Asynchronous Buck's output voltage question

[TripACT]

Hi all,

I have a question about a typical asynchronous buck's output voltage ?

the buck's output voltage is Vout=D*Vin

when connecting R load to a buck output, does it change the output voltage ? if so how come and why ?

from my orcad simulation it appears it does change the voltage although the output voltage should be depending on the duty cycle and the input's voltage only, no?

and about the output's current ? how can i calculate the output's current? ohm's law ?

changing the output resistor on the buck converter does change the current & voltage on the output.

Gary

 

[BradtheRad]

This assumes a filter capacitor is in parallel with the load, to average the current flow, and make it easier to read the volt level.

The buck converter has different modes to deliver voltage at the load, depending on how the coil is made to charge and discharge.

1. When the load is in a middle range, the proportional equation holds (Vout=D*Vin). The graph of current through the coil is a triangle wave which is above zero through the entire power cycle (more or less), and which touches the zero line (more or less) at the end of the power cycle.

2. As you reduce load impedance, it raises the time constant on the coil. The coil continually circulates a small amount of current around the output loop. Now it is acting to some degree as a choke, and less as a buck converter.

3. As you raise load impedance, volt level tends to rise to the supply V. The entire circuit starts behaving similar to a half-wave power supply, rather than a buck converter. The coil becomes idle through a greater portion of the power cycle.

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After you raise or lower load impedance, the volt level changes immediately, however you must allow several cycles for it to stabilize.

After you raise load impedance, the output voltage soars for a brief time, then settles downward to a stable level.

After reducing load impedance, the volt level drops for a time, then rises upward to a stable level.

 

[TripACT]

What do you mean by "load in the middle range"? What does consider a middle range?

 

[BradtheRad]

Below is a link which will open an animated simulation of three buck converters. It will play on your computer.

They are identical except for the loads.

The falstad.com website will open. Click Okay when asked to allow the connection.

https://tinyurl.com/7z825k5

Hover the mouse over an oscilloscope trace, to see which component it applies to.

The three modes of operation are evident from the yellow waveforms showing current through a coil.

You can change values by pressing ctrl (or alt) key, and clicking a component.

You can change the duty cycle to whatever you wish.

Press reset whenever you want to start from power-up.

None of the three modes is entirely unusable.

The two schematics at right show the load can range from 60 mA to 700 mA, and they receive the same voltage within a few tenths of a volt. You might call this the middle range.

 

[TripACT]

mA? Didn't you mean ohms?

Thanks for elaborating

- - - Updated - - -

i saw the link. great stuff !

thank you very much !

i didnt know buck can behavior very different according to the load and coil relationship

i always thought it must follow Vout=Vin*D law.

that explains alot now and does make sence !!!!


thank you !


BTW

i will be happy to understand better the mode choke. how can exactly can it act as a choke ?

 

[BradtheRad]

i will be happy to understand better the mode choke. how can exactly can it act as a choke ?

A choke in the sense that a coil is used to reduce AC ripple current, and produce a smooth DC power supply.

To further understanding, I have a Youtube video showing animated simulations of a coil under various DC waveforms.

It portrays the coil's ability to generate current briefly after supply current stops.

It depicts current bundles sized to proportion. It shows the flux field building and collapsing. It depicts counter-emf in the coil.

www.youtube.com/watch?v=LVNxrN4jgvs

Also a video depicting the inner workings of switched-coil converters (buck, boost, buck-boost).

Observe how the flux field builds during one half of the cycle, then collapses as the coil generates current during the other half.

www.youtube.com/watch?v=FT_sLF5Etm4

mA? Didn't you mean ohms?

Yes, it's strange how a load can be stated as 60 to 600 mA... or 9 to 90 ohms.