Calculate the RCD in the figure below

sky999

Calculate the RCD in the figure below [Copy link]

 

How should I calculate the RCD of this figure? I searched for a few days, but I was confused by the formula and didn't understand it. Here I used a 103 capacitor for C1, a 50R for R7, and an RS2M for D1.

I don't know why the MOS tube is so hot. It was hot in about 5 seconds. After about 1 minute, the transformer also got hot. The same thing happened when the inductor was removed.

maychang

Your circuit is not a flyback switching power supply circuit. It has been explained in another post that the output of this circuit is AC.

Since it is not a flyback switching power supply, there is not much point in setting up an RCD circuit.

sky999

maychang posted on 2023-8-7 21:52 Your circuit is not a flyback switching power supply circuit. It has been explained in another post: the output of this circuit is AC. Since it is not a flyback switching power supply...

I don't understand, but won't the core saturate? If there is no RCD, when the MOS tube is turned off, won't the capacity of the primary coil be given to the MOS tube, which will cause the MOS tube to heat up?

qwqwqw2088

Try removing the RCD.

It's a bit redundant

Is the purpose of setting up an RCD circuit to reduce heat?

RCD absorbs the peak value during switching.

sky999

qwqwqw2088 posted on 2023-8-8 07:30 Try removing the RCD. It is a bit redundant. Is the purpose of making an RCD circuit to reduce heat? RCD absorbs the peak when switching.

Because the MOS tube is hot after 5 seconds, but I can't find any other reason, so I guess it's the energy stored in the primary coil that runs to the MOS tube when it is disconnected, so it gets hot, so I added the RCD, and it's the same if I remove it.

sky999

This is the waveform of the MOS tube DS. Normally it should be red. In fact, at the moment when the MOS tube is disconnected, the peak voltage is very high. I don’t understand why it will quickly drop after being pulled up, and then slowly rise again.

maychang

sky999 posted on 2023-8-7 22:08 I don't understand, but won't the core saturate? If there is no RCD, when the MOS tube is turned off, won't the capacity of the primary coil be given to the MOS tube? Wouldn't it be...

There are so many things you don't understand.

As mentioned on the second floor: This circuit is not a flyback switching power supply. The secondary of the flyback switching power supply transformer is rectified by a diode, but the secondary of your circuit transformer is not rectified, and current flows in both directions of the secondary.

maychang

sky999 posted on 2023-8-7 22:08 I don't understand, but won't the core saturate? If there is no RCD, when the MOS tube is turned off, won't the capacity of the primary coil be given to the MOS tube? Wouldn't it be...

Whether the core is saturated or not does not depend on whether an RCD absorption circuit is added. What is more important is whether the magnetic flux in the core can be reset. In your circuit, there is a DC component in the primary of the transformer (because it is single-ended operation). Whether it will be magnetically saturated depends entirely on your transformer and circuit design.

maychang

sky999 posted on 2023-8-8 09:19 This is the waveform of the DS of the MOS tube. Normally it should be red. In fact, when the MOS tube is disconnected, the peak voltage is very high. And I don’t understand why it will be high after it is pulled up...

[This is the waveform of MOS tube DS, which should be red normally]

Why do you say "normal should be red"? Which book did you refer to for the waveform diagram? Most importantly: is your circuit the same as the reference circuit?

sky999

maychang posted on 2023-8-8 09:39 Whether the core is saturated or not does not depend on whether the RCD absorption circuit is added. What is more important is whether the magnetic flux in the core can be reset. In your circuit, the transformer is initially...

So how do we know whether the magnetic flux in the core can be reset? This diagram starts from the MOS tube, and it is indeed a complete circuit.

maychang

sky999 posted on 2023-8-8 09:56 So how should we know whether the magnetic flux in the core can be reset? This diagram starts from the MOS tube, which is indeed a complete circuit

[This diagram starts from the MOS tube, and it is indeed a complete circuit]

Since it is a complete circuit, don't you see that there is no rectification on the secondary side of the transformer? Isn't it a flyback circuit?

sky999

maychang posted on 2023-8-8 09:41 [This is the waveform of MOS tube DS, it should be red normally] Why do you say "it should be red normally"? Which book did you refer to...

Isn't it? I don't know, I think it should be like that. If the duty cycle is reduced, the peak voltage will be much lower, but the current will also be much smaller. The waveform of the secondary of the transformer looks very good.

maychang

sky999 posted on 2023-8-8 09:56 So how should we know whether the magnetic flux in the core can be reset? This diagram starts from the MOS tube, which is indeed a complete circuit

[How do we know whether the magnetic flux in the core can be reset? ]

Calculate based on circuit parameters and component parameters. Of course, your circuit is not a flyback, so it cannot be calculated as a flyback.

sky999

maychang posted on 2023-8-8 09:58 [This diagram starts from the MOS tube, it is indeed a complete circuit] Since it is a complete circuit, don’t you see that the secondary of the transformer is not rectified? It is not a flyback...

I don't understand. There is no diode on the secondary side of this circuit. The secondary side is as shown in the picture and it works normally. Except that the MOS tube is always hot, there are no other problems.

maychang

sky999 posted on 2023-8-8 09:56 So how should we know whether the magnetic flux in the core can be reset? This diagram starts from the MOS tube, which is indeed a complete circuit

You haven't even figured out the nature of the load connected to the secondary side of the transformer in the picture.

sky999

maychang posted on 2023-8-8 10:00 You haven't even figured out what kind of load is connected to the secondary of the transformer in the picture.

Piezoelectric ceramic ultrasonic transducer

maychang

sky999 posted on 2023-8-8 09:58 Isn't it? I don't know, it seems to be like that. If the duty cycle is reduced, the peak voltage will be much lower, but the current will also be too small...

[The secondary waveform of the transformer looks very nice]

Why don’t you post it and let us see?

sky999

maychang posted on 2023-8-8 10:07 [The waveform of the secondary of the transformer is very beautiful] Why not post it for us to see?

Ah, I posted this last time, you forgot again

sky999

maychang posted on 2023-8-8 10:07 [The waveform of the secondary of the transformer is very beautiful] Why not post it for us to see?

sky999

maychang posted on 2023-8-8 09:59 [How should we know whether the magnetic flux in the core can be reset? ] Calculate according to the circuit parameters and component parameters. Of course, your circuit is not...

My understanding is that the forward circuit is when the MOS tube is turned on, the primary transfers energy to the secondary, and the flyback circuit is when the MOS tube is turned on, the primary stores energy and transfers energy to the secondary when it is turned off, but I can't tell which one this is.