[Design View / Design Solution]
Develop Affordable Mixed-Signal Battery-Charger Designs
Using these techniques, your battery-charger designs can take advantage of the best of both the analog and digital worlds.
SWITCHING BATTERY-CHARGER DESIGN To illustrate these basic concepts, let's take a look at a specific battery-charger design. By partitioning the design into two parts, it's possible to develop affordable, "intelligent" power systems. Battery chargers are, by nature, mixed-signal systems. For example, the power train (in this case, the SEPIC regulator) is analog. Turning the power switch on and off at high frequency requires some type of analog driver circuit. On the other hand, charge termination timers, fault management, and on/off control typically are digital functions that use timers and programmable capability.
This example includes the following specifications:
Input voltage: 6 to 20 V
Output voltage: 0 to 4.2 V for one cell, 0 to 8.4 V for two cells
Preconditioning current: 200 mA
Preconditioning threshold: 3 V
Constant-current charge: 2 A
Charge termination threshold: 100 mA (current at which charge cycle is completed)
Features:
Overvoltage protection (battery removal)
Overcurrent protection (battery or load shorted)
Sense battery temperature for charge qualification
Using a two-part approach to the mixed-signal design, first select a microcontroller that can read the state of the battery pack (voltage and temperature, as well as programming the SEPIC regulator output current). This example uses the PIC12F683 eight-pin flash microcontroller. Next, add a high-speed, analog PWM controller with a built-in MOSFET driver, such as the MCP1630, to develop the "analog" programmable current source.
DESIGNING A SEPIC-PROGRAMMABLE CURRENT SOURCE As with all switching-regulator designs, the output is controlled by varying either the duty cycle or the percentage of switch on-time (Q1 in Figure 6). To regulate current going into the battery, charge current must be sensed. As the circuit diagram shows, there's no sense element in series with the battery.
The SEPIC regulator secondary winding (LS) carries the average output current. The primary winding (LP) carries the average input current. Secondary resistor RSENSE senses battery-charge current, while the high-speed, analog PWM reference input programs the desired battery-charge current.
Referring to Figure 6 again, the MCP1630 analog PWM controller and driver creates a "programmable" SEPIC current source. The PWM and driver supply the analog current regulation, MOSFET gate drive, and high-speed overcurrent protection. The microcontroller sets the SEPIC power-train switching frequency (500 kHz) and programs the SEPIC constant current.
The PWM and driver use the microcontroller hardware PWM to set the SEPIC switching frequency and maximum duty cycle. The hardware PWM frequency equals the SEPIC power-train switching frequency, while the hardware PWM duty cycle sets the maximum SEPIC power-train duty cycle.
A 500-kHz pulse with a 25% duty cycle out of the microcontroller hardware PWM sets the SEPIC switching frequency to 500 kHz, with a maximum duty cycle of 75%. A standard microcontroller I/O pin generates a software-programmable reference voltage using a simple RC filter. This programmable reference programs the constant-current SEPIC converter to a precise charge current.
At the non-inverting input (VREF), the programmable reference voltage sets the amount of battery-charge current. The MCP1630 PWM output duty cycle (VEXT) adjusts until the voltage at the VREF input equals the voltage at the FB input of the error amplifier. By adjusting the voltage at the VREF input, the battery current adjusts accordingly.
The PWM and driver can drive the MOSFET at frequencies greater than 500 kHz while monitoring the SEPIC switch current using an internal high-speed (12-ns typical) comparator. If the switch current is too high, the PWM duty cycle will terminate, limiting the battery current. Finally, the charge current is adjusted based on information such as battery voltage and temperature, received from an analog-to-digital converter (ADC).
To develop a constant-voltage charge phase, the microcontroller ADC reads the battery voltage and updates the programmable current source (SEPIC) to maintain the battery voltage at 4.2 V. This occurs much more quickly than the rate at which the battery voltage changes when it's subject to a constant current.
For Li-ion applications, the charge cycle terminates when the current necessary to maintain the battery voltage at a fixed 4.2 V reduces to some percentage of the battery C-rate (100 mA). This is set using firmware and is easily changed for different battery manufacturers' recommendations. In a typical analog charger, this termination charge current is a percentage of the charge cycle current, so it can't be changed easily.
For NiMH applications, the fast-charge cycle terminates when one or both of two conditions occur—either the battery voltage remains constant or drops with time, or the battery-pack temperature rise is higher than a predetermined value. When fast-charge terminates, a slow, timed top-off charge can begin. An ADC input and battery-pack thermistor together sense battery temperature. By reading the voltage at the "TEMP_SENSE" input, battery temperature can be determined.
Interrupting the PIC12F683 code when the sensed battery voltage is too high achieves overvoltage protection. The SEPIC converter shuts down in less than 1 µs, with minimal voltage overshoot occurring at the battery terminals.
The SEPIC converter diode prevents any path from sending battery discharge back to the system charger. The only quiescent current draw on the battery is from a battery voltage-sensing path, typically less than 5 A.
i need two chargers one multiple from 1.5volts dc to 110ac
Anonymous -May 30, 2009
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