Efficiency of constant voltage vs current in DC mode from fix ac supply

[Chirag134]

I have application whose resistance vary with time and required DC power supply. It can be operated in constant voltage or current mode. My question is if I have ac power source and convert to DC power, which will have more efficiency (or less losses ) constant voltage or current?

 

[FvM]

It depends. Switching mode power supply has constant high efficiency in both modes, linear power supply efficiency is proportional to output voltage.

 

[Chirag134]

If my load vary from 1 to 4 ohm and requirement of dc side is : cv mode at 4V and current vary from 4 to 1 A. CC mode operate at 2.5A and voltage in that case vary from 2.5 to 10 V, which mode has more losses while converting from ac to dc without considering resistive loss in load?

 

[KlausST]

Hi,

the question is not clear.
* whether switched mode or linear power supply
* if you include the load

Example:
You say varying load resistance.
On a constant current source the output power rises with increasing resistance
On a constant voltage source the output power rises with decreasing resistance.

Klaus

 

[D.A.(Tony)Stewart]

You have a good question which shows a range in output power.

An ACDC converter design may vary significantly with many output parameters, efficiency, cost, size, tolerance, complexity, etc.

However, sensing output and regulating the supply uses little or no power as these are just high-impedance inputs.

So the answer is there is no difference in choosing CC or CV for a SMPS because for any given topology the only difference in theory, is the feedback selected, namely voltage or current as the primary feedback. Sometimes secondary feedbacks are used for improving step response or disturbance stability which is still possible. Efficiency may change slightly with input or output changes in power as no-load becomes 0% efficiency. But, there may be no significant difference in %efficiency between CC or CV.

When a load can drift voltage with temperature by natural occurrence or design such as any PN junction or LED, it is wiser to use a CC supply to maintain constant brightness.

Learn more about SMPS below.

Power supply design seminar | Resources | TI.com

Resources for the power supply design seminar

www.ti.com

A linear supply often has a very low idle current, but efficiency drops immediately with the rise in drop voltage. So this would never be used unless there were exceptionally low ripple requirements. Then one might for some applications, consider a SMPS pre-regulator with a linear post-regulator with the minimum voltage drop. This was done in the early days using an SCR bridge pre-reg. and linear post regs by major brands such as Lambda.

 

[Chirag134]

Thank you for your responses. In addition to my above question I have following question: CV mode power is varying from 4 to 16 watts while in CC 6.25 to 25 watts. Doesn’t CC mode is at disadvantage compared to CV?

 

[KlausST]

CV mode power is varying from 4 to 16 watts while in CC 6.25 to 25 watts.

No. CV mode is 13.2W .. and CC is 16.7W.

Just joking.
Your numbers without context make no sense ... and so do my values.
--> Tell us where your numbers come from.

You talk about efficiency. It is defined as P_out/P_in .. so single values of power don´t make sense at all.
--> You always need ot give a pair of power values.

Klaus

 

[D.A.(Tony)Stewart]

Thank you for your responses. In addition to my above question I have following question: CV mode power is varying from 4 to 16 watts while in CC 6.25 to 25 watts. Doesn’t CC mode is at disadvantage compared to CV?

You are not following my advice! Why?

 

[D.A.(Tony)Stewart]

Normally CC will use a 50 mV FS @ max current to choose a low-side sense resistor and then amplify for the error amplifier to regulate the output just like voltage feedback. You can expect 9 or 10% losses over the given range.

 

[Chirag134]

No. CV mode is 13.2W .. and CC is 16.7W.

Just joking.
Your numbers without context make no sense ... and so do my values.
--> Tell us where your numbers come from.

You talk about efficiency. It is defined as P_out/P_in .. so single values of power don´t make sense at all.
--> You always need ot give a pair of power values.

Klaus

If my load varies from 1 to 4 ohm, the requirement of the DC side: CV mode at 4V, and the current varies from 4 to 1 A. If CC mode operates at 2.5A and voltage in that case varies from 2.5 to 10 V, which mode has more losses while converting from AC to DC without considering the resistive loss in load? CV mode power varies from 4 to 16 watts while in CC 6.25 to 25 watts. Isn’t CC mode at a disadvantage compared to CV?