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:
I'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/tps63020
http://www.ti.com/lit/ds/symlink/lmp8645.pdf
http://www.vishay.com/docs/34261/lp20bz11.pdf
Unfortunately, my I messed up the foot print of the XC61 voltage supervisor, so I had to leave that out 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.
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!
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 pcb
- 2A output current
- 3-4.2V Input voltage range for a single Li-I cell
- ~2.5-5V+ output voltage capability
- 90+% efficiency across most of the output range
- <15$ BOM for 10+ qty
I'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/tps63020
http://www.ti.com/lit/ds/symlink/lmp8645.pdf
http://www.vishay.com/docs/34261/lp20bz11.pdf
Unfortunately, my I messed up the foot print of the XC61 voltage supervisor, so I had to leave that out 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.
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!