Buck Regulator not Reaching Output Voltage

[Frenchie]

I'm struggling a bit with a switching pre-regulator design I'm working on. It's designed to track ~3V above the ouput voltage level and is based on the design Dave demonstrated in blog #329. All is working well at the lower end of the output range, it's happily delivering an Amp without issues. However, I'm struggling to get the thing to go up to it's datasheet max output of ~ Vin - 3V.

Schematic



I've made some progress with it, when it was first assembled I wasn't able to get more than about 15V from it. By tweaking the feedback resistor values and dropping R3 a couple orders of magnitude I've managed to get that up to 16.8V but I would like to at least get it up to 20V. I've played with the inductor value and it doesn't seem to make any difference to the output voltage with 33u, 47u or 68u.

I'm beginning to suspect the output capacitor might be a bit on the large side and is giving me my problem. This is followed by a linear output stage so ripple isn't the complete end of the world. Beyond that I'm really not sure where to go next.

Board Layout

[Kremmen]

For starters the output inductance and capacitance won't be the root cause of your problem, as long as you use values that are manageable by the 2596. The filter component values will have an impact on the controller dynamics but not to whether or not it will reach rated output voltage, at least with a reasonable load.
I suspect your feedback circuit. The internal reference of the 2596 is 1.23 V. To isolate the cause, you should measure if the voltage in the feedback pin (4) stays at  the reference voltage when you vary the feedback. If it does then your adjustment circuit (R2-R4, Q3) has some issues. If it doesn't, then the regulator is unable to do its job and something else is wrong.

[tszaboo]

You dont have feedforward capacitance on the feedback pin. Also, you might be saturating the inductor, please post the type. What is the type of D1? Is it maybe 15V Schottky? Did you measure it with a scope?
It might just be that the regulator cannot work with the same components through the whole region.

[Frenchie]

The inductor is a Bourns SRR1210 series http://www.digikey.com/product-detail/en/SRR1210-470M/SRR1210-470MCT-ND/2793214. Double checked the Schottkey is fine and no problems there. The feedforward capacitor was an oversight, forgot to mention the bodge cap that's currently soldered on the board. Thankfully this is just rev A  .

Scoping it out wasn't revealing anything too exciting, the switcher does seem to be functioning just fine and dandy. However, when I checked the reference pin and it never budges off 1.23V which as pointed out suggests that theres something up with my feedback circuit.

To be perfectly honest, I'm beginning to suspect that the linear regulator (LM317) might be latching ~ 15V on its way up and the pre-regulator is doing its job and holding just above that. I'll lift the base resistor and solder in a bodge and manually drive it up to 18V and see what happens. If that is the cause, I'm driving the adjust pin outside of the switcher output so I can always tap off that and bump the value of R2 slightly

[Frenchie]

Okay, it's not latching, trying to drive it any higher than 16V drops the output to nothing so there's definitely something up between Q3 and the resistors

[SeanB]

Q3 not breaking down or having high leakage at the higher voltages?

[Andy Watson]

What lies beyond Q3 and R3?

[Frenchie]

Okay, made some progress, I remembered that I bought a bunch of pre-made variable LM2596 boards off fleabay a while back. I fished one out of my parts bin, desoldered the trimpot off of it and modified the board to match my feedback circuit. Even went to the trouble of replacing the diode with the one off my own board.  What I wasn't able to pull across from my board were the BC857 (used a BC557 since they're nominally the same thing) and the switching components as they're in a different footprint.

Sure enough, the damned thing does what its designed to do, right up to 21V or so  .

I fear, that leads me back to one of a few possibilities either:

• My board layout is naff (it could be better, but I didn't think it was that bad)
• My selection of switching components is off
• I've got some damaged silicon, either the transistor or probably more likely the regulator
• There's some weird behavior happening due to the rest of the circuit (should have been ruled out by lifting R3, but it's still possible

I do have enough parts to hand to quickly throw the switching section of another board together which will give me an isolated test. I did test this regulator when I first built it but admittedly not over its full range. Think that's probably my wisest course of action at the minute.

What lies beyond Q3 and R3?

The output goes to the input pin of a LM317 and R3 goes off to the output of the same. Don't think the subsequent circuit is to blame as lifting R3 and driving the base with a voltage yields the same results.

[Frenchie]

Success! 

Almost certainly dead silicon, I'll lift the old Q3 later and test it properly to see which of the two is actually cactus. My money would be on the regulator if the transistor is working through at least some of its range. Of course, it might still be something in the rest of my circuit giving me the problems so celebration might be premature, at least I can rule out the board layout and component selection.



Note: my electronic load is only designed for about 12W sustained, so this test was only for a couple of minutes but it was happy enough to give an amp at least for that sort of duration.

Thanks all for the advice!