[moved] Charger - Buck Converter Stability

[rightmederek]

Hi all,

I have a question about using buck converter as a charger.

As charger, the loading cap should be really BIG (the loading is a battery), the complex pole of the buck would be very close to 0 (w = 1/(LC)^0.5). So how to make it stable?

Is it adding an even bigger compensation cap at the OUTPUT of error OTA?

Thanks,
DD

 

[KlausST]

Hi,

use a buck type charger circuit. They are made for this.

*****

Usually a battery is relatively high impedance with high frequency, and a buck controller works with high frequency. So the pole at the interfesting regulation loop frequency should be dominated by the ceramic cap.
But in detail it depends on circuit.


Klaus

 

[rightmederek]

Hi,

use a buck type charger circuit. They are made for this.

*****

Usually a battery is relatively high impedance with high frequency, and a buck controller works with high frequency. So the pole at the interfesting regulation loop frequency should be dominated by the ceramic cap.
But in detail it depends on circuit.


Klaus

Hi Klaus,

I thought battery is low impedance at high frequency.

For a voltage mode Buck Converter, the LC control-to-output would have 2 complex poles, which is unstable. And these 2 complex pole is close to 0 due to the big loading capacitor (battery).

I want to know how to compensate this situation.

Thanks,
DD

 

[KlausST]

Hi,

My recommendations:

* use a chrarging circuit, then you don´t have these problems
* if you need to use a buck converter, then use a current mode one, this should be less critical for low impedance loads.

Afaik, the battery is _relatively_ high impedance for high frequency, because it is a relatively slow chemical process to charge the battery. For better information on this please contact the battery manufacturer.

*********************
If you really want to go the hard way, then i recommend to use a series resistor (or series inductance) from charger to battery and an additional small capacitor from charger output to charger voltage feedback.
This should increase impedance (at loop frequency) to your battery and the buck regualtor output capacitor becomes dominant.

Klaus

 

[rightmederek]

Hi,

So you are saying that voltage mode buck converter as a charger is not a good choice?

If i want to build a USB-based battery charger, which means a DC input is available, what is the conventional charging circuit?

Thanks,
DD

 

[KlausST]

Hi

https://www.linear.com/products/Switching_Battery_Chargers
https://www.linear.com/products/USB_Power_Manager_(PowerPath,_Battery_Charger)

Klaus

 

[rightmederek]

No schematics in those links, what is a buck type charging circuit?

Thanks,
DD

 

[KlausST]

Hi,

sorry, i thought it is self-explaining...

the above links go to individual selection guides


Please first check the box "Buck"
then press the "search" button.

After that a list of parts appears:


If you press on the partnumbers you will see a page with a schematic of the selected part.

on the right side there are the complete pdf datasheets, application notes and additional information for download.

in every datasheet you will find schematics, and the information on how to calcualte and select devices.

I hope this helps.

Klaus

 

[rightmederek]

Hi Klaus,

Thanks for the help.

Based on the block diagram on the datasheet, it seems to me that the core circuit is a current-mode buck converter, isn't it?

if it is current-mode buck converter, i still have one more question, the loading cap (battery) is variable, the dominant pole and ESR zero built by the loading cap is undetermined, then how to compensate it?

Thanks,
DD