Remove Oscillations in SSR Design

[timi]

Hi Guys.

recently i am making a SSR controlled by micro.

i build it using a tc4427 and 2 irlr2905 the output from tc4427 is ok without load on the mosfets. when load is connected "light bulb" i have a very oscillating switch off. on switch on its ok.

attached are the schematics and oscilloscope print screen.

Please can any one help out.

PS. IN the PCB the mosfet are close together with tc4427 also. the power source for the bulb is a laptop adapter of 15V. on one screen shot i used 5V to drive the tc4427 on the last one 12Volt.

Timi.

 

[esp1]

hi,
Those are very fast pulses in the order of 50nSec, I would suggest a 'snubber' across each power FET, Drain to Source, say a 47R and 10nF in series.

How long are the wires from the FET's to the Lamp.?

E

 

[timi]

Hi ESP1,

Thanks for the reply. The wires for the Lamp are more than 140 cm away as i am planning to replace the car lamps, but on the test they are 40 cm long. Please can you send me a link or formula to calculate the RC snubber on each Fet.

PS. I needed to have electrical isolation from car electrical system as the pwm for the light need to be economical somehow

Timi

 

[timi]

Hi Guys,

i found this link for the snubber circuit:
http://aosmd.com/res/application_notes/mosfets/AN100_Snubber_Design_for_Noise_Reduction.pdf

is this correct more or less ?

Timi

 

[esp1]

hi timi.
That pdf link shows the snubber type I have in mind.
It may need a little experimentation to get the optimum suppression.

E

 

[timi]

Hi ESP1,

Thanks for the help. I will try it out on the car, with different values starting with yours to see if i can get close to it .

thanks again,

Timi