Simple Secondary Side Vcc Source for Low Power CVCC Power Supplies

AND8395/D
A Simple Secondary Side
Vcc Source for Low Power,
Constant Voltage, Constant
Current (CVCC) Power
Supplies
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APPLICATION NOTE
Prepared by: Frank Cathell
ON Semiconductor
Introduction
using high value current sense resistors. Most chargers that
fall in this category typically require an output voltage of 3
to 15 volts and are implemented with simple, low cost
flyback switchmode topologies.
The design of off-line constant voltage, constant current
(CVCC) power supplies for cell phone, hand tool, and
similar battery chargers can present several challenges if
low cost and circuit simplicity are necessary, yet good
performance and high efficiency are required. One typical
problem is associated with deriving a simple yet effective
Vcc source for the secondary control circuitry without
resorting to additional circuit complexity, more secondary
windings on the high frequency switching transformer, and
AC
input
18, 2W
4.7nf
”x”
D1
1A
600V
Figure 1 shows a typical CVCC charger implementation
which is described in detail in ON Semiconductor’s
Reference Design TND329/D (5 W CCCV AC-DC Adapter
GreenPointt Reference Design).
L1
820 uH
1
4.7uf,
400Vdc
x2
C2A
C5
1.5 nf
1 kV
C2B
T1
7,8
R2
150K,
0.5W
D2
D3
MURS160
R4 Is
0.62
1W
C4
1000 uF
6.3V
C7
0.1
50V
3
Q1
2N2907
C8
10uf
25V
D5
2
(4.3V)
C9
U1
_
Output
4
R7
MMSD4148A 10
NCP1014ST
+
5.1V @ 1A
5,6
D4
+
MBRS360T3
R6
(Vtrim)
0
1nF
”Y”
R8
2.2K
MMSZ5229B
R1
C1
Typical Application
R5
3
2
4
1
4
U2
1
3
opto
2
68
C6
+ C3
1 nf
10uf
25V
R3
200
NOTES:
1. L1 is Coilcraft part RFB0807− 821L (820 uH @ 300 mA)
2. U2 is 4 pin optocoupler with CTR of 50% minimum
3. See Magnetics Data Sheet for T1 construction details
4. U1 is 100 kHz version
5. D7 zener sets Vout: Vout = Vz + 0.85V
6. R4 set max current: Imax = 0.65/R4
7. R6 allows for Vout trimming (increase only)
8. Fuse resistor recommended for R1
9. Crossed lines on schematic are not connected
5V/1A CC/CV Power Supply with
Universal AC Input (Rev 3)
Figure 1. Simple 5 V, 1A Cell Phone Charger Schematic
© Semiconductor Components Industries, LLC, 2009
April, 2009 − Rev. 0
1
Publication Order Number:
AND8395/D
AND8395/D
50°C and was insufficient to cause any problems due to the
high value of the overcurrent sense resistor and the
accumulated overall circuit impedances.
Although this charger circuit meets previous energy
efficiency requirements, the constantly evolving standards
are moving toward even higher efficiency limits. On of the
largest sources of inefficiency in this circuit is the large value
of the current sense resistor R4, particularly when the
charger is operating in the constant current (CC) mode.
When the current reaches one amp, the voltage drop across
R4 turns on Q1 which bypasses zener D5 (which controls the
charger when in the constant voltage (CV) or float mode)
and regulates the current at a constant value. Note that the
voltage source which supplies current to the feedback
optocoupler U2 is just the output voltage of the supply and
this voltage will fall as the load resistance drops during
overload. At some point approaching a “hard” short circuit,
the output voltage level will fall below the forward drop of
the optocoupler’s photo diode. At this point the output
voltage is too low to power the opto and the V/I load line
profile will show a current “tail” in which CC operation is
not longer functional. Fortunately batteries and similar
loads do not drop this low in normal operation, however,
safety agencies may require testing with a hard short circuit
and if the current tail is excessive or can cause component
failure due to overheating or excessive current, the charger
will fail approval. In the case of the 5 volt, 1 amp reference
design, the current tail is shown in Figure 2 for operation at
6
V/I Profile at 50°C
5
Vout
4
3
2
1
0
0
0.2
0.4
0.6
0.8
1.0
1.2
Iout
Figure 2. Slight Current Tail Due to Hard Short
Circuit
For output of 12 or 15 volts this may cause a more
prominent current tail which may not necessarily pass a hard
short circuit condition.
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AND8395/D
to utilize a much smaller value sense resistor and some type
of operational amplifier with reference to do the current and
voltage sensing. A typical circuit is shown in Figure 3 below
using ON Semiconductor’s NCP4300A dual opamp with
internal 2.6 volt reference.
Despite the simplicity of the current sense circuit and the
minimal current tailing, the power dissipation in R4 at 1 amp
output is 620 milliwatts. Considering that the total power
output is 5 watts, this accounts for a 12% degradation in the
efficiency of the circuit alone. A better approach would be
T1
D2
+
MBR360G
C2
Flyback Xfmr
Secondary
0.1
1,000uF
6.3V
R1
R2
Gnd
Vcc
470
Vs
C4
0.1
1
R3
3K
8
2
D4
7
C6
1N4148A
0.1
1N4148A
U1B
−
R7
1K
6
Is
+
5
NCP4300A
R5
C5
0.1
D5
5 V/1 A
Output
−
0.1 ohm
0.25W
Isense
Feedback
Optocoupler
C3
Vsense
10K
1
U1A
4
−
+
2
3
R4 75K
Vref =
2.6V
internal
to U1
NOTES:
1. Crossed schematic lines are not
connected.
2. R8 sets Vout.
3. R4 sets Iout (set for 2A).
4. U1 Vref (pin 3) is 2.6V
R6
1.5K
C7
R8
3.0K
R9
3.3K
10nF
NCP4300A CVCC Feedback Control
For Flyback Converter
Figure 3. A Possible Solution for CVCC Charger Efficiency Improvement
voltage, and the specified minimum operating voltage for
the chip is 3.0 volts, this circuit will likely develop a
significant and unacceptable current tail in the CC mode
when the output drops below 3 volts.
This circuit may initially appear as a solution to the
excessive current sense resistor dissipation exhibited by the
circuit of Figure 1 since the value of this resistor has been
reduced from 0.62 ohms to 0.10 ohms, however, since
amplifier U1’s Vcc is derived directly from the output
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AND8395/D
A Better Solution
however, this would definitely impact the transformer cost,
winding construction, and pin-out requirements. Since what
we need here is a voltage that is not dependent on the flyback
output voltage, we can access the so-called forward voltage
of the transformer’s existing secondary flyback winding by
utilizing the capacitive charge pump circuit shown in
Figure 4.
A solution which could employ the circuit of Figure 3, and
provide a secondary Vcc that is independent from the
deterioration of the charger output voltage when in CC
mode, would be optimal as long as the overall circuit
complexity and cost is not significant. Adding an additional
secondary winding on the flyback transformer would work,
D1
C1
T1
0.1
50V
D3
1N4148A
1N4148A
D2
MBR360G
+
C2
Flyback Xfmr
Secondary
0.1
1,000uF
6.3V
R1
R2
Gnd
Vcc
470
Vs
C4
0.1
1
R3
3K
8
2
D4
7
C6
1N4148A
C5
0.1
D5
1N4148A
NOTES:
1. Crossed schematic lines are not
connected.
2. R8 sets Vout.
3. R4 sets Iout (set for 2A).
4. U1 Vref (pin 3) is 2.6V
5 V/1 A
Output
−
0.1 ohm
0.25W
Isense
Feedback
Optocoupler
C3
Vsense
U1B
R7
1K
6
−
Is
+
5
NCP4300A
4.7uF
35V
R5
10K
1
U1A
4
2
−
+
3
R4 75K
Vref =
2.6V
internal
to U1
R6
4.7K
C7
R8
3.0K
R9
3.3K
10nF
NCP4300A CVCC Feedback Control
With Charge Pump Derived Vcc
Figure 4. Secondary Vcc Bias Circuit Using Capacitive Charge Pump
Circuit Operation
capacitor C6 will be the sum of that on C1 and C2, namely
15 V plus 5 V = 20 volts. The actual value on this capacitor
will show some variation due to transformer leakage
inductance effects but will typically be in the range of 20 to
24 volts at 120 Vac input with the charger operating in CV
mode. Keep in mind that this Vcc voltage is AC line
dependent and will go to approximately 35 or 40 volts for
230 Vac input during normal CV operation. Under these
latter input conditions a small resistor in series with D3 is
recommended along with a 30 or 32 volt zener diode in shunt
with C6 to limit the maximum Vcc to below the specified 36
volts for the NCP4300A.
During the switching period when the primary side
MOSFET is on, flyback output diode D2 is non-conducting.
Charge pump diode D1 is conducting and charging capacitor
C1 to a voltage which is equal to the primary side dc bulk
voltage divided by T1’s turns ratio. In this case the
primary-to-secondary turns ratio is 11:1, so at 120 Vac input
the primary bulk voltage will be 165 Vdc and C1 will charge
to approximately 15 volts. When the primary side MOSFET
turns off and D2 conducts due to flyback action, D1 will be
reversed biased and the charge on C1 will be “pumped” via
diode D3 to Vcc capacitor C6. Because C2 and C1 appear in
series during the flyback period, the voltage on Vcc filter
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AND8395/D
Efficiency Impact
When the power supply is operating in the constant
current mode, the secondary Vcc will obviously be less than
the voltage when in CV mode because the voltage on C2 will
be lower due to the collapse of Vout. Under a hard short
circuit the theoretical output voltage could be zero, however,
there will also be some additional collapse of the forward
voltage of C1 due to the very short converter duty cycle
demanded by the short circuit. This voltage, however, will
still be sufficient (typically greater than 8 volts) to allow
proper operation of U1 and the rest of the secondary logic.
Figure 5 shows the CVCC V/I profile of the 5 watt charger
using the NCP4300A and charge pump Vcc bias circuit.
Note the virtual “textbook” V/I profile right down to Vout =
250 mV with no current tail.
The original 5 V, 1 A cell phone charger circuit of Figure 1
and TND329 is compared to the upgraded circuit using the
NCP4300A plus charge pump Vcc circuit of Figure 4.
Efficiency measurements were made for 120 and 230 Vac
inputs. The efficiency was measured per recent Energy Star
criteria for low voltage power supplies at 25%, 50%, 75%
and 100% of the charger’s rated load, and the average of
these measurements was calculated to determine the average
efficiency. The average efficiency results are shown in the
table below.
An efficiency improvement of about 5% was achieved
which is significant at this low of output power level,
particularly in light of the latest updated Energy Star
requirements. Lower efficiencies at 230 Vac input are
attributed to increased switching losses in the NCP1014’s
internal MOSFET due to the higher voltages involved.
6
V/I Profile with NCP4300A
5
Vout
4
3
2
1
0
0
0.2
0.4
0.6
0.8
1.0
1.2
Iout
Figure 5. CVCC V/I Load line Profile with NCP4300A
and Charge Pump Bias Circuit
Circuit Configuration
120 Vac Input Efficiency
230 Vac Input Efficiency
Original circuit (TND329 & Figure 1)
70%
65%
NCP4300A plus charge pump Vcc (Figure 4)
75%
69.4%
References
− Application Note AND8296/D: Increasing the Efficiency
of Low Power Converters with Resonant Snubbers.
− Application Note AND8134/D: Designing Converters
with the NCP101X Family.
− Design Note DN06009/D: 5 W, CCCV Cell Phone Battery
Charger.
− Design Note DN06013/D: 3.6 W Auxiliary Power Supply
for Appliances / White Goods.
(See ON Semiconductor website: www.onsemi.com)
− ON Semiconductor Datasheet NCP4300A/D.
− Reference Design TND329/D: 5 W CCCV AC-DC
Adapter GreenPointt Reference Design.
− Application Note AND8042/D: Implementing Constant
Current Constant Voltage AC Adapter by NCP1200 and
NCP4300A.
− Application Note AND8132/D: Performance
Improvements to the NCP1012 Evaluation Board.
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AND8395/D
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AND8395/D