Reliability of buck converters [LM2596 & MP1584EN]

Thread Starter

leolai88

Joined Nov 23, 2016
2
I am making an external portable "powerbank" for camera (rated @ 9V, 18W) using
1. DC coupler - http://www.ebay.com/itm/302072143563
2. 11.1V 3S2P Li-ion cells - extracted from a new laptop battery module
3. Battery protection board with balance control - http://www.ebay.com/itm/262714444457
4a. Buck converter with LM2596 - http://www.ebay.com/itm/221920170517
4b. Buck converter with MP1584EN (preferred option, since it is much smaller) - http://www.ebay.com/itm/122054336107

Datasheet: MP1584EN, LM2596

My questions are:
1. Are those buck converters reliable to output smooth constant voltage without any unexpected voltage spike?
2. Which voltage regulator is better in terms of stability and reliability?
3. Is there any overvoltage protection can be done?
4. Is there anything that I need to be careful about? since I am dealing with expensive camera

Thanks.
Leo
 

AlbertHall

Joined Jun 4, 2014
12,187
I can't comment on the rest, but for overvoltage protection, the best is a crowbar. When the voltage exceeds a limit an SCR is triggered to short circuit the supply and blow the fuse. Something like the circuit below with values adjusted for 9V+
 

Dodgydave

Joined Jun 22, 2012
10,591
I have used the Lm2596 loads of times, they're good stable output, has inbuilt thermal shutdown for shorts, the pcb needs an heatsink over 1.6Amps, you can get it in a 5amp version.

The Mp1584 runs at 1.5mhz which is better for reducing emi, whereas the Lm2596 is at 150khz.
 
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Papabravo

Joined Feb 24, 2006
19,576
The chips are probably the least of your worries. In order of importance I would be concerned about:
  1. The magnetic and current handling capabilities of your inductor.
  2. The quality and ESR of your capacitors
  3. The amount of copper, the length, and width of your PCB traces.
Please tell me your are not building one off on a proto board with point to point wiring.
 

ian field

Joined Oct 27, 2012
6,536
I am making an external portable "powerbank" for camera (rated @ 9V, 18W) using
1. DC coupler - http://www.ebay.com/itm/302072143563
2. 11.1V 3S2P Li-ion cells - extracted from a new laptop battery module
3. Battery protection board with balance control - http://www.ebay.com/itm/262714444457
4a. Buck converter with LM2596 - http://www.ebay.com/itm/221920170517
4b. Buck converter with MP1584EN (preferred option, since it is much smaller) - http://www.ebay.com/itm/122054336107

Leo
A buck converter is probably no less reliable than most other topologies. It usually comes down to the reliability of the solder joints.

The thing to remember about buck regulators is; if the series switch transistor fails SC - it dumps the full unregulated voltage into the load.

Early CRT VGA monitors used a buck regulator to cater for the different B+ requirements in different horizontal resolutions - buck failure was not exactly rare, and it usually wrecked the horizontal section.

Later models used a lower source voltage and boosted it with a flyback converter - if the transistor failed; it just tripped the main PSU shutdown.

Flyback is the safe way - if you need step down; use a transformer.

The alternative is; protect the load with a crowbar SCR - if you're charging a battery; isolate it with an anti discharge diode so crowbar activation doesn't endanger the battery.
 

Thread Starter

leolai88

Joined Nov 23, 2016
2
The alternative is; protect the load with a crowbar SCR - if you're charging a battery; isolate it with an anti discharge diode so crowbar activation doesn't endanger the battery.
Thanks for the advice, I think I will go with crowbar method with buck converter, seem to be easier.

By the way, mind to explain more about anti discharge diode? Thanks
 

MrAl

Joined Jun 17, 2014
9,633
Hi,

Yes the crowbar circuit mentioned in post #2 is a common way to make sure nothing expensive gets fried. I've seen this used in very expensive computer systems so that if the power supply acts up there is some secondary protection. Secondary protection is always recommended for expensive stuff. Testing is of course mandatory though, where you force the crowbar to react in order to make sure it works.

I've used the LM2576 which is similar to one of your chips and it's been running for years now.

One thing you have to look at with all converters is how the output reacts during initial turn on. Some converters may output a large surge during that time before they start to regulate. The best chips have a slow start mechanism to avoid this. You should make sure yours has that or it will keep triggering the SCR crowbar protect circuit.

Solid caps are also better than standard electrolytic caps. The standard ones degrade over time and then dont filter very well and have to be replaced. Solid caps have sort of become the defacto standard for computer motherboards too.

Part of the stability of a design like this depends also on the load transients encountered. For large transients, the inductor may be made smaller so that the natural response of the LC tank is minimal.

However, there would be no time when a circuit like this would be used for an expensive item without thorough bench testing. That includes but not limited to line transients, load transients, no load to full load testing, and a 24 hour full load infant mortality test. Without some testing any design no matter how simple could end up blowing out something much more expensive.
 
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ian field

Joined Oct 27, 2012
6,536
Thanks for the advice, I think I will go with crowbar method with buck converter, seem to be easier.

By the way, mind to explain more about anti discharge diode? Thanks
If the charger feeds into the battery via a diode - the current can't go back the other way when the crowbar fires.

A fully charged lithium battery contains a lot of energy and could well be warmed up some way toward critical temperature - letting it all go at once is likely to cause a bit of excitement.

If you're going to use a crowbar SCR - you have to include a diode so it only shorts the charger and NOT the battery.

The diode Vf means you may have to calibrate the output voltage to take that into account - terminal voltage is critical on lithium cells.

Crowbar pretty much dictates the use of a fuse - you can design the PSU with overcurrent shutdown, but if the buck transistor fails SC; that doesn't mean zip.
 

AlbertHall

Joined Jun 4, 2014
12,187
To avoid the diode drop you could use two fuses - charger to crowbar and crowbar to battery. Then the crowbar when activated would blow two fuses but it is only supposed to happen in extremis and is definitely better than the lithium exploding.
 
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