dc dc converter efficiency optimization issue

[viperpaki007]

I am designing a switched capacitor DC-DC converter and need to optimize the circuit for best possible power efficiency. I have heard that in order to optimize the power efficiency of the converter, the output voltage should remain constant for every change in the circuit parameters. Why is this restriction?. Can somebody elaborate..

thanks

 

[Miguel Gaspar]

I never heard of that.
For power eficiency one aspect is that the switching time of the transistor and diode must be very little compared to the periof of the switching frequency.
Another one is theRdc and Rac of the inductor and/or transformer.
The ESR of the capacitor.
And the Vf of Diode and Vsat of the transistor.
these are the most important

 

[leo_o2]

Do you mean DC-DC based on charge-pump?
For this kind of DC-DC, switching structure should be changed when VIN/Vo ratio is changed for best efficiency.

I am designing a switched capacitor DC-DC converter and need to optimize the circuit for best possible power efficiency. I have heard that in order to optimize the power efficiency of the converter, the output voltage should remain constant for every change in the circuit parameters. Why is this restriction?. Can somebody elaborate..

thanks

 

[viperpaki007]

@ Miguel Gaspar

So you mean that there is no requirement of making the output voltage constant in efficiency optimization?. I heard this statement, of keeping the output voltage constant, from one of my senior colleagues. He argued that while optimizing the circuit, you should only change one parameter at a time. so if you dont keep the output voltage constant and there is increase in output power. You will not be sure that the increase in output power is because of decrease in circuit losses or increase in Pin...
I know it seems confusing and i am not able to understand this idea. That's why i need help from you guys.

@ Leo_o2

You are right leo about the changing switching structure for SC DC-DC converters and i am already considering that. But my question was related with power efficiency optimization.

looking forward for your comments

 

[Teddy]

From application point of view - you want to keep voltage at its target value all the time (I do not consider startup and line/load transients) and efficiency should be optimized in steady state when there are no external variables cosidered. You measure efficiency when temperature is constant, load is constant etc. There you will be able to recognize what has biggest influence on total system.
Obviously in case you can program your output voltage and you Vin is changing the efficiency will be changing also. Then you have to decide what are the conditions (look at any dcdc datasheet) You can for example take Vin = 3.6V to Vout=1.2V which is very common spec.

 

[leo_o2]

I mean charge pump can be configered for different ratio like Vo=1/3Vin, or 1/2 Vin, or 2/3Vin, or 3/4Vin, or 3/2Vin, or 2Vin.
If suitable configeration is choosed as Vin is changes, the efficiency can be improved.

@ Leo_o2

You are right leo about the changing switching structure for SC DC-DC converters and i am already considering that. But my question was related with power efficiency optimization.

looking forward for your comments

---------- Post added at 03:06 ---------- Previous post was at 03:06 ----------

If you increase the number of flying capacitor, more ratio choice can be achieved.

 

[viperpaki007]

@Teddy
At this moment, My circuit is working in open loop condition. Making the output voltage constant at a certain value will be done lateron using a closed loop feedback. Right now i dont care about the output voltage and my main concern is just to optimize the circuit for best possible efficiency. My question is: Should i keep the output voltage constant while doing the efficiency optimization by changing circuit parameters. if yes, why?

 

[FvM]

By it's operation principle, the charge or discharge of capacitors in a SC converter will create energy losses. For this reason, any voltage control other than switching between rational voltage ratios, as mentioned by leo_o2, involves losses and affects the efficiency. So the efficiency optimization should be carried out with the intended operation conditions, including a regulated output voltage, if it's intended for the design.