Why is my circuit oscillating

Thread Starter

Rock Slate

Joined Jul 22, 2015
27
Hi guys

I am trying to build a power transfer circuit using an inductor and a capacitor. When I try to use the PULSE function(it is a constant frequency with a really short of time) , the circuit seems to be oscillating. Instead if I supply a DC voltage of 0V , it seems to reach a response of 12V, and if I supply an input voltage to the switch of 12V, it reaches 0V. I cant seem to understand when I supply a pulse with a large on time(almost the same as a DC voltage), it seems to oscillate.

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Thread Starter

Rock Slate

Joined Jul 22, 2015
27
Yes. There is something funny going on. The current through it seems to be in the order of kilo amp. Maybe I have to increase the input resistance of the switch. but this still doesnt explain the oscillation.

Second, the more ideal the switch, i though the better the circuit was. Is this because of the surges when the switch turns on?
 

MikeML

Joined Oct 2, 2009
5,444
Dont understand why you have two switches?

As configured, there is a short period when both switches are turned on at the same time (called shoot-through) due to overlap in the control signals.
 

Thread Starter

Rock Slate

Joined Jul 22, 2015
27
Yes. I didnt want to use a diode . That is why I used the NMOS instead. But thanks for linking to that thread. SOme really cool information which I didnt know.
 

MikeML

Joined Oct 2, 2009
5,444
You could use a second MOS switch in place of the Schottky, but you would have to generate non-overlapping gate drive signals...
 

Thread Starter

Rock Slate

Joined Jul 22, 2015
27
For the SW1, it is on whenever the input is low. It is an ideal switch so I would assume it would transition immediately and there is no rise time or fall time . I changed the circuit to generate non overlapping signals so that both the switches are not on simoultaneously. However I still see the huge currents. I have attached the circuit. Could you take a look? I forgot to split the source, so I am just specifying what I did instead . I split the voltages source V2, and changed the on time to a shorter value. However , it does not seem to work.
 
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Thread Starter

Rock Slate

Joined Jul 22, 2015
27
It is for current mode control of the buck converter. I was playing around with the one-shot values, however ideally it should be around 0.3-0.5V. I forgot to check it before uploading
 

crutschow

Joined Mar 14, 2008
34,281
How did you arrive at your inductor and output capacitor values?
Those values give a resonant frequency for the output L and C of about 13MHz, which is the oscillation you are seeing.
Normally the output resonant frequency of a switching regulator is in the low kHz range, not the MHz range.
 
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crutschow

Joined Mar 14, 2008
34,281
You are unlikely to get a real switching regulator to operate at 81MHz. :eek:
If you want to operate at that frequency, why is you simulation switching frequency 6.7MHz?
 
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