MOSFET output ringing
 

The following circuit is a MOSFET driver.

When I feed a 20KHz squarewave in, I get 20KHz out ... but as a
1.25MHz ring. This is as unloaded.

http://users.tpg.com.au/hburns/index.html

I assume this has to do with the air core coupling, but would like to
know more about how this happens. IOW what factors control the
presence and frequency of the ringing.

Can anyone please explain?

Jim Starret

MOSFET output ringing
 

Jim Starret wrote:
The following circuit is a MOSFET driver.

When I feed a 20KHz squarewave in, I get 20KHz out ... but as a
1.25MHz ring. This is as unloaded.

http://users.tpg.com.au/hburns/index.html

I assume this has to do with the air core coupling, but would like to
know more about how this happens. IOW what factors control the
presence and frequency of the ringing.

Can anyone please explain?

It is the self resonant frequency of the transformer
inductance in parallel with the inter winding and mosfet
output capacitance.

MOSFET output ringing
 

"Jim Starret"

The following circuit is a MOSFET driver.

When I feed a 20KHz squarewave in, I get 20KHz out ... but as a
1.25MHz ring. This is as unloaded.

http://users.tpg.com.au/hburns/index.html

I assume this has to do with the air core coupling, but would like to
know more about how this happens. IOW what factors control the
presence and frequency of the ringing.

Can anyone please explain?


** That secondary winding has almost no effect unless it is loaded.

Your primary winding is therefore a simple " tank circuit " - ie a
parallel tuned circuit, tuned by the stray capacitance between individual
turns and that mosfet driving it.

The 8 to 10 cycles of visible ringing indicates the " Q factor " of your
100t coil is around 8 to 10 also - a neat rule of thumb to remember.

Add a bit of ferrite inside the coil - see the ringing frequency take a
nose dive and the number of cycles go through the roof !!!

Real electronic devices, in a hand on situation, plus real time scope view
beats ANY simulation, anyday.




........ Phil

MOSFET output ringing
 

** That secondary winding has almost no effect unless it is loaded.

....... Phil


Thanks Phil.

Just out of curiousity, if I did load the secondary, say with some
kind of antenna, would the antenna be broadcasting the original
waveform, as per the 100T coil, or the waveform after it has been
affected by the load?

Sorry if this is a bit basic.

Jim Starret

MOSFET output ringing
 

"Jim Starret"

** That secondary winding has almost no effect unless it is loaded.


Thanks Phil.

Just out of curiousity, if I did load the secondary, say with some
kind of antenna, would the antenna be broadcasting the original
waveform, as per the 100T coil, or the waveform after it has been
affected by the load?


** Bizarre.

Try you Q on alt.radio-pirates.nut cases

Anytime.



.......... Phil

MOSFET output ringing
 

** Bizarre.

Try you Q on alt.radio-pirates.nut cases

Anytime.



......... Phil




All I was wondering, in a generally way, is what happens to the energy
in a tank circuit when it is detuned by loading?

Jim S.

MOSFET output ringing
 

"Jim Starret"

** Bizarre.

Try you Q on alt.radio-pirates.nut cases

Anytime.


All I was wondering, in a generally way, is what happens to the energy
in a tank circuit when it is detuned by loading?


** Then avoid using the terms like " antenna" and " broadcasting" in the
same sentence.

Makes your original schem look like a radio jammer.



........ Phil

MOSFET output ringing
 

Jim Starret wrote:


** That secondary winding has almost no effect unless it is loaded.

....... Phil


Thanks Phil.

Just out of curiousity, if I did load the secondary, say with some
kind of antenna, would the antenna be broadcasting the original
waveform, as per the 100T coil, or the waveform after it has been
affected by the load?

Sorry if this is a bit basic.

Jim Starret

You'd do better hanging a resistor on the secondary to simulate the
intended load.

- YD.

--

MOSFET output ringing
 

On Fri, 08 Jun 2007 09:12:15 +1000, Jim Starret wrote:
** Bizarre.

Try you Q on alt.radio-pirates.nut cases

Anytime.

All I was wondering, in a generally way, is what happens to the energy in
a tank circuit when it is detuned by loading?

It goes into the load and is transformed into heat. :-)

Cheers!
Rich