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Author Topic: Help required For Synchronous Multiphase Buck Converter Design Issues  (Read 2895 times)
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zab
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« on: May 31, 2013, 12:46:22 12:46 »

     Synchronous Buck Converter is known for High efficiency. It differs from normal buck converter. as Mosfet is used for switching as well as rectification.
There are many ics available in the market for this purpose. But I want to make this with pic or dsPic controllers. I have gone through many application notes for this purpose.
    In simple circuits single pwm drive a special Synchronous Buck drivers having complementary outputs doning the trick. You have only to vary the duty cycle.But the problem arises when you want to have multi phase options.
In dsPic You can have have this problem solved. You can have  both Synchronous along with Multiphase Buck configuration options.
The pwm output is configured an complementary for both pairs have phase difference of your choice. In complementary configuration hi and lo transistor works in alternately in pair. If 1 is ON the 2 will be OFF.and vice versa.
      If the converter is used with simple load .it does works from low duty cycle to max duty cycle.but when the converter is used with battery as a load.then at low dutycycle lower transistor is need to swith OFF before the completion of Pwm Period. So that current from battery may not flow back to ground through lower side transistor.
    The Question here is how I can decide that there is a need to switch off this transistor and when at what stage? The converter is required in CCM as well as In DCM mode.
    One point I missed is PWM pulse can be stopped with the Fault option.It means I have play with this option to stop lower transistor pulse.in every cycle.But question still remain How to get status of this stop need?
   All who can Do help, are requested to please come forward from help. and share their valuable skill and knowledge.
Thanks ,

« Last Edit: May 31, 2013, 03:00:48 15:00 by zab » Logged
MTong
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« Reply #1 on: June 01, 2013, 01:25:23 01:25 »

Hi Zab,

I believe that what you described (current from battery flowing back through low side transistor) can happen only as you get close to DCM.
That is, while the low side transistor is on, as long as there is a positive current in the inductor (flowing to the load/battery), you should be ok. If you are in DCM instead, when the inductor current goes to zero, if you keep the low side transistor on, the battery will start charging the inductor (negative current in the inductor).

Anyway in battery charging, I think you should do some kind of current mode control, so in CCM no problem, but in DCM you should shut down the low side transistor when current ~0A

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Tekno1
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« Reply #2 on: June 02, 2013, 05:41:41 05:41 »

In complete IC circuits of syncronous buck there are two current sense circuitries one for measuring high side switch for cycle-by-cyle current limit and the other measuring current through low side switch to make sure low side switch does not conduct to gnd as you were mentioning.
One needs to monitor current so that reverse current flow will not occur in the inductor.
I am not familiar with your setup so I will not be much help here but in power management IC's these circuitries comes implemented in hardware.  Looks like your hardware is software + driver transistors type. In that case old discrete implementation of current sense circuitries may be help to you. I believe some old Texas Instruments app notes (TI acquired Unitrode and get the knowledge) could be found TI's web page and could be useful for you.  Good luck.
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zab
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« Reply #3 on: June 07, 2013, 02:12:08 14:12 »

My DSL connection had some problems .so I can not respond it time. I am planing to use allegro current sensor in series with coil to sense both forward  and reverse current. My circuitry is just in planing phase at present. I will share it when it take some concrete shape.
At present I want to know only if some body if have done this before or not. some suggestions may be shared to give it practical shape.
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