Hello everyone,
I am trying to make DC - DC converter using SG3525. (fig. circuit diagram). in circuit diagram i make some changes. i remove full bridge rectifier, change mosfet to IRF3205 and remove feedback circuit which is connected to inverting input (pin 1). i connect a 220 volt bulb (load) to secondry side of ferrite core trasnformer to check every things work fine or not. I check the gate waveform and drain source waveform which show in attachment figure pic_gate waveform and figure pic_drain waveform. when i connect load ( 220v bulb) on secondry side of transformer waveform gate waveform are distorted which can see in figure gate waveform.
I dont have any idea what happen. please tell me how can i resolve them.

 

Ringing is caused by parasitic L & C. So common solution is to put a small
valued R in series to gate of MOSFET. In the order of 10's of ohms,
experiment by trial as your layout affects this.

https://toshiba.semicon-storage.com/info/docget.jsp?did=59460&prodName=TK5Q65W

https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.DM00068312.pdf

Also make sure your scope probing techniques are not affecting the observed
waveforms -

http://literature.cdn.keysight.com/litweb/pdf/5992-2848EN.pdf

https://cdn.testequity.com/documents/pdf/probes-affect-measurement-an.pdf

http://web.mit.edu/6.101/www/reference/ABCprobes_s.pdf


Regards, Dana.

 

Because pins 1,2 are not used correctly to alter the feedback path.

 

What are you using for a transformer?

I suspect the ringing is likely almost entirely due to layout. I've done lots of switcher design and never seen anything even remotely as bad as that gate waveform. Can you post photos of your actual circuit?

When presenting waveforms, you need to learn to use your oscilloscope to actually show some reasonable detail. The scale factors need to be changed so not much more than one complete switching cycle fills the screen horizontally and the peak to peak voltage nearly fills the screen vertically. As it is, the gate waveform is full of quantization artifacts that obscure what is actually happening.

Thank you for naming your scope images sensibly!

 

Ringing is caused by parasitic L & C. So common solution is to put a small
valued R in series to gate of MOSFET. In the order of 10's of ohms,
experiment by trial as your layout affects this.

https://toshiba.semicon-storage.com/info/docget.jsp?did=59460&prodName=TK5Q65W

https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.DM00068312.pdf

Also make sure your scope probing techniques are not affecting the observed
waveforms -

http://literature.cdn.keysight.com/litweb/pdf/5992-2848EN.pdf

https://cdn.testequity.com/documents/pdf/probes-affect-measurement-an.pdf

http://web.mit.edu/6.101/www/reference/ABCprobes_s.pdf


Regards, Dana.

Thanks dana
I already try to add 10 ohm reister in mosfet fate but nothing changes in waveform.

 

Because pins 1,2 are not used correctly to alter the feedback path.

I connect to feedback path but nothing happen . Problem remain same.

 

Thanks dana
I already try to add 10 ohm reister in mosfet fate but nothing changes in waveform.

Have you tried as high as 50 ohms ? Say 20, 30, 40, then 50 ohms ?

What type of Rs are you using, not wire wound I hope. Should be Carbon or
film types.

What does layout look like of prototype board ? Picture ....

https://www.analog.com/en/technical-articles/short-ground-leads-make-better-scope-photos.html

Regards, Dana.