Design
Ideas

January 16, 1997

Switching current pump makes better charger

Alexander Belousov, Rego Park, NY

Many currently proposed
battery-charger designs are simply voltage regulators redesigned to be
current sources. Unfortunately, this design approach is far from optimal
because it leads to unnecessary hardware redundancy and complexity. This type
of charger also raises a stability problem because of the additional active
device in the feedback loop.

The alternative design in
Figure 1, which is based on a
switching-inductive current pump, is much simpler and easier to build because
it uses common discrete devices. Also, the primary design goal of a switching
voltage regulator is to keep the constant output voltage under the different
load conditions, but a current pump is designed to maintain constant load
current under varying terminal voltages. Thus, a current pump is inherently
suited to the charging requirements of NiCd and NiMH batteries, and even the
constant-current requirements of Peltier Coolers.

FIGURE 1

Combining
a switching-inductor current pump with a temperature-sensing circuit
produces a battery charger that's simpler and more inherently suited for
charging requirements than are switching-voltage-regulator designs.

The design in Figure 1 combines a switching current pump
with a temperature-monitoring design formerly proposed in Kerry Lacanette's
Design Idea, "Temperature
sensor doesn't discharge battery" (EDN, Aug 1, 1996, pg 106). The current pump in Figure 1 aims to keep the current constant
while the battery voltage varies. The circuit uses a typical switching
inductor similar to the boost topology in voltage regulators; thus, the
design formulas are similar. The main design formula for discontinuous mode
is

where ILOAD is
the average load (charge) current; VBAT and VCC are the
battery and supply voltages, respectively; D is the duty cycle equal to TON/TON+TOFF;
FSW is the switching frequency; and h is the overall efficiency.
The efficiency term is the source of most uncertainty in this equation. The
efficiency of switching regulators based on currently available n-channel
MOSFETs is typically 85 to 95%. So, using an efficiency of 0.9 when solving
this equation produces acceptable results.

Note that the equation
assumes full inductive discharge during TOFF, which implies
discontinuous-mode operation. The mC must provide the proper duty cycle to
ensure that the energy stored in the inductor completely dissipates during
the TOFF interval. This requirement means that

where VF is
the forward voltage of catch-diode D1, approximately 0.5V.
Continuous-mode operation is not recommended for this application.

Note also that for this
current-pump topology, VBAT has to be greater than VCC
to prevent dc flows directly through the forward-biased catch diode. A VCC
of 5V well suits the charging of camcorder and other batteries. For lower
voltages, you can modify the design in Figure 1 for 3.3V operation or change the
topology of the current pump (Figure 2). In Figure 2b, for boost-mode operation, the
control circuit has to turn on MOSFET Q2 continuously; for buck
mode, the control circuit has to turn off Q1.

In Figure 1, zener diode DZ
protects the output from the short-term voltage spikes that may occur if the
load disconnects during the operation. The digital control algorithm should include
a subroutine that checks the terminal voltage and stops the current-pump
operation when the terminal voltage rises above the predetermined threshold.
In this design, CH0 of the ADC10732 performs continuous monitoring
of the terminal voltage. Output capacitor C1 improves stability
and reduces ripples, which is an important criterion for driving a Peltier
Cooler.

Use the following
equations to select the inductor:

and

where IL(PEAK)
is the peak inductor current and E·T is one of the inductor's basic
specifications. The inductor, D1, and Q1 must be able
to carry the inductor's peak current. You can digitally adjust ILOAD
by controlling D, FSW, or both. It is a good design practice to
keep FSW within 50 to 500 kHz because both analog and digital
parts of the circuit can easily handle frequencies in this range, and the
inductor does not "buzz."

Two control algorithms
are possible for switching-inductor circuits: pulse-width modulation (PWM)
and pulse-frequency modulation (PFM). For certainty and simplicity, this
circuit uses the PFM control algorithm and discontinuous operation. First,
the A/D converter samples VBAT, and the mC calculates the moving
average (10 to 100 samples are reasonable). The system also checks the load
temperature. If there is no reason to terminate charging, the mC then
calculates and adjusts FSW to produce the necessary load current.
The system also must check that, for the calculated frequency, the inductor's
E·T and IL(PEAK) are still in the safe area.

The component values in Figure 1 produce a 6V/0.1A battery charger
with D=0.2 and FSW=50 kHz. For higher currents, such as 0.25A, you
can choose a smaller inductor, such as 15 µH, or reduce FSW.
You can increase D, but keep D<DMAX, or you will convert the
system into a continuous-conducting mode. You can upgrade the 10-bit ADC10732
to the 12-bit ADC12032 for higher resolution. Q1 features low RON
providing higher efficiency under operation to VCC as low as 2.7V.
Any similar MOSFET is also applicable. This circuit uses VCC and
battery temperature for charge-termination criteria. If you use this circuit
to drive a Peltier Cooler, you can use the LM235 temperature sensor to
measure the surface temperature of both sides. (DI #1975)

FIGURE 2

For
low-voltage operation, you can modify the current pump to include a
transformer (a) or create a universal buck/boost design (b).

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