How to limit output current on switch mode power supply? - Page 1
Forum section: electronics / repair

author: carl_lab
author_rank: Frequent Contributor
timestamp: October 24, 2016, 02:35:27 pm
content: I have some small notebook-power-supply-like SMPS I want to use to charge 48V LiIon batteries.These batteries have internal battery managment systems for safety.I need a voltage of 13*4.2V=54.6V and limited current of about 1A (constant current mode).I know how to change output voltage of SMTP (by changing a voltage divider in feedback loop) to fit the 54.6V I need.But:When I connect the power supply to battery, current is about 2A for about a second, then power supply switches off (because 2A is over spec.).I need to add some circuit, that limit output current to about 1A.Can someone show me a circuit diagram of a current limiting (constant current) SMPS?I think LED light power supplies may be suitable, but I don't want to buy new power supplies, I just want to use these ones I already have.The additional circuit maybe a shunt resistor, OP amp, reference Z diode,... something similar this way, that controlls the opto coupler in feedback loop of SMPS.Could this work?

author: Zucca
author_rank: Unknown
timestamp: October 25, 2016, 09:23:20 am
content: Can you tell us which IC is using this SPS? I would look into the datasheet of that IC a as a starting point, normaly the key note or the circuit examples are very useful. If you are lucky there is an example on how to limit the current...

author: bktemp
author_rank: Super Contributor
timestamp: October 25, 2016, 09:35:55 am
content: Most notebook power supplies output around 20V. Going to >50V does not work very well, because of either the fixed turns ratio of the transformer (for forward converters) or the switching transistor will also see a much higher voltage and fail (for flyback converters).You circuit could work, but may require a compensation network for loop stability. Instead of connecting R3 to Q1 you could also connect it directly to TL431.

author: carl_lab
author_rank: Frequent Contributor
timestamp: October 25, 2016, 11:04:09 am
content: Quote from: zucca on October 25, 2016, 09:23:20 amI did not open the power supplies yet.But yes, you're right, datasheets are helpful mostly.Problem is: I need a constant current limiter but the power supply shuts down if current exceeds limit (foldback mode):Quote from: bktemp on October 25, 2016, 09:35:55 amIt's actually 48V power supplies. So additional 4-5V should be not the problem."notebook-like" refers to formfactor/case.Quote from: bktemp on October 25, 2016, 09:35:55 amNo. I don't think so.Q1 works as an inverter.If current exceeds limit, opto's LED will shine to stop oscillation of the primary oscillator (same as in over voltage case).

author: bktemp
author_rank: Super Contributor
timestamp: October 25, 2016, 12:00:02 pm
content: Quote from: carl_lab on October 25, 2016, 11:04:09 amTL431 also works as an inverter:When the voltage at its input goes over 2.5V it turns on. Q2 simply simulates overvoltage at the feedback point when the current is too high.Since there is already a compensation network around TL431, it may be easier than adding a completely seperate control loop.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2016, 10:36:38 pm
content: This may be of interest?It's made for CC operation primarily, with CV as a protection feature (hence, poor regulation in CV). But the lesson should be easily applied either way.Youdefinitelydon't want two transistors cascading, collector to base, I mean, unless "astable multivibrator" is what you're going for there!Tim

author: carl_lab
author_rank: Frequent Contributor
timestamp: October 26, 2016, 08:12:35 am
content: Quote from: bktemp on October 25, 2016, 12:00:02 pmQuote from: T3sl4co1l on October 25, 2016, 10:36:38 pmOK, then this way should work:

author: carl_lab
author_rank: Frequent Contributor
timestamp: October 26, 2016, 08:21:54 am
content: Quote from: T3sl4co1l on October 25, 2016, 10:36:38 pmI think for Li-Ion battery charging stability of max. voltage is much more important than stability of charging current.Limiting the current is primary for overload protection of charger and battery.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2016, 02:29:28 pm
content: Quote from: carl_lab on October 26, 2016, 08:12:35 amNot quite.Actually, that may be even worse, in a sense. The TL431 acts like a transistor with a low fT, high hFE, and very stable Vbe (~2.50V). The low fT means, not only will it still oscillate -- it will do so at a frequency where the power supply is hammering on and off, hard!Managing gain and phase shift is the important part here.Note that Vin can't be pulled any lower than 2.5V (if R3 = 0), or in general, whatever the (R3 || R5) + R6 voltage divider comes out as. If you don't need the current limit to work effectively down to low output voltages, then no problem. But then, if such a crude limit is acceptable -- would it be acceptable to implement this on the primary side instead?Tim

author: carl_lab
author_rank: Frequent Contributor
timestamp: October 26, 2016, 05:40:44 pm
content: I want to charge 13-cell-Li-Ion-battery-packs, so 13*3V = 39V is the lowest battery voltage if fully discharged.

author: oldway
author_rank: Super Contributor
timestamp: October 26, 2016, 07:24:31 pm
content: Why not only change the output voltage of the SMPS (that's easy) and add an external linear current limiter (also very easy, 2 pnp's transistors with heat sink, a 0R6 resistor more two other resistors).Power dissipation will be max. 15W, that's acceptable.

author: carl_lab
author_rank: Frequent Contributor
timestamp: October 26, 2016, 10:51:15 pm
content: Quote from: oldway on October 26, 2016, 07:24:31 pm15W of heat extra is 15W too much. The power supplies is fanless design.Tomorrow I'll break the case open of one of the power supplies, we'll see...

author: carl_lab
author_rank: Frequent Contributor
timestamp: October 29, 2016, 03:07:23 pm
content: Quote from: zucca on October 25, 2016, 09:23:20 amDAP008http://fa.itservice-bg.net/baza/ic_power/sm_data/DAP008_data.pdfI fear, there is no TL431, so voltage regulation will be not that good...But there are two opto couplers in my device, I'll try to retrace the circuit diagram.

author: Zucca
author_rank: Unknown
timestamp: November 02, 2016, 08:56:11 am
content: Just skimmed the Datasheet.Quotethat's your problem.QuoteQuoteNot bad for charging batteries. On the other side it looks to my poor eyes that the output voltage is set by using the trafo on the right side. The rest is current business.Interesting.Can you give us some board pics?

author: xavier60
author_rank: Super Contributor
timestamp: November 02, 2016, 10:10:51 am
content: Quote from: carl_lab on October 26, 2016, 08:12:35 am

author: xavier60
author_rank: Super Contributor
timestamp: November 02, 2016, 10:28:55 am
content: placing a capacitor between B and E of Q2 will slow the current loop response more. R3 needs to be kept high to limit the voltage range the current loop has control over.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2016, 04:39:17 pm
content: Even better is an R+C from base to collector. The series collector resistor R3 generates a proportional voltage drop, so that there is an "output" voltage from the transistor to draw feedback from (otherwise, the REF pin will have little voltage change on it, being centered at 2.50V for obvious reasons). The series base resistor R2 allows feedback to be applied; otherwise the "input" (base) impedance would be shorted out by R1, making feedback impossible to apply.Likewise, C1 should be an R+C. In both cases, the R+C is a pole-zero network, that significantly increases the phase margin when dimensioned properly.The C1 response will stack with the Q2 response, making phase margin worse for current regulation than for voltage regulation. You'll need to test actual values in circuit, and play around until both operating modes behave well (i.e., step load transient testing).Tim

author: carl_lab
author_rank: Frequent Contributor
timestamp: November 02, 2016, 07:29:39 pm
content: Quote from: carl_lab on October 29, 2016, 03:07:23 pmHere is it:

author: carl_lab
author_rank: Frequent Contributor
timestamp: November 02, 2016, 07:48:28 pm
content: Quote from: T3sl4co1l on November 02, 2016, 04:39:17 pmSo, this is what you mean?BTW:What is the 2nd feedback loop for (ZD54, ZD55, IC3 -> ADJ pin)?Is it just a redundant over voltage protection?Which resistor has to be changed for voltage adjustment? R55+R58 (one of these)?What is D51 for?For temperature compensation of Q51?