Funny waveform on 12V->30V SMPS

[simingx]

Hi all,
I am building the 12V->30V SMPS described here: http://sound.westhost.com/project89.htm

One modification was to include Schottky diodes (SB160) across the gate resistor for faster turn off.
I am also using a single SPP80N06 MOSFET per leg to drive the primary instead of 2 parallel FETs.

However I am getting some strange waveforms at the primary... What's causing this?
If I remove the diode it goes away but then I have a slight overshoot to 40V instead.
Which is the lesser evil?

 

[simingx]

Here's the signal at the gate of the MOSFET.

 

[Hendrik]

What you are looking at is dead time.

 

[Eva]

A faster turn-off usually results in slightly less heat but more EMI radiated by the transformer.

The spikes are due to transformer leakage inductance. They are ok if they appear only under load and they don't exceed Vds rating at full load.

 

[simingx]

Hi,
Thanks for all the replies.
I rewound the transformer on a toroid core and the funny waveforms are gone now... Yes good coupling is absolutely essential!
Also I found I had too much output inductance before the smoothing capacitors... realized you don't really need any if the design is unregulated (which it is).