SMPS transformer thought experiment. - Page 1
Forum section: electronics / beginners

author: Milmat1
author_rank: Contributor
timestamp: July 15, 2025, 02:45:16 pm
content: Hopefully I can explain this well enough. Looking a a transformer circuit such as in a SMPS.The primary winding current is DC being switched on and off at say 100khz. OK this produces the relative motion required for electromagnetic induction. The pulsed DC through the primary winding is always the same polarity. Positive going pulses at frequency.So isn't the induced secondary current is also always the same polarity. So you get a pulsed output of a different voltage at the same frequency, but it too is always positive going.Why does it then need rectified by diode(s) ? Wouldn't a simple capacitor filter smooth this into a decent DC ?(these are the stupid thoughts that keep me awake at night....)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 15, 2025, 03:22:13 pm
content: I advise you stop with thinking your questions are stupid. Firstly it's not a supid question at all. Second, when someone starts by saying they have a stupid question, then I usually immidately close the thread, because I have no intention of wasting my time on reading stupid questions. The world is too short for that.And concerning your question... It is quite common, espcially for small SMPS circuits to only have a single rectifier diode on the output coil. See for example the attached schematic.But there are disadvantages too. When the magnetic core is only magnetized in one direction, you use only half of it's capability. So for big power supplies, where the ferrite core is substancial, this would make it more expensive. But for a small SMPS the simplicity and the low part count offsets the extra cost of the bigger ferrite.

author: Slh
author_rank: Regular Contributor
timestamp: July 15, 2025, 03:41:42 pm
content: I guess that you're looking at flyback or forward converters? There's lots of isolated SMPS topologies.In flyback converters, the transformer/coupled inductor/"whatever else people like to call them " stores energy when the switch turns on. It is then released through the output winding and diode when the switch turns off. If the diode isn't there then it will act just like a transformer.Forward converters are more like a buck converter with a transformer as the input source.In both cases, assume that the output capacitor is charged and a no leakage inductance transformer. The voltage across the transformer output winding is now the output voltage. Essentially, the transformer will always have a fixed DC voltage on it. Sadly, transformers hate this! Well engineers hate this anyway, transformers aren't sentient. The transformer magnetising current will build up until the transformer core saturates and it stops acting as a transformer. This is likely to happen in a few cycles.When it saturates, the transformer inductances will plummet to that of air coils and the coupling factor will plummet as well so where previously most of the energy was transferred, now almost none of it is. Also, your switch will probably go pop.

author: xvr
author_rank: Frequent Contributor
timestamp: July 15, 2025, 03:48:38 pm
content: QuoteThat is correctQuoteThat is wrong. If you omit diode that current flow will alternate. Do not forget about output capacitor. It will keep output voltage for some value (for load), and when voltage from transformer will be less that one of capacitor you will get reverse current flow.P.S. This is a very simplified and incomplete description of how SMPS works, but it is still correct for output circuit.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 15, 2025, 03:59:31 pm
content: Quote from: Milmat1 on July 15, 2025, 02:45:16 pmFaraday's law says that it is not possiblehttps://en.wikipedia.org/wiki/Faraday%27s_law_of_inductionYou can apply pulsed DC, and yet the DC component on the coil will be 0V. Faraday's magic

author: Milmat1
author_rank: Contributor
timestamp: July 15, 2025, 04:27:15 pm
content: THANK YOU ALL!!That is what I love about asking questions here. Straight honest answers without ridicule or smart comments etc.THANK YOU!

author: Milmat1
author_rank: Contributor
timestamp: July 15, 2025, 04:30:38 pm
content: Quote from: Doctorandus_P on July 15, 2025, 03:22:13 pmYes the schematic is exactly what I was thinking of. I had not thought it through about the capacitor discharging.Thank You!

author: CaptDon
author_rank: Super Contributor
timestamp: July 17, 2025, 10:42:24 pm
content: Without at least a single diode the capacitor would always be shunted by the D.C. resistance of the secondary coil and would appear to be shorted.