Questions about power

Decim

Questions about power [Copy link]

Hello everyone! I want to ask a question about power. Here is the picture: First of all, I want to ask, V10 is a DC input, the size is 15v, the current is about 2A. But why is the power on coils L1 and L6 greater than the input? Is it because capacitor C18 has energy storage? Or does V2, which controls the MOSFET, also input a certain amount of power? Another question is that I have made this model, but I don’t know how to measure the power. I only have an oscilloscope and a multimeter. I connected a 1Ω resistor in series with the inductor, and used an oscilloscope meter to connect it in parallel to the inductor and the resistor. The waveform measured has an effective value. This effective value is used as the voltage of the inductor power. When the other two test leads are used to measure the effective value of the voltage on the resistor, the voltage value is divided by the 1Ω resistance. The power on the inductor is calculated by the formula P=UI. I don’t know the feasibility of this method, and I don’t know whether the effective value on the oscilloscope can be used to calculate the inductor power. Please answer!

maychang

"But why is the power on coils L1 and L6 greater than the input?" This is a single-ended flyback switching power supply circuit. It is possible that the power of the inductor stored energy is greater than the DC power output power in certain time periods during a working cycle of the switching power supply. However, it is impossible for the average power of the inductor stored energy to be greater than the DC power output power during a working cycle.

maychang

"…the power on the inductor is calculated using the formula P=UI…" I'm afraid your understanding of "power" is not accurate enough.

qwqwqw2088

Why is the power on coils L1 and L6 greater than the input? How do I know that I have made a physical object but don't know how to measure the power?

Decim

qwqwqw2088 posted on 2018-3-19 17:38 Why is the power on coils L1 and L6 greater than the input? How do I know? I made a physical object, but I don’t know how to measure the power and test what...

Just like testing how much energy is stored in the inductor, use the above method, connect an ohm resistor in series, and measure the voltage and current of the inductor through the effective value displayed on the oscilloscope, which is the ammeter internal connection method. There is a little error, but why is the power greater than the DC input?

Decim

maychang posted on 2018-3-19 17:05 "But why is the power on coils L1 and L6 greater than the input?" This is a single-ended flyback switching power supply circuit. In a working...

I just want to know if I am using this method to measure power correctly, and where is the understanding of P=UI wrong? Please point it out.

maychang

Decim posted on 2018-3-19 18:40 I just want to know whether my method of measuring power is correct, and where is the mistake in my understanding of P=UI? Please point it out

Your method of measuring power (measuring the voltage across the inductor and the current through the inductor) measures the apparent power, not the active power.

gmchen

P=UI, what is wrong? The mistake is that we did not consider that voltage and current are both vectors, or in other words, there is a phase relationship between them.