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Specialized DC to DC converter

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Dodgecobra

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I have a relatively good background in electronics and would like to build a DC to
DC converter that takes in 12v and outputs the following: 3v,4.5v,9v,12v,19v [50w max. with a max current of 5A @ voltages below 12]. I know this would best fall under SMPS. I have been able to exactly know what diode and inductor I need, switching frequency, ripple ratio and compensational counter measures against various losses but havent been able to find the info that will give me what optimum output capacitor and switching mosfet (that will be controlled by a PWM signal) that I require. I have chosen to go with Continous conduction mode and I have gone through switching power supplies A to Z by Maniktala according to the relevant chapters and the best I can get is its voltage rating should be at least 20 - 50 % above max output voltage. My best guess for the capacitance would be sth like CR=t where C is the capacitance and R is the load resistance which makes t a value that should be far greater than the switch's off time . As for the switching mosfet all I think is it has to brave 5A average current all the time. What capacitance and mosfet should I work with? How do I get it by calculation?
 

OlPhart

Member
Have you looked at National Semiconductors' WebBench? Or, if efficiency isn't an issue, a linear regulator controlling a bigger power transistor? There're app notes on that, too. G.H <<<)))
 

indulis

New Member
What topology are you considering? Isolated or non-isolated? Is it a multiple output DC-DC or a single output?
 
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Dodgecobra

New Member
Have you looked at National Semiconductors' WebBench? Or, if efficiency isn't an issue, a linear regulator controlling a bigger power transistor? There're app notes on that, too. G.H <<<)))
I would prefer to work with a mosfet as I have plenty of PMW microcontrollers at my disposal, I'll try Webench, I hope it gives the soln. + how it arrived at the soln.

What topology are you considering? Isolated or non-isolated? Is it a multiple output DC-DC or a single output?
I wouldnt want the load to determine the output current so I'd have to go with Continous Conduction Mode,multiple output. I thought the fact that its a buck boost is enough to tell that it works off-line hence isolated or what do you mean?
 

indulis

New Member
I wouldnt want the load to determine the output current so I'd have to go with Continous Conduction Mode,multiple output. I thought the fact that its a buck boost is enough to tell that it works off-line hence isolated or what do you mean?

What does CCM have to do with the output current? Did you mean current mode control?
If it's a multiple output, your average primary current is going to be a lot more than 5A.
With 12V in, how do you consider that "off-line"?

You never mentioned in the original post that you were going with a buck-boost.
 

Charantj22

New Member
Hi. . . . . . Actually am searching for a converter that converts variable voltage(4-14v dc ) to constant 12v DC. . . . . . . . If you could know about this,please help me
 

indulis

New Member
How much current? 4 volts might be hard, but converters that run from 4.5-9Vin with 12Vout are available from many DC-DC manufacturers.
 

Dodgecobra

New Member
Hi. . . . . . Actually am searching for a converter that converts variable voltage(4-14v dc ) to constant 12v DC. . . . . . . . If you could know about this,please help me
Start by reading Maniktala's book, it will give you exactly what you need right upto where the switching buisness comes in which is where am stuck

What does CCM have to do with the output current? Did you mean current mode control?
If it's a multiple output, your average primary current is going to be a lot more than 5A.
With 12V in, how do you consider that "off-line"?

You never mentioned in the original post that you were going with a buck-boost.
Sorry I went through my first post and true I havent mentioned buck boost... so ya it is a buck boost. Its off-line by the fact that during the ON period of T the inductor is the only thing that is charging and the load is off, during the OFF period of T the inductor is discharging and the source is off hence offline in operation i.e. the load is not being driven by the source but purely by the inductor

How much current? 4 volts might be hard, but converters that run from 4.5-9Vin with 12Vout are available from many DC-DC manufacturers.
If you have looked at SwitchingPowerSuppliesAtoZ Maniktala's book you will realize that the equation V(on)*t(on)=V(off)*t(off) where V(off) is your desired output voltage, with V(on) being the input 12v you can manipulate the duty cycle to get your desired V(off) which will be the inductor discharge cycle voltage to the load and knowing its operating in Continous Conduction Mode which means you set t(on) and t(off) where T=t(on)+t(off) hence the inductor current never drops to 0. Essentially I will get all my desired voltages by just manipulating the duty cycle using a microcontroller connected to a sensor. So getting the 4v will be a breeze. Mind you I am making it not buy it because it will fit my needs perfectly. The current should be limited to 5A

Let me try to make it more clear with a diagram:
**broken link removed**


Now a question to everyone who can help, in place of the BJT shown I would like to use a mosfet due to better switching speeds and lower parasitic losses, How do I go about selecting an optimum mosfet and capacitor for the job through calculation? I would also like a current limiting circuit too
 

Dodgecobra

New Member
I have everything all figured out after much research: Am using an 50N06 as my mosfet, a custom made inductor, a 470uF filter cap and an additional EMI filter. Now my problem is I need a floating mosfet driver, I have looked at MCP1406 and can see a circuit showing it requires the source to be grounded. I need it to float bcoz am using an N-channel mosfet to feed the inductor with current during the onstate. Can someone help me with a floating configuration of it or any other that floats?
 
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indulis

New Member
Originally Posted by indulis
What does CCM have to do with the output current? Did you mean current mode control?
If it's a multiple output, your average primary current is going to be a lot more than 5A.
With 12V in, how do you consider that "off-line"?

You never mentioned in the original post that you were going with a buck-boost.
Sorry I went through my first post and true I havent mentioned buck boost... so ya it is a buck boost. Its off-line by the fact that during the ON period of T the inductor is the only thing that is charging and the load is off, during the OFF period of T the inductor is discharging and the source is off hence offline in operation i.e. the load is not being driven by the source but purely by the inductor


Originally Posted by indulis
How much current? 4 volts might be hard, but converters that run from 4.5-9Vin with 12Vout are available from many DC-DC manufacturers.
If you have looked at SwitchingPowerSuppliesAtoZ Maniktala's book you will realize that the equation V(on)*t(on)=V(off)*t(off) where V(off) is your desired output voltage, with V(on) being the input 12v you can manipulate the duty cycle to get your desired V(off) which will be the inductor discharge cycle voltage to the load and knowing its operating in Continous Conduction Mode which means you set t(on) and t(off) where T=t(on)+t(off) hence the inductor current never drops to 0. Essentially I will get all my desired voltages by just manipulating the duty cycle using a microcontroller connected to a sensor. So getting the 4v will be a breeze. Mind you I am making it not buy it because it will fit my needs perfectly. The current should be limited to 5A


The term “Off-Line” is used with switch mode converters that are powered from “line voltage” i.e. 120VAC or 220VAC depending which side of the pond your on. If it’s powered from a DC source then it’s called a DC-DC switch mode converter.

Yes, volt seconds must balance and CCM means is that the inductor is never emptied of current.

You may find that your control loop will not be as easy as you think to compensate given your output voltage range.
 
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