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Micro controller for Half bridge converter

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deer

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half bridge converter

Hi Everyone,

I am planning to design a 500W battery charger for my UPS system with halfbridge converter. I am new to the converter design so it took me sometime to finalise the converter topology. Have i selected the right topology for my appln?

I am planning to use a microcontroller to control the PWM switching signal to the Power switches. I need to monitor the output voltage and current and hence need to decide upon the duty cycle for the PWM switching signal to the switches.

First of all is it recommended / possible to do this high freqeuncy switching by Microcontroller or do i need to go with any dedicated swithcing ICs?

I selected this method because it would simplify my life to control the output voltage and current as per the need of the battery charging characteristic rather than go with the complex analog circuitry.

Also what is instability in this kind of converters? Will it not be possbile to avoid the instability by sensing and controlling through Microcontroller?

Thanks in Advnace.
Deer
 

bobcat1

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HI

You can use Freescale DSP56F8xx or TI TMS320F28xxx as a controller who can control the main power loop

Other wise use any micro controller in the outer loop and a PWM controller in the inner loop

All the Best

Bobi

The microcontroller specialist
 

deer

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Hi Bobi,

First of all i thank you for your reply. I have been waiting for some one's reply for a long time.

I am planning to use a single microcontroller only for this application. I am not sure about the control theory of this power converter and i am very new to this.

I am planning to sense the output voltage and current through ADC and apply corrections to the duty cycle through the same controller. Will it sounds good?

I am not aware of the inner loops and outer loop details of this converters. Could you please brief about what it means?

Lot of Thanks in advance,
Deer
 

svhb

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Hi,

For 500W output, maybe a full-bridge design can be interesting. Please post your schematic, or a block diagram.

For a battry charger, normally slow control loop can do the job, since the battery itself is not a very dynamic load, so a simple microcontroller with a pwm can do the job. Just be sure you foresee some hardware for protecting the power electronics and the battery in case of software bugs or microcontroller issues (like missing clock, ...).

Another simple option is to use a simple TL494 (or newer versions of it) and set up the internal opamp-circuit for voltage and current limit. You can drive the reference for these control loops by two pwm outputs of the microcontroller. This is probably the safest and easiest solution. (I used it in a battery charger some years ago).

Stefaan
 

deer

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Hi Stefan,

Thank you very much. I am really sorry about my belated thanks to you. I went on vaccation so could not access the internet and see the reply.

You gave me confidence in proceeding with the circuit having the switches controlled by PWM.

I will reply here once i suceed in my way.

Many thanks again.

Regards,
Deer
 

eem2am

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you need fast ADC like in dspic to do digital control.

you need knowledge of PID algorithm too...and dsp programming.

what about analog chip doing it , with ucontroller to supervise?
 

deer

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Yes many people suggest the idea of having analog chip to control the converter and the micro to monitor it.

But i could not find or understand the concept of varying the output voltage and current of the converter output as the analog IC does not take any input from the external IC's (Micro) to vary its PWM for voltage and current control

Am i not looking the right component correctly?

Regards,
M.Nagarajan
 

svhb

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suppose you have some analog smps ic that drives power electronics needed, the smps switch uses somewhere a reference for its internal control loop. The trick is to vary this reference with a micro controller.

For this strategy, you need an smps design that has an external reference input. And if possible for both voltage and current. If I may refer back to my post about using the TL494 (very old chip, ...), if you look into some datasheets and application notes, you will find some possibilities to make a voltage and current controlled system. There are two control loops, one for current, and one for voltage, both control loops are tied together with a diode, so the lowest request wins the game (voltage regulation or current regulation). You can best compare it to a proper bench top power supply that has a maximum voltage and a maximum current regulation knob.

If you first make a design that fully works in all desired conditions with only the power stage hooked up (by setting the reference voltage with potentiometers, ...), you can start thinking on adding the micro-controller for the battery-charging process. The micro also has to measure the momentary output current and voltage, and can decide what to do with the references for the desired maximum current and voltage.

Since the micro controller does not have to do high speed duty-cycle settings, but only some slow-changing reference values, the reference can be made with a PWM-output from the micro controller. Hook up an appropriate RC to the micro controllers PWM/Timer peripheral and there the reference is.

Stefaan
 

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