sync buck convrter problem

[BradtheRad]

The waveform looks better without the capacitor.

I have been trying in various ways to get a waveform like you get with the capacitor. It looks like ringing due to LC oscillations. However the cap also appears to produce a volt level which interferes with operation. It's as though the active component(s) lose their reference. This might occur (for example) when the bias can't turn on a transistor/mosfet due to the emitter leg being at too high a volt level.

One thing I often see in simulations, is high-frequency ringing after the coil has discharged. These go away as soon as the transistor turns on. I have found it helps to install a 1k resistor across the diode. Try lower values as necessary.

---------- Post added at 12:54 ---------- Previous post was at 12:22 ----------

Also remember, a charged coil must never be isolated by sudden high resistance, even momentarily. The current must have somewhere to go.

Otherwise the collapsing flux field will generate a high voltage kick which will go through neighboring components, quite possibly ruining them.

 

[haywire]

Are you trying to select randomly ?? with events ??? it isn't design . you should learn , how to design . this way isn't correct and your value isn't correct too .

i got a question though. why have written that i need 100uF bootstrap capacitor in one of the pictures?
the switch voltage (yellow) and the diode voltage (blue).
the duty cycle/gate (yellow) and the output voltage (blue)

i'm sorry. because of the limited supplies that's the reason why i did that. but don't worry, i've followed the way you have calculated and tried varying other values to fit each other. this is what i got

The waveform looks better without the capacitor.

I have been trying in various ways to get a waveform like you get with the capacitor. It looks like ringing due to LC oscillations. However the cap also appears to produce a volt level which interferes with operation. It's as though the active component(s) lose their reference. This might occur (for example) when the bias can't turn on a transistor/mosfet due to the emitter leg being at too high a volt level.

One thing I often see in simulations, is high-frequency ringing after the coil has discharged. These go away as soon as the transistor turns on. I have found it helps to install a 1k resistor across the diode. Try lower values as necessary.

---------- Post added at 12:54 ---------- Previous post was at 12:22 ----------

Also remember, a charged coil must never be isolated by sudden high resistance, even momentarily. The current must have somewhere to go.

Otherwise the collapsing flux field will generate a high voltage kick which will go through neighboring components, quite possibly ruining them.

i've tried the experiment again. apparently the inductance wasn't enough and the capacitance is too high. so, i have calculated and simulated before carrying out the experiment. i've uploaded the picture. is that right? and no more ringing, i think.

by the way, i tried with 10 ohms..it burnt. i guess the current was too high.