Isolated Flyback converter in boost mode  - Page 1
Forum section: electronics / projects

author: shangaoren
author_rank: Regular Contributor
timestamp: December 19, 2018, 02:18:55 pm
content: Hello everyone,I'm looking to make an isolated power supply for a jbc tip (isolated to not interfere with thermocouple in the tip)I would like to output around 160W at around 30V with an input going from 9 to 25V, it's for a heating element so the stability is not a critical part.As far as i know isolated flyback seems to do the stuff but i'm not an expert in that domain :Is there some already done design that match my needs ?If not what should i choose an all integrated controller or design my own with a microcontroller ?What transformer should i take ?

author: nick_d
author_rank: Regular Contributor
timestamp: December 20, 2018, 10:28:22 am
content: It's really not too easy, I know having spent many hours trying to do it (switched to other projects, will return to it).The best advice I can give is buy a PS matching those specs from China. Although, do look inside before installing it.Otherwise, this is what you do:1. In Digikey parametric search choose your controller. Many have built-in power MOSFET but I think with your power requirement you may have to use an external MOSFET.2. Design your transformer, I used a procedure from a book and wrote a Python program to carry it out. It involves basically figuring out how much energy the transformer needs to store and your maximum DC resistance and so on, then choosing a bobbin, core and wire of sufficient capacity.3. Read datasheet for controller carefully and build one of the examples in it. It will help you calculate resistors etc.4. Pay particular attention to the feedback network. This is rather tricky as it has an opto-isolator driven in its linear region. There is a part that is used for this, something like a buffered reference with error amplifier or similar. Being clever I tried to make do with a less complicated circuit. Bad!5. The SMPS IC and the feedback IC will have to be compensated, since there is delay through the loop and without feedback it will oscillate. In your application you can tolerate slow response to step-changes in input voltage or load conditions, so be generous with the compensation. Otherwise it's tricky and perhaps best done by experiment, as I don't know of a good mathematical procedure so far.Still keen? Keep us postedcheers, Nick

author: shangaoren
author_rank: Regular Contributor
timestamp: December 27, 2018, 12:59:30 pm
content: Thank you for your clear explainations.I need to fast results so i will not use flyback for this iteration but i'm curious, i will try later

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 28, 2018, 07:22:09 pm
content: Your requirements are a bit... much! Especially the input voltage range considering the output voltage and power required.Did you realize that getting 160W output power from a 9V source at say, 80% effiency will draw over 22A! Better have big cables/copper traces/connectors... here. If you don't want to set your house on fire.You can take a look at Murata, RECOM or XPPOWER isolated DC/DC modules for instance. (Eg.:https://power.murata.com/en/products/dc-dc-converters/isolated.html)You could find something that fits but you may have to settle for a more reasonable input voltage range.Note: I'm assuming a DC input and output voltage here. You didn't clearly state that's what you were after. Was it AC to AC instead?Anyway, why don't you just use a separate mains transformer (or a transformer with two separate secondaries) instead?

author: ocset
author_rank: Super Contributor
timestamp: December 28, 2018, 10:52:03 pm
content: you could use say 2 or 3 parallel boosts to boost your 9V-25V up to 40V, say.Then from there, use a full bridge to get your 30V, 160W.At these low voltages, you could use a couple of bootstrap high side drivers for the hi side fets.Or you could just use a flyback to go from the 40v to 30v, 160w.Use a couple of paralleled schottky diodes in the secondary, or a sync fet.I reckon for your app you could get away with on/off control and just pass the (digital) feedback signal through a digital isolator.As you say, its just a heater so who cares about some all singing and dancing dynamic feedback loop.So it would be in burst mode.......vout above demand......primary side switches away......vout below demand....primary side does not switch....youll end up with some hysteretic ripple on the output, but does that matter?

author: shangaoren
author_rank: Regular Contributor
timestamp: January 03, 2019, 08:59:01 am
content: Quote from: SiliconWizard on December 28, 2018, 07:22:09 pmI know that is a lot of Amps but the idea is to be able to feed the iron with a 12 or 24V standard power supply or a 3 to 6 cells Li-Po battery, i do some modelism and Li-Po battery can handle easily very high currents. In a perfect world the flyback should be settable to modify voltage output to match the power limit of the input.I saw on an another thread that heaters should be fed by alternating current so the output can be AC.

author: nick_d
author_rank: Regular Contributor
timestamp: January 03, 2019, 11:05:04 pm
content: You can simply connect the secondary of the flyback transformer to the heater, I don't think you would even need a rectifier diode here. Just give it a whack (charge primary currenr for a while then abruptly stop it) whenever too cold and you need some heat. Or if you wanted continuous heat output then possibly a forward converter topology might be better (smaller transformer?). Anyhow, this is a pretty undemanding application. But why not save yourself a lot of trouble and get an element of correct specs to drive with no converter. I understand you want the option to run with a LiPo battery. But this usage seems highly questionable to me. A high performance battery to drive a heater?Nick

author: shangaoren
author_rank: Regular Contributor
timestamp: January 04, 2019, 09:24:01 am
content: Without a transformer the ground of battery will be the ground of the heater and the ground of the instrumentation. If you take a look at the wiring of a JBC tip you will see that heater is wired in series with thermocouple and the thermocouple is wired to tip body which should be wired to permanent earth and permanent earth will be wired to supply ground. The result will be current flowing through thermocouple if there is no isolated supply on the heater.Maybe this picture will be more clear than my explanation :https://www.eevblog.com/forum/reviews/for-jbc-fans-out-there-jbc-tips-anatomy/?action=dlattach;attach=31171