[SOLVED] Squarewave Misformed From MOSFET Fed From Wellformed Squarewave.

[Zak28]

Im employing an outstanding silicon oscillator from Linear Technology, its +5v into a 2v-4v into a power mosfet gate low and behold without desire I found distortion. The images posted are

1. Oscillator output (no visible distortion)

2. Vds square wave output visible distortion

3. Distortion zoomed to visibility range


I TRIED attaching Zeners across FET Vds however I found a requirement for zener breakdown to be about 80% of Vds output! I cannot employ Zeners although if power and heat weren't an issue I would since they literally flatten the distorted squarewave when the zener is 80% less than Vds which is a nice thought but I would like another circuit recommendation to remedy this.

 

[hamidmoallemi]

where the output of MOSFET is connected to ?
this distortion is because of parasitic components , but I think it's not a big problem because the overshoot is not very large, its normal

 

[FvM]

It's a well expectable effect of MOSFET gate-drain capacitance Cgd.

Rising Vgs edge will first transfer some charge to the drain node before the threshold voltage is exceeded and the drain is pulled low. Can't be avoided by a single ended FET stage.

Possible solutions depend strongly on your application. You may want to feed a compensating charge to the output node.

 

[Zak28]

It's a well expectable effect of MOSFET gate-drain capacitance Cgd.

Rising Vgs edge will first transfer some charge to the drain node before the threshold voltage is exceeded and the drain is pulled low. Can't be avoided by a single ended FET stage.

Possible solutions depend strongly on your application. You may want to feed a compensating charge to the output node.

Were about FET amp does one position capacitance? Perhaps I should employ FET with paralleled across Drain-Source Zener built in but I might be mistakenly taking TVS diodes for voltage regulation diode.

Strangely FETS with Rds >1Ω literally have NO visible overshoot. I might employ such a FET since I'm only passing about 28mA dissipation might workout if package is TO-220 or TO-247 surely I will find something applicable, its a thru hole project.

 

[Okada]

Low Rds(on) N-Channel Mosfets.

 

[FvM]

Zener diodes are off-topic if you want to handle a few volts overshoot.

Strangely FETS with Rds >1Ω literally have NO visible overshoot.

Not particularly strange, MOSFET capacitances vary largely with chip size and are roughly related to Rdson.

I would expect something like a TO92 transistor for an 28 mA switch.

I notice that your results are from simulations only. Although the principle effects are usually correctly represented, it's not guaranteed that all MOSFET parameters are closely modeled. You should look at the real circuit, too.

To better understand your results, we would need the exact simulation circuit.

 

[Zak28]

Applied transistor with likable characteristics no more overshooting.