Phase cut rectifier switching erratically...why?

[Flyback]

Hello,

Please can you help solve the erratic switching that’s happening in my phase cut rectifier?

I have a 50KHz (sinusoidal) rectifier to which I have added phase cut circuitry, (to reduce the average output current)

Schematic………………………
https://i49.tinypic.com/34rucgj.jpg
(can be seen clearer in schematic attached as below...)

Output current waveform with no phase cutting………….
https://i45.tinypic.com/kcl3z9.jpg


Output current waveform with phase cutting………….
https://i47.tinypic.com/14y3iw9.jpg

..how can I get rid of the bit where the waveform switches erratically near time = 70us?

the LTspice file is attached in .txt, (converts to .asc) and the schem in pdf



please note in the below schem, that "GATE" should be connected to the gates of the FETs , and not to the ground

 

[Flyback]

By the way, here is a clearer schematic..........

View attachment scm COUPLER phase cut1.pdf

 

[alec_t]

I don't have an answer, but for future reference you can post .asc files here as attachments.
Also, .jpg format is poor for schematics; gif, png or pdf is preferable.

 

[Flyback]

sorry, i did look for the panel of attachment icons when i first posted but didnt see them........then i saw some when i looked at the "reply" window after posting...so used it there to post the schem........i'll see if i can edit things neater.....OK , managed to do as you kindly advised.

 

[kubeek]

The main problem is that D1 and D2 will allways conduct regardless of the transistors, so there is no point to the rest of the circuit.
If you are trying to do something like synchronous or active rectification like it seems, you should look up how it´s usually done.

 

[Flyback]

D1 and D2 are simultaneously shorted out by the FETs......doing this diverts current away from the LEDs...and hence dims them.

The circuit works well, but has this "blemish" 70us after start up.................after this it behaves beautifully....(on the sim).............but i wonder of the blemish is the harbinger of future poor operation?

I am definetely not trying to do syncronous rectification here, or active rectification.

...i am simply dimming the leds by periodically diverting the current away from the leds, and through the loop contained by the two fets

 

[kubeek]

The circuit works well, ...

I seriously doubt that. Did you notice that GATE is connected to ground, so the transisitors can never turn on, being N-fets? And if I delete that ground, that 5k1 resistor will keep the transistors in linear mode as the RC constant on the gate is way too high for this frequency. With 22 ohms it sort-of starts doing something, but changing V1 and V3 doesn´t affect the output current much.

 

[Flyback]

Hello, Thanks Kubeek, sorry i misconnected.

Please find the corrected schem here...

Right, it wont allow me to upload the corrected image...i will think of something.

GATE should not be connected to ground...it should go to the gates of the fets

 

[Flyback]

Hello, Thanks Kubeek, sorry i misconnected.

Please find the corrected schem here...

Right, it wont allow me to upload the corrected image...i will think of something.

GATE should not be connected to ground...it should go to the gates of the fets