How is SMPS output of transformer stable regardless of its input - Page 1
Forum section: electronics / projects

author: abdulbadii
author_rank: Frequent Contributor
timestamp: August 12, 2022, 07:41:50 am
content: How is a decent SMPS (its circuit) that can ensure transformer output being stable or nearly even regardless of the transformer unstable input?brief yet clear explaining a roughly outlined circuit functions phases

author: tunk
author_rank: Super Contributor
timestamp: August 12, 2022, 01:49:06 pm
content: I think diodegonewild@youtube has done some videos about how SMPSs work.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2022, 04:22:58 pm
content: Quote from: abdulbadii on August 12, 2022, 07:41:50 amThe transformer is driven by PWM. A TL431+opto-coupler is used to sense when the voltage on the secondary has reached the required level and the duty cycle of the PWM is reduced, or increased accordingly.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 12, 2022, 05:28:00 pm
content: An smps / buck boost dc-dc converter is supposed to have what we call a feedback line / sense, it must lower the output voltage if it tend to goes up and vice versa to obtain what we call regulation / voltage stabilizationSame for linear psu's tooYou see a lot of tl431 devices in them, normaly they are used for a voltage reference, but ive seen some starnge tricks with themAnd normally for the voltage / sense feedback a good psu have an optocoupler to isolate this line to get in the end an isolated psu between the input and outputBuck boost or buck down dc-dc for most of the time are not isolated between input and output , and i try to avoid them, when they fail, the damages are more extensive ..

author: jonpaul
author_rank: Super Contributor
timestamp: August 12, 2022, 06:42:07 pm
content: feedback

author: ejeffrey
author_rank: Super Contributor
timestamp: August 12, 2022, 07:04:01 pm
content: Quote from: abdulbadii on August 12, 2022, 07:41:50 amYour question isn't exactly clear. Are you asking how a transformer used in an SMPS can have an output set by a regulator rather than the input voltage * turns ratio?Technically it can't. Regulation of this kind requires energy storage and at least according to modern taxonomy transformers don't store energy. If you look at a forward converter, you have an output inductor after the transformer, which is necessary for it's operation. A forward converter is basically a standard buck converter with a transformer between the switches and the LC filter. The transformer provides isolation and impedance transformation but is not responsible for the voltage regulation. If you look at the output voltage at the transformer windings it will just be the input voltage * turns ratio with the duty cycle varying as the feedback loop adjusts to regulate the output voltageYou will see the same if you look at most of the other isolated SMPS topologies. They use a transformer to do the bulk of the voltage transformation via the turns ratio and an inductor for regulation.The main exception is the flyback converter. A flyback converter does not use a separate output inductor. Instead it does store energy in the transformer. While that component is called a "flyback transformer" it is actually considered a coupled inductor and the core is built with a gap just like a regular power inductor.

author: CaptDon
author_rank: Super Contributor
timestamp: August 12, 2022, 07:26:18 pm
content: The answer is in almost every datasheet of SMPS controller I.C.'s as well as every online design guide for building SMPS's as well as many of the application notes for the transformer magnetic properties supplied by the transformer core manufacturers.

author: magic
author_rank: Super Contributor
timestamp: August 13, 2022, 05:49:27 am
content: Quote from: abdulbadii on August 12, 2022, 07:41:50 amFlyback converters, for example, employ half-wave rectification, conducting alternately on the primary and the secondary, so input and output pulse amplitudes can be varied independently of each other.