(EE project) psu - Buck Converters Provide a Battery Charger and System Power.pdf

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Buck-converter Charger also Provides System Power
Robert Hanrahan - National Semiconductor
11/16/96
Many systems require long time operation during periods of power loss. Often a Lead
Acid battery (gel or wet-cell) is found to be the best solution because of the high capacity
and relative low cost. The battery is charged during normal operation, and used to power
the system during power loss. These systems require a circuit to charge the battery as
well as regulate voltage for the system Vcc. In these systems one must provide a current
limited voltage to the battery for charging, while developing system Vcc in both the
charge or discharge condition.
Many older designs would utilize inefficient linear regulators to provide these functions.
These designs require a large heat sink for regulating the battery voltage to the system
Vcc (typically 5V).
One may utilize switching regulator technology to provide a much more efficient design
at about the same relative cost as a linear regulator design. Many of these designs utilize
low voltage AC power that is usually provided by a low cost wall transformer. Because
of the switching technology utilized in this design, one could accommodate a wide input
voltage range and thus may be used for power line voltages from 100V to 240V without
any circuit changes.
One of the best approaches to the design is a current limited voltage source that sources
current into the battery until the battery voltage reaches a voltage setpoint. The charger
then operates in a constant voltage mode, supplying the current required to maintain the
voltage.
Most lead acid batteries have a voltage setpoint of 13.8V at 25
o
C. The current limit is set
depending on the exact battery and charge time requirement.
The design shown in Figure 1 employs two Simple Switcher Buck converters from
National Semiconductor. The first regulator U1 is an LM2576 or LM2596 Simple
Switcher used to efficiently step down the unregulated input voltage from the output of
the rectifier. This buck converter generates the input voltage for the battery while also
providing voltage to the second regulator. Both buck regulators may utilize either a
slower 52 kHz converter or a higher frequency device marked respectively. The higher
frequency devices employ added features such as sync input and soft-start. The second
regulator U2, is a small DIP or SO LM2574 or LM2594 capable of providing up to 0.5A
system Vcc. One must consider the system current requirements when setting the current
limit value of the charger. The current limit value set by the gain of U3 must be increased
by the current required to supply power to the system.
1
+
Vbatt
12-13.8V
470uH
MRB350
R5
0.1
Ω
MRB350
0.01uF
D5
330uH
Vout
5VDC
@0.5A
L1
D1
330uF
+
C3
R3
100K
Ω
R6
Vbatt
10K
Ω
D2
LMC7101
U3
+
10K
Ω
-
1N914
0V
R7
100K
Ω
R4
C2
0.001uF
R2
22K
Ω
D3
C4
U2
LM2574-5
or LM2594-5
on/off
+
1N5817
D4
330uF
C5
U1
BR1
Vin
16-40VAC
C1
330uF
+
LM2576-ADJ
or LM2596-ADJ
on/off
R1
2.1K
Ω
The first regulator provides the charge voltage setpoint with current regulation
while the second regulator provides the system Vcc
FIGURE 1.
U1 is regulated at the Battery charge voltage with the feedback network R1/R2. These
resistors are chosen by Vout=1.23(1+R2/R1). The diode D3 provides current switching
between U1 and the battery during power loss.
The shaded area is used to measure and regulate the current flow into the battery during
battery charge. The circuit utilizes a shunt resistor to measure the current to the battery,
and amplifies it via the amplifier U3. The LMC7101 shown for U3 is a National
Semiconductor CMOS OP-Amp that provides an output voltage inversely proportional to
current. The LMC7101 provides a BW of 1 MHz and is available in a very small SOT23
package. Other Op-Amps such as the National LMC6482 will also do the job, and are
available in standard DIP and SO packages.
With the gain of 10 provided by the Op-Amp, the diode D2 will forward bias and pull up
the feedback voltage when the output current is about 1.6A (Vref+diode drop). During
normal voltage regulation the diode is reverse biased.
The second regulator U2 is used to provide 5V to the system. This buck regulator
efficiently provides system power when the input is at its highest voltage of about 13.8V
or at the lower voltage that will be generated when U1 is current limiting .
2
+
Vbatt
12-13.8V
470uH
MRB350
R5
0.1
Ω
MRB350
0.1uF
D5
330uH
Vout
5VDC
@0.5A
L1
D1
330uF
+
C3
R3
100K
Ω
R6
Vbatt
10K
Ω
D2
LMC7101
U3
+
10K
Ω
-
1N914
0V
R7
100K
Ω
Current Limiter
R4
C2
0.001uF
Vbatt
10K
Ω
R8
D3
C4
U2
LM2574-5
or LM2594-5
on/off
+
1N5817
D4
330uF
C5
U1
BR1
Vin
16-40VAC
C1
330uF
+
LM2576-ADJ
or LM2596-ADJ
on/off
R2
22K
Ω
R1
2.1K
Ω
R10
100K
Ω
Vbatt
-
U4
LMC7211 470
Ω
+
0V
R12
1M
Ω
R11
D4
LM4041 10K
Ω
R9
-1.2
OPTIONAL
Charge Detector
An additional SOT23 comparator and voltage reference provides a battery charge
indication.
FIGURE 2.
Some systems may need an indication of charge complete. In a system that utilizes a
microcontroller with on chip A/D (such as the National COP8ACC), one could connect
the output of U3 into the input of the A/D, and read the charge current directly.
Depending of the voltage reference used for the A/D, and accuracy needed, one may need
to add another Op-Amp stage prior to the A/D. Figure 2 shows the circuit with an added
SOT23 voltage comparator. The output of the current amplifier U2 is compared to the
voltage set by the potentiometer R9. This voltage can be set to represent the current flow
that takes place at the end of charge.
Simple Switcher designs can be analyzed and verified by utilizing a software package
called “Switchers Made Simple” from National Semiconductor. By splitting the design
into two buck voltage regulators, one may utilize the software to obtain component values
with vendor part numbers, junction temperatures, stability, and lots more design
information.
The software package is available free of charge by calling National at 1-800-272-9959 or
through the Web site at www.national.com.
3

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