Synchronous buck converter having output voltage ringing during switching

[schmitt trigger]

"There are probably newer seminar papers covering the same topic."

Most likely true.
However, there is something to be said about the old Unitrode app notes...they were classics.

 

[FvM]

Some links that seem to work now, although not found by a search TI.com

The archives have a size of 2 to 18 MB each, covered period is 1983 to 2003

Sem100 https://focus.ti.com/lit/ml/slup209/slup209.zip
Sem300 https://focus.ti.com/lit/ml/slup215/slup215.zip
Sem400 https://focus.ti.com/lit/ml/slup216/slup216.zip
Sem500 https://focus.ti.com/lit/ml/slup217/slup217.zip
Sem600 https://focus.ti.com/lit/ml/slup218/slup218.zip
Sem700 https://focus.ti.com/lit/ml/slup219/slup219.zip
Sem800 https://focus.ti.com/lit/ml/slup220/slup220.zip
Sem900 https://focus.ti.com/lit/ml/slup221/slup221.zip
Sem1000 https://focus.ti.com/lit/ml/slup210/slup210.zip
Sem1100 https://focus.ti.com/lit/ml/slup211/slup211.zip
Sem1200 https://focus.ti.com/lit/ml/slup212/slup212.zip
Sem1300 https://focus.ti.com/lit/ml/slup213/slup213.zip
Sem1400 https://focus.ti.com/lit/ml/slup214/slup214.zip
Sem1500 https://focus.ti.com/lit/ml/slup208/slup208.zip

 

[The_Babatian]

I have read some snubber design stuff before but I think considering my desired power rating which is 8V input and 4V output, there would be no need to add additional snubber circuits. But after trying four boards with various versions of layout and combination of components, I started to consider the necessity of snubber circuits. Thanks for your help!

 

[schmitt trigger]

Something I like to do when designing prototype boards, is to leave placeholders for snubber components.

I leave them at first unpopulated, but if during the testing I see some ringing which requires mitigation, I can add or remove components easily.

 

[The_Babatian]

That is great, I would also do that next time. I am attaching my newest layout below, let me know if you have any advice regarding the placement of the components. Note that for this layout, the ringing of the output during switching is much smaller, less than 100mV, but since my following experiments would have a stringent requirement of the ringing, I am still expecting to reduce it to a lower level. Also, I figured that there is also a small ringing in the input with a similar shape as that in the output, any idea about that? Do you think additional input filter needed?


And also a sample waveform, red trace is high gate signal, blue trace is low gate signal, yellow trace is output voltage (AC coupled),and followed by the switching node signal.

 

[FvM]

I don't think that you'll achieve much lower switching "noise" without in- and output filters.

 

[schmitt trigger]

When you consider what capacitor is best for lowest broadband noise, it is usually advisable to have different types of capacitors in parallel.

For instance, the primary filter may be a tantalum, followed in parallel by a pair of 1 uF ceramics, followed in parallel with several 0.01uF ceramics.