DC/DC Converter Design (Vout = Vin + 6.5V) - Page 1
Forum section: electronics / projects

author: schwarzrmsu
author_rank: Contributor
timestamp: June 04, 2015, 12:54:53 pm
content: Hello All,Huge fan of the blog, but just getting into the forum. Thought this would be a great place to post a question:I have been working on a design that requires me to create a fixed rail between Vin and a voltage that is always 6.5V above Vin. The challenge lies in the fact that my Vin can change quiet regularly between 3V-24V (Automotive...). A quick sketch of the scenario is shown below:In most cases, designing a fixed output boost converter is sufficient, but this design requirement has left me scratching my head. Please share me your thoughts or possible chip sets that may help me through my design process. If a similar thread has been published I will be more than happy if you share the link with me.Thank you everyone. I look forward to contributing more to the forum in the future.

author: Scrts
author_rank: Frequent Contributor
timestamp: June 04, 2015, 01:13:21 pm
content: Can you define the application a little bit more? Why do you need that changing range?

author: schwarzrmsu
author_rank: Contributor
timestamp: June 04, 2015, 01:31:24 pm
content: Quote from: Scrts on June 04, 2015, 01:13:21 pmThis design is for an automotive application. Below I will define the scenarios for the spectrum of voltages:3V - Worst Case Scenario Cranking Battery Voltage (May be acceptable to use 4V-5V for this worth case)12V- Nominal Battery Voltage13.5V - Nominal Alternator Voltage24V - Double Battery Protection For Jump Start (Yes people sometime hook batteries in series instead of parallel...)I should also mention that the design as a whole will draw around 100mA at a Vin of 12V.Thank you for the quick response!

author: halexa
author_rank: Regular Contributor
timestamp: June 04, 2015, 01:44:02 pm
content: Hi,Crank issue is often solved with a capacator bank after the regulator to hold the voltage during crank. In my experience with automotive application they often specify that the ECU shall have function between 8-16V input.

author: schwarzrmsu
author_rank: Contributor
timestamp: June 04, 2015, 01:49:38 pm
content: Quote from: halexa on June 04, 2015, 01:44:02 pmThis is a good point, that is why I mentioned I have a little leeway in this minimum voltage requirement. Unfortunately I do not have a ton of real estate for a significant capacitor bank, but will be adding bulk capacitance to the output of the DC/DC converter.

author: Kalvin
author_rank: Super Contributor
timestamp: June 04, 2015, 02:27:51 pm
content: Sounds a boost-converter and an optocoupler connected across the output and input (and one zener + resistor), and the optocoupler's output connected to the feedback input.

author: schwarzrmsu
author_rank: Contributor
timestamp: June 04, 2015, 02:53:48 pm
content: Quote from: Kalvin on June 04, 2015, 02:27:51 pmHi Kalvin,Do you mind providing a quick block diagram of this idea? It sounds like we want to create an isolated voltage reference using an optocoupler/zener and then route that back to the boost converter feedback pin. I am having a hard time taking these pieces and arranging them into a working concept.Please let me know if you can help. Thanks!

author: Kalvin
author_rank: Super Contributor
timestamp: June 04, 2015, 03:41:13 pm
content: Basic idea would be something like this.ps. Now that I drew this, maybe a PNP would do in place of the optocoupler.

author: Scrts
author_rank: Frequent Contributor
timestamp: June 04, 2015, 04:03:53 pm
content: I am also working in automotive and the jump-start is sometimes 28V, however depends on OEM. Do you really need to be functional at all conditions?Anyway, you didn't tell us what is the load? What are you trying to power? Why that device must have a variable input voltage? Why it cannot be fixed at certain voltage, so the solution would be buck-boost or SEPIC.

author: schwarzrmsu
author_rank: Contributor
timestamp: June 04, 2015, 05:02:56 pm
content: Kalvin,This is a brilliant concept. This will give me a great starting point, just need to tweak some parts of my boost converter and I should be on my way. Thank you very much.Scrts,Right now I am working on a development type project, not married to a particular OEM just yet. Totally understand how requirements can vary drastically OEM to OEM. Right now we are trying to generate a working concept that covers a good portion of the OEM specs and then tweak the design in order to be fully compliant if not already.I do not feel comfortable releasing the other portions of the design. If it were my personal development project I would completely open up and release all of the details, unfortunately that is not the case. The purpose of this thread was to help me understand (and hopefully others as well) how to design a unique type of DC/DC converter.Others,Please feel free to add other ideas that may accomplish the Vout = Vin + Fixed_Voltage type goal.Thank you.

author: jolshefsky
author_rank: Regular Contributor
timestamp: June 04, 2015, 05:16:32 pm
content: Would it work to just use an isolated buck-boost converter to deliver 6.5V and wire the output in series with the input?

author: schwarzrmsu
author_rank: Contributor
timestamp: June 05, 2015, 12:39:13 pm
content: Here is a simplified sketch of Kalvin's circuit, but using a PNP transistor instead of an opto-coupler:I simulated this design using LTspice over the range of my inputs and seems to regulate just great. A little noisy, but I am going to play around with some filtering or voltage regulation to see if I can null that out. Now I will spend the day looking for the perfect boost converter. Thanks again.Jolshefky,Are you suggesting a second buck-boost converter that is isolated to create the 6.5V reference? Or are you suggesting a solution that is a single switching regulator?

author: Kalvin
author_rank: Super Contributor
timestamp: June 05, 2015, 01:55:20 pm
content: Glad to hear that you have made progress.Btw, you should get a bit better regulation if you disconnect the resistor from the output, disconnect the transistor's base from the zener/resistor junction and then connect the resistor directly to transistor's base so that the current flow is zener - resistor - base.

author: jolshefsky
author_rank: Regular Contributor
timestamp: June 05, 2015, 03:34:59 pm
content: Quote from: schwarzrmsu on June 05, 2015, 12:39:13 pmJust one buck-boost converter. Something like this (note the polarities)...Code:+V --*--------------* *-- Vout| | || _________ | |*-|+ B/B -|--* |*-|- +|-----*| |_________||0V --*-------------------- 0V(That took longer than I intended ...)

author: schwarzrmsu
author_rank: Contributor
timestamp: June 05, 2015, 08:18:19 pm
content: Quote from: Kalvin on June 05, 2015, 01:55:20 pmhmm, I am getting better results with the orientation I posted. I will play around with some values and see if I can get it to work.jolshefsky,Very cool. I like this idea too, but I do not need isolation for my particular design and these isolated DC/DC converters tend to be in larger packages. Also they require a minimum current in the order of mA which is very high for my application. Appreciate the input though!