step-up converter doesn't start with small solar cell - Page 1
Forum section: electronics / beginners

author: FrankBuss
author_rank: Unknown
timestamp: November 21, 2015, 02:18:15 pm
content: I've built this circuit:The BPW34s charges C1 up to 1.2V in daylight, if IC1 is not connected. But when I connect it, it charges C1 only up to 0.7V, then stops and the output at C2 stays at 0.5V. I guess the problem is that the BPWs can only generate some 100 uA, which results in a very slow startup voltage ramp for IC1, because if I disconnect IC1, wait until C1 is charged, then connect IC1 again, it works, the output at C2 is 3.3V. And it stays at this output, and C1 stays charged, too, at the same 1.2V. It doesn't help to add more BPW34s, I tried it with 6, too. Changing C2 doesn't help either, I tried different values from 100nF to 100uF.How can I fix it?Or is there another cheap step-up converter which works with very slow startup voltage ramp and has low quiescent current? I don't need a stable 3.3V output, anything from 3V to 3.6V would be sufficient and efficiency doesn't need to be very good.How can I build it discrete, with transistors or FETs?

author: Seekonk
author_rank: Super Contributor
timestamp: November 21, 2015, 02:26:41 pm
content: First get rid of D4, it serves no purpose. That will get you a hair more voltage. Did you try this in the sun? It would be worth trying each cell with a pot and note at which voltage it starts to take a nose dive and calculate the current. Parallel cells would work better. Cells are current sources.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 21, 2015, 02:29:46 pm
content: What happens when you attach IC1 with SHDN low?Or make sure SHND is not >800 mV until BATT is >910mV.Maybe you also need to replace the BAT54 with a Vf of 0.4 with a somethinglikea BAT60A which has a forward voltage of 0.12 at 10mA. Or remove it completely. You only need a diode when you intend to put them in parallel, which you have not.The bat60 has a high reverse leakage, finding diodes for this is tricky.

author: madires
author_rank: Super Contributor
timestamp: November 21, 2015, 02:39:10 pm
content: Try to use the SHDN pin to delay the start-up (by a RC combination for example).

author: AndyC_772
author_rank: Super Contributor
timestamp: November 21, 2015, 03:09:10 pm
content: What's the SHDN threshold voltage?Perhaps you could put a forward biased silicon diode between C1 and the SHDN pin, to allow C1 to charge to a higher voltage before SHDN goes high.

author: FrankBuss
author_rank: Unknown
timestamp: November 21, 2015, 03:34:07 pm
content: Quote from: Seekonk on November 21, 2015, 02:26:41 pmI used the circuit as shown in step 2 fromthis site. They say the diode is required, because when C1 is charged and then it gets dark, the current would flow back to the diodes. Is this right? Sorry, I'm a beginnerQuote from: Seekonk on November 21, 2015, 02:26:41 pmI tried it with a bright desk lamp, should be the same as sun (very cloudy outside at the moment, so can't try it in real sun). Good idea to make a diagram. I measured voltage and current at different pot positions:I'll try now to delay startup with the SHDN pin. Would be still interesting to use a discrete pure transistor solution, because it would be cheaper (the MAX1724 costs a few Euros, if you get it at all, because you know, Maxim: cool parts, but you don't get many of them at Digikey, if you don't order 1000s of it).

author: Aodhan145
author_rank: Frequent Contributor
timestamp: November 21, 2015, 03:41:44 pm
content: Bright desk lamps are not the same as the sun! Try it outside Ive done this before.

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2015, 03:45:50 pm
content: Quote from: Aodhan145 on November 21, 2015, 03:41:44 pm+1

author: FrankBuss
author_rank: Unknown
timestamp: November 21, 2015, 04:08:05 pm
content: No sun outside at the moment, but it works now with two silicon diode from batt to shdn (from a parts bin, can't read the marker, I guess something like 1N4148).But I wonder if this is reliable if I would mass produce it, because the SHDN voltage threshold can be from 500 mV to 800 mV and the diode voltage drop can vary, too. And of course, for mass production the MAX1724 is too expensive and overkill for my project, I just need something between 3V and 3.6V.

author: edavid
author_rank: Super Contributor
timestamp: November 21, 2015, 04:45:24 pm
content: Quote from: FrankBuss on November 21, 2015, 04:08:05 pmYou could poke around AliExpress and see what is popular:http://www.aliexpress.com/wholesale?SearchText=+boost+3.3vLooks like the AP1603 is a possibility.

author: FrankBuss
author_rank: Unknown
timestamp: November 21, 2015, 08:21:07 pm
content: Quote from: edavid on November 21, 2015, 04:45:24 pmThanks, the AP1603 looks good, and it is cheap, I guess I couldn't build it cheaper with discrete transistors with this performance. But I can't find the quiescent current in the datasheet. Output current will be much less than 1 mA for my application, maybe with some peaks in the mA range. If the quiescent current is too high, it wouldn't last long when it is dark. And it might have the same problem with the slow startup voltage ramp as the Maxim part.