tiny DCDC with many isolated output rails? - Page 1
Forum section: electronics / projects

author: hlavac
author_rank: Frequent Contributor
timestamp: April 14, 2018, 07:53:15 pm
content: I need a tiny cheap DCDC power supply to provide many (maybe up to 16) wimpy floating 15V gate supply rails for high side N-MOSFET switches from 24V main rail.I was thinking forward converter with a transformer with many windings, each output having simple diode and cap per winding, made from toroid core and wound with a bunch of wires twisted together... running sorta open-loopSwitching frequency should be probably pretty high (1MHz-ish?) to keep the transformer core small and unsaturated...Any tips for a chip to drive the primary side? Ability to sense that output caps are charged enough to stop pumping for a while to save energy would be a plus.. (reflected voltage sampling? not sure what the multiple windings would do to it though ...)

author: james_s
author_rank: Super Contributor
timestamp: April 14, 2018, 08:32:26 pm
content: I think I would try a simple Royer oscillator, I've built DC-DC converters that way that worked very well. No IC required, just a couple of transistors.

author: hlavac
author_rank: Frequent Contributor
timestamp: April 14, 2018, 09:18:20 pm
content: Good point. I think I will try that.

author: jbb
author_rank: Super Contributor
timestamp: April 14, 2018, 09:24:47 pm
content: Texas Instrumens have written some white papers on what they call the 'fly-buck' topology. It's a way of tacking isolated outputs onto a buck converter. If you've already got a buck converter for your control supply, this could be a good option. The main output is properly regulated, and the isolated outputs are ... sort of regulated.When it comes to implementation, the stray capacitance of a multi-winding transformer can be troublesome. How fast are the switching edges going to be in this circuit? (High dV/dt levels push more current through stray inter-winding capacitance.)Also, have you looked at a bootstrap supply for the high-side FETs? They have some restrictions (most importantly, duty cycle < 99%) but are very cheap and simple.

author: BrianHG
author_rank: Super Contributor
timestamp: April 14, 2018, 09:28:42 pm
content: I don't know speed or current you want, but, have you considered optocoupler photovoltaic MOSFET drivers. You get isolated gate drive with a built in isolated DC supply. Many are only 8v out, so, you need 2 in series for 16v.

author: BrianHG
author_rank: Super Contributor
timestamp: April 14, 2018, 09:37:18 pm
content: If changing to logic level mosfets, IE 5v, these guys are 75cents each:https://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: BrianHG
author_rank: Super Contributor
timestamp: April 14, 2018, 09:41:01 pm
content: Also take a look at solid state switches. 'Optocoupler ACorDC mosfet output relays'. This replaces your mosfets, and they have come down in price far enough with high enough output currents that they may end up costing less and giving you less of a hassle to make your project function. You can now get 20mOhm on resistance for AC apps, 10mOhm on resistance for DC apps, or 10 DC amp switch for under 4$ in a DIP6 package. Under 3$ for 25pcs. Though this is only for 30v switching. Higher voltages exist.

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2018, 10:32:30 pm
content: Quote from: hlavac on April 14, 2018, 07:53:15 pmA flyback lets you sense voltage on the secondaries, which approximately balance.

author: ocset
author_rank: Super Contributor
timestamp: April 14, 2018, 10:37:10 pm
content: With forward type converters you need to discharge the magnetising flux after every switching cycle.I woudl just do a flyback, and regulate an auxiliary rail, which woudl be coupled to all the other rails...and you thereby regulate them....just put in an rc filter on each rail so you dont get too much overcvoltage on each secondary due to leakage current ringing.

author: hlavac
author_rank: Frequent Contributor
timestamp: April 14, 2018, 11:06:20 pm
content: Ok looked at the Royer, it seems to me it gets out of control too easily wrt amplitude and frequency as it is resonance based, which is a problem for transformer saturation (runaway current -> mosfet kaboom) and output voltage stability (gate>Vgsmax -> mosfet kaboom) for the intended forward converter useMaybe just a drive winding driven by a NPN transistor from a digital PWM output and transformer reset winding with a diode to ground...?Bootstrap supply yes looked at it, but it kind of worries me it needs to be charged first, while I shouldn't have 100% duty cycle typically due to winding current control there may be times I need to for some period of time, then the caps would have to be big, then they would charge slower... complicates things. Thats why I wanted real gate voltage rails, to not have to worry about it.. maybe I'm just paranoid sorryAs for the tiny DCDC modules, yes I have been lazy and used something like this (but 15V) in the prototype, but they werent cheap like this

author: BrianHG
author_rank: Super Contributor
timestamp: April 14, 2018, 11:21:06 pm
content: Just use a Vishay LH1262CACTR Optoisolator Photovoltaic Output. It has 2 channels, 15v output on each. You'll need 8 of them to drive 16 mosfets. Just tie the 2 outputs to your MOSFET's gate & source. Nothing else. Small, compact, completely isolated. Though, it's designed for ON/OFF operation at it takes 35us to turn on, 90us to turn off. Rapid pulse switching would have to be done on the low side if you are making a high frequency switcher.

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2018, 06:53:56 pm
content: I would use an inverter with an isolation or gate drive transformer. If you need more outputs, then add more transformers.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: April 15, 2018, 07:19:25 pm
content: How about an LT3999 (push-pull drive)? You would need to have full-wave rectifiers at the output to avoid core saturation. The driver takes a duty cycle input, and you could use it to implement a feedback loop. If efficiency is not a concern, run open-loop and stick a 15V LDO on the output of the rectifier.For prototyping purposes, use the Versa-pac transfomers (six independent coils), that would give you four outputs per transformer. As David mentioned above, add more transformers for more outputs. These could be swapped for custom magnetics in your final production version.

author: hlavac
author_rank: Frequent Contributor
timestamp: April 16, 2018, 08:55:17 am
content: LT3999 looks like an excellent candidate for what I want to do. Will have a look