Boost converter won't Soft-Start - Page 1
Forum section: electronics / projects

author: Mad ID
author_rank: Regular Contributor
timestamp: February 03, 2015, 08:55:35 pm
content: Hi,I have developed a boost converter which gives 4V@600mA out from 2.5-3.6V input voltage using Semtech SC4501 converter. The converter uses current-mode control.The problem is that I'm having trouble with the soft-start which causes high inrush current from the battery (>2A). In does not work no matter what capacitor I put on SS pin. The output ramps quickly in 100 us. When I scope the SS voltage it ramps slowly depending on the capacitance value! I need the soft start of at least 5 ms or longer to keep the boot-up current below 0.5AWhat I have noticed is that soft-start for this particular controller depends on the control loop. Why is that?On my previous design with different output voltage & compensation network soft-start worked. My phase margin is at least 55-60deg or more (500mA current step gives no overshoot). I have tried dozens of different combinations for the compensation network and frequency and the result is the same, it won't soft start!Any clues what could be the problem? I believe it's related to the this particular controller, they messed something up.The schematic is below

author: langwadt
author_rank: Super Contributor
timestamp: February 03, 2015, 10:15:44 pm
content: you are not just seeing the peak from the output cap being charged via the diode and inductor?

author: Mad ID
author_rank: Regular Contributor
timestamp: February 04, 2015, 12:12:29 pm
content: Quote from: langwadt on February 03, 2015, 10:15:44 pmYes, it looks that way.Datasheet says that the soft start controls the current through the inductor, and it simply doesn't work.I saw in TI datasheets that their boost converter use the same soft-start principle via PNP transistor clamping the error amp so it should work...any ideas?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2015, 12:37:50 pm
content: So you want a soft start from zero but you chose a boost that's got a piece of wire and a diode straight from input to output? DUH...You need a SEPIC or something like that.Tim

author: Mad ID
author_rank: Regular Contributor
timestamp: February 04, 2015, 12:45:42 pm
content: Quote from: T3sl4co1l on February 04, 2015, 12:37:50 pmI would say stupid me, BUT, there are converters with same piece of wire which clearly have soft-start listed in the datasheet together with start-up waveforms.Take a look at TI datasheet, Figure 16.http://goo.gl/OJBpKXTo be honest I don't see how it works, the best the converter can do is to turn the low side FET on which will pump the current into the inductor..and it must release at some point which will charge the output capacitor very fast.What's your opinion? How can it work?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2015, 12:59:19 pm
content: In Fig.16 they just show voltage going up... whoop de do, they don't say what it's going up from! No "0V" mark. I hate it when they do that. But yeah, it's probably obvious what it's coming up from, in this case.Tim

author: Mad ID
author_rank: Regular Contributor
timestamp: February 04, 2015, 01:35:11 pm
content: Quote from: AcHmed99 on February 04, 2015, 01:10:06 pmYes, everything is clear now..soft start works from Vin up, no way to limit inrush in the output capacitor charging to Vin-Vd.Thanks guys!

author: MrAl
author_rank: Super Contributor
timestamp: February 04, 2015, 01:43:05 pm
content: Hi there,There may be an inrush current even though there is a 'soft start' mechanism for the chip, depending on how they define "soft start".For this chip, it looks like they define 'soft start' as the capability of reducing the OUTPUT overshoot, not necessarily the input current surge.The theoretical input current surge peak for your chosen values is around 2.5 amps with a 2.5v input (not counting the diode drop). The peak surge is roughly equal to Vin, so for Vin=3 it would be 3 amps for example.Probably the only way to reduce this without adding additional components is to increase the inductor value and maybe also decrease the capacitance value.The input peak surge goes up with the square root of the capacitance ratio, but goes down with the square root of the inductance ratio, so for 20uH you would see sqrt(10/20)=0.707 of the peak you see now, which is Ipeak=Vin. So instead of 2.5 amps you'd see about 1.8 amps peak, and for 40uH you'd see about 1.25 amps peak.The peak goes up with the square root of the capacitance so for double the capacitance you'd see about a 41 percent higher peak surge, so with half the capacitance you'd see about 71 percent of the peak, so with double the inductance and half the capacitance you'd see about half the peak you see now, which would be around 1.25 amps.If you can get away with 40uH and 2.5uF you would see about 1/4 of the peak you see now, which would bring it down to about 0.7 amps with a 2.8v input.These calculations are based on an equivalent series resistance of 0.01 ohms, but unfortunately the peak does not come down that much until this resistance increases beyond what we would probably want to use in a converter circuit (like 1 or 2 ohms).You'll have to look into this further. You can modify the circuit of course, or just use a different converter. If you go over about 50uH with your present circuit you'll have to test very carefully.

author: Mad ID
author_rank: Regular Contributor
timestamp: February 04, 2015, 02:21:10 pm
content: Quote from: MrAl on February 04, 2015, 01:43:05 pmHi,thanks for the helpful answer.Unfortunately, I cannot increase the inductance easily because SC4501 is a current mode control regulator. By increasing the L, ripple current decreases which causes the control loop to operate in noise. At first I had 20uH and I could see sub-harmonic oscillations regardless of compensation components. At 700kHz 10uH is optimal inductor for this regulator. I will use this new knowledge on future projects

author: Alex30
author_rank: Regular Contributor
timestamp: February 04, 2015, 02:25:51 pm
content: If you have the headroom just go for a mosfet with a RC time based circuit on the gate between VCC and C37

author: Mad ID
author_rank: Regular Contributor
timestamp: February 04, 2015, 02:28:50 pm
content: Quote from: Alex30 on February 04, 2015, 02:25:51 pmYes, that's my plan for a revision, slew down the input voltage.

author: MrAl
author_rank: Super Contributor
timestamp: February 04, 2015, 02:58:15 pm
content: Quote from: Mad ID on February 04, 2015, 02:21:10 pmHi,Oh sorry to hear that. I wasnt sure if that would be enough anyway though.But your new reply is a little surprising too, because i quoted those values only after reviewing the data sheet, which told me that they tested up to 47uH. Not sure what frequency that was though, or maybe something else with the layout is not optimal. But yeah, it looks like a circuit mod is in order here. That will get it over with pretty quick :-)Good luck with it.

author: Mad ID
author_rank: Regular Contributor
timestamp: February 04, 2015, 03:25:52 pm
content: Quote from: MrAl on February 04, 2015, 02:58:15 pm47uH would be fine for a new board with bigger inductor footprintThanks again.