I would like to ask, how is the transformer in a single-tube transformer-coupled amplifier magnetically reset?

xztl

I would like to ask, how is the transformer in a single-tube transformer-coupled amplifier magnetically reset? [Copy link]

 

As shown in the figure, this single-tube power amplifier is coupled through a transformer, and the transformer works in the first quadrant. How is it magnetically reset? If Vi is a square wave with a duty cycle of 0.5, and T is equivalent to a switching state, will the transformer be saturated?

xztl

The square wave is shown in this picture (picture found on the Internet). Generally, the forward power supply has a reset winding or other RCD reset. How is the reset shown in the above picture? I saw that the actual product did this, but I didn’t understand the principle.

maychang

xztl Published on 2021-6-26 09:52 The square wave is shown in this picture (picture found on the Internet). Generally, the forward power supply has a reset winding or other RCD reset, and the above picture shows...

"The square wave is shown in this picture (picture found on the Internet)"

Things found on the Internet need to be carefully distinguished. This picture is only a schematic diagram, not a complete circuit. Moreover, the input of the circuit is marked as a square wave, but the output is marked as a sine wave. This is not a simple amplifier, but includes a filtering circuit.

maychang

In this transformer-coupled amplifier circuit, the core must be reset, whether you like it or not. This single-ended circuit, as you said, works in the first quadrant, and the core is reset because it works in the first quadrant.

xztl

maychang posted on 2021-6-26 10:46 In this transformer-coupled amplifier circuit, the iron core must be reset, whether you like it or not. This single-ended circuit, as you said, works in the first quadrant, and the iron...

Thank you for your answer. Working in the first quadrant, if the duty cycle of the square wave does not exceed 0.5, this transformer can be magnetically reset, right?

xztl

maychang posted on 2021-6-26 10:46 In this transformer-coupled amplifier circuit, the iron core must be reset, whether you like it or not. This single-ended circuit, as you said, works in the first quadrant, and the iron...

Where does its demagnetizing energy come from?

maychang

xztl Published on 2021-6-26 09:52 The square wave is shown in this picture (picture found on the Internet). Generally, the forward power supply has a reset winding or other RCD reset, and the above picture shows...

There is a slight difference between the first floor diagram and the second floor diagram: the emitter of the bipolar transistor in the first floor diagram has a resistor to the ground. This resistor has a negative feedback effect, while the source of the MOS tube in the second floor does not have such a resistor. Therefore, the tube in the first floor does not work in a switching state. If it works in a switching state, this resistor is unnecessary and harmful.

maychang

Whether the transformer will be saturated has nothing to do with the core reset. Reset is a must, but saturation is not. Whether it is saturated depends on the specific circuit working state.

However, transformers in single-ended circuits are prone to saturation. To prevent the core from saturating, an air gap is usually added to the core of a transformer working in a single-ended circuit.

xztl

maychang posted on 2021-6-26 11:08 There is a slight difference between the first and second floor pictures: the emitter of the bipolar transistor in the first floor picture has a resistor to ground. This resistor has a negative feedback effect, while the MOS on the second floor ...

The actual circuit is shown on the second floor. The first floor is an ordinary transformer-coupled amplifier. The input is generally analyzed as a sine wave, but the second floor diagram is a switch state. I have never seen it used like this before. Now I find that I can't understand its magnetic reset.

maychang

xztl posted on 2021-6-26 10:58 Thank you for your answer. Working in the first quadrant, if the duty cycle of the square wave does not exceed 0.5, this transformer can be magnetically reset, right? ...

I have already said that the tube in the first floor diagram is not working in a switching state. So there is no problem that the square wave duty cycle does not exceed 0.5.

I also said: In any case, the core will always be magnetically reset. As for what the result of the reset will be, that is another question.

xztl

maychang posted on 2021-6-26 11:08 There is a slight difference between the first and second floor pictures: the emitter of the bipolar transistor in the first floor picture has a resistor to ground. This resistor has a negative feedback effect, while the MOS on the second floor ...

Generally, a single-ended transformer-coupled MOS tube drive circuit has a DC-blocking capacitor, so the problem of magnetic reset can be solved.

xztl

maychang posted on 2021-6-26 11:21 I have already said: the tube in the first floor picture is not working in the switch state. So there is no problem that the square wave duty cycle does not exceed 0.5. I also said: no...

Well, I actually want to ask about the picture on the second floor. How to magnetically reset when working in the switch state?

maychang

xztl posted on 2021-6-26 11:06 Where does its demagnetizing energy come from?

"Where does its demagnetizing energy come from?"

When the iron core is demagnetized, it is not that the circuit transmits energy (does work) to the iron core, but that the energy stored in the iron core is released into the circuit, and the iron core does work on the circuit.

In addition, I don't know which circuit the "it" in this sentence refers to.

xztl

maychang posted on 2021-6-26 11:23 "Where does its demagnetization energy come from? " Iron core demagnetization is not the circuit transmitting energy to the iron core (doing work), but the energy stored in the iron core is released...

I mean the picture on the second floor. Is there a path through the body diode of the MOSFET?

maychang

xztl posted on 2021-6-26 11:23 Well, I actually want to ask about the picture on the second floor, how to magnetically reset when working in the switch state

I think you are confusing three different circuits. The first is a single-ended linear power amplifier circuit, the second is a resonant power amplifier circuit, and the third is a forward switching power supply circuit.

As I said in the second floor diagram, the input is a square wave, but the output is a sine wave. This circuit must include a filter circuit. The filter circuit may be a resonant circuit composed of L3, L4, C5, C6, and C9. In this circuit, when the tube is turned off, the voltage across the primary of the transformer may be much larger than the power supply voltage, and the direction is to output energy to the outside. It is the transformer (both the primary and the secondary) that outputs energy to the outside, which resets the core magnetic field.

maychang

xztl posted on 2021-6-26 11:14 The actual circuit is shown on the second floor. The one on the first floor is an ordinary transformer-coupled amplifier. The input is generally analyzed as a sine wave, but the diagram on the second floor is open...

"The actual circuit is as shown on the second floor...but the diagram on the second floor shows the switch status. I have never seen it used like this before."

It is estimated (not sure) that this is a resonant power amplifier, that is, a power amplifier circuit with a resonant circuit as the load. The tube works in Class C state. The tube works in a switching state, periodically replenishing energy for the resonant circuit, and the resonant circuit outputs a sinusoidal signal to the load.

maychang

xztl posted on 2021-6-26 11:30 I mean the picture on the second floor. Is there a path in the body diode of the MOSFET?

"Is there a path through the body diode of the MOSFET?"

The body diode inside the MOS tube has nothing to do with reset. The body diode of the bridge circuit (half bridge or full bridge) is related to magnetic reset. When one side of the upper and lower tubes is turned off, the body diode of the other side may be turned on, and when it is turned on, it will conduct current into the DC power supply.

xztl

maychang posted on 2021-6-26 11:33 I think you are confusing three different circuits. The first is a single-ended linear power amplifier circuit, the second is a resonant power amplifier circuit, and the third is a...

Thank you for your patient reply . It can be said that the principle shown in the second post is fine, right? What if there is no LC circuit at the back? Will the magnetic core be saturated? (The transformer is directly connected to RL or the secondary is open, and the drive is still 0.5 duty cycle square wave)

maychang

xztl posted on 2021-6-26 11:21 Generally, the single-ended transformer-coupled MOS tube drive circuit will have a DC blocking capacitor, so the problem of magnetic reset can be solved

Yes, a single-ended switching power supply (forward or flyback) may use such a capacitor. This capacitor also blocks DC. However, without this capacitor, the transformer core still needs to be magnetically reset. With this capacitor, the transformer core can be smaller without saturating.

maychang

xztl posted on 2021-6-26 11:40 maychang posted on 2021-6-26 11:33 I think you are confusing three different circuits. The first is a single-ended linear power amplifier circuit...

"Is it true that the principle shown on the second floor is not a problem? 】

I don't know. I'm just guessing, not judging.