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Boost OR Flayback (DC to DC)

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arvinfx

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I need about 300W 24V-12A DC From 12V DC

Which topology can provide me upper power in output? BOOST or Flyback
 
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audioguru

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I don't know what FLAYBACK is. I think you mean Flyback whick is for making very high voltage at low current.

A boost converter will use 30A from 12V.
 

Ubergeek63

Well-Known Member
I need about 300W 24V-12A DC From 12V DC

Which topology can provide me upper power in output? BOOST or Flayback
Boost is a specialized form of a flyback. You can get more power at modest voltage increases from a boost for a given magnetic since the inductor only has to supply the difference between the input and output voltage.

Dan
 

tcmtech

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what are you trying to power and where are you using it?
 

arvinfx

Member
what are you trying to power and where are you using it?
must turn on the DC motor on my car whit this:

24V 2A on without load
24V 2-12A with load

I hear the boost topology can only 150W power provide to us!!!
And flyback can up to 1kW power!

It is right?
 

dknguyen

Well-Known Member
Most Helpful Member
Boost has less functionality than flyback so physics dictates that boost is better at what it does. In fact, this is the case. Components in a boost circuit have less peak voltages and current stresses on them than flyback so can use cheaper components with lower ratings and have less noise. They can also boost by a larger amount than flyback (but can't step down). They are also capable of higher power because of all the aformentioned points.

So usually, if you don't need the features of flyback (isolation, or multiple tapped outputs from the transformer, step-down and step-up) you go with the boost. Boost does have the major disadvantage of inverted voltage though. A SEPIC converter is another kind. It's can do everything a flyback converter can do (you can think of the circuit it like a buck-boost converter combined into one circuit, or a variation on the flyback converter), but with lower noise and peak stresses with no voltage inversion like the boost, but no isolation like the flyback. It's also harder to figure out how it works.
 
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arvinfx

Member
Boost has less functionality than flyback so physics dictates that boost is better at what it does. In fact, this is the case. Components in a boost circuit have less peak voltages and current stresses on them than flyback so can use cheaper components with lower ratings and have less noise. They can also boost by a larger amount than flyback (but can't step down). They are also capable of higher power because of all the aformentioned points.

So usually, if you don't need the features of flyback (isolation, or multiple tapped outputs from the transformer, step-down and step-up) you go with the boost. Boost does have the major disadvantage of inverted voltage though. A SEPIC converter is another kind. It's can do everything a flyback converter can do (you can think of the circuit it like a buck-boost converter combined into one circuit, or a variation on the flyback converter), but with lower noise and peak stresses with no voltage inversion like the boost, but no isolation like the flyback. It's also harder to figure out how it works.
I no need isolation, or multiple and only need make 24V and high current power from Lead-Acid .

Thanks
 

Oblong

New Member
I don’t know who told you a boost is only good for upto 150W but you’re wrong. A flyback max power is about 150W before it becomes impractical unless you interleave.

The boost topology is used into the kW range and is typical used at the input to high powered SMPS’s for power factor correction (limiting harmonics). Google “active PFC” you will see probably some boost APFC ‘s in the kW range.

For your 300w maybe halfbridge ,push-pull or full bridge topology,would be best.

Didn't you have a thread for a boost 12V to 24V 3A?

Heres a 250w Boost useing synchronous rectification.

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1032,C1082,P38055,D25148
 
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Boost has less functionality than flyback so physics dictates that boost is better at what it does. In fact, this is the case. Components in a boost circuit have less peak voltages and current stresses on them than flyback so can use cheaper components with lower ratings and have less noise. They can also boost by a larger amount than flyback (but can't step down). They are also capable of higher power because of all the aformentioned points.

So usually, if you don't need the features of flyback (isolation, or multiple tapped outputs from the transformer, step-down and step-up) you go with the boost. Boost does have the major disadvantage of inverted voltage though. A SEPIC converter is another kind. It's can do everything a flyback converter can do (you can think of the circuit it like a buck-boost converter combined into one circuit, or a variation on the flyback converter), but with lower noise and peak stresses with no voltage inversion like the boost, but no isolation like the flyback. It's also harder to figure out how it works.
A boost does not invert the voltage. A buck-boost has two varieties - inverting & non-inverting. The application the OP stated would lend itself to a boost. A boost & a SEPIC have quiet inputs but noisy outputs. A SEPIC has the same problems as a buck-boost, more parts and lower efficiency. The SEPIC's ability to step down as well as step up has a price. The inductors carry more per cycle energy. Also, a SEPIC is slower than a buck-boost, and much slower than a boost.

Just a summary of the tradeoffs involved with different topologies.
 
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tcmtech

Banned
Most Helpful Member
Depending on duty cycle it could be cheaper and easier to simply put a second battery in. Use a relay to switch it from series (for charging) to parallel when running the motor.
Thats how the old 6 volt systems worked on larger trucks years ago.

They used two batteries in parallel at 6 volt but used a pair of SPDT starter solinoids to boost the starter up to 12 volts. worked well too. A few farm tractors still do this to use 12 volts systems to start on 24 volts.
 
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dknguyen

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A boost does not invert the voltage. A buck-boost has two varieties - inverting & non-inverting. The application the OP stated would lend itself to a boost. A boost & a SEPIC have quiet inputs but noisy outputs. A SEPIC has the same problems as a buck-boost, more parts and lower efficiency. The SEPIC's ability to step down as well as step up has a price. The inductors carry more per cycle energy. Also, a SEPIC is slower than a buck-boost, and much slower than a boost.

Just a summary of the tradeoffs involved with different topologies.


Oh, yeah that's right. BOost doesn't invert voltage. The buck-boost does (the unisolated version of the flyback).
 
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Ubergeek63

Well-Known Member
Oh, yeah that's right. BOost doesn't invert voltage. The buck-boost does (the unisolated version of the flyback).
Actually flyback really only refers to when you take the energy.

It is either forward or flyback. When the switch is on it is forward modem, when it is off it is flyback.

Push-pull, bridge, half bridge, etc are all special cases of forward mode. In forward mode the input rail is translated directly to the output by the transformer ratio. It is for this reason that all forward mode systems need an output inductor as well as a capacitor.

Flyback mode stores the energy and dumps it to the output. The transformer is itself the inductor. You do not see the transformer effect on the output leading some to incorrectly claim that it is just coupled inductors.

You actually see the the transformer on the primary side of the the isolated flyback converter. When designing a flyback the term "reflected voltage" pops up. The power switch has to withstand the sum of the supply voltage, the spike from the leakage inductance (the reason for the snubber), AND the reflected voltage.

I actually used it to my advantage. I had 7V and needed 15V for gate drive and 300V for an ultrasonic transducer. I drove a ADSL transformer with a switcher chip into a voltage multiplier. Transformer drove the multiplier alternately in forward and flyback mode to get the regulated 300V while the primary boosted to get the 15V gate drive supply.

Dan
 

arvinfx

Member
I think forward ( Boost) topology can provide to us 300W maximum and Full-Bridge can um to 1KW in output! but i can`t find the IC for generate PWM wave for this topology .!?
 
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