push-pull SMPS datasheet mistake ? [TL494CN]   [SOLVED] - Page 1
Forum section: electronics / beginners

author: Mate_Well
author_rank: Contributor
timestamp: April 01, 2020, 09:21:35 am
content: For a project i will need to step up voltage from 12v battery (to respectively +17 v and - 17v rail) i'm left with creating smps becouse of this, found an ic that has all features (if not more) that i need.The ic is TL494CN (datasheet :https://www.gme.cz/data/attachments/dsh.322-001.1.pdf) i have tryed to understand the basic functions, calculated most of the component values, transformer etc....Either i understood it's function wrong or there is mistake in the datasheet, i feel like the bases of the stransistors should not be conected together (since it switches only 1 of them at time, that part is also "highlighted" in light red)in attachments there should be 2 pictures (scheme from datasheet, and one that i used for understanding it)any help would be appreciated <3, don't know if i should trust datasheet or my gut feeling in this case

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2020, 10:02:16 am
content: Yeah, obvious goof.Much better anyway to use N-MOSFETs; even the contemporary IRF540 will perform better than the TIP32s shown. The gates will need a bit of a driver; the TL598 can be used directly, or a dual gate driver IC added.Here's a basic scheme:Notable improvements that should be made:- Note that the output is stacked on top of (i.e., in series with) the supply; this saves on capacity, but also means the output can never be unpowered (even when the converter is disabled -- if you need that as an option -- 12V flows out through the diodes). This connection is easily enough changed to a regular common-ground secondary, like shown above.- And of course, for +/-17V, you'll need a FWB and two chokes, instead of a FWCT rectifier and one choke. (The secondary CT will be ground, no choke there.)- The snubber is awfully aggressive (the 50 ohm 5W), probably that much won't be necessary. Depends on the transformer's leakage inductance, which depends on how it's wound.- Current feedback can be compensated by bringing an R+C from pin 3 to pin 15, and reducing the capacitance on pin 16. Probably as shown, it would oscillate or chatter when current reaches the threshold; probably not destructive, but not ideal.- Even more preferable, current feedback should be used as a primary feedback option, and a separate error amp used for voltage feedback. This changes the controller to average-current-mode control, a dramatic improvement over the voltage-mode control shown.- The capacitance between pins 2 and 3 needs to be an R+C as well. Typical starting value 100k + 10nF and adjust up or down from there until good stability and load step response is found.- An R+C from "VFB" to pin 1 may also be desirable, particularly for voltage-mode control (combined with the R+C between pins 2-3, this is called lead-lag compensation).- Note this shows how to use a TL494 with a discrete gate driver. Using a TL598, the 1k pulldowns on pins 9 and 10 are not necessary; the drive diodes and 2N4403s may not be necessary either.- If isolated output is required, that "VFB" connection won't be possible; instead, you need an optoisolator with a TL431 or similar voltage reference / error amplifier.- Depending on your source, protection may be desirable: transient protection, additional filtering, UVLO/OVLO, "hiccup" current limiting, overtemp, etc. Some of these can be added on easily (e.g., TVS diodes, filter caps and chokes), others require additional circuitry; the main downside is the circuit simply grows as you tack on more features. At some point you're probably better off using a more modern controller that contains all these features.- And for that matter, if you're not drawing much current from it after all, you may be better off with a flyback type supply, even a ready-made off-the-shelf converter/isolator module.As for transformer winding, it's worthy of its own topic, really.Tim

author: Mate_Well
author_rank: Contributor
timestamp: April 01, 2020, 10:36:19 am
content: Huge thanks <3the part behind secondary will have to be redone, i will use 2 FWB with both chokes (and "big" capacitors), i will use the overcurrent preatty much as only hiccup mode (if the amplifier part that will be behind, get's shorted or something like that) the plan for the project is learn a lot, and ideally also make 25~40W (x2, one for left and one for right chanel) car amplifier, primary will have to handle somewhere around 5~6A (worst case)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 02, 2020, 03:57:34 am
content: Only need one FWB, no need for extra voltage drop or extra transformer windings. FYI.Tim