2. Function Description
2.1 CTRL Pin
For some applications, the system requires external
programmable protection function. The CTRL pin
has two kinds of modes to trigger the protection: high
level trigger and low level trigger. The low threshold
voltage is 0.5V and high threshold is 2.5V. When the
CTRL pin voltage is lower than 0.5V or higher than
2.5V, latch or auto-restart protection will be triggered
(different versions of 3105 offer different protection
combination, which is shown in Table 2).
Version
AP3105
AP3105
VCC OVP
Auto-
Autorecoverable
Frequency
Adjustable
Fixed at 65kHz
4.5kΩ
10kΩ
AP3105V
Latch
Better
Best
AP3105L
Latch
External Protection
NA
By “CTRL” pin
AP3105R
VCC OVP
Auto-recoverable
OLP & FOCP
Auto-recoverable
Standby
Latch
Autorecoverable
Latch
CTRL(low)
Latch
CTRL(high)
Autorecoverable
Latch
Latch
Latch
Latch
Auto-
Auto-
Auto-
recoverable
recoverable
recoverable
Latch
Table 2. Version Classification of AP3105
/Auto-recoverable
Latch
CTRL pin voltage maintains 1.6V if the pin is
floating, so leave CTRL pin open if the designer does
not need this function. Once the latch protection is
triggered, the bulk capacitor will provide the energy
to the IC through start-up resistor to ensure the IC
disable the output signal (latch mode). This mode
will not be released until the AC input is shut off.
Therefore, the de-latch time is mainly depending on
the value of HV startup bulk capacitor. If the system
needs a short de-latch time, it is better for the startup
resistor to take power from the point before the
rectifier bridge. Typical application of CTRL pin is
shown in Figure 1.
/Auto-recoverable
Table 1. The Difference between AP3103/AP3105
The AP3105 integrates a lot of functions such as the
Lead Edge Blanking (LEB) of the current sensing,
internal slope compensation and several protection
features which include cycle-by-cycle current limit
(OCP), fast OCP (FOCP), VCC over voltage
protection, OTP, OLP protection. The “CTRL” pin is
designed for customers to add external protection
functions such as OVP and OTP.
The AP3105 is specially designed for off-line
AC-DC power supply, such as LCD monitors,
notebook adapter and battery charger applications. It
can offer the designers a cost effective solution while
keeping versatile protection features. The IC uses
SOT-23-6 package type to realize its compact size.
This application note includes detailed explanation of
the IC’s major functions, some considerations about
Apr. 2011
FOCP
recoverable
VFB Resistor
Performance
OLP&
Note:
1. The sink current to the CTRL pin should be lower
than 5mA by selecting a proper pull up resistor.
2. If the designer needs to apply a bypass capacitor
on CTRL pin, the capacitor should not be more than
1nF.
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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Application Note 1062
OVP and OTP
OVP
2.3 VCC Maintain Mode
Under ultra light load or load transient condition, VFB
will drop to lower than 1.4V, thus the PWM drive
signal will be stopped, and there is no more new
energy transferred due to no switching. Therefore, the
IC supply voltage may drop to the UVLO (off)
threshold and the system may enter the unexpected
restart mode. To avoid this situation, the AP3105
holds a so-called VCC maintain mode which can
supply energy to VCC. When VCC decreases to a
setting threshold (10.1V), the VCC maintain
comparator will output a drive signal to make the
system switch and provide a proper energy to VCC
pin. When VCC increases to 10.6V, the gate signal
will be stopped. The VCC maintain function will
cooperate with the PWM and the burst mode loop to
make the output voltage variation be within the
regulation. This mode is designed for reducing
startup resistor loss and it will achieve a better
standby performance with low value VCC capacitor
and larger startup resistor. The VCC will not reduce
to the UVLO (off) threshold during the startup
process and under ultra light load or load transient
condition. To avoid the “VCC maintain mode”
triggering in normal operating condition, it is
suggested to design the VCC value higher than VCC
maintain threshold under minimum load condition.
The processing of VCC maintain mode is shown in
Figure 4.
OTP
Figure 1. CTRL Pin Application
2.2 Fast OCP Function
When the load is short-circuited, the power converter
can be protected by OLP protection. But if the output
filter inductor and the secondary Schottky is
short-circuited, the transformer will be immediately
saturated resulting in the breakdown of the MOSFET
due to high voltage stress. The AP3105 bears built-in
fast OCP function to alleviate the saturation of the
transformer and reduce the voltage stress of
MOSFET. The FOCP position and FOCP waveform
are shown in Figure 2 and Figure 3. When the
secondary Schottky and the output filter inductor is
short-circuited, the power converter can trigger latch
or auto-restart immediately within several switching
cycles with fast OCP. The FOCP threshold on FB pin
is 1.8V.
Fast OCP
Burst mode
VCC maintain mode
FB
1.4v
vcc
10.6v
10.1v
Figure 2. FOCP Position
Gate
Diode shorten
Load decreasing
Figure 4. The Process of VCC Maintain Mode
3. PCB Layout Consideration
VCC
3.1 High Frequency Loop Consideration
As shown in Figure 5, there are four major high
frequency current loops:
FB
SENSE
1. The current path from bulk capacitor, transformer,
MOSFET, RCS returning to bulk capacitor
2. The path from GATE pin, MOSFET, RCS returning
to the ground of IC
Figure 3. FOCP Waveform
Apr. 2011
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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Application Note 1062
3. The RCD clamp circuit is a high frequency loop
In addition, the IC should not be placed in the loop of
switching power trace, and in some applications, the
power ground could be crossed over by the control
signal (low current and low voltage), but the
switching power trace with pulsating high voltage
should not be crossed over.
4. Transformer, rectifier diode, and output capacitor
is also a high frequency current loop
The loops must be as short as possible to decrease the
radiate area for a better EMI, and if the MOSFET and
Schottky diode have heat sink, the heat sink should
be connected to their ground separately.
3.2 ESD Consideration
Electro-Static Discharge (ESD) is an important
testing item for switching power supply. The
system’s ability for bearing the test could be
improved by designing a path to release the electric
charge to the ground.
As shown in Figure 6, the red line represents the
proposed path to release the charge. The copper tips
for discharging should be placed between primary
side and secondary side, but the distance between two
copper tips should be consistent with the requirement
of the safety specification.
The input common mode filter and differential mode
filter will affect the effect of transient discharging, so
the copper tips should be added and their distance
should be as short as possible. Another way is placing
a resistor paralleled with the inductor to replace the
copper tip and the resistor’s value is about 1kΩ to
5kΩ. A smaller resistor is helpful to ESD but has bad
effect on lighting surge.
Figure 5. High Current Loop
F1
RT1
BD1
AC
L1
CX1
R1
L2
R2
C6
T
R3
C1
R5
C2
R6
R12
R7
C3
5
VCC
U1
NTC
3 CTRL
D3
C8
C7
RLoad
D2
Q1
GATE 6
R8
R9
AP3105
1 GND
L3
D4
D1
R13
SENSE 4
R10
FB
2
C4
R11
R16
C9
R14 R15
CY1
C5
U2
R17
U3
Figure 6. The Path of Release Charge of ESD
Apr. 2011
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
3
Application Note 1062
3.3 Ground Layout Consideration
A proper “Ground” layout is important to decrease
unknown noise interference and EMI issue in the
switching power supply.
smaller. A smaller voltage on “FB” pin will result in
a lower operating frequency. It is good for achieving
low standby power, but it will also make the OLP
point larger.
A so-called “Star” connection is highly recommended
for primary GND. As shown in Figure 7, the ground
of MOSFET, auxiliary winding, Y-cap and control IC
are separated and finally connected together on bulk
capacitor ground. The width of these grounds should
be kept as large as possible. The primary side of
Y-cap could also be connected to the high voltage pin
of the transformer.
4.4 The Output Voltage Dividing Resistor
The value of output voltage dividing resistor should
be as high as possible, but the maximum value of the
resistor connected to GND (R17 in Figure 6) should
not exceed 15KΩ.
4.5 Primary RCD Clamp Circuit
To get a better standby power, the RCD clamp circuit
could be replaced by a Transient Voltage Suppressor
(TVS) and a diode (Figure 8). The advantage of the
TVS clamp is that it only conducts when necessary
and it is independent of the switching frequency.
Compared to a RCD clamp, it reduces no-load power
but increases costs and EMI. Besides, a lower value
of RC is contributed to standby power, while the
voltage stress on MOSFET should be in the spec.
Bulk ground
MOSFET
ground
Y-CAP
ground
IC ground
Auxiliary
winding ground
Figure 7. Star Connection of Primary GND
4. Standby Power Loss Reduction
Some methods are recommended here for reducing
the standby power loss.
4.1 X-capacitor and X-resistor
A good quality X-capacitor will be helpful to save the
standby power, and a low value X-cap could also
decrease the X-cap loss. According to IEC 60950, for
the X-cap exceeding 0.1µF, the voltage will be
decayed to 37% of its original value during an
interval equal to one constant, and after calculating,
the RC value is determined by the formula “R×C<1”.
Therefore, for a low value X-cap, a higher value
X-resistor could be used, and the losses on X-resistor
will be reduced.
Figure 8. Clamp Circuit with TVS
4.6 Secondary Diode RC Snubber
A low value RC which parallel with Schottky could
be helpful to the low standby power. The value
should be adjusted to make the voltage stress on
Shottky not exceed spec at turn-on.
5. Demo Design of 12V 1A Adaptor
12V 1A DEMO using flyback topology is designed,
and the system specification is as follows:
4.2 Current Sampling Resistor
The value of current sampling resistor could affect
the standby power. A lower value CS resistor is good
for low standby power. But it also has effect on the
OLP result, a lower value CS resistor will make a
larger OLP point.
Output voltage and current: 12V/1A
„
Input voltage range: 90Vac~265Vac
Table 3 is the test result of the standby power, and
Table 4 shows the demo board components list.
When the load is 12V/0A, the standby power is less
than 60mW in the input voltage range under 230V.
The power is measured by a power meter Chroma
66202. Figure 9 shows the application circuit
schematic.
4.3 “SENSE” Pin RC Value
The value of “SENSE” pin RC could also affect the
standby power. A larger value of RC can make the
I-peak sense signal and the voltage on “FB” pin
Apr. 2011
„
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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Application Note 1062
Input Voltage
90Vac/60Hz
115Vac/60Hz
180Vac/50Hz
230Vac/50Hz
264Vac/50Hz
Input Power
0.032W
0.0345W
0.0463W
0.0605W
0.0742W
Table 3. Test Result
R1,R2
RES,1206,510K,J
2
C1
CAP,Y1,1000pF,250V
1
R3
RES,1206,10Ω,J
1
C2,C3
CAP,AL,10µF,400V,Φ10mm,
Rubycon, YXA, 105°C
2
R4
RES,1206,0Ω,J
1
C4
CAP,CD,222M,1KV
1
R5
RES,1206,1.5Ω,F
1
C5,C6
CAP,Low ESR,
AL,470µF,16V,Φ8mm, CapXon, LZ
2
R6
RES,1206,10Ω,F
1
C7
CAP,AL,2.2µF,50V,Φ6.3mm,
Rubycon,YXF
1
R7,R10
RES,1206,1.5M,J
2
C8
CAP,AL,22µF,50V,Φ6.3mm,
Rubycon,YXF
1
R8
RES,0603,1K,J
1
C9
CAP,0805,102,100V,J
1
R9
RES,0603,10K,J
1
C11
CAP,0603,151,50V,J
1
R11
RES,0805,10Ω,J
1
C13
CAP,0603,683,50V,J
1
R12
RES,0603,24K,F
1
C14
CAP,0603,222,50V,J
1
R13
RES,0603,5.6K,J
1
D1~D6
DIODE,1N4007,1A,1000V,DO-41
6
R14
RES,0603,100K,J
1
D7,D8
DIODE,1N4148,0.15A,100V,LL-34
2
R15
RES,0603,91K,F
1
D9
DIODE,SCHOTTKY,SB3100,
3A,100V,DO-201
1
R16
RES,0805,0Ω,J
1
U1
IC,PWM,AP3105,SOT-23-6,BCD
1
R18
RES,1206,3K,J
1
U2
IC,OPTO
COUPLER,PC817B,DIP-4
1
J1
BASE,3.96mm,3PIN,3N2
1
U3
IC,AS431I,TO-92,BCD
1
J2
BASE,3.96mm,2PIN
1
F1
FUSE,CUBIFORM,SLOW,1A,250V
1
T1
X'FM,EF-20,102Ts,1.6mH
1
L2
CHOKE,FER,DR6*8,1.5mH, 0.3A,
Φ6mm
1
Q1
MOSFET-N,
1A,600V,TO-251,STD1NA60
1
L3
CHOKE,2.2µH
1
Table 4. BOM of DEMO BOARD (12V/1A)
Apr. 2011
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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Application Note 1062
F1
D1~D4
L2
R1 R2
C3
C2
AC
T1
R7
R3
C4
12V…1A
R10
L3
C9
D9
R4
D5
C5
D7
C7
C8
C1
5 VCC GATE 6
AP3105
C11
R14
R15
R11
R9
3 CTRL SENSE 4
FB
RLoad
D8
U1
1 GND
C6
D6
R8
C13
R5
R6
2
R18
R13
R12
U2
U3
R16
C14
R17
Figure 9. Application Circuit Schematic
Apr. 2011
Rev. 1. 0
BCD Semiconductor Manufacturing Limited
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