Boost converter - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: December 15, 2015, 11:55:30 am
content: Boost converters run out of puff if you try and get more than double the input voltage out of them at any decent power level. Too many volt/seconds discharge for the inductor. Here's an idea I cooked up that I want to take 10VDC input and give 24VDC out at 10 amps for automotive use. Each mosfet has 45% max on time. The idea is that as each mosfet switches on to charge it's respective inductor it also pulls the inductor supply voltage up to 2x input voltage. The inductors discharge into (Vout - Vin). Haven't tried it out yet. Diode voltage drop may end up being a concern. Any opinions?

author: max_torque
author_rank: Super Contributor
timestamp: December 15, 2015, 02:18:09 pm
content: Surely for a simple voltage doubler, you just take an isolated 1:1 source and tack it onto the original voltage? (ie 12(original) + 12(isolated) = 24v??) That way your DC-DC only requires a boost of 1?(obviously, if you need output galvanic isolation from the supply, you can't do this)

author: Circlotron
author_rank: Super Contributor
timestamp: December 15, 2015, 09:01:57 pm
content: Just a voltage doubler is no good - output needs to be regulated to a fixed 24V even though the input goes down to 10V, as per the drawing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2015, 09:03:20 pm
content: Strip away everything but the CT choke and the MOSFETs:Each drain cannot be above 2*VCC + Vf because the opposite side body diode shunts to GND.Boost for 10 to 24, even 36V isn't a big deal. The duty cycle is irrelevant, as long as the design is appropriate (CCM with average current mode control, or DCM with peak or average current mode control).You can also do a PP forward converter, with a 1:1 transformer (actually, slightly more than 1:1 to account for regulation tracking), stacking the output on top of the supply.Tim

author: Someone
author_rank: Super Contributor
timestamp: December 15, 2015, 10:44:44 pm
content: Quote from: Circlotron on December 15, 2015, 11:55:30 amGet better suited inductors, 10:1 boost ratios are easy enough.

author: Circlotron
author_rank: Super Contributor
timestamp: December 16, 2015, 01:29:07 am
content: In the existing design the two mosfets are driven from each side of a SG3525 hence the < 50% duty cycle. It's CCM and voltage control mode. Really basic. At this point I'm trying to use the existing board as a basis for experiments. In the meantime I thought up another topology. Whenever an inductor is charged it gets Vin x 2 as before, and when it is discharging, if the opposite side mosfet is conducting, the inductor discharges into Vout - (Vin x 2). Also used one less pair of diodes. Downside is each mosfet sees 3x the current of the existing design, but shorter duty cycle....Edit -> Just realised both new circuits won't output less than 2x input voltage.

author: Fungus
author_rank: Super Contributor
timestamp: December 16, 2015, 06:32:36 am
content: Quote from: Someone on December 15, 2015, 10:44:44 pmAt 10 amps?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2015, 10:08:57 am
content: Yeah, ratios over 3:1 are pushing it, but under there is quite doable.For higher ratios, you should really consider a tapped inductor, or a transformer. Make the magnetics do the work -- that's what they're there for!!!Tim

author: DanielS
author_rank: Frequent Contributor
timestamp: December 16, 2015, 04:50:55 pm
content: Quote from: Fungus on December 16, 2015, 06:32:36 amA boost converter with 10A output might get a bit touchy (and the ripple current would be murder on the caps) but a double-forward converter could easily handle it with a suitable transformer core and windings - that's how many electronic inverters work: boost 12-24V battery voltage to 160-300VDCand then use an output bridge to switch output polarity.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 16, 2015, 05:02:30 pm
content: I have no problem whatsoever in a 1:3 to 1:4 boost converter. It's high power (100W, from 2.5...4.5 to 9...14V, 27A), and only single phase. Peak efficiency about 96%, full power efficiency about 92%. I think the idea of 1:2 limit is a severe misunderstanding. I think it starts getting more problematic after about 1:4 or 1:5. Below 1:3 is 100% normal everyday boost converter stuff. If you have problems at 1:3, then you have some fundamental issue which is going to wreak havoc at below 1:2 too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2015, 09:16:53 pm
content: 1:2 is ideal for a number of reasons:- Duty cycle being ~50% is theoretically appealing- The increased peak current (supply current is double the output current) and increased voltage rating (the peak/output voltage is double the input voltage) is equal with other topologies, e.g., push-pull and bridge (which are voltage limited by their supply to this ratio)- Therefore the SOA and Rds(on) are optimal in a certain sense, one which extends not only from switching converters, but amplifiers as well. The 1:2 ratio (and 1:4 in V*I) also appears in the linear power transfer theorem.Basically, if you're about, oh, 1:2 beyond 1:2, you'll be having that much harder of a time (i.e., SOA needs to be 16 times the power handling!). Not at all impossible, especially with transistors these days, but everything gets that much more difficult: the duty cycle limit goes way up (>75%), even suggesting a new control circuit is desirable.For example, a peak current mode converter needs to be dimensioned so it just reaches peak current, just as the duty cycle is topping out -- which is generally impossible given tolerances of input, output and component values! If it's too conservative on duty cycle, then it's wasting huge periods of time where it's not delivering power at all, and the V*I has to bethat much higherto compensate (try 1:32 or worse!). If it's not conservative enough, duty cycle peaks above 75%, it begins to skip cycles (ripple suddenly ~doubles), it goes into CCM and chaotic behavior ensues (ripple increases AND becomes erratic!).Tim