Switching Buck regulator Help needed

[Frosty_47]

Good evening ladies and germs,

Just tested my battery charger circuit at 6V and 700mA of current. Well, I should have listened to people when they told me that the regulator will drop the current to protect itself. The 700mA current slowly dropped to 400mA. So now I see my problem. Took me a while to understand what every 1 of you been telling me all this time (guess I learn better by experimenting instead of theory). Anyway, can someone please tell me how I can replace the Voltage regulating LM350 with a Buck switching regulator ? I want to leave my current limmitng LM350 as it is. My input voltage stays the same (30V). The output Voltage will vary from 3V-28V. Maximum current draw is 1A.

Thanks a lot

**broken link removed**

 

[ericgibbs]

hi frosty,

Its important to note, if you apply a constant charge voltage to a battery, the current will reduce as the battery charges up to the constant charge voltage. This effect will occur with SMPS as well as Linear chargers.

This effect is increased when you have a series diode in the output of the charger, as the current thru the diode falls, the forward drop across diode is non linear with current.

If you want to 'force' charge your battery, you require a constant current charger with a upper voltage cut off.

 

[Frosty_47]

Thanks for the reply

What do you mean when u say "upper voltage cut off " ?
The current draw dropped from 700mA to 400mA almost instantly (in 30seconds). I retested the circuit when LM350 cooled down and received the same results. I can actually set the current higher than 700mA but it still drops to about 400mA after 30 seconds. LM350 does this to protect itself from overheating. Overnight I actually though of something different. I plan to add transistors to my circuit in order to take the load of the LM350 regulator. This way I can still charge my battery at desired current without putting to much load on the LM350 itself. Do you what kind of transistors I should use ? I want to use 8 total…

Thanks

 

[ronsimpson]

heat

Are the LM350s running HOT?

 

[Gayan Soyza]

Hi frosty

The calculated current 700mA is available only for few seconds after that it’s decreasing the calculated value. When the battery voltage increases the current decrease.

If you want to charge the battery using the calculated value (700mA) then remove the second voltage adjusted LM350 & charge only with first LM350 (constant current circuit) but with this method batteries will overcharge & destroy so its not a good idea.

But in that charging time with calculated (700mA) current will be higher than your circuit.

I’m using constant voltage/current limited type for SLA batteries. The circuit is in the datasheet of LM317T.If you like you can replace to a high current regulator.

In that I calculated the current at 0.1C rate. It’s charging with the calculated current only within few seconds after that decreasing the current & increase the battery voltage.

I would suggest you charge your SLA batteries with constant voltage/ current limited type & tell us
* How long your battery works?
* How long can your battery supply current for your circuits?
* How long did you use your battery with this circuit?

If the battery stays with your charger longtime & gave good results then it is a good charger otherwise……….?

After built my charger correctly (calculated 450mA) I didn’t go to measure its current again. I know its charging with a very low current when the voltage increases.

 

[Frosty_47]

Ic....

Thanks for the reply.

My question is simple. What types of pass transistors should I use in order to split the current ?

Thanks

 

[Hero999]

My input voltage stays the same (30V). The output Voltage will vary from 3V-28V. Maximum current draw is 1A.

That's a big problem.

The current regulator has too higher dropout voltage to give 28V with an input voltage of 30V

You won't be able to draw much current at 3V as the power dissipation will be too much for the regulator to handle.

You need a switching regulator.

A switching regulator would be a better option.

Have a look at MAXIM and Linear Technoloy's websites; they do a variety of ICs that are targeted at this kind of problem.