Hello,

The input to my buck converter MOSFET switch is a PWM signal with a certain duty cycle. However, upon observing the voltage waveform of the MOSFET, the voltage is a bit distorted.

The input to MOSFET (PWM signal) and output of MOSFET is observed and pictures are attached.

1. PWM input to MOSFET -- Blue waveform


2. MOSFET output voltage (Duty Cycle) -- Blue waveform


Can anyone please explain why is there a difference in the attached 2 pictures. The duty cycle should be same and it shouldn't be distorted post coming out of the MOSFET.

Thanks

 

hi 356,
Please post a circuit diagram of the Converter.
E

MOD: Deleted your duplicate Thread

 

hi 356,
Please post a circuit diagram of the Converter.
E

It is actually a university assignment but I am confused as to why they both differ. I am attaching the picture of the circuit.



The first picture (PWM input to MOSFET -- Blue waveform) in the question was measured at 8 (Vpwm) and the 2nd picture (MOSFET output voltage (Duty Cycle) -- Blue waveform) was measured at 12

 

Since is for school think about what's connected to the drain of that mosfet and what effects they can have...

 

Since is for school think about what's connected to the drain of that mosfet and what effects they can have...

one theory that I was working on was that there is an inductor connected to the MOSFET drain, but I am confused. The inductor influences current.

The rise time and fall time of the current gets impacted due to the presence of an inductor. But I have the waveform of the MOSFET's voltage.

I don't know where I am going wrong.

 

You're on the right track... Now figure out why it looks the way it does... By the way they do ring not just spike...

 

Remember there is stray capacitance to ground at the connection between the MOSFET and the inductor.
What does an inductor and a capacitor form?

Can you look at the inductor current?

 

if i am not getting wrong, (and I really appreciate your help)

As the PWM signal goes HIGH - MOSFET switch is closed, inductor current starts to increase linearly until the point that the inductor current is maximum (i.e. inductor acting as a short-circuit).

During the time when the inductor current is increasing linearly, inductor voltage depicts unwanted spikes.

I can't seem to understand the reason behind these spikes.

 

Remember there is stray capacitance to ground at the connection between the MOSFET and the inductor.
What does an inductor and a capacitor form?

Can you look at the inductor current?

an LC oscillator circuit?

if I am not wrong,

An oscillation network is formed and leads to parasitic oscillation of a MOSFET. The ringing voltage between the drain and the source during turn-off could return to the gate terminal through a positive feedback loop via the gate-drain capacitance Cgd and cause the gate voltage to oscillate.

 

read up on inductors... why they do they do they do.

 

Spiking background and scope probing -

https://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdf

https://download.tek.com/document/02_ABCs-of-Probes-Primer.pdf


Regards, Dana.

 

an LC oscillator circuit?

Not an oscillator, but damped oscillations.

You really need to look at the inductor current.
I suspect its operating in a discontinuous mode where the current stops flowing through the inductor and freewheeling diode about halfway through the MOSFET off period.