Constant current buck converter - Page 1
Forum section: electronics / projects

author: bjcuizon
author_rank: Frequent Contributor
timestamp: May 06, 2017, 09:40:44 am
content: Hi everybody, I wanted to connect my 19v laptop charger/power supply into a dc-dc buck converter. It has a current limiting feature and it is set by shorting the output. When shorting the output, does it overload my laptop power supply? Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: May 06, 2017, 01:41:35 pm
content: Relying on the overload protection to limit the current to a certain value is bad practise because:There's no guarantee the PSU will be able to reliably operate in constant current mode, for extended periods. It's designed as a protection system, rather than a mode of normal operation.The current limit might not be tightly defined. The tolerance could be very wide and stability poor.There is probably a large capacitor on the output, which will drastically reduce the output impedance at AC, which is bad for a current source, that ideally should have an infinite impedance. A very large current may flow (possibly as much as 100 times the rating) before the power supply goes into constant current mode.The correct way to do this, is to add a buck regulator circuit to the output of the laptop power supply.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: May 06, 2017, 10:50:00 pm
content: I'm sorry for not clarifying what I said. Yes I will add a buck converter (lm2596 based) to the output of the laptop psu. To set the buck converter's current limit, I need to short the output of the buck converter. My question is: Will the laptop psu overload when I will short the buck converter to set its current limit?

author: Zero999
author_rank: Super Contributor
timestamp: May 06, 2017, 11:20:16 pm
content: Quote from: bjcuizon on May 06, 2017, 10:50:00 pmThat's a bit difficult to answer when you haven't said what the current rating of the laptop PSU is. Hopefully it will include some form of overcurrent protection and won't be damaged if overloaded.Re-reading your original post:Quote from: bjcuizon on May 06, 2017, 09:40:44 amThat doesn't make and sense. Normally the current limiting feature is set when the circuit is designed, for example by choosing the value of current sense resistor or it might be fixed. Don't you mean that the current limiting feature is enabled when the output is short circuited or the set current limit is exceeded?A buck regulator doesn't draw the most current when its output is short circuit and it's operating in constant current mode. The input current is dependant on how much power is being derived to the load, which is close to zero, when short circuited. It will draw the most current when the output voltage is close to the input and the maximum current is drawn, just before it switches over to constant current mode.

author: langwadt
author_rank: Super Contributor
timestamp: May 06, 2017, 11:35:23 pm
content: Quote from: Hero999 on May 06, 2017, 11:20:16 pmI suspect he means like you would often do when setting the current limit on a bench supply, short the output turn the current limit pot until the ammeter shows the desired current limit

author: Zero999
author_rank: Super Contributor
timestamp: May 06, 2017, 11:42:22 pm
content: Quote from: langwadt on May 06, 2017, 11:35:23 pmI wouldn't use such a procedure. The bench supplies I use, all have a display for the current limit, so there's no need for short circuiting it with an ammeter.In the first post he mentioned that the power supply is LM2596 based. I had a quick flick through the datasheet and there was no mention of how to set the current limit: if there is, I missed it (I didn't read it thoroughly) but I suspect it's fixed. I suppose it's possible to make an LM2596 supply have a variable current limit by adding other components but no schematic was given, so I assumed it was the bog standard datasheet reference design.

author: richard.cs
author_rank: Super Contributor
timestamp: May 07, 2017, 09:06:50 am
content: It's one of those ebay LM2596 modules with 2 or 3 pots, right?*Generally I would expect you to be able to short-circuit it's output and draw minimal power from the laptop adaptor because when the output is short-circuit the output voltage is low, therefore the input current is low. It will be kinder to the module to short it's output before you apply power, that way you're not discharging the output capacitor into the short (and through your meter).*These often have a current sense circuit based around a PCB track and an LM358 op-amp feeding back into the voltage divider. Sometimes the other op-amp in the package is used as an output voltage comparator to light an LED for "battery charged" or similar., this will be the third pot if there is one.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: May 08, 2017, 08:03:29 am
content: Yeah those are the ones I found. And i'm trying to build a home made bench supply with it. Thanks for your reply.

author: Zero999
author_rank: Super Contributor
timestamp: May 08, 2017, 12:30:40 pm
content: Quote from: bjcuizon on May 08, 2017, 08:03:29 amIt should work then, as I said, a buck regulator will not draw much current, when short circuited.Do you have a datasheet/schematic/ebay link for the LM2596 module and a the full specification for the laptop PSU you intend to use?For future reference please provide as much information as possible, when asking questions on the Internet. If you'd provided more information about the buck module (e.g. a link to the shop you bought it from, in absence of a schematic) and laptop PSU (e.g. the make and model number, if the output current wasn't specified) then you would have had the answer on the first or second response to this thread.

author: james_s
author_rank: Super Contributor
timestamp: May 08, 2017, 06:19:19 pm
content: I used a module like that in a little bench supply I built around a 32V HP power brick. It works pretty well, although the current limit oscillates if you look at it on a scope. I haven't reverse engineered the circuit but I suspect they are driving the shutdown pin on the regulator with a comparator monitoring a sense resistor on the output. It's not as good as a proper linear bench PSU but it's a handy portable unit and the current limit is sufficient to keep the magic smoke in. You will want to glue additional heatsinks on the regulator and diode though if you want to use the full rated current. Those Chinese modules are mostly rated rather optimistically.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: May 10, 2017, 04:54:41 am
content: The buck converter is Lm2596 based and here is the link...http://www.ebay.com/itm/Boost-Buck-Voltage-LM2577S-LM2596S-DC-DC-Step-Up-Down-Power-Converter-Module-/322041999051?hash=item4afb32eecb:g:q~wAAOSwnbZYFurgThe laptop psu is a Targus 19.5V 90W power brick.Thanks.

author: james_s
author_rank: Super Contributor
timestamp: May 10, 2017, 03:48:09 pm
content: I would be shocked if you managed to overload a 90W power brick with that little module. Go ahead and try, if you do overload the brick it will simply shut down.

author: DBecker
author_rank: Frequent Contributor
timestamp: May 10, 2017, 04:34:55 pm
content: I have several of those that I use as 'bench' power supplies.Short-circuiting them is the recommended procedure to set the current limit. They draw only modest power while short circuited -- the freewheeling diode is providing almost all of the current.If you set the current limit after start-up you'll get a little surge from the capacitor discharge, but it doesn't seem to do damage. It's likely safer than the unpredictable behavior of a unpredictably-ramping source supply feeding the switching supply which is starting up into a dead short. But I've not killed a module with either approach. A reverse voltage connection, the other hand, will kill them instantly. As will pulling the wires so that a running board drops down onto trimmed-off component leads.As bench supplies, they are great for most things. They have some switching noise, but that's to be expected. They are switching supplies after all. You should be designing your stuff to work with a regular power supply, not a linear regulator with two filter stages.And, like many FleaBay modules, their ratings are a 'never exceed' valve, not a continuous operating point. Even if it says "continuous ...", they mean it has a chance of still functioning after 1 msec at that value. The 3A supplies are good for well over an amp, and the 5A supplies might work at 2A continuous. Anything higher and you should add a heat sink, use heavy wires to draw heat away from the connectors, and ponder your operating conditions i.e. conversion ratio so that power is split between the switch and freewheel diode.

author: Zero999
author_rank: Super Contributor
timestamp: May 10, 2017, 06:23:53 pm
content: Quote from: bjcuizon on May 10, 2017, 04:54:41 amThank you for posting that. Yes, it should be fine to short circuit it, without tripping the laptop PSU's current limit.Quote from: DBecker on May 10, 2017, 04:34:55 pmDo you have a schematic for the module?It contains both the LM2596 and LM5577 and the ebay link says it can both step up and step down. Is it something you have to set i.e. whether it's a boost or a buck converter or does it do it automatically?If it's automatic, I'd be interested in how it decides whether the LM2596 or LM2577 is activated.How does the current limit part work? Is it stable or does it oscillate as james has experienced?

author: james_s
author_rank: Super Contributor
timestamp: May 10, 2017, 06:32:35 pm
content: I'm betting they're just in series. First it steps up the voltage to some preset value then it steps down the voltage to the desired output.

author: Zero999
author_rank: Super Contributor
timestamp: May 10, 2017, 10:26:19 pm
content: Quote from: james_s on May 10, 2017, 06:32:35 pmPossibly but that would be very inefficient.

author: james_s
author_rank: Super Contributor
timestamp: May 11, 2017, 12:38:05 am
content: Sure but what do you expect of a $3 module out of China? It's not going to be top notch in any way, I can promise you that.

author: bjcuizon
author_rank: Frequent Contributor
timestamp: May 11, 2017, 03:37:24 am
content: Thanks for your replies guys. I should probably start building it as I'm now assured I will not overload my laptop psu.