Hacking Chinese TL494 supply for battery charging (CC CV) - Page 1
Forum section: electronics / projects

author: ttyridal
author_rank: Contributor
timestamp: March 24, 2019, 12:21:37 pm
content: Hi,I have one of these TL494 based supplies laying around and was thinking about modifying it to do battery charging.The supply is already reverse engineered here:http://imajeenyus.com/electronics/20151028_smps_variable_voltage/s-400-12_supply.pdfNow, what worries me a bit is this passage from the application note (http://www.ti.com/lit/an/slva001e/slva001e.pdf)QuoteSo, the current limit, as implemented, will potentially switch the outputs multiple times during a period. Is that going to be a real problem?(and why is this not considered a problem on the voltage feedback?)Alternatively, I was thinking about replacing the TL494 with a mcuBad Idea?

author: Benta
author_rank: Super Contributor
timestamp: March 24, 2019, 02:04:02 pm
content: Using the TL494 in CC mode was one of the most painful experiences in my life (first job just out of university).There is a failure mode in CC operation that is in no way obvious and cost my employer quite some money back then (80s). We had to buy a storage 'scope (very expensive in those days) to track it down.The TL494 has two error amps, one for voltage feedback, the other for current feedback through a sense resistor in the ground lead. We tested the design extensivley, both CV operation and CC operation was perfect and very precise, loops were stable and the converter ran like a dream.After a couple of months, units with blown power transistors began coming back from the field.The problem was, that because the CC error amp measures the DC output current, the sense resistor sits after the output cap. This means that if the converter output is short circuited, it goes into CC mode. So far so good. But when the recovering from the short, the output cap has to be charged from 0 V to operating voltage. And as the sense resistor is outside the loop, current through the output cap and transistor switch goes through the roof, also because the inductor saturates.We solved it using a current limiter at the switch transistor (just a Zener and a resistor) and transistors with larger SOA.If the TL494 had had dynamic current limiting, this would never have been an issue.

author: ttyridal
author_rank: Contributor
timestamp: March 24, 2019, 08:37:04 pm
content: That would be a problem, yes..Now, this circuit has the sense-resistor before the caps. It also has a short circuit protection that needs a power cycle to reset. The later could have given me trouble in CC mode, but I will be charging 12v batteries - so the output should be in 11-15v range, even when current limiting, thus not triggering the short circuit protection.I guess I would still see the problem you mention if I disconnect the battery while charging in CC phase.(fixed wrong link to schematics in first post btw)

author: coromonadalix
author_rank: Super Contributor
timestamp: March 24, 2019, 10:49:29 pm
content: I would use any specialised beattery chargers or proven circuits, i would not take any chances to overheat or overcharging a battery ? i've seen busted, exploded, overheated cells in batteries etc ... i had one who almost blew up in my face, it was slightly hot on the plastic case but hot as hell in the cells, the charging curve spec was bad.Your smps circuit would be a great supply for a charger circuit, but i would not tamper with it.Even adding a mcu would or could be a pain, sometimes the mcu could or can be too slow to react etc... you may end with designs issues and safety ?

author: ttyridal
author_rank: Contributor
timestamp: March 25, 2019, 01:25:30 pm
content: Thanks,I fully appreciate your experience, but can it really be that bad?A ready made charger has maybe 2-3 settings (sla, flooded, lifepo4) and there is no way to enter the spec's for a particular battery. As long as I closely monitor the voltage and current going into the battery I recon my charger would be no worse at following the curves.Also, this is marine/automotive batteries where the primary charge source is an alternator. Shouldn't be too hard to beat that setup with regards to stable voltage, current, absence of spikes etc.The critical point being able to keep the source under control at all times of course. And in that regards, I was looking for experience on constant current behavior on TL494 and the lack of latch behavior on the error amplifiers.For the record I am planning tomodifythe smps so it can have different current / voltage settings for the different charge stages, together with end of charge shutdown - if that wasn't clear from the initial post.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: September 17, 2020, 07:01:24 pm
content: I am thinking to build up 40/36 v 10A batery( Lithium, Acid or whatever) charger used in Indian auto rickshaw or e scooters. I need to know the protection first. Not sure SMPS or flyback or any other topologies could work. MCU base could be very expensive to start with. If you have any experience you can share.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2020, 11:08:29 pm
content: It's not everything I'd like to see but it has the basics: average current mode control, shared with voltage mode control. This will control output current, and as mentioned, it's before the output caps so it's there all the time, no startup issues. (What would be nice to have, is cascaded loops, rather than wired-OR loops. That way, current controls PWM, and voltage controls current. You split the LC output filter in two halves, and can compensate each individually. No impossible two-pole compensation, no dependency on filter cap ESR. This can be added with an external op-amp, rather annoying but certainly still doable here.) On top of that, there's the startup circuit, which is powered by peak voltage, so as long as the transistors are switching at all, the controller is limited to low duty until the output voltage comes up.Which, maybe isn't great, a hiccup circuit might be better. The low throttle startup might get stuck on a resistive load, for example. Maybe not, dunno. I can't see at a glance what duty it can go down to, or if it'll be unstable (so the startup is more in surges, kind of a jank hiccup mode). I don't see how it could possibly be latching though.Also, delivering much current at low duty cycle can dissipate a heck of a lot of power. If it's always in a startup condition, it might not be very fail-safe.This should be pretty easy to check on a real unit, though.As for battery charging, what battery? A basic CC/CV to cutoff (good for lead acid and most lithium) only needs proper settings for the respective limits, and a charge controller which merely consists of a hysteresis comparator (probably with a current sense amp, since the cutoff will be a relatively low current). You may want a selectable shunt or CC setting to allow for varying size batteries. More complicated cycles (NiMH for example) you may want a proper controller, just to manage the dV/dt or temperature sensing or whatever is needed.Tim

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: September 18, 2020, 04:24:04 am
content: Hi T3sl4co1l!Your explanations are great, seems like you have practically use TL494. It's been always a challenge to control current and voltage at the same time. I haven't go through TL494 datasheet, interesting to see how it deals with CC or CV. Does it control current first? Then you said voltage helps the current? What's the benifit to use cascaded loops? Looking at the primary and secondary winding arrangements for protection is understandable. Dont you think shorter duty current can be controlled? Self starting winding is problematic?Cant we think an anti spark circuit at the output?At last for battery voltage level indicator or 12V fan does not help any thing? Shunt sansing cant be possible here?Without MCU , what solution you can predict?