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3 Phase charging using 3-Half Bridges with PIC

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Hello everybody. I have designed 3 Phase inverter using 3 half bridges with a PIC microcontroller and a step up transformer is used to raise the voltage. Now for charging the batteries we supply mains back to the transformer. The 3 MOSFETs which are connected to the positive terminal of the batteries are kept off, their body diodes work as rectifier and the mosfets which are connected to the negative terminal of the batteries are given pulses. That's the theory I read somewhere.
Now my questions are-
How to the pulses should be? All ON at the same time or will it short the circuit? Or ON one after other?
And if the pulses should be with delay and without overlapping, do I need to synchronise them with mains or should I send the pulses with duty cycle control according to the charging current and not consider synchronisation? Thanks!

This is the bridge configuration I used-
https://www.google.co.in/search?q=3...AUIBigB&biw=1366&bih=662#imgrc=GULXzZmg_X2rUM:
 

tcmtech

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Nothing turned on ever. You already have full voltage DC coming back through the bridge assy as is.

Think about the current path between any two phases if any switching device is turned on.
 

kubeek

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Unless you turn any of the LOW-SIDE MOSFETs ON there is no current path formed and no current flows. Am I right? And that's how charging current is controlled!
That is not true, current will flow through the body diodes and you can´t do anything about that.
 

kubeek

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No mos will be active, unless you want to do synchronous rectification. Then it would be active at the same times its diode was in the three phase rectifier.
 

Inquisitive

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Please upload image in post #4 to ETO. Unable to see image.
Links are soon broken when linked to outside websites.
Thank you.
 

tcmtech

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How can I control current to the batteries ( connected across DC bus)?
With a control system independent from the three phase bridge.

Being you are using it as a three phase AC to DC rectifier it's no different than any other three phase bridge rectifier and thusly follows the same electrical rules and actions.

Any voltage regulation you want to do has to be done either on the AC side or after the DC comes out of it.
 
While reverse engineering I didn't find anything except IGBTs and a control card. A shunt resistor is used to measure current to the batteries. And there is a pot which controls the current from 5 Amps to 20 Amps. Right now I don't have access to the UNIT. If ever got I will measure pulses to the IGBTs in mains mode to see if how the charging is done. Until then if you can please help me out.
 
No mos will be active, unless you want to do synchronous rectification. Then it would be active at the same times its diode was in the three phase rectifier.
What is synchronous rectifier? Also difference between MOSFET and IGBTs? Answer could lie between the difference?
 

kubeek

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Difference is quite big. MOSFETs will allways have the inherent body diode that creates the rectifier, but IGBTs don´t neccesarily need one, so it is possible to control the amount of current flowing back from the transformer into the DC source.

Synchronous rectifier is like a normal rectifier, but uses active elements like transistors that you intentionally turn on instead of diodes that work by themselves.
 
Thanks for your help.
So I need to send pulses to IGBTs to switch them on and let the current flow through them. Should the pulses be synchronised with AC mains?
 

kubeek

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Yes they need to be synchronized, that´s why its called synchronous recification. If they are not you will most likely burn the expensive igbt modules.
 
I surfed some forums. They say ignoring the high side IGBTs and sending pulses to the low side IGBTs at the same time we short the transformer windings, energy is stored in the transformer and released when signals removed (through high side IGBTs' body diodes). Right?
 
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