Hello everyone,
I have a problem with understanding the power factor of a switch mode power supply. More specifically, a phone charger.
So it all started with me trying to measure the efficiency of my phone charger. I just measured the voltage, then the current after loading the charger at nominal amperage. I have multiplied the voltages and currents (in and out) and calculate the efficiency with Eff=Pout/Pin. The results where 45% efficiency. I start thinking that I'm doing something wrong. I know that by multiplying voltage x current it gave me apparent power (AC side), but in my idea, a phone charger should not consume reactive power and I write later why.
I will try to explain what I know about power factor and I hope that somebody will correct me where I'm wrong.
I know that usually the energy in an electric circuit it flows from the source to the load. Especially in DC circuits, but in some cases this may not be always true. I understand that a capacitor or a coil can absorb energy from the AC line while the sine is ramping up and then pushing that energy back in the lines when the sine is descending (or I guess it can push energy back anytime some kind of component from the circuit can generate a bigger voltage then the source). The same thing apply for the negative part of the sine.
In a pure DC restive circuit, measuring the current between the load and the source can give us a clear indication where the energy is heading, in an AC circuit on the other hand, the current and voltage is continuously changing direction. In order to establish where the energy is heading in an instant moment, we need to take the voltage in consideration so that any multiplication of the current and voltage that gives a negative value, will conclude that the load is pushing energy in to the source.
In my mind is clear that by having a sinusoidal current and voltage that are out of phase, there will be periods where their product will be negative. Integrating that products for a period of time and we get the reactive energy.
My conclusion is that in order to have reactive power, somewhere in time there has to be a negative V*I and the load must have some kind of battery (capacitor, inductor, chemical battery or other things) that can store the energy and released it later in the source.
Taking the clear case of a capacitor connected to AC, it is clear that when the sine goes up the capacitor charges, and when the sine goes down the capacitor release his energy back. A motor is like an inductor so is easy to understand, or an ac transformer.
I do also know that a bad power factor is also considered when a load is not consuming energy in a sinusoidal fashion. A SMPS may consume energy only when the sine reaches 90% of is amplitude, it get charged and after that is consume zero till the next 90% is reached in the negative alternance. That is why you want to use an active PFC, which act like a DC to DC converter to charge the SMPS main cap even at lower voltages.
Now, here is what I do not understand. Given the fact that we have a classic SMPS with a full bridge rectifier on the input that feed the main capacitor. How can the energy flow back to the source if the diode should prevent any reverse flow?
Is either they are leaking current when the SMPS MOSFET is switching the transformer and some high voltage spikes appear or, the diodes is not closing perfectly or fast enough and some energy can escape back.
I have only a one channel oscilloscope home and I can plot only the current of my phone charger. But still I can presume where the voltage is based on the huge spike current gets when the cap begin to charge.
I CAN SEE NO REVERSE FLOW FOR THAT CURRENT.
I found some kind of explanation on the internet about some kind of harmonics but, I can't imagine how harmonics works in this case, need more explanation here.
I attach here some images with what I measured and my presumption where the voltage should be. The yellow plot is the voltage measured across a 10 ohm shunt resistor mounted on a 230V 50Hz supply connected to a phone power adapter of 5V 2amps running at 2 amps. The orange plot is the voltage that I presume it looks like.
So, how can 15% of the energy stored in the phone charger get back in the grid? (assuming a pf of 0.85)
I have a problem with understanding the power factor of a switch mode power supply. More specifically, a phone charger.
So it all started with me trying to measure the efficiency of my phone charger. I just measured the voltage, then the current after loading the charger at nominal amperage. I have multiplied the voltages and currents (in and out) and calculate the efficiency with Eff=Pout/Pin. The results where 45% efficiency. I start thinking that I'm doing something wrong. I know that by multiplying voltage x current it gave me apparent power (AC side), but in my idea, a phone charger should not consume reactive power and I write later why.
I will try to explain what I know about power factor and I hope that somebody will correct me where I'm wrong.
I know that usually the energy in an electric circuit it flows from the source to the load. Especially in DC circuits, but in some cases this may not be always true. I understand that a capacitor or a coil can absorb energy from the AC line while the sine is ramping up and then pushing that energy back in the lines when the sine is descending (or I guess it can push energy back anytime some kind of component from the circuit can generate a bigger voltage then the source). The same thing apply for the negative part of the sine.
In a pure DC restive circuit, measuring the current between the load and the source can give us a clear indication where the energy is heading, in an AC circuit on the other hand, the current and voltage is continuously changing direction. In order to establish where the energy is heading in an instant moment, we need to take the voltage in consideration so that any multiplication of the current and voltage that gives a negative value, will conclude that the load is pushing energy in to the source.
In my mind is clear that by having a sinusoidal current and voltage that are out of phase, there will be periods where their product will be negative. Integrating that products for a period of time and we get the reactive energy.
My conclusion is that in order to have reactive power, somewhere in time there has to be a negative V*I and the load must have some kind of battery (capacitor, inductor, chemical battery or other things) that can store the energy and released it later in the source.
Taking the clear case of a capacitor connected to AC, it is clear that when the sine goes up the capacitor charges, and when the sine goes down the capacitor release his energy back. A motor is like an inductor so is easy to understand, or an ac transformer.
I do also know that a bad power factor is also considered when a load is not consuming energy in a sinusoidal fashion. A SMPS may consume energy only when the sine reaches 90% of is amplitude, it get charged and after that is consume zero till the next 90% is reached in the negative alternance. That is why you want to use an active PFC, which act like a DC to DC converter to charge the SMPS main cap even at lower voltages.
Now, here is what I do not understand. Given the fact that we have a classic SMPS with a full bridge rectifier on the input that feed the main capacitor. How can the energy flow back to the source if the diode should prevent any reverse flow?
Is either they are leaking current when the SMPS MOSFET is switching the transformer and some high voltage spikes appear or, the diodes is not closing perfectly or fast enough and some energy can escape back.
I have only a one channel oscilloscope home and I can plot only the current of my phone charger. But still I can presume where the voltage is based on the huge spike current gets when the cap begin to charge.
I CAN SEE NO REVERSE FLOW FOR THAT CURRENT.
I found some kind of explanation on the internet about some kind of harmonics but, I can't imagine how harmonics works in this case, need more explanation here.
I attach here some images with what I measured and my presumption where the voltage should be. The yellow plot is the voltage measured across a 10 ohm shunt resistor mounted on a 230V 50Hz supply connected to a phone power adapter of 5V 2amps running at 2 amps. The orange plot is the voltage that I presume it looks like.
So, how can 15% of the energy stored in the phone charger get back in the grid? (assuming a pf of 0.85)
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