How to Improve Efficiency of Inverting DC-DC SMPS? - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 19, 2020, 04:45:14 pm
content: i'm building inverting DC-DC from 5V to -15V. i read somewhere, efficiency of this kind (invert and boost) is 80% and more. but when i tried simulating, the efficiency is only 50 - 60%...from picture and tina-ti sim circuit below, at output -15V and 30 ohm load (0.5A), output power is 7.5W (P = VI), but probing on the input current its 3A @ 5V ie 15W input, factoring 80% PWM, its 12W, so efficiency at best is 7.5/12 = 63% only, on real circuit i suspect it will be much lower. so the question is, is there any systematic way on inductor/capacitor selection to improve efficiency? i tried few inductor and capacitor values on the sim (20KHz PWM) but efficiency doesnt go up significantly. so please help..

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 19, 2020, 05:40:08 pm
content: Use Schotky diode instead of usual silicone D1, and be sure that T1 fully opened by control signal.IRF9240 have Rds = 0.5 R - it is too much.Find T1 with smaller rated voltage (<=100V) and smaller Rds (for example IRF9540, or even IRF4905 with Rds = 0.02R).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2020, 09:54:25 pm
content: Keep in mind that efficiency depends on how, and when, the calculation is performed. If startup transients are still settling out, their energy will be included in the calculation, giving an incorrect answer. When the simulation is only a few milliseconds long, these can have a large impact on the result, compared to an IRL test that extends over whole seconds.I forget, does TINA-TI support cycle-average or quasi-periodic analysis? If so, there should be a way to set initial conditions to avoid transients.Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 19, 2020, 11:40:14 pm
content: Quote from: Vovk_Z on February 19, 2020, 05:40:08 pmdiode loss is around 0.3V * 3A = 0.9W, there are 11W more to nail down... the problem with tina-ti is finding the right mosfet model, the parameters shown are just mind boggling, all the mosfets selection showing like 100V Vds speccant find the right one. i will be using AO3401 p-mosfet for the real job, its meant for lower voltage work, Vgth low enough that it will fully open with 5V supply, much more lower Rds-on. so yes i agree, parts selection in the sim is quite abysmal.Quote from: T3sl4co1l on February 19, 2020, 09:54:25 pmi dont know what is support cycle-average nor quasi-periodic analysis. but what i know is tina provides DSO like view simulation, where traces are like moving in real time, so when the display settled down, i took a screenshot, so i believe what i showed is steady state.i was expecting there is calculation/analysis for inductor/capacitor value selection in academic/systematic way, but maybe there isnt. i'm a bit dissapointedmy google foo also turned me down.. i have to continue with mosfet/diode tuning. i did change to schottky, didnt improve much. changed mosfet in sim, it got worse. what improving a bit is by tuning/reducing PWM duty cycle (attached) i can reduce Iin from around 3A to 2.5A while maintaining -15V output level. i guess this is why switching controller ICs are limited to like 15 - 85% duty cycle. too much duty cycle is not anyway (reduced efficiency curve?) o well i guess i'll go the normal way, just do it and try and error in real life circuit.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 19, 2020, 11:44:38 pm
content: one logical thinking i came up why this circuit topology is that the poor performance is mainly due to some current (power) has to be dumped to ground (during p-mosfet ON) in order to keep the inductor's (current) momentum to maintain voltage inversion on the output. but my reading contradicts with the belief so... i dont know what i dont know, or i know/read/understand it wrongly

author: twospoons
author_rank: Frequent Contributor
timestamp: February 20, 2020, 12:16:46 am
content: Try picking a MOSFET thats fully enhanced with a gate drive of -5V. The one shown in your circuit probably needs closer to -7V to be fully on. I assuming 5V PWM?I assume Tina accepts standard spice models (I don't like it - I use simetrix) so it should be easy enough to find a model for the part you want to use in real life

author: ejeffrey
author_rank: Super Contributor
timestamp: February 20, 2020, 05:33:26 am
content: Step away from the simulator.You can work out the bulk of the losses manually. Rds-on, diode losses, and inductor DCR can be calculated on a napkin, no need for a simulator. There is certainly no point running a simulator with the wrong components.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 20, 2020, 09:44:46 am
content: agreed. i guess this is my mistake by using wrong parts in sim. anyway, sim helped me in inductor and capacitor size selection, so i dont have to deal with 1st order derivative math on paper. punch in the pwm freq, see the current in virtual dso is not topping up indicating saturating inductor, tweak the value until it seems right. capacitor the same, reduce the value until sagging within acceptable level. such a thing.btw, parts arrived from lcsc. the smd 470uH inductor is big and heavier than what i expected (12x12mm footprint) for a compact and mini psu. maybe i can rip that apart and reduce the coil bit by bit until i get the sweet spot on real circuit. as i said, i hope there is systematic way on this, but i guess there is isnt. i'm ready for the next phase... build on real circuit. thanks all.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 21, 2020, 09:24:45 am
content: Quote from: Mechatrommer on February 20, 2020, 09:44:46 amUsual rated inductance is 47..100 uH max (with that size). So try to stay in 47 uH inductor, for example.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 21, 2020, 10:53:19 am
content: the problem when i want to be quick is i ordered 220,470 and then 2.2 and 4.7uH. now i missed 22 and 47uHmaybe on next revision.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 21, 2020, 01:20:22 pm
content: You don't need explicit derivatives for inductor sizing, any DC-DC converter datasheet will have the design formula that is accurate enough for any reasonable purpose.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 21, 2020, 06:59:16 pm
content: indeed! i'm going to use me2109F ordered from lcsc as switch controller at 300KHz it only need 22uH as per datasheet, now i'm screwed. i cant sim greater than 50-100KHz as the mosfet went flat off. so i ordered what is in the simagain o well, maybe i can also try to play with my original plan ie using attiny10 mcu as custom switch controller. and i guess too big inductance wont affect much @ 300KHz, better bigger than small to avoid saturation, right?btw i also ordered me2139 @ 1MHz switching. it should be able to use much smaller L and C but i'm not comfortable with the speed, maybe it will create lot of high frequency noise affecting analog opamp that it will be powering up (even though there will be LM337 as post linear regulator). with slower switching freq, i can increase gate resistance to make slower rise time, hence reducing switching noise, that the idea...but maybe its just my noob, the plan is also as.. experimentation and gain some experience in this. cheers.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 21, 2020, 08:44:00 pm
content: Quote from: Mechatrommer on February 21, 2020, 06:59:16 pmDepends how you mean, but I would say the reverse. Lower value inductors have higher saturation current for a similar core. But if you get too low the ripple current and peak current go up. But that is normally more a problem for your switch than the inductor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2020, 11:43:33 pm
content: Regulators with internal compensation simply use whatever inductance they are designed for. Using too large a value will only make them oscillate.Larger values are desirable in general -- when you don't have restrictions like fixed control parameters -- because the current ripple fraction is smaller, and therefore core and winding losses are lower, and a cheaper material, like powdered iron, can be used.These don't happen to be strong motivations at low power levels, so the typical regulator uses a fairly high ripple fraction (say 50-100%), which usually requires a ferrite or composite inductor (which are cheap in those sizes), allowing a simple control type (peak current mode control), and easy design.Ed: FYI, ME2139 appears to be a voltage mode controller, which is a pain to compensate (it also has internal compensation, good luck with that) and doesn't indicateanyprotection features whatsoever. It will likely explode during extreme conditions: startup, transient load, high or low input voltage, etc.Tim

author: Yansi
author_rank: Super Contributor
timestamp: February 21, 2020, 11:53:13 pm
content: Why don't you use any of the thousands of available stepdown (buck) converters for this job?

author: twospoons
author_rank: Frequent Contributor
timestamp: February 22, 2020, 06:23:08 am
content: Your LM337 post reg is not going to do much filtering above 100kHz. Check the ripple rejection vs frequency curve. You'd do better with a passive LC filter.Or you could get really keen and switch to the Cuk topology.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 22, 2020, 07:44:29 pm
content: thanks tim, and jeff, and others... , pack of infos....Quote from: T3sl4co1l on February 21, 2020, 11:43:33 pmi'm not aware of differences between/among the switching controllers. i did read the title of some datasheets about "current mode" or "voltage mode?" but i didnt give a rat arse about them, i thought they are all quite the same. what matters is the internal circuitry diagram of the drive pin, the compensation or feedback/correction circuitry however is simply a blackbox to me, i only expect it to work as it should ie output low PWM % when overvoltage, increase PWM when undervoltage. and i havent seen a SMPS that needs external compensation correction so extensively, they are all just simple voltage divider to feed some ref voltage like 1.25V to feedback pin, so its maybe my limited sense of experience. any reference to what are the differences among them regarding this? and i guess the slower ME2109 is its brother, so should be a PITA too? i'll see when i put them in the circuit. thanks for the insight, i'll watch over it.Quote from: T3sl4co1l on February 21, 2020, 11:43:33 pmi specifically choosed the ICs that dont have internal power mosfet so i will not be limited by its internal power limit/spec (freedom to do more things outside such as adjustable Vout beyond limit of the IC etc). those choosen are gate control type, not the Vout type (the already regulated output on silver plater). and both of them can drive 1A gate current. with slower freq, and lower output PSU current spec of 0.5-1A, i can further reduce the gate current limit by increasing the resistance to reduce the IC's weight lifting. if anything to blow, its the external mosfet (AO3401) used. correct? thats why i didnt care much about temperature protection, i'll diy outside the chip if i want to.. the smallness footprint of the controller is more important too as this project is mini-size in mind (albeit being scalable in power and adjustability), hence sot23. featurefull is not really that important as the load will be predictable in this project, ie to power some opamps and additional circuitries if necessary, within the PSU limit budget.Quote from: Yansi on February 21, 2020, 11:53:13 pmthis again, i'm not sure the difference and why its so important? i guess any controller that has gate control output can be used in either buck, boost, invert as long as the drive circuitry is suitable. its just a matter of mosfet/inductor/diode/capacitor arrangement/topology used as explained in basic literatures, no?however, i have another circuit from previous year that i havent got to correct or find a reason why (see attachments), its still sitting somewhere waiting for its queue... its a larger adjustable bipolar psu project for general purpose in the lab. it uses 34063 as controller in buck mode pre-regulation (the actual L and C value that i built on circuit may differ from what is shown in the diagram). the upper portion of the snippet picture (SMPS1) is normal buck from higher VCC+ voltage bucked down to VSS+. the lower portion (SMPS2) is mashed up or hack to get negative voltage VSS- from larger magnitude negative source VCC-. both ±VCC are from transformer secondary ±20V and the final output is lower ±10-15V iirc (top right in the attached zipped pdf diagram below, ±VO using my poorman diy linear regulation method). the poorman linear part is another story (i should already expect it to be "poor" performing as its name implies, nevermind)... the positive portion of SMPS1 in the snippet worked correctly, PWM dance nicely to get flat SMPS output. but the negative part, although it can produce the requested negative output VSS-, but its visibly sagging even at low load, probing the SWC pin of SMPS2 indicated quite retarded PWM output compared to the positive side. my initial conclusion was that i blame on the mashed up -ve side push pull BJTs that maybe doesnt match properly with the 34063 driver circuit (SWC pin). and/or also maybe the opamp arrangement that goes to ref/FB/CII pin probably gave slower feedback respond or such. but as i said, i havent got to debug those circuit, i lost interest due to circuit complexity and much work was done to correct solder bridges on the diy etched pcbthat i got tired. and some other projects came in the way (as usual... never complete a project when new project comes). so if you have sharp eye maybe you can spot some error or foolishness there, i'll appreciate it. thanks and cheers.Quote from: twospoons on February 22, 2020, 06:23:08 amnot sure what you meanby LC, bandpass? or mabe i can put RC as well to attenuate high freq noise. 0.1 ohm and 220uF should cut down anything beyond 7KHz. i know LM337 is a bit old, havent updated my knowledge on more modern negative adjustable linear regulator. and i still have many LM337/317 in stock.Quote from: twospoons on February 22, 2020, 06:23:08 ami did try building this on sim. it seems we need the right caps and L values, otherwise its oscillating, there must be systematic way at LC pair values selection. and it requires 2xL and 2xC, that will increase the circuit size... maybe i'll look in more details later, for now, not sure whats the real benefit if i have to compromise increasing board size... thanks for your idea anyway. havent heard of Cuk topology before.

author: twospoons
author_rank: Frequent Contributor
timestamp: February 22, 2020, 09:47:17 pm
content: LC filter: low pass. Series L, parallel C. Gives you a second order filter, with twice the roll off of an R C filter. This will be easier to make work than your 0.1R and 220uF, as you will struggle to find a 220uF capacitor thats effective at 1MHz (ESL, ESR and all that). This is true for any filtering of course, which is why manufacturers publish impedance curves for capacitors and inductors.The advantage of the Cuk is the continuous input and output currents - which makes filtering easier. Yes its more complex, but that balances against the filtering that can be required for other topologies. The two inductors can be wound on the same core, which can save space and cost.But its "horses for courses" - you need to look at the priorities for your application to decide what is best.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2020, 01:02:31 am
content: Quote from: Mechatrommer on February 22, 2020, 07:44:29 pmWhen they're quite clearly telling you they're different? Come on now.You can read up on control methods in appnotes likehttps://www.ti.com/lit/an/slua110/slua110.pdffor example. There are more simplified explanations if you look around.Regardless of control method, the part that glares at me is the complete absence of any current sensing of the driven transistor. That thing is toast. Actually the datasheet indicates as much. Referring to the ME2109https://datasheet.lcsc.com/szlcsc/1808091834_MICRONE-Nanjing-Micro-One-Elec-ME2109D515M5G_C236787.pdfpage 10:QuoteThey expect you to limit the output brute force with the transistor's resistance.On the upside, IRF510 are cheap, but don't forget the heatsink!They give a range of acceptable inductances but it's not clear at what point compensation is a problem. Again, compensation is internal, fixed, there's no adjustable range, it simply responds at whatever rate it does. Presumably, it'll squeal like a piggy if you put 470uH on there, or even more likely, the wrong caps.Note that the output filter cap is part of the loop, and needs specific ESR. You'll probably use a tantalum, or a ceramic with a resistor in series. (Page 10, paragraph 3.)QuoteWhich they don't specify -- don't know where you got "1A gate drive" from, maybe IEXT (abs max)? (Which is labeled as "EXT Pin Voltage", nice copy paste error.) Data table seems to give it as something like a 74HC bus driver output pin -- IEXTH and IEXTL are -35/+55mA for a 0.4V drop. (Also, that drop seems to be from VOUT/FB, not from VDD -- yet they do not show this connection on the block diagram. Which means Sneaky!) Extrapolating to 5V, we might hope for up to -437/687mA (i.e., short circuit to the opposite rail), but ~200mA is more likely into a real gate.Which is perfectly adequate for a tiny thing like this, you wouldn't want to drive more than a few watts. But I wouldn't bother when there are reliable, quality regulators available for prices well within my design budget.Note that regulator = integrated switch, controller = external switch. This is a controller. You can get regulators that only need... maybe four external parts, very compact.You can also use a BJT with this, using a resistor divider from EXT to base to GND. The short circuit current could perhaps be set a little more accurately this way. They don't give a diagram or formula, unfortunately.QuoteSure, but therein lies your problem. It is perfectly within the scope of your above request, that the switch turn on or off, hard. 0 and 100% are still PWM%'s. But a glance at the circuit will show that's not a good approach.That's the fundamental problem with voltage mode controls: they can only look at the voltage, but the inductor current could be anywhere. You can at least limit PWM%, but now it's a matter of degree: what do you limit it to, 99%? 90%? 50%? There will always be some situation where that's not enough. Into a low voltage or short circuited load, PWM might have to drop to 1% to protect the switch.Can we interest you in a NCV3063 instead?...QuoteRight, many controllers integrate that for you. You don't have any control over them, you have to hope they've done their homework and chosen the right values. In turn, you need to choose Lsw and Cout such that the loop response is complementary to the internal compensation.That's really what you're after, compensation is a time constant for the error amp which is complementary to the output filter.Current mode controllers have the advantage that inductor current is basically compensated for free. The voltage error amp sees a simple 1st order response, and it doesn't much care about cap ESR.Voltage mode controllers go straight from PWM% to Vout, the inductor current is unknown -- and the filter is 2nd order with excess phase shift, so they have to add hacks like the capacitor across the VOUT/FB voltage divider, and Cout ESR, to make it even usable in the first place.They usually end up hacking in extra circuitry to deal with short circuit conditions, like sharply reducing PWM and frequency when Vout is low (assuming a short), or using a current sense resistor anyway (but stopping short of using it directly for control). They all end up being equivalent in some manner, they're just lumpy hacks approximating the straightforward and preferred method.Or by analogy: when you press the throttle in an automobile, do you expect its position to instantly correspond to velocity? Or do you expect it to correspond to acceleration, so that you apply and release throttle to get up to speed?QuoteTypically transistors fail as three-way shorts. At these low voltages, the controller might survive anyway. In general, it's a good bet that the transistor, gate resistor, controller and maybe some other support circuitry gets toasted. Very common failure mode in offline power supplies (which are typically current mode, e.g. using the old UC3842 peak current mode controller -- they're a lot more reliable, but still a far sight from indestructible!).QuoteHow about TPS560200, modestly low voltage, sync rect (no external diode needed), just add inductor, capacitors and resistor divider? LMR14203XMK, wide voltage, SOT size. TI, ON Semi, LT/AD ($$), AOS, Richtek, ST, Microchip, etc., all good brands with various offerings, likely to be of better quality than LCSC dregs -- but still need to be inspected with a critical eye towards any gotchas. Most of their websites are easy enough to use, and have better selection tools than even Digi-Key.QuoteSort of. If PWM goes to 100%, that's fine for a buck, but a boost will never get any output voltage, until the switch lets out its magic smoke.But the buck needs either a bootstrap driver for NMOS, or a PMOS which has much lower performance. The bootstrap driver will eventually run out of charge, and turn off, limiting PWM to a bit shy of 100%.You can use a bootstrap driver near GND all the time, that's fine, but a non-bootstrap driver can only work with respect to GND.And the invert needs either PMOS, or an isolated driver for NMOS -- bootstrap won't cut it here because the source pin goes below GND. You can't actually find gate driver ICs that do this, they only serve voltages above GND.Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 23, 2020, 10:50:52 am
content: Quote from: T3sl4co1l on February 23, 2020, 01:02:31 ampage 5 in datasheet you linked, both ME2109 and ME2139 are the same ±1000mA though they made copy paste typo by tagging it as EXT Pin "Voltage" but i highly believe its should be EXT Pin "Current" (how can a voltage has mA unit?) the circuit is also provided in the page (black box Buffer Driver) since it can drive both current direction with the plus minus sign, i believe it already has internal push pull bjts or fets which is a nice thing for my need, i only need to put one external power mosfet.Quote from: T3sl4co1l on February 23, 2020, 01:02:31 amthe PWM is limited internally by the IC's conventional wisdom 15-78% (page 1) so i have no control over it, i just have to work around that range. maybe this is one distiction between buck and boost/invert mode. with buck controller, it can go to 100% PWM if it likes to, without destruction, but a no no in boost/invert topology.Quote from: T3sl4co1l on February 23, 2020, 01:02:31 amdont worry mate, i already have Seek Compact thermal camera, before this i have "the finger". even if i dont have datasheet, it will tell when a fet/bjt needs a heatsink or not. it deals with pretty much any thermal issues, in the circuit or in the house, neat!Quote from: T3sl4co1l on February 23, 2020, 01:02:31 ami already have bootstrap IC (soic8) in stock to drive high side NMOS if needed. but i'm not using it in this project. i can accept the loss of lower performance PMOS in exchange for circuit simplicity. and i trust the loss compared to using NMOS+bootstrap will not be that much in this little project. just a wild example, using 65mOhm PMOS (AO3401) vs 33mOhm NMOS (AO3400) at 3A, thats 0.3W of extra loss, i can accept that.Quote from: T3sl4co1l on February 23, 2020, 01:02:31 amthanks for suggestion, maybe good option for another project i'll keep that in mind. they only capable to around 1-2A (internal power fet). the circuit in OP is boost (inverted) spec at least to 0.5A @ -15V ie 7.5W. assuming 100% efficiency, input current is already at the limit of the IC 1.5A @ 5V. optimistically 80% efficiency, thats 1.8A and according to 50% of the sim, the IC needs to output 3A, beyond the spec. what will really entertain me is if there is a chip with push pull buffer driver like ME2109 (±1A max) with FB and/or Isense pin, and whats more important is another pin to adjust the PWM frequency, so i can choose what frequency for a particular circuit. some time i need to lower the freq, to increase inductor size, hence more current can flow through it, in for example, a larger PSU project. searching few parts out of hundreds in lcsc didnt give me much hope for that, i dont have time to read datasheets for all of them and mostly they are similar with only FB pin (sot23-6 or soic8 package) there are many more 10/12/16/24 pins packages, but i dont really like the big footprints for this project.anyway, all of these infos will help me put the puzzle esp when i get the circuit running on real pcb. currently still fixing my diy pcb maker machine. i cant wait to make another pcb order from china, that will take too long. thanks all.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 04, 2020, 03:29:07 pm
content: Quote from: T3sl4co1l on February 21, 2020, 11:43:33 pmi decided to use the faster ME2139 due to small L+C size (which i already ordered) and LC filter idea from twospoons to knock out any > 100KHz noises or so. i built a rough circuit on diy pcb and both +ve and -ve rails are working yay! but there are more noises to knock out in later revision, currently LC or any filter is not implemented yet, probing was made directly on the boost/invert output..attachment 1ramping up and down of both rails... yellow = +ve rail, blue = -ve rail (not really -15V due to quick and crude resistor divider installed). dummy loads of 470 ohm present on both rails... ironically, the -ve rail which uses TL431 ref hack/bias to the FB pins is less noisy than the +ve rail.attachment 2this ME2139 seems to have an intelligent function, where on the -ve rail, it made a "startup pulses" (FB > 1.25V) and then idle for a moment, and then do the real pulsing later on. i didnt expect this kind of behaviour i thought it will just pulsing continuously when power is applied. so this china made IC is rocking. (yellow = -ve rail, blue = EXT (SW) pin of S2)attachment 3Quote from: twospoons on February 22, 2020, 09:47:17 pmi think i will proceed with LRC + RC filter since it looks ok in the sim... stacking 2 LC network, i still get undesirable notch due to resonance nature of the LC filter. the LRC + RC will use 2x 0.1ohm resistor to do the low pass effect, at 3A, there will be 0.6V drop (1.8W loss) across them which i think acceptable. i can always use 2x 0.05ohm (4x 0.1ohm parallel series) if i want to reduce power loss but at expense of increased low pass frequency. what do you think?attachment 4ps: i made rough calculation on the efficiency.. 470 ohm load on both rails (around +15V and -10V), 5V power supply showing 0.25A, so its around 55% low, quite close to the original simulation

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 08, 2020, 06:34:45 pm
content: this bloody thing. i already was so happy that i made revision to include filtering and got only about 30mVpp noise. everything worked fine at about 30mA current output (470 ohm into 15V output)... until i made the load test. its not much, just 200-400mA test, the outcome is burnt 2x AO3401 and just now the +ve rail AO3400 gone (both ME2139 survived). this bloody thing is f*cking inefficient. at 200mA 15V output (3W), it draws about 1.4A from 5V power supply (7W) even lower efficiency than the simulation. and the regulation is as poor as road potholes.. the -ve rails 2nd AO3401 did take few abuses (latched to fully on) before giving up, the 1st one dead instantly due to surge current (AO3401 is ON during startup) when connected to battery pack (now i added NTC during 2nd AO3401, but now demised as well, NTC is random 1ohm R25 from my pile of stock) i thought its my stupidity and the FB circuitry hack using TL431 -ve rail as bias, ok, so now without -ve rail active (removed), i thought things will be much better on +ve rail, i was so wrong, the +ve rail is just as poor less than 50% efficiency. testing 0.5A, AO3400 is toasted without mercy 1st time, now i'm out of idea, burnt mosfets on both rails, its time to sleep.some boring chronology of the load test... if somebody want to help diagnosing...1st - 3rd attachment is the latest schematics and pcb top and bottom (how its done and probed, ie 0.1 ohms are added to each rail to sense current)test on -ve rail (inverting converter)purple CH3 = 5V supplyyellow CH1 = 5V supply after NTCblue CH4 = Q2's (AO3401) gatecyan CH2 = V- (-ve output rail -15V)1.png: (4th attachment)load pulse 0.2A is applied, the -ve rail is pulled to the GND like hell, the blame is on the slow respond NTC as seen on yellow trace.2.png: (5th attachment)larger load 0.5A iirc is applied, the -ve rail is fully floored and my electronic dummy load unit tripped open (automatic), and the Q2 is latched ON afterward.. ME2139 tried its best to do some intelligent by pulsing off and then on again to recover (i guess), seen as blue fill. but to no success as Q2 is already latched ON, not sure why, and -ve rail goes to the floor afterward (not shown in 2.png)3.png: (6th attachment)steady state latching, while ME2139 still pulsing, but -ve rail is already floored. Vusb (after NTC) is read as 2V only(PSU in current limit mode 1-3A) due to shorted AO3401. i managed to do 4 or 5 times this latched mode abuse madness and screencapture it before the AO3401 dies permanently (not regulating)... during latched mode, AO3401 is cooking oil flux that i put on it as "toast indicator".4.png: (7th attachment)shape of Q2's gate during undershoot recovery (HI = Q2 off, LO = Q2 on). i dont think its any problem (nice squarish), isnt it?test on +ve rail (boost converter)some changes in probing...yellow CH1 = 5V supplycyan CH2 = 5V supply after NTCblue CH4 = Q1's (AO3400) gatepurple CH3 = V+ (+ve output rail +15V)a.png: (8th attachment)load pulse 0.2A is applied, the +ve rail is pulled to the GND like a pothole, Vusb after NTC goes to about 3V. but i dont think its enough for excuse to not regulate properly, ME2139 keep pulsing until overshoot when load is released. when overshoot, pulses goes off no surprise there as its designed for boost topology, low side NFET should be turned off to let the +ve rail goes down again.b.png: (9th attachment)Q1's gate voltage (pulses) during 30mA idlec.png: (10th attachment)Q1's gate voltage (pulses) during load test 200mAclosing: maybe the circuit, the ME2139, or the dc-dc parts used dont like transient load. or some other mistake i made, or some theory i missed, i dont know, cant think rationally right now, its time to sleep, wait for another idea tomorrow after 24 hours work straight (i slept 14 hours yesterday in the weekend just for this failure to happened, my bad dayMurphy is always right)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 08, 2020, 07:05:55 pm
content: Obligatory "told ya so"...200mA each rail, and you're going to power this from USB? What?Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 08, 2020, 07:40:44 pm
content: not from USB as in a PC USB. but from an appropriate power bank or wall adapter that can adapt to itmy concern is, why a load of just 0.2A at one rail will consume more than an Amp and resulting mosfets destruction. as a teaser... my mark is 0.5A on each output rail (15W) 1A if possible.. that make it 60W (5V 12A) on the input at least for 50% efficiency but thats still a long way to go, from what i experienced today. (i thought i'm that close)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2020, 12:02:19 am
content: USB chargers are still only 10W.60W through a USB connector might make a poor cigarette lighter.Did you measure the actual switching waveforms?Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 09, 2020, 12:45:12 am
content: Quote from: T3sl4co1l on March 09, 2020, 12:02:19 amthen i have to find more expensive charger or make one myself, but that is for later...Quote from: T3sl4co1l on March 09, 2020, 12:02:19 ami'm using USB-C connector (5A capable) maybe asking 1A on each output rail is too much. 0.5A maybe reasonable @ 50% efficiency will require 6A on USB input. but but... i'm trying to figure out how to achieve 70-80% efficiency, that will make input current much lower and so i can spec the output current much higher. reading this, maybe this is not a simple one line black and white formula to deal with... so i guess i have to figure out (hack) and explore the grey/uncharted area for myselfhttps://electronics.stackexchange.com/questions/131698/boost-converter-efficiency-goes-down-as-duty-cycle-goes-upQuote from: T3sl4co1l on March 09, 2020, 12:02:19 amyou mean the mosfets' gate voltage? its in the attachment above 3,4,b,c.png if you mean switching current (voltage measured on 0.1ohm shunt, I = 10V) then i have them as well but didnt post it, they are much more noisier.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2020, 01:28:48 am
content: Those are pretty slow gate waveforms. Drain (and current) is where it really matters, but it's not looking good, also the supply voltage looks to be drooping heavily under load I think?Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 09, 2020, 01:46:43 am
content: i'll take note of those in the next probing session thanks. maybe i will reduce the gate resistance. the voltage droop is mainly due to the NTC used during transient load and 5V PSU went into current limit (set at 1A and have to crank up to 3A for 0.5A output load test), i'll try to figure out to get more solid supply next time. these droop and transient load is 2 undesirable recipe for disaster here, esp on inverting dc-dc (pmosfet will turned on when ME2139 EXT pin goes LO, ie default during startup and recovery phase). i'll concentrate on improving +ve rail boost dc-dc alone next time and see where the problem came from.

author: twospoons
author_rank: Frequent Contributor
timestamp: March 11, 2020, 12:51:45 am
content: I suspect the ME2139 is simply too weak to drive the AO3400 / AO3401. I'm estimating (crudely i admit) switching times around 150ns on / 250ns off mark. On a 1MHz converter thats way too slow.Basis for estimate is the 6nC gate charge of the AO3400, and the +40mA/-25mA drive of the ME2139.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 11, 2020, 01:24:57 am
content: yup i guess so.. the max recommended gate capacitance is 700pF (ME2139 datasheet). AO3400 has 800 - 1000 pF, so clearly beyond that. anyway, i removed 10 ohm resistance to the gate, made the feedback closer and tested each rail one at a time. i managed to get 80-90% efficiency on +ve rail up to 300mA before AO3400 is heating up real bad. transient load respond also improved greatly. but when testing the -ve rail alone, thats where the problem is, efficiency real bad, AO3401 heating up even at light load 30mA, at around 200mA, another AO3401 is burnt again. rail stabilization is just as before road pothole bad. so i declared my hacked up feedback system on -ve rail is a failure, i guess ME2139 is not designed to compensate for the (inverted) loop. so i move on to next revision, i made some further study and i found charge pump topology, so i'm using it right now. if mosfets still heating up, maybe i have to switch to the slower ME2109 controller to reduce switching loss which responsible for the heating up.