Ultra low cost Isolated DC/DC Convertor - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: March 23, 2018, 11:26:50 am
content: Hi,I have this idea of this circuit for making a 12V 10ma isolated DC/DC module, what's your opinion about it? is it doable? How can I calculate the galvanic isolation? Do you have a better and cheaper idea? voltage regulation is not a concern, since we can use Low cost linear Reguators for the job

author: BrianHG
author_rank: Super Contributor
timestamp: March 23, 2018, 11:54:30 am
content: You can buy isolated DC-DC converters for 75cents US each, except, to get that price, you need to buy 10 at a time for 7.50$.5v to 5v, 1 watt (200ma). 1kv isolationhttps://www.aliexpress.com/item/10pcs-isolated-dcdc-power-module-5V-to-5V-1W-DIP-dc-dc-converter-B0505D-1W-quality/32813792229.html?spm=2114.search0104.3.99.181832a5v6Oxt4&ws_ab_test=searchweb0_0,searchweb201602_3_10152_10151_10065_10344_10130_10068_10342_10547_10343_10340_10548_10341_10696_10084_10083_10618_10139_10307_5711215_10313_10059_10534_100031_10103_10624_10623_443_10622_10621_10620,searchweb201603_25,ppcSwitch_5&algo_expid=242e9ff2-65c4-4e3a-bbff-153cbf76f9a6-16&algo_pvid=242e9ff2-65c4-4e3a-bbff-153cbf76f9a6&priceBeautifyAB=0

author: Zero999
author_rank: Super Contributor
timestamp: March 23, 2018, 01:20:41 pm
content: To answer the original question: yes, that will work but there are far superior ways of getting an isolated DC:DC converter.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: March 25, 2018, 08:05:22 am
content: Thanks Hero999,Quotewould you please share some of them?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 25, 2018, 08:46:20 am
content: QuoteRead the datasheet for the CM inductorCM chokes are certainly a potential source of cheap magnetics, but isolation voltage is likely not well specified, or if specced will be a few tens of volts at most. If you just need to break a ground loop it's probably fine but not a good idea if any significant voltage is involved.Ethernet magnetics or ISDN transformers might be worth a look for where you care about isolation voltage

author: nctnico
author_rank: Super Contributor
timestamp: March 25, 2018, 01:39:02 pm
content: Quote from: ali_asadzadeh on March 23, 2018, 11:26:50 amThe PWM duty cycle will set the input / output voltage ratio but you will probably need a small load resistor. I have used all kinds of transformers for this purpose. A cheap telephone line transformer will also do if you need a certified barrier.

author: Marco
author_rank: Super Contributor
timestamp: March 25, 2018, 07:04:28 pm
content: Quote from: mikeselectricstuff on March 25, 2018, 08:46:20 amA CM choke for mains use will obviously specify isolation voltage.Electronic design used to havea circuit idea postabout this very topic, but actual schematic has been lost in time. Unless you have specific requirements, like say regulated output at low power consumption for which you still have to pay through the nose, it hardly seems worth the effort any more.PS. a little googling foundthe schematic.PPS. I think he build it like a flyback converter BTW.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 25, 2018, 08:32:40 pm
content: Quote from: Marco on March 25, 2018, 07:04:28 pmBut any mains-rated choke is likely to be way oversized for something that only needs 10mA

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2018, 09:49:33 pm
content: Parts like this,https://www.digikey.com/product-detail/en/taiyo-yuden/TLF9UA103WR23K1/587-2793-ND/2573880typically hi-pot in the 10kV range(!). Transformer characteristics are crap. Expect about 1W with 20% regulation (open loop) at 12V, 50kHz.Tim

author: ovnr
author_rank: Frequent Contributor
timestamp: March 25, 2018, 10:41:56 pm
content: Quote from: T3sl4co1l on March 25, 2018, 09:49:33 pmWell, the price is right at least, and I trust that + a driver more than a mystery unit from Aliexpress. Any rough numbers on how crap they are, efficiency-wise?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2018, 10:47:30 pm
content: Efficiency is fine, loss is by leakage inductance.Tim

author: Marco
author_rank: Super Contributor
timestamp: March 26, 2018, 12:18:19 am
content: I think the two switch flyback is ideal for a simple converter. Just like push/pull (and unlike the simply flyback from my link) it doesn't waste too much energy, but it can't flux walk and it only needs two FETs instead of a full bridge.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: March 26, 2018, 05:26:11 am
content: Guys thanks for your inputs, I think the main pricey part is the transformer, As BrianHG has suggested an off the shelf product is under 1USD, we can use gates or mux's to drive the transformer in full bridge mode, like using this part SN74LVC1G19 , Since I only need 10mA we should find a cheap way of making the transformer, as we can use a spare timer on our MCU to generate the Clock pulse for removing the oscillator too, So what cheap options are available?