Buck converter: why not do it like this? - Page 1
Forum section: electronics / projects

author: tbj
author_rank: Contributor
timestamp: December 06, 2012, 11:17:38 am
content: I believe that by taking a PWM signal and passing it through a low-pass filter, one ends up with an output voltage proportional to the pulse width of the input signal, right?Why do we not see more buck converters that work in this way? Surely it's much simpler than using an inductor as an energy storage device?If this idea will work, I want to build a variable output buck converter.Cheers,R

author: Simon
author_rank: Unknown
timestamp: December 06, 2012, 12:35:33 pm
content: your low pass filter will have a resistor in it that will loose a lot of power, so you might as well have a linear reg

author: tbj
author_rank: Contributor
timestamp: December 06, 2012, 01:24:49 pm
content: Not if you use an LC filter like on the output of a class D amplifier.

author: JVR
author_rank: Regular Contributor
timestamp: December 06, 2012, 01:28:14 pm
content: Quote from: tbj on December 06, 2012, 01:24:49 pm!=Quote from: tbj on December 06, 2012, 11:17:38 am

author: tbj
author_rank: Contributor
timestamp: December 06, 2012, 01:34:28 pm
content: OK. Point taken.However... let's say I decided to build a power supply like this, instead of the conventional buck converter which uses the inductor as an energy storage element and not as a filter. Purely as a 'what-if' excercise. Would it work? Would it work well?This strikes me as easier to build for the hobbyist than the traditional design.

author: mrflibble
author_rank: Super Contributor
timestamp: December 06, 2012, 01:34:54 pm
content: Quote from: tbj on December 06, 2012, 01:24:49 pmQuote from: tbj on December 06, 2012, 11:17:38 amLoL!

author: mrflibble
author_rank: Super Contributor
timestamp: December 06, 2012, 01:42:33 pm
content: Quote from: tbj on December 06, 2012, 01:34:28 pmI guess that depends on your localized definition of "easier". Easier what way? Easier to understand? Easier to design? Easier to afford? Easier to get good efficiency?If that power supply is supposed to source any decent amount of current I find that a buck converter is pretty easy.Besides, if you want easy + poor efficiency, just grab a 3-pin linear regulator and burn away. ;-)

author: Short Circuit
author_rank: Frequent Contributor
timestamp: December 06, 2012, 02:46:34 pm
content: 1 draw the schematic for a (synchronous) buck converter2 draw the schematic for a PWM driver followed by an LC filter

author: Simon
author_rank: Unknown
timestamp: December 06, 2012, 03:25:42 pm
content: you can use a fixed PWM driver and a low pass filter using an inductor and the rectifier diode and you will have an outoput, but it will not be regulated.

author: alanb
author_rank: Frequent Contributor
timestamp: December 06, 2012, 03:27:35 pm
content: I suppose the answer to this type of question is to build it and see if it works as you expect it to.

author: IanB
author_rank: Super Contributor
timestamp: December 06, 2012, 03:38:17 pm
content: Quote from: tbj on December 06, 2012, 11:17:38 amThis only works if the input PWM signal is an active off signal, not a passive signal. In other words, the PWM cannot be a switched on/off supply, it must a supply that sinks current at 0 V in order to pull the voltage down to zero. This arrangement is necessarily going to be feeding power in to the filter during the on part of the cycle and then draining power from the filter during the off part of the cycle. Essentially an RC filter is a power consumer. Since it is a power consumer it should work with signal levels of power rather than large levels of power.QuoteBecause a buck converter is a power converter. It is meant to take input power and transform it to output power with minimal losses. But as we see above a low pass filter is a power consumer, not a power converter.Now you could arrange for the load side of the low pass filter to drain the power out of the filter rather than the source side, but then you would need regulation, and almost immediately you have a regular buck converter. A buck converter is actually a suitably controlled low pass filter with a PWM input. You have just reinvented the buck converter.

author: Simon
author_rank: Unknown
timestamp: December 06, 2012, 03:41:14 pm
content: I think we are over complicating, if you take the feedback loop out of a SMPS you have a fix PWM power output going through the same LC circuit with the rectifier diode that allows back feed during the PWM off cycle. But the voltage will be a fixed % of the input and subject to the effects of the load with no means of correction.

author: hlavac
author_rank: Frequent Contributor
timestamp: December 06, 2012, 04:36:37 pm
content: The big difference is that buck converter converts POWER (which remains constant, minus conversion losses) to a lower voltage and higher current combination. All the power still goes to the load.With low pass filter the power will bypass the load partially and is wasted.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 06, 2012, 05:25:15 pm
content: Quote from: Short Circuit on December 06, 2012, 02:46:34 pmThis. The only difference is that the buck converter normally uses a freewheeling diode as the low-side drive while a PWM driver uses transistors for both sides (push-pull).

author: Simon
author_rank: Unknown
timestamp: December 06, 2012, 06:12:48 pm
content: Quote from: hlavac on December 06, 2012, 04:36:37 pmso isn't the LC output circuit of a SMPS a low pass filter ? your statement is too generic, it depends on the elements in the filter.

author: Simon
author_rank: Unknown
timestamp: December 06, 2012, 06:15:20 pm
content: Quote from: ejeffrey on December 06, 2012, 05:25:15 pmHigher efficiency and power SMPS use a low side switching transistor too, the extra circuitry needed to avoid shoot through and time the low side switch don't justify the cost and complexity for low cost low power SMPS, why do you think we also have pull up resistors ? - it makes little sense to add another transistor and control logic to a circuit that outputs such a small output that it can use a resistor.