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# Questions about DC DC converter, especially buck converter

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#### Jonas1893

##### Newbie level 4
Hello, I am a student and I am trying to understand how the Buck converter works. I have a few questions that I do not have an answer to. Can you help me?
1. In the professor's script, we first considered a buck converter with a resistor at the output without a capacitor to derive the formulas. I know that the current in the coil increases with an e-function and then decreases again with an e-function and settles at some point. Now we have a capacitor at the output and no more resistor: How does the current in the coil change when a reverse voltage or a large capacitance is connected to the output?
2. Sketch the coil current when the reverse voltage is greater than the output voltage set via the duty cycle. Additionally, sketch the current in the switch and the free-wheeling diode. I'm not sure how it looks like
3. Explain when you absolutely need a closed loop control for your your buck converter.
Under which conditions is control via the pulse pause ratio?
4. Is it possible to operate a buck converter also with 10GHz?

Thank you so much for your help!

Hi,

I recommend to do some internet search for application notes and tutorial videos.

1) Without capacitor it makes no sense. With capacitor the current is no e-function anymore.
How does the current in the coil change when a reverse voltage or a large capacitance is connected to the output?
in ideal case: I_L = integral (dV/dt) / L

Difference with / without capacitor:
* without C: dV/dt depends on current --> e function
* with capacitor: V is about constant --> almost linear

2) again: there are a lot of really good videos around that explain every current in detail and waveform.

3) consider input voltage variation, output current variation, expected output voltage variation.

4) what could be the problem of very high frequency?
Hint: Many years ago buck converters worked with a couple of 10kHz then more than 100kHz, now many work above 1MHz

Klaus

Here is Buck simulation in the free download LTspice
Youtube vid can show you how to use LTspice
Or you can just hit the running man icon and watch and click to see V and I.

When you ve sim'd enough bucks, youll find that for small low power bucks, you can pretty much design them by quickly hacking them together in LTspice

#### Attachments

• Buck 24v to 12v at 3a.zip
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