Hej,

Let:

• be the mean value of X over a full period;
• IL be the inductor current;
• Id be the drain current of the power switch;
• Io is the output current;
• Vi and Vo be the input and output voltages;
• dc be the duty cycle of the main switch;

Then :

• The transfert function of the boost converter is Vo=Vi/(1-dc)
• The power consumed by the converter is: Pi=Vi*;
• The output power of the converter is Po=Vo*;
• assuming that Pi=Po and using the relation between Vi and Vo yields: *Vo*(1-dc)=Vo* then : *(1-dc)=
• Since Id=IL when the switch is 'on' and that the mean value of IL when the switch is 'on' is equal to the mean value of IL when the switch is 'off: =dc*
• Finally: Io=*(1-dc)/dc

Measuring can thus be used to determine =Io. Note that this relation is not very accurate, in particular due to our neglecting the efficiency of the converter, as well as tothe noise in the drain current. Though, this should be accurate enough to charge a battery.

Since no sensor on the output stage, The simple way is used the matimatical manipulation. You should used the correlation between the current input and the current & voltage output at the same time. You can derive it from the basic equation of boost converter. Good luck. :)

Thanks a lot for all helps and recommendation. Since I'm new in this field, I have some basic question at first. 1- As I know the duty cycle is used for determining the output voltage. However, in this case we want to charge a battery with minimum voltage of 380 Volt.  So we should choose the duty cycle to have a voltage greater than 380 V and also to fulfill the condition of ( Io

1. You cannot choose independently the current and the voltage applied to the load (i.e. the battery). There is a relation between U and I, this relation dependant on the type of load. For a resistance, for instance, U and I are proportional. For a battery, a motor... the U and I are not proportional, but are related.

You want to control the current charging your battery, so this is I that you must regulate: you must constantly adjust the duty-cycle so that the current in the battery is equal to the correct value. Then, the voltage across the battery will be set by the circuit, you will not have any control over it.

The duty cycle must be constantly (i.e. dynamically) calculated and updated to ensure that the behaviour of the converter will be correct.

If this is not obvious, you should start by reading on basic DC-DC power convertion, starting with output-voltage regulated converters, and then switching to current-regulated ones.

2. Charging lithium batteries is far from trivial. You should read about it before starting. http://batteryuniversity.com/

You will see that depending on the phase, you wil either regulate the current flowing through the battery or the voltage across it. You will also sea that you must constantly monitor the voltage and current, in order to know when to change phase.