Power consumption calculations including efficiency

Hi All,

I did the below calculations but i`m not sure if i figure them correctly especially when i need to add the DCDC converters efficiency and the external ACDC PSU.

So my example:

Board 1: 12v x 2A = 24w
Board 2: 28v x 1A = 28w
Board 3: 5v x 2A = 10w

Each board is power by a different DCDC converter (12v, 28v, 5v) and each converter has 90% efficiency.

The whole system is powered by an external ACDC PSU with output 30v.

My calculations:

Total system power consumption: 24w + 28w + 10w = 62w
DCDC converter efficiency 90%, 62/0.9= 6.8w
3x DCDC converters 6.8wx3=20.6w (due to efficiency)

New total system power consumption=62w+20.6=82.6w

For the external ACDC PSU which runs on 30v:

I=82.6w/30v =2.75A is required

Questions 1: Is the efficiency calculation correct?
Question 2: I found somewhere that for the efficiency calculation i need to use this approach "multiply the input current by 1 + (100/efficiency)" but the values i get are not logical (*4.34A vs 2.75A)

*1+(100/90)=2.1
Input current = 62w / 30v = 2.06A
2.06A * 2.1 = 4.43A

Thanks,
Christos D

DCDC converter efficiency 90%, 62/0.9= 6.8w

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Perhaps just a typo of yours, but the corretc value is without the decimal dot:

• DCDC converter efficiency 90%, 62/0.9 Input Power = 68w

I did not check anything else after this point, the error should be propagated from then on.

stoui333 said:

...
DCDC converter efficiency 90%, 62/0.9= 6.8w

Click to expand...

No, Pin_tot_converters = Pout_tot_converters/efficiency = 62W/0.9 = 68.89W

3x DCDC converters 6.8wx3=20.6w (due to efficiency)

Click to expand...

No, you've calculated the total input power to the three converters as Pin_tot_converters already.

New total system power consumption=62w+20.6=82.6w

Click to expand...

No way. This part is not supposed to be there.
The calculated Pin_tot_converters = 68.89W is Pout_tot_converters + Plosses_tot_converters already.

For the external ACDC PSU which runs on 30v:

I=82.6w/30v =2.75A is required

Click to expand...

No, Iout_acdc = Pout_acdc/Vout_acdc = 68.89W/30V = 2.296A
N/B: Pout_acdc = Pin_tot_converters.

Hi Akanimo,

Thanks for your reply, i did understood the logic.