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[SOLVED] Whast is DC to DC Converters

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tahir4awan

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What is DC to DC Converter

What is DC to DC converter. I have read them on two places first they are used in ADC to chop the analog signal for sampling and known as choppers.

Second in Power Electronics to extract multiple DC voltages from one DC supply.

Correct me if I am wrong.
 
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Re: What is DC to DC Converter

What is DC to DC converter. I have read them on two places first they are used in ADC to chop the analog signal for sampling and known as choppers.

Second in Power Electronics to extract multiple DC voltages from one DC supply.

Correct me if I am wrong.

Hey tahir4awan,

I'm a little more proficient in power supply design than the subtleties of ADC front ends, so I'll comment on that one:

A DC/DC converter is a circuit that generates a DC output voltage from another. While a linear regulator technically falls under that broad description too, DC/DC converters generally refer to switching power supplies, where a (generally) electronic (or rarely) mechanical commutation (or chopping) process is employed.

What are we chopping? The input voltage (or more precisely, power). A DC/DC converter employs (generally) inductors/transformers (or in low power applications) capacitors to efficiently achieve a specified voltage transformation. These elements only operate on AC waveforms (i.e. those constantly changing with time) or require seqential switching operations (such as the "flying capacitor" converter). We thus chop the input DC supply to generate an intermediate AC supply, that is then smoothed and filtered back to the target DC output voltage. A closed-loop feedback control circuit typically performs the required regulation by modifying the DC/AC conversion process (such as frequency, duty cycle etc). The frequency of the chopping process is kept high to ease the output filtering requirements and permit the use of small inductance/capacitance values within the converter.

Hope that helps - technically these converters should be called "DC/AC/DC converters" but we settled on "DC to DC converter" instead :)

Cheers,
James.

P.S. Come to think of it, I have never heard of a DC/DC converter used in an ADC. They are often seen powering them for reasons of galvanic isolation, but not in the front end. Perhaps you mean a chopping "sample and hold" circuit?
 
In short the input and output of the converter are isolated from each other means input side voltage and ground are seperate from output side voltage and ground just like an optocoupler.
You apply a DC input and get the DC output (may be step up or step down).
No concept of ADC exists there.
 

Well since Dc voltage can't be step up but it can only be step up from the source,therefore a high frequency inverter is used to step up the voltage from one potential to another,then the out put is been rectified and filtered this is called swich mode power supply,also known as Dc to Dc converter so I can agree with your second answer,ADC is not applicable here.
 

Re: What is DC to DC Converter

Hi thylacine1975,
I have a question about the performence differences between the linear regulator and the swith regulator:
It seems that there is less ripples and noises in the output of your DC/DC converter when you use the linear regulator instead of the swith regulator,So why?Dose the ripples and noises just come from the swithing of the MOSFET?
If the input DC supply have lot of ripples and noises,then we use the linear regulator to convert it to another DC supply,dose the output have the same ripples and noises?
TKS!
 

Re: What is DC to DC Converter

Hi daraemon_liu,

Yes, there certainly is more noise present in the output of a switching regulator than from a linear regulator, and you are correct - this arises as a consequence of the switching action of the power transistor (BJT, MOSFET or IGBT). The switching action induces noise via 3 main mechanisms:

1. Discontinuous power transfer from input->output, as the energy is tranferred through the converter (through inductors/transformers/capacitors/etc) in "bursts". This is reduced by the output filter, typically a few-pole LC configuration, with the corner frequency as far below the switching frequency as practical (given constraints of cost, size and saturation of the magnetics). Residual noise may be (typically) a few hundred millivolts in a crappy converter, to microvolt levels in a careful, low power design.

2. Capacitive coupling. Stray capacitance between switching and output nodes can couple energy during the switching transitions of the transistor. This is reduced by limiting switching rise/fall times, reducing the voltage excursions on switching nodes (via snubber networks, low-self inductance winding techniques etc) and minimising stray capacitance (via electrostatic shielding, geometry etc).

3. Magnetic coupling. This is particularly insidious as effective magnetic shielding is hard to accomplish, particularly at low frequencies. In this case, flux from switched inductors couples into output filter inductors/PCB traces, inducing ripple voltages. Shielded inductors (potcores, toroids etc), geometry and careful PCB layout reduce these effects.

Some of these mechanisms are bigger/lesser problems in the various converter topologies, but are fairly universal nonetheless.

While switching noise is absent from a linear regulator, they can have noises of their own which may or may not matter, depending upon their application. Be aware that even the ubiquitous LM7805 can oscillate horribly (2Vpp output ripple!) if they are inadequately decoupled (yes, you _do_ need the input/output 'stability' capacitors!). Input ripple will generally be attenuated (and converted to heat) and substantially reduced with a linear regulator, but only if its frequency lies within the loop bandwidth of the (active) regulator. The datasheet will give you some clues as to how much attenuation you'll get - look for something like "ripple rejection" - values of ~50+ dB (at 100Hz) are typical. In the case of a zener diode, the ripple is (to first order) reduced by the voltage divider formed by the series resistance and the diode's equivalent series resistance (and further filtered by any shunt capacitance).

Cheers!
 
Re: What is DC to DC Converter

Hi thylacine1975,
Thank you for your reply.
I am also confuse about the definition about the output ripple and the output noise. It seems that output Ripple is caused by the change of the logic status of the chips which need this voltage to be supplied.While the noise comes both from the power converter itself (as you described above )and the circumstance noises which may effect the power converter.so can we draw a conclusion that the output Ripple just depend on the load the of the power convert while the output noise mainly depend on the converter itself ?
I also did't know what is the input ripple ?does it just eqiuals to the another output ripple which was used as an input?
TKS again for your helpful advices!
 

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