Can anyone Explain this Battery charger concept

[samson.bunty1986]

Hi all,

Here am attaching the datasheet of Batttery charger..Kindly go through it and explain me how they form boost circuit??and how they calculated the duty cycles?? If i would like to get 30V output to charge 24V,45AH battery how to calculate the duty cycle?? The mains waveform Has Positive and negative half cycles so how they calculated the Inductor value,duty cycle,charging current??they are saying that this circuit will form boost circuit in +ve and -Ve half cycle...In this circuit how to control the current?? I would like to have 7A charging current at initial.

One more thing,as we know the mains voltage has positive and negative pulses..so how to take instant voltages here to operate mosfets?? Since,here each mosfet will work in each cycle..

So kindly help me out to understad this circuit...

Thanks in advance

 

[godfreyl]

That is not a datasheet. The paper just describes an idea for a new kind of charger. Page 2 explains how it would work. You need to first understand how a normal boost converter works, then this one will not be difficult to understand.

 

[samson.bunty1986]

Hi Godfreyl,

Thanks for your reply..

I know about the Boost converter..Since the boost converters are come under DC-DC converters there is no need to calculate for -Ve voltages.. And also the input voltages will not come to Zero and -ve in boost converter..
We will calculate the Inductor value ,duty cycle and all by taking the minimum DC input voltages...BAsed on this we will vary the duty cycle..
But in case of AC input we will get Zero voltages as well as negative voltages..So here how to vary the duty cycle?? And how to calculate the inductor vaue..

Based on the input we will vary the dutycycle and keeps the output voltage constant..Now here how to make the output volatge constant??

Hope you understant my doubt..

 

[godfreyl]

The duty cycle will have to vary continuously. They propose to use a microcontroller to control that. The charging current to the battery will probably not be continuous. They wanted PFC, so the current will change as the input voltage changes, with no current when the voltage is close to zero.

I think it would be much easier to just rectify the AC, and use a normal converter, or other charging circuit.

 

[samson.bunty1986]

godfreyl,

Thanks for your reply..

So,they have taken all instantaneous voltages(0to peak) and varying the duty cycle?? How to do this variation?? For 24VAC input, to get 30VDC for charging Will the duty cycle varies from 20% to 100%??As per normal calculation (Which we do for DC Converter) for 24V input and 30V output the duty cycle am getting is 20%...Kindly go through the attached sheet

 

[goldsmith]

Dear Samson
Hi
I think this formula can help you to understand : Vout=Vin/1-D.C .
Best Wishes
Goldsmith

 

[samson.bunty1986]

Dear Goldsmith,

Thanks for your reply..

I know that formula..Used in Boost converter when the input voltage is DC.. In DC the Vin(min) and Vin(max) are at certain voltages..so we can easy find the duty cycle.. But here,the input is 24VAC..Varies from 0to24V and 0to-24V..So how to vary the duty cycle here...Will i have to do this with sampling??

 

[godfreyl]

But here,the input is 24VAC..Varies from 0to24V and 0to-24V.

There are 2 switching transistors - one for each half of the wave, so each transistor only has to deal with 0 to +24V. The transformer secondary is floating, so there is never any negative voltage at A or B.

During one half cycle of the mains, point A is at zero volts and M2 is switching a positive voltage at point B.

During the other half cycle, point B is at zero volts and M1 is switching a positive voltage at point A.

 

[samson.bunty1986]

Dear Godfreyl,

Thanks for your reply..

When the secondary voltage varying from 0to 24V, we need to vary the duty cycle right?? because, in bosst circuit as per the input voltages we need to vary the duty cycle.. So how to vary the duty cycle,when the voltage varying from 0to 24V

 

[godfreyl]

Because they want PFC, they will need to control the duty cycle in such a way that the current drawn from the transformer is proportional to the voltage. They said that they would do that with a microcontroller, but I don't know the details.

The result, though, is that the battery is not charged with a constant current. If you want an average charging current of 5A, you will actually get a current varying sinusoidally between zero and 10A at 100Hz (or 120Hz depending on your mains frequency).

 

[samson.bunty1986]

Dear godfreyl,

Thanks for your reply.
I have a doubt..Can we vary the PWM signal for mosfets by comparing sine wave and triangular wave??As we will do in inverters.. Can we do like that??

If i would like to charge the battery with 5A max for some time how to do that??Do you have any idea??

Can you suggest me if you know through hardware,how to control the duty cycle in such a way current drawn from transformer is propoirtional to voltage??

How to limit the current here say 5A??

 

[godfreyl]

Your circuit has to monitor the current and adjust the duty cycle accordingly.

From the PDF:

The input current, input voltage and output voltage are measured and presented to a micro-controller, which provides a voltage control reference accordingly to the control chip L4981B from ST microelectronics. By this control strategy, the output current can be controlled by the micro-controller. Thus, customer desired charging profiles can be easily implemented with embedded software.

 

[samson.bunty1986]

Dear Godfreyl,

If we measure the Input current,input voltage,output voltage and given to the controller as well as PFC controller will the PWM pulses vary automatically as per the Output current?? Why am asking because,here the Gate driver is getting pulses from PFC IC but not from Microcontroller..Correct me if i wrong..

 

[D.A.(Tony)Stewart]

²Samson, I quickly looked at the article schematic and understand how it works. Let me explain, how I see it and hopefully that will make sense to you too.

The power supply consists of a step-down transformer with controls for PWM to vary the current pump charge that is programed to keep constant current and vary slowly as charge approaches 100% hence reflecting a 100% PF constant load.
The feedback consists of;

- primary current ( via CT1 )
- DC output to a comparator such that if below full charge then Vref is enabled and thus PWM output and I pump charge to battery. I suspect that threshold is 14.2V
- internal Vref is a classic ultra-stable bandgap diode @ 1.25V
- an active full-wave bridge (AFWB) which is activated at PWM rates and provides 2 of the 4 bridge legs and the other two with passive Schottky diodes.
- a filtered bridge called an RCD filter that is "before" the output stage of the AFWB hence,
- I call this latter signal a "DC potential" feedback which two signals;
- instantaneous FW rectified signal potential "A" and average DC voltage potential "C"
- the former, DC output actual, which also provides zero-crossing clock to sync. the PWM
- the latter average potential is simply a smooth or LPF'd full wave.
- the actual AFWB pumped output Voltage to the battery is also sensed / compared to enable or disable the Vref in original statement and becomes signal "B"
- lastly the PWM signal uses a multiplier / divider ratio analog circuit to define a signa, Current Regulator = AB/C²
- This control signal is compared with a triangle wave synchronized to the AFWB signal rectified DC out of "potential voltage" to yield a PWM sliced signal to correct the pulse width.
- These PWM signals being fairly high f (> 20KHz) are then synchronized with the incoming AC zero crossings to make a sync'd PWM.

I did not bother with the Math to caclulate values, but I understand how it works.
-


Any questions?

 

[samson.bunty1986]

Dear Sunny,

Thank you so much for your brief explanation.It really helped me ..

But i have some questions here.

Here,Will the PWM control is done by PFC IC??Correct me if am wrong.. Then what does the controller is doing here??

Will The Controllerer sense input voltages,input current and output voltage and give Vref to PFC IC??

As i studied about PFC IC, this PFC IC also sense the Feed Forward voltage( Output feedback Voltage)...Will this sense voltage is given by controller??

The PFC has error amplifier and Current amplifier and PWM comparator..So will this PFC IC sense Voltages and current and then control the PWM signals???

How can we say that controller is controlling the output current if the battery is getting charge??? Will this work done by PFC IC??

What does "INTERNAL VREF IS CLASSIC ULTRA-STABLE BANDGAP DIODE" means which you mentioned??

How to detect zero crossing ???

Regards,
Samson

- - - Updated - - -

Dear Sunny,

Thank you so much for your brief explanation.It really helped me ..

But i have some questions here.

Here,Will the PWM control is done by PFC IC??Correct me if am wrong.. Then what does the controller is doing here??

Will The Controllerer sense input voltages,input current and output voltage and give Vref to PFC IC??

As i studied about PFC IC, this PFC IC also sense the Feed Forward voltage( Output feedback Voltage)...Will this sense voltage is given by controller??

The PFC has error amplifier and Current amplifier and PWM comparator..So will this PFC IC sense Voltages and current and then control the PWM signals???

How can we say that controller is controlling the output current if the battery is getting charge??? Will this work done by PFC IC??

What does "INTERNAL VREF IS CLASSIC ULTRA-STABLE BANDGAP DIODE" means which you mentioned??

How to detect zero crossing ???

Regards,
Samson

 

[samson.bunty1986]

Dear Sunny,

Can you tell me how the gate driving circuit done...Can we use any gate driving IC?? The PWM from PFC controller will be in Pulses..So,how can we switch ON one mosfet continuously for half cycle when other mosfet switching ON/OFF... how to do this???

 

[D.A.(Tony)Stewart]

All the circuits I described are in their block diagram design in Fig 3.
No PFC IC