mypooka said:
Hi im looking for a schematic
that uses a pic micro to do it
with a PWM, if not ill just do it
schematics no problem for me.
code, got PMW, regulating duty cycles
is the problem, via code.
Dear Friends
Excuse me for lating.I was busy two past weeks.
A little about The Charger:
it is an off line flyback charger designed for charging 24V lead-acid batteries @ 4A.Switching freq. of
75Khz reduces the size of charger and increases quality at the cost of EMI which must be filtered.
Unitrode UC3844 controller is selected for several benefits:
-it works in current mode thus is more stable than voltage mode SMPS
-it has short circuit protection
-due to internal under/over voltage look out there is no need to special supply for the IC
-also it is a low cost controller IC
for switch a high voltage NMOS (2SK794) is selected , thus MAX input voltage can be higher than 250V.
also the ckt works @ MIN input voltge of 170V.
input and output are isolated by a tight CTR optocoupler.
The core is TDK
Q3230 ,it has a small size ,good heat sinking and good shielding.
Primary: 42 turns,0.7mm,in 2 layers
Scondary:7turns,2*0.9mm,in 1 layer
Primary aux :6 turns,4*0.5mm,1 layer
Secondary aux :6 turns ,4*0.5mm,1 layer
Output voltage @ 25°C=27.6V
Output voltage TC=-3.9mV/°C
Short circuit current=4A ;Duration=infinite
there are 4 controlling signal in secondary side:
CSEN=current sense,it is linearly proportional to output current (Io=0A:CSEN=0)(Io=4A:CSEN=2.5V)
VSEN=output voltage,it is linearly proportional to output voltage(Vo=0:VSEN=0)(Vo=27.6V:VSEN=
2.5V)
CCON=current control,by this signal u can control output current according to :
Io=0.4+1.44*CCON ; 0V<CCON<2.5V => 0.4A<Io<4A
VCON=voltage contrlo,by this signal u can control output voltage according to :
Vo=13.8+5.52*VCON ; 0V<VCON<3.5V => 13.8V<Vo<33.12V
if the board works stand alone or its control pins are open,it works in simple constant current/constant
voltage mode.Exciting VCON & CCON by a micro controller or other means can change output
characteristics ;also there is a DIPSWITCH in secondary side which can be used for controlling output
voltage level .RESERVED resistors must be calcuated for this reason.
As u see your external control is not in closed loop , thus there is no unstability problem.Also converting DIGITAL PWM to ANALOG can be done by a simple RC low pass filter.
REFERENCS :
Unitrode/TI App. Notes (
www.unitrode.com):
SLUA055.PDF :
"Simple Switchmode Lead-Acid Battery Charger " contain useful info. about lead acid
batteries and charging algorithms.
SLUA058.PDF :
"AN OFF-LINE LEAD ACID CHARGER BASED ON THE UC3909 " useful hints for
offline charger designing and closing the feedback loop.
SLVA061.PDF :
"Understanding Boost Power Stages in Switchmode Power Supplies" for flyback
basics.
Power Integrations INC. App. Notes (
www.powerint.com):
All of App. Notes Are Recommended,specially for flyback transformer design and construction,EMI and
safety.