Why do offline switchers in flyback dc-dc circuits require bias/aux winding? - Page 1
Forum section: electronics / general

author: triedgetech
author_rank: Newbie
timestamp: February 16, 2021, 07:11:11 am
content: I've looked at different offline switchers, and the typical application schematics all suggest to use additional secondary winding for powering IC only, usually called "aux/bias". Why? Why can't just those ICs use the secondary winding that is the output?For example, let's look at this Viper22A flyback circuit:Vdd: 9V <-> 38V.So it can be supplied with 12VDC.Why can't you just use the single secondary winding, which will be used both for output power and to power IC? There's galvanic isolation between secondary winding and a primary one.Why do you need additional, secondary winding called aux/bias winding? Seems wasteful to me.Is this somehow related to the fact that offline switcher should be powered up from a winding that doesn't have any load? E.g. you don't have high current drawing from that winding??

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2021, 07:39:11 am
content: How pray tell will you power a mains-referenced regulator from an isolated secondary winding?Gotta be isolation somewhere!Now, there are some ways around it:1. It can be fully self-powered, like the TNYxxx's and others. These are small and inefficient (erm, >70%? I actually can't find a typical figure offhand, weird), but that's acceptable for small loads (a few watts).Note that the Viper does this at least for startup -- there's no resistor from V+ to pin 4 to start it, as others might need. Erm, assuming the above schematic is correct, I haven't checked, probably fine.2. It can be independently powered -- a chicken-egg problem of course, and usually would just be a cost adder. Now, sometimes you need another power source anyway, like for standby to operate a "soft power" button. Might as well tap off some of that for the main switcher, eh?3. When it can be fully secondary powered, and some other means provides isolation, gate drive transformer for instance. Ye Olde ATX PSU was done this way, where either an aux supply provides 5V for the motherboard and either the same 5V, or another voltage, runs the TL494 or similar controller (in other words, the situation of #2).The other way was, the power inverter would initiallyself-oscillate,then once secondary power comes up, the controller kicks in and throttles it down. This was relatively easy to do with the BJT switching transistors used. The base drive transformer, with positive current feedback to assist driving, was key to this dual mode startup. Similar circuits are (were? heh) used in CFL lights, without the regulator of course.Tim

author: Berni
author_rank: Super Contributor
timestamp: February 16, 2021, 08:11:16 am
content: Why can't you power the switcher IC from the output windings?Because the switching IC is connected to the primary side, if you also connect it to the secondary side then the transformers isolation is made useless by the chip being connected to both sides. Chips don't have magical galvanic isolation inside. See how pin 1 of the chip is connected to the mains side GND?But but.. there is also R10 and C12 connected to both the output GND and mains side GND! Yes those are there to give high frequency switching noise a return path and to keep the output common mode close to mains to prevent the transformer isolation breaking trough. The value of both of those is chosen so that only a harmlessly small amount of current can flow between mains and output, making the output still safe to touch.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 16, 2021, 01:23:02 pm
content: In some designs, the aux winding is also used for feedback so the optoisolator circuit is not needed.

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2021, 08:35:16 pm
content: The auxiliary secondary winding allows the primary side circuits to efficiently power themselves while leaving the secondaries galvanically isolated from the primary.Quote from: T3sl4co1l on February 16, 2021, 07:39:11 amThe early Tektronix switching power supply used in their 7000 mainframe and 875 portable oscilloscopes did something even stranger. The control circuits are isolated from the primary side switching transistors with transformers as expected, but are powered by the primary side current sense transformer which also delivers the phase information necessary to control switching. This allows the control circuits to directly monitor the secondary output voltages but they are not powered by the secondary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2021, 09:11:09 pm
content: Quote from: David Hess on February 16, 2021, 08:35:16 pmHah, that's either a rather large CT or a carefully optimized (low power) control. (S'pose I should flip through the schematics some time just for kicks.)Also, just to round things out -- another semi-esoteric method was often used by Sony, in Trinitron monitors especially. The primary inverter was a self-excited oscillator with current feedback, and the secondary side controller varied bias current in the feedback transformer, which was a special multi-leg core being used as a saturable reactor. Thus the feedback transformer saturates sooner or later depending on control current, varying the switching frequency; a resonant load thus causes output voltage/power to be controlled. Only one IC needed, a TL431 (not counting any protection circuitry, that is).Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2021, 10:44:19 pm
content: Quote from: T3sl4co1l on February 16, 2021, 09:11:09 pmThe current transformer is about the size of the lid of a TO-3 Transistor. It is T1235 shownhere on page 19. T1230 is the base drive transformer.

author: ocset
author_rank: Super Contributor
timestamp: February 20, 2021, 06:35:26 pm
content: If you want you can scrap the aux coil and just power the controller via a LNK302 HV buck converter.But yes its common to use a aux coil. Yes you do have to watch it doesnt get loaded down too much.But as long as the aux coil gets up to operation voltage before the startup capacitor discharges too much, then you are ok. There is a "handover from the startup cap to the aux coil.Of course, you can use a cheap isolated DCDC module to power the controller from the secondary if you want. But most people tend to think the aux coil is the cheapest way of doing it.It can be a problem if the startup cap gets loaded down too much....because it wont power the controller for long enough before the aux coil kicks in...in such cases you can have an opto switched by a secondary side comparator, which only switches the load on to the aux coil when the sec (and therefore aux too) has got up to operation voltage...This stops you seeing the smps never start and the startup cap charging up and down repeatedly....down below the controllers start up threshold.Here is a free smps course for you, that gives a section on "startup of an offline smps"https://drive.google.com/file/d/0B7aRNbu3Fes4TU92Mkw3YlA3ams/view?usp=sharing