pulses of Buck converter issue

crutschow

Joined Mar 14, 2008
27,173
That's likely due to stray inductance and capacitance in the circuit.
A small resistor in series with a small capacitor from the MOSFET output to ground should minimize that.
Start with about a 50Ω resistor in series with a 100nF capacitor.
Adjust their values for minimum spike amplitude.
 

crutschow

Joined Mar 14, 2008
27,173
what is the output that you mean? from Mosfet Drain to ground?
The output to the inductor, which is shown as the MOSFET Source in your schematic.
The Drain goes to the supply so why would you consider that an output?
Would that capacitor reduce the ripple in output voltage, the ripple is high 1.5 v?
No.
The output ripple is determined by the switching frequency, the value of the inductor and the output capacitor, and the load current.
 

ronsimpson

Joined Oct 7, 2019
1,227
Would that capacitor reduce the ripple in output voltage, the ripple is high 1.5 v?
No it will reduce the ripple on the input. You need a capacitor close to the MOSFET.

The ripple seems high. It is a function of Frequency L1, C6 and Rload (R1). The LC makes a low pass filter to reduce the ripple.
I don't know the voltage or current. Increasing C6 will help the ripple. Adding a second cap will cut the ripple to 1/2.
 

Thread Starter

mah

Joined Mar 15, 2010
393
The output to the inductor, which is shown as the MOSFET Source in your schematic.
The Drain goes to the supply so why would you consider that an output?
but this will increase stability of the input voltage and i don't want to increase stability by adding capacitor, i increase it by controller design
 

Thread Starter

mah

Joined Mar 15, 2010
393
The ripple seems high. It is a function of Frequency L1, C6 and Rload (R1). The LC makes a low pass filter to reduce the ripple.
I don't know the voltage or current. Increasing C6 will help the ripple. Adding a second cap will cut the ripple to 1/2.
why is the ripple high, it should be very small according to calculation , it should be 4.2e-4
1606922797093.png
 

ronsimpson

Joined Oct 7, 2019
1,227
You know that L and C are in the formula. Also F^2.
Please post a scope picture of the ripple.
If C6 is low quality there will be ripple. C6 has ESR which will increase the ripple. (enteral series resistance) Also it has ESL. Normally C6 is chosen for its ESR and "current rating at 100khz". Some caps have a 60hz ripple current rating and a 100khz ripple current rating.

If the inductor is under rated for current it might be saturating. This will increase the ripple!
 

Thread Starter

mah

Joined Mar 15, 2010
393
If the inductor is under rated for current it might be saturating. This will increase the ripple!
the load draw around 3.5 A while the inductor didn't get hot so i think it is rated. also i reduced the current to 1 A the ripple decreased but not too much
 

crutschow

Joined Mar 14, 2008
27,173
but this will increase stability of the input voltage and i don't want to increase stability by adding capacitor, i increase it by controller design
No.
Those small values of R and C for a snubbing circuit at the MOSFET output will have no significant effect on the loop stability.
 
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