High power DC-DC switching converter - Page 1
Forum section: electronics / projects

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: December 20, 2016, 03:11:03 am
content: HelloI am wanting to design a relatively high power boost converter that has the following (simple) specs:Vin = 13.4V to 14.4VVout = 15.5VIout = 30AWould anyone be able to point me to a resource that talks about the advantages/disadvantages of various topologies (SEPIC/CUK/ZETA...) or recommends which topology to use for such a power level?Cheers

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: December 21, 2016, 02:49:24 am
content: Thanks for the help evb149I don't think using a transformer based DC-DC converter would be right for me since the input/output voltages are so close together, have had a poke around the TI & LT website for some converter and am going to try simulate the LTC3786 to see what sort of losses i'll have to deal with, hopefully then will know if it's worth going to a multi-phase design.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 21, 2016, 03:25:04 am
content: I think a multiphase boost converter would be best for you, you use multiple lower power interleaved boost converters for cost and efficiency reasons.Here is an examplehttp://www.ti.com/tool/PMP7969you could use something very similar to this to achieve your goal.

author: mariush
author_rank: Super Contributor
timestamp: December 21, 2016, 04:51:10 am
content: A couple of LTC3862 should do the trick (check page 37 of datasheet, easily scaled to 30a) :http://www.linear.com/product/LTC3862Expensive though...

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 21, 2016, 05:54:33 am
content: QuoteThis is an ideal application for a transformer, one winding handles 12v , the other handles 3v , you can select really low loss 20v MOSFETS for the 3v side , while the mosfets on the 12v side only carry 1/4 of the current, and the duty cycle is close to 50%.With a transformer, the volt-amps in the core will be 1/4 of that required in any other type of converter inductor.Any non-synchronous power stage will have massive losses, figure in 1v drop at 30 A = 30W , the cost of these diodes and the heatsink will be more than the cost of Sync MOSFETs and the drive.Most non-transformer designs will have a very low duty cycle and higher losses in the switching transistors.You only need a simple push-pull driver (eg UC2825/UC28025 is one I've used), and you can drive the 12v and 3v MOSFETS with the same gate signal by capacitive coupling.At light load/hi input volts there will some freewheeling schottky diodes required (20v rating, Vf <= 500mV) , although sync rectification with another MOSFET and an IR1166 will be cheaper than the heatsink for the diodes, and can all be SMD.A recent inverter transformer design took the output from a series string of concentrating PV cells, so the available voltages were 0, +2.5,+5,+7.5,+10 , the unit could take upto 40A from the 10v, and sink/source 20A to/from the taps at 2.5, 5, 7.5v, the unit could output to a 20v bus (could be wound down to 15v if needed) and/or 240VDC to a bus. Efficiency was 98% for the 20v output and 96% from 240VDC (at 200W throughput). There were 12Mosfets (typically PSMN2R6-40Y) driven from one UC2825 (using capacitors) , and a freewheeling "diode" MOSFET driven by an IR1166. The 240VDC output used a quasiresonant voltage doubler. The transformer was an ETD34 and ran at 60kHz, at 2.5volts/turn, the primary was a copper strip 0.5mm x 20mm. All MOSFETs were SMD and used no additional heatsink apart from about 20x10mm of copper foil. The PCB was in a sealed box with the CPV elements, and an ambient air temperature of 80C, and a design dT of 20K, energy was recovered from the snubber circuits to power a small fan to keep the transformer cool (core losses were fairly high in order to get 2.5volts/turn).A better design would be a phase shifted resonant transformer, again it's a 12v to 3v transformer that is needed.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: December 21, 2016, 08:25:57 am
content: Quote from: salbayeng on December 21, 2016, 05:54:33 amSorry am a bit confused, are you suggesting using a buck converter to step the voltage down from ~13V to 3V and then use a transformer to increase it to 15.5V?

author: rx8pilot
author_rank: Super Contributor
timestamp: December 21, 2016, 08:45:54 am
content: Joining........Sent from my horrible mobile....

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: December 21, 2016, 09:21:05 am
content: Quote from: rx8pilot on December 21, 2016, 08:45:54 amStill confused ; - ;The thing is if I use a buck to first step the voltage down does that not mean my inductor/mosfets will have to be rated for a higher current?

author: salbayeng
author_rank: Frequent Contributor
timestamp: December 21, 2016, 09:56:38 am
content: QuoteNo , use the transformer with 12V input , and 3V output , then rectify the 3v to make 3v DC , and put the 3v in series with the 12v.So input =12v output = 12+3 =15v .It works like an autotransformer. only 1/4 of the power goes through the transformer, the other 3/4 come directly from the input.Imagine having a 12v battery, and putting a 3v battery on top to make 15v.Now replace the 3v battery with a 3v power supply, powered by the 12v.Then use feedback to adjust the 3v so that the output voltage stays at 15v.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: December 21, 2016, 10:22:45 am
content: Quote from: salbayeng on December 21, 2016, 09:56:38 amHeh, interesting, are you able to post an example circuit?

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2016, 10:23:45 am
content: Quote from: peter.mitchell on December 21, 2016, 03:25:04 amThis is a typical boost converter with a synchronous rectifier. At the 500W power level the OP is asking for you don't have to go for multi-phase. Just use a couple of low RDSon MOSFETs in parallel for the switch and rectifier. Judging from the input voltages I smell automotive which may lead to a design which needs to handle higher input voltages. Also 14.4V may be too low. Some newer car battery chemistries use higher charging voltages combined with fast charging systems.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: December 21, 2016, 10:30:49 am
content: Quote from: nctnico on December 21, 2016, 10:23:45 amYea should have mentioned this is for charging a secondary battery, reason why I want to boost the voltage is to try and mitigate the voltage drop of the cable run.

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2016, 05:32:33 pm
content: In that case don't bother with the circuit! If you use cables with a reasonable thickness the voltage drop won't be an issue at all. You just have to calculate the maximum current and make sure the wires can handle that current. Also put a fuse in between for safety.