DC-DC current source peer review - Page 1
Forum section: electronics / projects

author: Obi_Kwiet
author_rank: Regular Contributor
timestamp: July 31, 2012, 02:16:41 am
content: Howdy folks! I've spent the last few months teaching myself power electronics, and I designed a constant current DC-DC converter to use with laser diodes. I'm running into some issues that I am at a loss to understand, so I'd really appreciate some any insight that any of you might have.It turns out that my design choice was not particularly novel; I discovered that several other people were using the same IC as me for a laser driver, but nevertheless, it's been pretty instructive. This is my first real full blown PCB/electronics design, so I've spent a lot of time just learning the ropes. My goal was to design something with the following specs:16-17mm diameter pcb2A output current3-4.2V Input voltage range for a single Li-I cell~2.5-5V+ output voltage capability90+% efficiency across most of the output range<15$ BOM for 10+ qtyI'm using the TPS63020 with a differential amplifier for current feedback. The resistor sitting out there all by it's lonesome is an alternative current sense resistor that can be swapped out to allow access to the driver's full current range. The .05 Ohm resistor covers 200mA to 2.5A and the .5 Ohm resistor covers 20mA to 250mA.Datasheets:http://www.ti.com/lit/gpn/tps63020http://www.ti.com/lit/ds/symlink/lmp8645.pdfhttp://www.vishay.com/docs/34261/lp20bz11.pdfUnfortunately, my I messed up the foot print of the XC61 voltage supervisor, so I had to tear that off and short EN and PS/SYNC to Vin. According to the data sheet this is ok, as long as I have the .1uF capacitor on the input.PCB Layout:Top:Bottom:Hopefully I did a decent job on the layout. It's the only layout I've ever done, and Eagle was all kinds of terrible to work with. I followed all the advice I could find on the internet, within the constrains of the package.Anyway, I ordered the parts, soldered the front in a griddle with some solder paste, and hand soldered the back. Did a continuity test, and everything checked out. Solder paste is magical.Results:I wired it up to a dummy diode load, with a Vf of about 3V. I set my output current to about 300mA using the .05 Ohm sense resistor. (Turns out those tiny trim pots are miserable to work with. I'm going to have to come up with a better solution. Maybe a small voltage programable digi-pot?) Anyway, efficiency was horrible. I set my power supply to 4.1V, and showed an output current to the driver of about 600mA. Using the sophisticated "touch different parts of the driver to see what's getting hot" method, I determined that most of my power loss was coming from the inductor. I found this odd, because it met the specs recommended by the datasheet. I hooked up the thing to my O-scope and probed the input rails. I immediately noticed a large ~2Vp-p voltage ripple. More interesting than that, however was the fact that it has at a frequency of of ~25KHz, which is about two orders of magnitude smaller than the TPS63020's 2.4MHz switching frequency. The switching noise can still be seen on small enough time scales, and has a much more reasonable amplitude.I thought it might be my power supply, so I hooked it up to a battery with the same result. At some point I mixed up the polarity on the input terminals and fried the thing, so I soldered another one up, and got the exact same waveform, so it wasn't a fluke. Does anyone have any idea what could be causing this? I'd really appreciate some help, because I don't know anyone who can help me with this, and I have no idea where a frequency this low could be coming from.O-Scope traces:10us/division, 1v/division:1us/division, 1v/division:20ns/division, 1v/division:I'd really appreciate any light that anyone could throw on this. Thanks!

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 31, 2012, 07:08:51 pm
content: What comes to mind is what happens if you power up the circuit without a load, or when the LED fails in normal use.There will be no current, input FB will be at zero volts and the TPS will probably kill itself trying to raise the voltage above limits.Did you power it without load? Then it's time for prototype #3 I think, and something to prevent this from happening again, like a zener from the output to FB.

author: Obi_Kwiet
author_rank: Regular Contributor
timestamp: July 31, 2012, 07:36:34 pm
content: It definitely has a load. I didn't turn it on until it was soldered into the dummy load, consisting of four N5404 diodes in series followed by a 1 Ohm resistor. I haven't done a trace of the output yet, but measuring across the 1 Ohm resistor indicated that current is being properly regulated through the load.The TPS also has built in over-voltage and over-current protection. It should turn off in an open terminal scenario.

author: nctnico
author_rank: Super Contributor
timestamp: July 31, 2012, 08:47:13 pm
content: The switcher isn't made for regulating current so you'll have loop instability. Put the 47uf capacitor across the load and use a 220nf capacitor from Vout to ground. You can also use a larger current sense resistor and feed the voltage across it directly into the FB pin. The downside is that the load is floating from ground.

author: gscsxys
author_rank: Newbie
timestamp: August 01, 2012, 11:04:47 am
content: ok

author: Obi_Kwiet
author_rank: Regular Contributor
timestamp: August 01, 2012, 10:08:03 pm
content: Quote from: nctnico on July 31, 2012, 08:47:13 pmMy practical understanding of control systems is unfortunately pretty limited, but from a practical standpoint, shouldn't the LPF on the amplifier fix this? Perhaps the LPF is too large, and it's not reflecting the response in the output quickly enough causing oscillation with the overcurrent/overvoltage protection? I'll see if I can't test it in voltage mode, and if that works, I'll try moving the -3db point of the LPF close to 2.4Mhz.Why would your suggested capacitor configuration help? This seems like a really useful thing to understand.EDIT: I reread the datasheets description of the voltage control loop. It must have some way of internally rejecting output voltage ripple. I bet the LPF is unnecessary. I'll try getting rid of it.

author: nukie
author_rank: Frequent Contributor
timestamp: August 02, 2012, 12:18:35 am
content: That is one shit pcb house. Where did you get your PCB made? The drilled holes are not centered.

author: Obi_Kwiet
author_rank: Regular Contributor
timestamp: August 03, 2012, 04:02:46 am
content: http://dorkbotpdx.org/wiki/pcb_order/They use a PCB fab in the USA, so it's a little surprising. Still, The tracks were good down to the 6/6mill spec they promised and due to the size, I got ten copies for like 5$ shipped, so it's hard to complain. For a larger order, I'd go elsewhere.

author: nukie
author_rank: Frequent Contributor
timestamp: August 05, 2012, 05:48:16 am
content: That's very cheap if you dont mind waiting. I assume its ENIG looking at the picture. The drill holes in the pictures on the website you linked seems centered.

author: nctnico
author_rank: Super Contributor
timestamp: August 05, 2012, 05:30:55 pm
content: There is nothing wrong with the holes. There are drilling tolerances so once in a while you get a PCB with the holes off-centre. No problem as long as the hole is within the pad.