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參數(shù)資料
| 型號(hào)： | 80C751 |
| 廠商： | NXP Semiconductors N.V. |
| 英文描述： | 80C51 8-bit microcontroller family(80C51 8位微控制器系列) |
| 中文描述： | 80C51的8位單片機(jī)系列（80C51的8位微控制器系列） |
| 文件頁(yè)數(shù)： | 1/12頁(yè) |
| 文件大小： | 106K |
| 代理商： | 80C751 |
Philips Semiconductors
Application note
AN439
87C751 fast NiCad charger
1
June 1993
DESCRIPTION
This application note describes a portable
standalone, automatic constant current
NiCad battery charger using the Philips
Semiconductors 87C751 microcontroller. This
unit will fast charge NiCad batteries from a
12V source such as an automobile battery or
a DC power supply. The charge current is
2.5A which is suitable for charging NiCad
cells of 1200mAh capacity in little over 1/2
hour.
Use of a microcontroller provides complete
flexibility of design parameters such as the
number of cells, their capacity and charge
rate requirements. A number of key
microcontroller techniques described in this
application note are a state machine, a
low-cost, high-resolution single slope
analog-to-digital converter comprised of the
microcontroller and a comparator as well as
an analog control system.
As shown in the block diagram (Figure 1), the
charger consists of the 87C751, a switching
charge current regulator, an analog-to-digital
converter and some LED indicators.
DESIGN OBJECTIVES
The goal of this design is to achieve a
maximum charge rate without damage to the
NiCad cells. To determine the requirements
for such a charger, we need to examine the
most important characteristics of NiCad cells.
As a NiCad cell charges, gas bubbles are
released from the electrolyte and accumulate
on the plates, reducing the effective plate
area and increasing cell impedance. When
the cell gets near full charge the rate of gas
generation and temperature rise increase,
since the charge current produces gas rather
than stored charge. At that point the process
goes into thermal runaway. The cell pressure
rises sharply causing the case to vent. A
large enough venting can destroy the cell
immediately. If the venting is of a lesser
magnitude, as would be the case with a
relatively low charge current, the cell capacity
is reduced.
The standard technique is to charge the cells
at such low current that there is no risk of
thermal runaway. The electrolyte then
reabsorbs the gas bubble at the same rate as
they are generated. Usually this implies a
charge current of 0.1 times the cell capacity
and a charge time of 16 hours. We want to
achieve full charge in about half an hour.
Slow charging also increases the likelihood of
dendrite formation. Dendrites are crystalline
fingers that can propagate through the plate
separators and short the cell internally. Fast
charging tends to clear these shorts before
they have a chance to become significant.
CHARGE TERMINATION
There are two methods commonly used for
terminating NiCad fast charges: delta-peak
voltage detection and delta-temperature
detection. This charger uses the delta-peak
voltage method. It is simpler to implement,
especially if interchangeable battery packs
are to be charged, because the temperature
sensing method requires a temperature
sensor to be attached to the battery pack.
The temperature change in the battery pack
depends on how well the pack is thermally
coupled to its surroundings, which also
makes temperature sensing somewhat tricky.
The voltage on a NiCad pack rises during
charging, steeply at first, and then at a lower
rate. When the pack is nearly full, the voltage
rate of rise increases a little, then falls to zero
as the voltage peaks. As the pack goes into
over-charge the voltage starts to drop and the
internal temperature and pressure rise. A
typical voltage drop used for charge
termination is 1% of the peak voltage as
shown in the battery voltage waveform
(Figure 2). This charger uses 1% of the
measured peak voltage as the threshold,
which means that it can charge any number
of cells from 1 to 6 without any external input
to select a number of cells.
THE 87C751
MICROCONTROLLER
The Philips Semiconductors 87C751 contains
a 2k byte ROM, a 64 byte RAM, 19 I/O lines,
a 16 bit auto-reload timer, a five-source
fixed-priority interrupt structure, a
bidirectional I
2
C serial bus interface and an
on-chip oscillator.
In this application note, the timer is used as a
‘tick-timer’, scheduling events and measuring
periods. The ports are used to control and
monitor the external analog circuitry. The
software is written in C using the Franklin C
compiler.
Source
8XC751
PWM
CONTROLLER
ADC
LEDs
L
Rsense
P-FET
Load
SU00413
Figure 1. Block Diagram
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相關(guān)代理商/技術(shù)參數(shù) |
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| 80C86-2 | 制造商:undefined 功能描述: |
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