# How to design the feedback loop for the current-mode buck converter?

#### Zhang Jian

Joined Dec 31, 2014
10
Dear all:

I am building a buck converter to charge a battery, and the battery and the buck converter have been built, but I need a current mode feedback to charge the battery in a constant current.

I will upload some files to help you to understand.

In the first file, you can see the feedback is composed of compensator and modulator (comparator). But this document tells me how to build the feedback in voltage mode not current mode. I know I also need compensator and modulator in my current feedback. and I am not sure the difference of the two modes, because I need keep the current constant. What is more, I think you can understand what I want to build from the first document, because I think it is quite similar.

And my circuit is like file2, and I will upload the Ltspice file too, if you can download the Ltspice which is free you can check it out.

Thanks and Regards,
ZJ

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#### Zhang Jian

Joined Dec 31, 2014
10
The whole circuit with voltage mode feedback and battery.

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#### ronv

Joined Nov 12, 2008
3,770

#### Zhang Jian

Joined Dec 31, 2014
10
Hello ronv:

Thanks, but I need to build the circuit in LTspice, and I cannot find the Ltspice file like ZXTC1009.

Regards,
ZJ

#### Zhang Jian

Joined Dec 31, 2014
10
Hello ronv

Thanks, but I think this document cannot conclude the feedback loop I need or I cannot understand this document.

Regards,
ZJ

#### crutschow

Joined Mar 14, 2008
27,007
That circuit seems to be rather complex for just charging a battery.
What is the supply voltage, the battery voltage and charging current?

#### Zhang Jian

Joined Dec 31, 2014
10
Hello

The supply voltage is 40V, but the output voltage can be controlled by the duty cycle of PWM, so the output voltage is equal to the input voltage multiply the duty cycle.

the battery voltage is set to be 3300MAh. And the charging current should be controlled under 1A.

Thanks.
ZJ

#### crutschow

Joined Mar 14, 2008
27,007
3300mAh (I assume you mean milli not Mega Ah) is the battery capacity, not its voltage.

#### Zhang Jian

Joined Dec 31, 2014
10
Hello

Yes, it is milli and the voltage is around 5V.

Thanks

#### Zhang Jian

Joined Dec 31, 2014
10
Hi

I think it should be built by compensator and comparator, but I have no idea how to set the value of components.

And what is the difference between the voltage mode and current mode when I build the feedback?

Thanks.
ZJ

#### crutschow

Joined Mar 14, 2008
27,007
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And what is the difference between the voltage mode and current mode when I build the feedback?
As implied by their name, voltage-mode feeds back the output voltage for regulation, and current-mode feeds back the output current (typically from a small resistor in series with the load) for regulation.

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#### crutschow

Joined Mar 14, 2008
27,007
Below is the simulation of a simple hysteretic (bang-bang) current feed-back converter. It shows an average battery charging current of 1A with a 11 mA ripple current.
An advantage of the hysteretic converter is that it requires no compensation in the feedback loop.

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#### Zhang Jian

Joined Dec 31, 2014
10
Hello crutshow:

Thanks, and I think the difference between the hysteretic converter and buck converter is the hysteretic need the capacitor, right?
then you choose another way to build the feedback.

Regards,
ZJ

#### crutschow

Joined Mar 14, 2008
27,007
Thanks, and I think the difference between the hysteretic converter and buck converter is the hysteretic need the capacitor, right?
then you choose another way to build the feedback.
What capacitor?
The converter I posted is a (hysteretic) buck converter.

#### Zhang Jian

Joined Dec 31, 2014
10
Hi crutschow :

Thanks I understand, and I mean in the buck converter it usually has a capacitor in the output.

Regards,
ZJ

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#### crutschow

Joined Mar 14, 2008
27,007
Thanks I understand, and I mean in the buck converter it usually has a capacitor in the output.
You don't need an output capacitor for a current-mode hysteretic converter.

#### MrAl

Joined Jun 17, 2014
8,150
Hi,

My question would be, how accurate do you really need this charge current to be? Normally for charging a battery you dont need a super accurate current level such as you get from a full blown error amplifier/integrator type feedback system. In fact, if it varied from 80 percent to 120 percent of nominal it would probably still be ok. Normally only the voltage needs to be regulated with any pseudo high accuracy. A relaxed specification on the charge current accuracy means a simpler circuit is possible.