Transformer primary multiple winding - Page 1
Forum section: electronics / beginners

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 10, 2019, 07:18:16 pm
content: Why this transformer has two separate windings on the primary side?As far as I know, if they used for 110V or 220V selection, they should have to be connected about in the middle of the same winding, but these are totally separate between each other (infinite resistance). The resistance between connected pairs is about 0.8 Ohms for both of them.Regards,

author: jmelson
author_rank: Super Contributor
timestamp: September 10, 2019, 07:28:06 pm
content: Most likely, you connect the outer two pins to your mains, either 120 or 240 V. For 240, you connect the two inner pins together. For 120 V, you connect each inner pin to the outermost pin near it. It is done this way to make the bridging jumpers simplest on a PC board or terminal block.Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2019, 07:54:40 pm
content: That looks more like a ferrite core transformer from an SMPS, in which case the two windings are either the primary, and a "reset" or auxiliary winding.Measure the turns ratio using a function generator set at 100kHz+.Tim

author: FreddieChopin
author_rank: Regular Contributor
timestamp: September 10, 2019, 07:59:06 pm
content: Quote from: T3sl4co1l on September 10, 2019, 07:54:40 pmJudging by the large and thick isulated prong on table I'd say it's more likely a HV trafo from old kinescope TV.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 10, 2019, 07:59:58 pm
content: Quote from: T3sl4co1l on September 10, 2019, 07:54:40 pmYep, and if you look real closely at the wire terminations you can see that one pair uses a much thinner gauge of wire than the other, so almost certainly a reset winding for a forward transformer. Assuming the turns ratio is 1:1, that is*.* - it's technically possible to use a non-unity turns ratio, to, for example, extend the duty cycle range at the expense of subjecting the switch to a higher reset voltage, but I've never seen this done commercially.

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 11, 2019, 11:03:07 am
content: Quote from: T3sl4co1l on September 10, 2019, 07:54:40 pmTim,Yes, you have right, this part is from an SMPS.Question: What's the purpose of the "reset" winding and how this affects the transformer and SMPS itself?Another Question: You write, "Measure the turns ratio using a function generator set at 100kHz+." I know the input and output voltage. In=220V and Out=12V so the turns ratio is = 220/12 = 18.3. So, why and how should I measure the turns ratio with Function Generator and why specifically at 100Khz+..... Sorry for the tons of questions !!Regards,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2019, 02:36:34 pm
content: Not quite! The output voltage has to be higher, because it's pulsed and filtered by an inductor (which you should also have, and a dual diode, or two loose diodes).The inductance of the transformer is fairly low (~mH primary, ~uH secondary) so you need a high frequency to do much with it; it's not like you can plug it into 50Hz mains and do anything but blow a fuse!Tim

author: ArthurDent
author_rank: Super Contributor
timestamp: September 11, 2019, 03:35:31 pm
content: Quote sdancer75 - “I know the input and output voltage. In=220V and Out=12V…”That is not correct. The input to the SMPS may be 220V but that voltage is rectified to maybe 340VDC then a PWM controlled power oscillator circuit feeds the ferrite core high frequency transformer at perhaps 100Khz with some varying voltage. Using high frequency allows a small light ferrite core transformer to be used and ripple filtering is much simpler.A linear supply of the same rating would have a much more massive iron core transformer that would have a 220V line frequency input directly to the primary winding but this isn’t a linear supply.https://www.autodesk.com/products/eagle/blog/linear-regulated-vs-switch-mode-power-supply/

author: sdancer75
author_rank: Frequent Contributor
timestamp: September 11, 2019, 03:41:26 pm
content: What's the role of the reset winding?

author: mikerj
author_rank: Super Contributor
timestamp: September 11, 2019, 03:58:05 pm
content: Quote from: sdancer75 on September 11, 2019, 03:41:26 pmIn a forward converter the reset winding is used to reset (i.e. zero) the magnetic flux in the core when the primary winding is turned off. Without this feature the residual flux would increase with each cycle until the transformer saturates (which is bad news in a switching converter).The additional winding may not a reset winding though, it depends on the topology of the converter. Some designs have an auxiliary winding which is used to power the main controller IC.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2019, 04:04:43 pm
content: Again, if indeed it was a forward converter (which sounds likely at this point) -- then, the switch and primary are arranged much as Arthur's schematic above. But that's a flyback supply: note that, when the switch turns off, the primary voltage (the non-dotted side of the winding) shoots up (flyback), and the secondary voltage (non-dotted) shoots up in sync, until it's clamped by the output diode.In a forward converter, the output filter is a choke-input type, which means there's no clamp on the inductor voltage. The secondary is phased oppositely, so that the diode conducts while the switch is on, charging the inductor. (The inductor maintains its current during off-time, with a second clamp diode from GND.)That means the transformer's flyback is unconstrained, which will destroy the switch.A note about the transformer itself: in the flyback supply, the transformer is made with a low inductance (it's better thought of as several coupled inductors; inductors store energy, transformers merely transform it). In the forward converter, the transformer is made with a high inductance, so as not to store much energy, and this means the flyback pulse tends to be weak, only delivering a few watts perhaps. (The excess voltage alone is enough to destroy the transistor though.)There are three typical ways to handle the transformer flyback:One is a clamp diode into a capacitor. This acts like a boost converter on the primary side, creating a high voltage supply (say 600V). The supply is just a few watts, and isn't useful for anything else, so we burn it off in a load resistor. This is called an RCD clamp snubber: it uses a resistor, capacitor and diode, and the capacitor is relatively large, so that it acts to clamp the voltage (the change in capacitor voltage is small during a cycle). (This would be D1-C4-R8 in the above schematic, except with a larger value for C4.)Another is to add an auxiliary winding on the transformer, which we can connect to one supply rail (GND or +V) on one side, and with a diode to the other rail. Now the flyback is clamped at exactly +V, just as in the above schematic, but instead of delivering useful output power or burning it in a resistor, it is "stirred" or recycled back into the supply, costing minimal losses. Great, huh?The last is to use two switches and a single winding. This seems over-the-top, but semiconductors as a group are one of the cheapest parts of a power supply design, so we can afford to use more of them if it offers other savings. In this case, we can switch the top and bottom ends of the primary winding, so that the whole winding itself is reversed in polarity during the off cycle -- it serves as its own reset winding, greatly simplifying the transformer design.This is called "2-switch forward", and also works for flyback with the bonus that the transistors can never be over-volted by a suddenly open-circuit load, and that the transformer's leakage inductance is safely controlled (and indeed recycled), whereas the energy stored in leakage is usually just burned off in a one-switch flyback (which is the intended function of D1-C4-R8 in the above schematic). The downside is, the flyback voltage obviously can't exceed the supply voltage, so some adjustments need to be made relative to the traditional flyback design (a somewhat lower nominal duty cycle).Tim