Hi all

I want to design charger with smps transformer
I will use tl494 for pwm and mosfet driver for primary side of transformer for range 150 to 240 v AC
I have some questions here , in secondary side I will get constant voltage let say 14.5 volt to charge 12 volt battery,this method use feedback from output voltage
As i know 12 volt charger output over 14 volt then after connect to battery it will decrease depend on battery voltage, and increase constantly to 14.7 then the charger will disconnect (I assume one stage charger)
here i'm confusing if I used feedback from output the pwm will set to pulse width that output 14.5 ,it's mean feedback will compensate drop voltage

I asked to use correct feedback for smps charger?????

i want to ask if i made isolated auxiliary turns on transformer and set it to fixed voltage let's say 5 volt ,then use high ampere secondary for charging ,did the voltage on load drop in each output?

 

Designing offline SMPS is difficult and dangerous.
Battery chargers are cheap. Don't build one, buy one.

 

Hi, Study attached file for all you need to know.

 

Designing offline SMPS is difficult and dangerous.
Battery chargers are cheap. Don't build one, buy one.

Well If the proper safety precautions are adhered to, and someone is prepared to learn, they are only as dangerous as say a valve amplifier, tv etc.I as much as anyone promote safety above all else, but we are in danger of stifling initiative if everytime someone enquires about smps they are advised to steer clear and buy one. At some point we have to say yes, by all means study the design and learn, As long as we provide the necessary safety guidence required.

 

hi all

my question about feedback in battery charger , I have good information about smps

 

Hi,

It sounds unclear about what you are asking. Feedback is always used to correct the output, either for voltage or current or both. In a battery charger you usually need to monitor both so you can limit the current and regulate the voltage when needed.

Maybe post a schematic.

 

Hi,

It sounds unclear about what you are asking. Feedback is always used to correct the output, either for voltage or current or both. In a battery charger you usually need to monitor both so you can limit the current and regulate the voltage when needed.

Maybe post a schematic.

I want to make 1 stage charger, in iron normal transformer,if I put 220 to primary I will get 14.5 to secondary , when put this output to battery the voltage decrease under load and over the time the voltage will increase during charging.

In smps use pwm with duty cycle , so how to set this duty cycle to get 14.5 volt, if I use feedback from voltage the feedback will compensate voltage underload, so battery will not charge

 

Hello again,

Well the simplest explanation is you use PWM and the actual implementation depends on what PWM controller chip you are using. If you use one of the chips like the TL494 then you look on the data sheet and find the "typical application" and that will give you most of the circuit you need. The two error amps are used for voltage and current control. The current control can limit the current to a safe level while the voltage control can regulate the output once the voltage gets to the right level.

The typical circuit is a buck circuit controlled by the TL494 and that controls the charge. You just change the values of the components to fit your needs for whatever battery you are using.

There are probably also application notes for the TL494 also you can look at.

 

so how to set this duty cycle to get 14.5 volt

Read the datasheet for the TL494. See Fig 14 in particular.

 

Much easier to modify an Atx or laptop psu to give out 14 V than to attempt to build your own,..

The pwm on the TL494 is set by the on board oscillator on pins 5,6 and the voltage feedback on pins 1,2 or 15,16, it's an easy chip to work with.

 

Thanks, all
I will try using tl494

 

Designing offline SMPS is difficult and dangerous.
Battery chargers are cheap. Don't build one, buy one.

I agree - some discount stores have (allegedly) smart chargers that bulk charge and then relax the voltage to minimise gassing.

Stores like Lidl & Aldi usually have them in as limited time offers that come around every few months.

There may be stores that keep them as regular stock, but probably not as cheap.

 

Much easier to modify an Atx or laptop psu to give out 14 V than to attempt to build your own,..

The pwm on the TL494 is set by the on board oscillator on pins 5,6 and the voltage feedback on pins 1,2 or 15,16, it's an easy chip to work with.

Usually only the 3.3 & 5V rails are monitored for regulation - you have to load those rails to keep the PWM spinning.

They often use 6.3V electrolytics on those rails - so watch the voltage as you turn the wick up.

A more elegant approach is to strip out the 3.3 & 5V secondary side and re wire the voltage sense divider to the 12V rail.

 

Designing offline SMPS is difficult and dangerous.
Battery chargers are cheap. Don't build one, buy one.

AFAICR: the Optimate uses an iron cored transformer with about 40V secondary. can't remember which chip - but its a common off the shelf heat sink tab variety.