Hi,

I am testing a 48W 220Vac-24Vdc flyback converter that I designed. I am trying to calculate the efficency of the converter but got stuck at what values to use. I measured the input current using a clamp meter to be 110mA and the input voltage to be 228Vrms. The output voltage is 24V and the current is A with an led strip as a load. I am using an LT3798 controller that includes PFC which I am utilizing so in my calculations I assumed a pf of 0.95.
What I am not sure about is when calculating the input powerdo I multiply the rms current and voltage together and then use that as the input powerin the calculations or do I have to convert the rms values to peak values?

 

Voltage RMS value and current RMS value. Evaluate the cos phi as well. because the EMC filter has its influence.

 

Okay so in power calculations I use rms values right? no need to convert anything? Thanks btw

 

Okay so in power calculations I use rms values right? no need to convert anything? Thanks btw

RMS V x RMS A is Volt Amps (V-A) is 'apparent power' but isn't the 'true power' in Watts which is what you need for efficiency calculation (Watts out/Watts in). You have to factor in the phase angle. If you measure the phase of input I wrt input V that gives you the power factor (assume 18.2deg = 0.95PF) so your 0.11A @ 228v = 25.08VA @ 0.95PF = 23.83W

https://www.omnicalculator.com/physics/power-factor is a useful calculator.

 

RMS V x RMS A is Volt Amps (V-A) is 'apparent power' but isn't the 'true power' in Watts which is what you need for efficiency calculation (Watts out/Watts in). You have to factor in the phase angle. If you measure the phase of input I wrt input V that gives you the power factor (assume 18.2deg = 0.95PF) so your 0.11A @ 228v = 25.08VA @ 0.95PF = 23.83W

https://www.omnicalculator.com/physics/power-factor is a useful calculator.

Thanks so much.

 

Thanks so much.

That is only true if the input current is a sinewave.
In your case, with a PF of 0.95 you can assume it is. At a guess the less than unity value of PF will be caused by distortion of the current waveform, not in a phase shift.
However, if your circuit has no PFC, you will have to evaluate the true power, which is the average of the instantaneous power. i.e. you sample I and V, at more than 2ksamples/second multiply them together and THEN average

 

If I recall correctly, a plain vanilla full wave rectifier with a straight capacitor filtering has a power factor of around 0.6
But this is from my memory, please search for the actual value or even better, measure it.