boost converter overshoot / undershoot - Electronic Circuits Projects Diagrams Free

justDIY · 20th December 2006, 02:09 PM

I thought all was well with my little boost converter, but then I decided to measure things, to see how the efficiency was doing. What I found was worse than I expected.

Test setup:

Four white leds, wired in series.
Sense resistor 38.3 ohm 1% smt
Requested output ~30mA (32 to be exact)

Voltage source 1:

Two weak AA batteries, approximately 2.5 volts.
Voltage at sense resistor 1.0 volts, output current 26 mA
Voltage across the leds 12.9

Voltage source 2:

USB port, approximately 5 volts.
Voltage at sense resistor 2.05 volts

V/R=I = 53 mA output
Voltage across the leds 13.4

So my regulation is terrible for some reason. Aside from a larger sense resistor and pwm input disabled, my design follows the reference design exactly. (Figure 22. White LED Supply)

I've tried different amounts of output capacitance, in case ripple is messing with the feedback. perhaps I need to decouple the feedback pin to ground with a small cap (i've seen that in some designs)? did I miss something?

switcher datasheet:
http://www.ti.com/lit/gpn/tps61040

schematic:
http://projects.dimension-x.net/pict..._boost_sch.png

pcb layout:
http://projects.dimension-x.net/pict...n_cc_boost.png

assembled gizmo (show with 10uf output cap):
http://projects.dimension-x.net/pict...61040_cc_1.jpg

OutToLunch · 20th December 2006, 03:28 PM

Why such a large output cap? Just one of thise 4.7uF caps you have on the input and put it on the output.

Regarding your layout - I would have done some things differently... I would have rotated the sense resistor 90 degrees counterclockwise and then moved it up so that the external connection to it goes directly to the left (when looking at the layout pic you gave a link to). The connection to the FB pin would just go straight down then to the left to the IC pin. The ground connection on both terminals would be connected (currently the output ground is not even tied to input ground on the board). The existing connection between ground copper and the IC ground pin is thin - I would fatten it up as much as possible and then (with the sense resistor being rotated) just continue the ground connection from the ground pin of the IC to the output ground.

I think the biggest problem here is that the output capacitor ground is just floating. Even if you have it tied around externally to the input ground, it is too long of a path - introducing parasitics. Make it more direct - if need be, just solder a wire from the cap ground to the ground trace on the board.

justDIY · 20th December 2006, 04:58 PM

the ground that seems to be floating is connected to the input ground on the breadboard, with a jumper on the breadboard (picture was taken pre-wireup)... revision two takes care of that, however, I just want to make sure everything is the way it should be, before committing to another pcb panel.

I can see now that the weak ground for the chip might be a problem, I just assumed since the trace was so very short that the resistance wouldn't be a problem. I'll try soldering a jumper from that pin to the nearby ground.

I cannot use the 4.7u cap like I have on the input, as it's only rated for 16 volts. When this project is complete, I hope to drive 8 leds in series which will require 26 volts. I have a 35v ceramic on order, waiting to see if it makes it before xmas.

here's a link to the revision two board, the big IC is a pic micro.

http://projects.dimension-x.net/pict...artlight_2.png

Roff · 20th December 2006, 07:22 PM

Are you using the recommended inductor? (Looks like you probably are). Saturation current is a critical parameter.

justDIY · 20th December 2006, 07:51 PM

I'm using the 10uH inductor specified by the reference design, also listed as "typical" by the datasheet

Quote:

Since the PFM peak current control scheme is inherently stable, the inductor value does not affect the stability of the regulator. The selection of the inductor together with the nominal load current, input and output voltage of the application determines the switching frequency of the converter. Depending on the application, inductor values between 2.2 µH up to 47 µH are recommended. The maximum inductor value is determined by the maximum on time of the switch, typically 6 µs

I haven't had a chance to improve the grounding yet ... I hope that's all it is.

I'm also going to hook up the scope and make sure the switch is running smoothly and not "bursting".

Quote:

Some applications require a very tight line regulation and can only allow a small change in output voltage over a certain input voltage range. If no feedforward capacitor CFF is used across the upper resistor of the voltage feedback divider, the device has the best line regulation. Without the feedforward capacitor the output voltage ripple is higher because the TPS61040/41 shows output voltage bursts instead of single pulses on the switch pin (SW), increasing the output voltage ripple. Increasing the output capacitor value reduces the output voltage ripple.

justDIY · 20th December 2006, 10:49 PM

alrighty, using that left over copper strip along the top of the pch, I bonded the 'floating' ground pin to the input ground. I used three bridges, one at the input, one where the ground for the chip connects, and one at the far end, where the output capacitor is.

I removed the 1uf electrolytic, and replaced it with a 0.1 uf ceramic

at first, I was excited, my meter showed 1.20 volts across the sense resistor ... but then it started dropping, and was hovering around 1.00 when I shut it down. figuring my batteries might be sagging, I connected the circuit to a 5v supply. Sense resistor voltage was over 2 volts!

the switch output looks clean on the scope, all the pulses are clear and evenly spaced, although I'm not exactly sure what output voltage and multiple pulses would look like.

here's some photos on what the scope says, first is the switch output, second is the feedback pin
http://projects.dimension-x.net/pict...40_waves_1.jpg
http://projects.dimension-x.net/pict...40_waves_2.jpg

Styx · 20th December 2006, 11:16 PM

why havn't you setup the feedback with a potential-divider? the cct has it just tied to GND

Also what are you hoping in trying to "create" a differential power supply via taking the +VE at the output of the switcher and the -VE at the feedback - anything the load does will mess up the switcher since it will put a voltage on that pin?

this is a single-ended booster chip

Roff · 21st December 2006, 12:02 AM

Quote:

Originally Posted by Styx

why havn't you setup the feedback with a potential-divider? the cct has it just tied to GND

Also what are you hoping in trying to "create" a differential power supply via taking the +VE at the output of the switcher and the -VE at the feedback - anything the load does will mess up the switcher since it will put a voltage on that pin?

this is a single-ended booster chip

I think the idea is to create a current source for the LEDs. Look at fig. 22 in the datasheet.

justDIY · 21st December 2006, 04:16 AM

i tried soldering a jumper from the ground pin on the device directly to the ground plain, giving it a very strong ground ... no change ... when the input voltage is under 3 volts, the output current is below the set-point ... input voltage over 3 volts, output current is way above the set-point.

justDIY · 21st December 2006, 04:35 AM

I found the design manual for their constant current led driver evaluation module, and was looking at the waveforms they have in there.

http://www.ti.com/litv/pdf/slvu068a

my led current waveform looks very similar to theirs (figure 2-2), perhaps not as exaggerated ... hard to hold the probe, camera and fiddle with dials at the same time

but my switching waveform looks nothing like theirs (figure2-3), unless it's a fake ... mine doesn't ring like that at all ... isn't ringing generally a bad thing anyway?

Roff · 21st December 2006, 05:25 AM

Your layout looks like ENABLE is grounded through RPD (I'm assuming the PIC is not installed). I guess I'm missing something.

hjames · 21st December 2006, 08:09 AM

The "ringing" in 2-3 of the app note is indicative that the switching regulator is operating at less than full capacity - Essenially every cycle ends with the feedback voltage above 1.2V, so the chip doesn't try to immediately begin the next cycle. Instead it waits, and what you see is the end of the inductor flapping around with it's parasitics. The next cycle starts when the fb voltage drops below 1.2V, and the chip "charges" the inductor up to 400mA.

Also, according to the app note (pg 2-6) there's ~8mA of ripple at 4.2v. In your circuit this would end up being 300mV worth of ripple, which is what you're seeing. BTW, in their circuit, it would end up being .8V worth of bouncing up and down...

Ocelaris · 21st December 2006, 08:25 AM

Sorry to ask such a simple question, but what and where did you get that copper trace? Is it a etching kit or just tape?

Looks good to me none-the-less! SMD wonder works! I'd just be happy to get one of those going.

justDIY · 21st December 2006, 12:41 PM

Quote:

Originally Posted by Ron H

Your layout looks like ENABLE is grounded through RPD (I'm assuming the PIC is not installed). I guess I'm missing something.

Ron, you are correct. However, I'm not using the revision two pcb yet, still working with the one in the first post.

My thoughts with RPD (pull-down) is a 'safety' measure ... if the PIC pin driving enable goes floating for some reason, RPD will hold it low until the PIC drives it high.

justDIY · 21st December 2006, 12:45 PM

Quote:

Originally Posted by Ocelaris

Sorry to ask such a simple question, but what and where did you get that copper trace? Is it a etching kit or just tape?

Looks good to me none-the-less! SMD wonder works! I'd just be happy to get one of those going.

I etched it using using the photolithography method

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Title	Starter	Forum	Replies	Latest
Overshoot and ringing	doofus32	General Electronics Chat	3	28th August 2006 04:04 AM

20th December 2006, 02:09 PM	(permalink)
justDIY	boost converter overshoot / undershoot I thought all was well with my little boost converter, but then I decided to measure things, to see how the efficiency was doing. What I found was worse than I expected. Test setup: Four white leds, wired in series. Sense resistor 38.3 ohm 1% smt Requested output ~30mA (32 to be exact) Voltage source 1: Two weak AA batteries, approximately 2.5 volts. Voltage at sense resistor 1.0 volts, output current 26 mA Voltage across the leds 12.9 Voltage source 2: USB port, approximately 5 volts. Voltage at sense resistor 2.05 volts V/R=I = 53 mA output Voltage across the leds 13.4 So my regulation is terrible for some reason. Aside from a larger sense resistor and pwm input disabled, my design follows the reference design exactly. (Figure 22. White LED Supply) I've tried different amounts of output capacitance, in case ripple is messing with the feedback. perhaps I need to decouple the feedback pin to ground with a small cap (i've seen that in some designs)? did I miss something? switcher datasheet: http://www.ti.com/lit/gpn/tps61040 schematic: http://projects.dimension-x.net/pict..._boost_sch.png pcb layout: http://projects.dimension-x.net/pict...n_cc_boost.png assembled gizmo (show with 10uf output cap): http://projects.dimension-x.net/pict...61040_cc_1.jpg __________________ If you don't have a planet, what good are gold bars? want to contact me directly? gmail gordonthree check out my project website: http://projects.dimension-x.net Favorite numbers: 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0 Last edited by justDIY; 20th December 2006 at 02:11 PM. Reason: changed title

20th December 2006, 03:28 PM	(permalink)
OutToLunch	Why such a large output cap? Just one of thise 4.7uF caps you have on the input and put it on the output. Regarding your layout - I would have done some things differently... I would have rotated the sense resistor 90 degrees counterclockwise and then moved it up so that the external connection to it goes directly to the left (when looking at the layout pic you gave a link to). The connection to the FB pin would just go straight down then to the left to the IC pin. The ground connection on both terminals would be connected (currently the output ground is not even tied to input ground on the board). The existing connection between ground copper and the IC ground pin is thin - I would fatten it up as much as possible and then (with the sense resistor being rotated) just continue the ground connection from the ground pin of the IC to the output ground. I think the biggest problem here is that the output capacitor ground is just floating. Even if you have it tied around externally to the input ground, it is too long of a path - introducing parasitics. Make it more direct - if need be, just solder a wire from the cap ground to the ground trace on the board.

20th December 2006, 04:58 PM	(permalink)
justDIY	the ground that seems to be floating is connected to the input ground on the breadboard, with a jumper on the breadboard (picture was taken pre-wireup)... revision two takes care of that, however, I just want to make sure everything is the way it should be, before committing to another pcb panel. I can see now that the weak ground for the chip might be a problem, I just assumed since the trace was so very short that the resistance wouldn't be a problem. I'll try soldering a jumper from that pin to the nearby ground. I cannot use the 4.7u cap like I have on the input, as it's only rated for 16 volts. When this project is complete, I hope to drive 8 leds in series which will require 26 volts. I have a 35v ceramic on order, waiting to see if it makes it before xmas. here's a link to the revision two board, the big IC is a pic micro. http://projects.dimension-x.net/pict...artlight_2.png __________________ If you don't have a planet, what good are gold bars? want to contact me directly? gmail gordonthree check out my project website: http://projects.dimension-x.net Favorite numbers: 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

20th December 2006, 07:22 PM	(permalink)
Roff	Are you using the recommended inductor? (Looks like you probably are). Saturation current is a critical parameter. __________________ Ron (aka Rube)

20th December 2006, 10:49 PM	(permalink)
justDIY	alrighty, using that left over copper strip along the top of the pch, I bonded the 'floating' ground pin to the input ground. I used three bridges, one at the input, one where the ground for the chip connects, and one at the far end, where the output capacitor is. I removed the 1uf electrolytic, and replaced it with a 0.1 uf ceramic at first, I was excited, my meter showed 1.20 volts across the sense resistor ... but then it started dropping, and was hovering around 1.00 when I shut it down. figuring my batteries might be sagging, I connected the circuit to a 5v supply. Sense resistor voltage was over 2 volts! the switch output looks clean on the scope, all the pulses are clear and evenly spaced, although I'm not exactly sure what output voltage and multiple pulses would look like. here's some photos on what the scope says, first is the switch output, second is the feedback pin http://projects.dimension-x.net/pict...40_waves_1.jpg http://projects.dimension-x.net/pict...40_waves_2.jpg __________________ If you don't have a planet, what good are gold bars? want to contact me directly? gmail gordonthree check out my project website: http://projects.dimension-x.net Favorite numbers: 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

20th December 2006, 11:16 PM	(permalink)
Styx	why havn't you setup the feedback with a potential-divider? the cct has it just tied to GND Also what are you hoping in trying to "create" a differential power supply via taking the +VE at the output of the switcher and the -VE at the feedback - anything the load does will mess up the switcher since it will put a voltage on that pin? this is a single-ended booster chip __________________ Nothing is impossible. Once a problem is realised, the rest is just details

21st December 2006, 04:16 AM	(permalink)
justDIY	i tried soldering a jumper from the ground pin on the device directly to the ground plain, giving it a very strong ground ... no change ... when the input voltage is under 3 volts, the output current is below the set-point ... input voltage over 3 volts, output current is way above the set-point. __________________ If you don't have a planet, what good are gold bars? want to contact me directly? gmail gordonthree check out my project website: http://projects.dimension-x.net Favorite numbers: 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

21st December 2006, 04:35 AM	(permalink)
justDIY	I found the design manual for their constant current led driver evaluation module, and was looking at the waveforms they have in there. http://www.ti.com/litv/pdf/slvu068a my led current waveform looks very similar to theirs (figure 2-2), perhaps not as exaggerated ... hard to hold the probe, camera and fiddle with dials at the same time but my switching waveform looks nothing like theirs (figure2-3), unless it's a fake ... mine doesn't ring like that at all ... isn't ringing generally a bad thing anyway? __________________ If you don't have a planet, what good are gold bars? want to contact me directly? gmail gordonthree check out my project website: http://projects.dimension-x.net Favorite numbers: 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

21st December 2006, 05:25 AM	(permalink)
Roff	Your layout looks like ENABLE is grounded through RPD (I'm assuming the PIC is not installed). I guess I'm missing something. __________________ Ron (aka Rube)

21st December 2006, 08:09 AM	(permalink)
hjames	The "ringing" in 2-3 of the app note is indicative that the switching regulator is operating at less than full capacity - Essenially every cycle ends with the feedback voltage above 1.2V, so the chip doesn't try to immediately begin the next cycle. Instead it waits, and what you see is the end of the inductor flapping around with it's parasitics. The next cycle starts when the fb voltage drops below 1.2V, and the chip "charges" the inductor up to 400mA. Also, according to the app note (pg 2-6) there's ~8mA of ripple at 4.2v. In your circuit this would end up being 300mV worth of ripple, which is what you're seeing. BTW, in their circuit, it would end up being .8V worth of bouncing up and down...

21st December 2006, 08:25 AM	(permalink)
Ocelaris	Sorry to ask such a simple question, but what and where did you get that copper trace? Is it a etching kit or just tape? Looks good to me none-the-less! SMD wonder works! I'd just be happy to get one of those going.