# Buck Converter MOSFET Output Voltage distorted (PWM)

#### Rango356

Joined May 2, 2019
5
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

#### ericgibbs

Joined Jan 29, 2010
9,805
hi 356,
Please post a circuit diagram of the Converter.
E

#### Rango356

Joined May 2, 2019
5
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

#### Wolframore

Joined Jan 21, 2019
1,742
Since is for school think about what's connected to the drain of that mosfet and what effects they can have...

#### Rango356

Joined May 2, 2019
5
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.

#### Wolframore

Joined Jan 21, 2019
1,742

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...

#### crutschow

Joined Mar 14, 2008
24,711
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?

#### Rango356

Joined May 2, 2019
5
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.

#### Rango356

Joined May 2, 2019
5
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.

#### Wolframore

Joined Jan 21, 2019
1,742
read up on inductors... why they do they do they do.

#### crutschow

Joined Mar 14, 2008
24,711
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.