Help with DC-DC power supply, poor regulation & output coupling - Page 1
Forum section: electronics / projects

author: jmw
author_rank: Frequent Contributor
timestamp: January 12, 2018, 03:50:23 am
content: I'm trying to build a 5V/1A to 140V/10mA DC-DC power supply to power Nixie tubes from common USB phone chargers. I settled on using a boost topology with a TI LM3478 controller, since it has no limit on the duty cycle, which must be quite high (.9643...). After many simulations in LT-Spice with transient models from the parts makers, I had something that looked reasonable and simulated with a 133 V output across a 14k load. Made a 4-layer layout and sent it to OSHPark for manufacturing, got it back, assembled it, and was pretty happy when it put out 139V across the outputs from an iPhone charger, and was able to light a IN-13 bargraph Nixie tube from it. However, it's not able to hold voltage and I noticed some noise coupling at the controller, and could use some help.1. The output voltage starts to droop with any current draw. I wasn't expecting great efficiency with this boost ratio and high switching frequency (750 kHz to keep the inductor and output cap to a reasonable size); the 5V input draws 180 mA quiescent current and then a flat 260-280 mA for outputs up to 10 mA, so at 78 V/10 mA it's 60% efficient. From simulation, I expected input current draw to be closer to 450-500 mA, not sure why it's so low.I-V curve of the output-> For this, why might I be getting such poor regulation? What limits the input current to boost power supply (both a phone charger and bench supply can put out more than 280 mA)?2. Using a scope I probed the input and output capacitors, and the voltage and ground leads to the LM3478 controller (using the little ground spring instead of the loop wire). The input ripple from a phone charger is 60 mV P-P and the output ripple is 100 mV P-P with a 15k load, good enough for me. But there's nasty coupling of this ripple to the Vin pin on the controller - almost 500 mV P-P @ 750 kHz! Even the controller's ground pins show a 30 mV P-P blip at this frequency.-> Are there any big PCB layout mistakes that are coupling noise around the whole circuit? The only thing I thought of immediately is to use a power plane instead of making layer 3 a signal layer to carry the 5V input voltage around - but this didn't help with output coupling to ground even though layer 2 is the ground plane.100 mV output ripple with a 15k load - I can live with thisOutput coupling to the controller power pin - yuck!Input cap ripple - the higher frequency spikes are output coupling around 35 mV p-pSchematic, omitting the charger detection part for brevityTop side layout with the power componentsPrototype board

author: jmw
author_rank: Frequent Contributor
timestamp: January 12, 2018, 04:34:47 am
content: Could you explain a little more the idea behind wanting to use DCM? Everywhere I look the conventional wisdom is use CCM, the math is easier, etc etc. The biggest problem I had simulating designs with lower inductance was running out of slope compensation headroom. Lower inductance = faster current ramp, but I can look if there's adjustments to my 55uH design attempt that might simulate ok.And C6 - the LM3478 datasheet optionally recommends 100p across FB and ground "to reduce noise", and since I don't have to have bean-count my BOM, I figured why not...

author: jmw
author_rank: Frequent Contributor
timestamp: January 12, 2018, 04:38:58 am
content: My thought was inductors are easier to source than transformers in any and all quantities, that's why I went with boost.

author: jbb
author_rank: Super Contributor
timestamp: January 12, 2018, 05:13:41 am
content: Also, in DCM the inductor current goes to zero each cycle, which means it ‘forgets about’ the previous cycle, which reduces the phase lag of the system and makes control easier. (In control system speak, it reduces he order of the system by 1).Maybe a voltage doubler stage could help with the ratio, too.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 12, 2018, 06:18:44 am
content: I used this chip to go from 12V (battery) to 130V to run a couple of tubes. Load current was about 65 mA.You definitely need to use a lower inductance value. I used about 6.8uH and ran the chip at 100KHz, D.R =35%. It seems to me that the circuit cant pump enough current because the inductor cannot store enough energy each cycle.As for C6. I put it in and it does just fine, but then the time constant created by MY C6 and my lower resistor (R5 in your schem.) was at about 180 nS which is way out of dangerous ground. Your compensation pole is in the region of 50 uS, same as me so is quite removed from the TC imposed by C6 and should make C6 quite safe.By the way Jim Williams had some excellent advice for tuning the compensation network by empirical methods in his application note AN-25 Appendix B. It worked quite well for me.

author: jmw
author_rank: Frequent Contributor
timestamp: January 12, 2018, 08:14:47 am
content: Quote from: basinstreetdesign on January 12, 2018, 06:18:44 amControl theory is mostly out of my background, but it sounds like this needs to be studied carefully for this to work. Do you have a pointer to AN-25? I only read AN-1286 "Compensation for the LM3478 Boost Controller" which was helpful in building this design, even though I didn't feel like I walked away with a deeper theoretical understanding.

author: jmw
author_rank: Frequent Contributor
timestamp: January 12, 2018, 08:19:22 am
content: Think I found ithttp://cds.linear.com/docs/en/application-note/an25fa.pdf