Full Bridge Converter - Have I done it right?  - Page 1
Forum section: electronics / projects

author: SaabFAN
author_rank: Frequent Contributor
timestamp: November 29, 2016, 08:38:06 pm
content: After it was pointed out here that my idea of using a Half-Bridge Converter at low voltages and high currents wouldn't really work, I have redesigned the circuit as a full bridge converter and would like your opinion on it before I order any boards.Do you notice anything wrong with my design? Or the layout? I have verified that the TL494 control chip is working with this configuration and I have some experience with synchronous and asynchronous DC/DC-Converter circuit. But I have only one more or less failed isolated converter on my record so far^^Schematic and Layout are attached.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2016, 10:58:22 pm
content: The gate drive traces are pencil thin, and the driver chips are quite far from the transistors.If you were doing this with the TL494 alone, it wouldn't be a problem at all, because its outputs transition in the 100-200ns range. Dead slow. But with drivers in there, you have to mind how powerful and how fast they are.The isolation isn't offline grade, but it would be okay for functional, and probably basic, insulation types, at modest voltages (a few hundred volts peak, including transient/surge). The voltages sound roughly automotive, so that's fine.The feedback loop is guaranteed to oscillate under some conditions. Under voltage regulation conditions, it's very likely to oscillate as-is; the TL494 is wired with over 10x voltage gain, which breaks therule of bounded ranges.But even with loop gain managed, the circuit is very likely to be unstable, or extremely slow (thus requiring massively more than C6 to filter the output!), because L1 and C6 are inside the voltage feedback loop.In current regulation, the loop gain is maximum, untamed by compensation, and delayed by L1 and C6 (and U4, and U5's slew rate and GBW), so is guaranteed to oscillate. Not only that, but it's probably not going to oscillate in the periodic sense, but chatter chaotically when slightly too much load current is drawn. And, importantly, this occurs on the load side of C6, not the converter side, so every little gulp of current the load draws, beyond that threshold, will cause it to hiccup.And the voltage error amp doesn't know about any of these goings-on, so a current-limit hiccup will spawn a huge windup and rebound in the voltage loop. Very messy indeed!Note: these problems aren't so much your fault -- these are problems that have been around as long as the TL494 (and its predecessors), which is to say,decades. Like, since the 80s, late 70s maybe even. I mean, yes, they're solved problems, thatyou should know about... But even more importantly, these aresolved problems that application notes should know about. And most of the time, ignore completely.What's the solution?Current mode control.The structure should be modified so that:1. The TL494 is just a PWM box. Nothing more. 0-5V input, 0-max PWM% output. It doesn't know anything else that's going on. So, one error amp is strapped to "off" (VREF and GND on the inputs), the other as a follower (-IN to COMP, +IN to opto).2. U4 goes in series with L1. Now, unless there's been a fault in T1 or the diodes, you have a precise measure of inductor current, at all times. And therefore switching current, so you can prevent your transistors from winding up into a short circuit.And that measurement is only modestly delayed, because the ACS711 has modest bandwidth -- not really enough for a 100kHz converter, but if you use a somewhat excessive value inductor, maybe a powdered iron toroid instead -- big dumb and cheap -- you can limit how far the peak inductor current can rise (or fall) in the time it takes to respond to that event. Which leads to...3. Inner current loop. You are building a constant current source, first and foremost! Yes, this is better! This will need an additional op-amp, maybe something simple like half an LM358. This drives the optoisolator, setting PWM% to control current.4. And, what do we set the current to? Now we apply the TL431. (Or, use the other half LM358 as voltage error amp, and use the TL431 as a regulator only. That works, too.) Sense output voltage, doing all the same stuff -- Rvd1, Rvd2, R4 and C10. But instead of driving the opto (as U3 does now), the voltage error amp drives the current error amp. You have one loop inside the other!As bonus, the current can never even becommandedto reach a higher current than the voltage-error-amp's output -- so by clamping that voltage, you can also get precision current limiting!Pretty slick.One last return to the circuit. I see things I actually like! It would be a shame not to call them out!- D3 is well chosen for what it's protecting (U1, U6 being the most sensitive I think, at 25V max). It's also current limited (R3). As long as R3 is, well.... it might need to be pretty fuckoff massive to handle an automotive load dump condition, but, as long as it doesn't fail shorted, everything will bepretty okay!- Although the transistors are pretty massive (way oversize, given the 10A-ish capacity of the circuit), the drivers aren't (~200mA), so switching will be pretty slow (not much faster than the TL494 alone). You may end up with a lot of switching loss, but you can always drop the switching frequency.- You've planned for current limiting at all, which is a far sight better than many beginners!- And you've got a TL431 with pole-zero compensation (i.e., C10 in series with R4). The values probably aren't right, but hell,I put in placeholders, most of the time.- AND you've got a bias resistor across the opto LED, so the TL431 won't be starved for bias when lightly loaded (which occurs when the output is heavily loaded). This means the knee (as the supply transitions from constant voltage to constant current) will be sharper. Notwithstanding my earlier comments about the current limiting, but with that solved, you bet!Cheers,Tim

author: SaabFAN
author_rank: Frequent Contributor
timestamp: November 30, 2016, 12:51:19 am
content: Thanks for the detailed answer.I'll incorporate the suggestions into the circuit and post the result here.You're right, this is meant to be for "automotive" (actually on a boat) use with a maximum sustained voltage of less than 20V.The converter supplies a 10€ microprocessor controlled solar-charger that only controls the negative rail. So the input to the charger has to be isolated from the rest of the power-grid. Otherwise I would have used one of my proven StepUp-Converters with a Linear Technology Chip in the middle of itMy idea of current limiting was that if more than 20A is drawn, the circuit runs into a "wall" and is turned off hard until the current is less than 20A again. Oscillation was no concern there, just a hard limit on the maximum output current to ensure nothing overheats or the batteries are cooked because of overcurrent and a failed charger. Fuses will break the connection if currents are in excess of 25A.But having this inside the control loop instead of using it as a limit-switch is much betterThe reason why I chose big transistors and through hole ICs is that I wanted the converter to be reliable and field-repairable. The TO-220 MOSFETs are more than overkill for this application and the chips are socketed so they can be easily replaced should they fail and are operated well below their maximum ratings in terms of supply-voltage.Also these components are dirt-cheap on ebay and available in every electronics-storeBtw.: Is the Ti App-Note SNVA555 "Understanding and Applying Current-Mode Control Theory" a good source about this topic?Also, I have read a few publications about load-dumps and designed the circuit accordingly. But because this is a boat, there can be load-dumps of up to 150A if the bow-thrusters are operated. The Alternator is able to deliver up to 180A. So I guess that there's quite a bit more power behind a load-dump spike than in a car. R3 therefore is a through hole-resistor so I can put a bigger one in there if necessary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2016, 01:30:06 am
content: Quote from: SaabFAN on November 30, 2016, 12:51:19 amAh...hrm. Two problems there:1. If it should "hard limit", or "turn off" for a while, then you should have a currentfaultcircuit. A small shunt resistor on the output would be fine (no need to waste the money for a Hall effect device!), with a comparator that sets a flip-flop. It either stays latched (until the user hits a reset button), or a timeout period elapses before it resets itself (auto-restart).This, by the way, is cooked into the nature of most offline switchers -- the controller is self-powered, so as long as the secondary is delivering useful voltage, the controller is also powered usefully. Too much load and the voltage drops out; the controller disables itself (undervoltage lockout UVLO), and power stops. Nothing happens until the startup resistor recharges the controller's supply to the turn-on threshold, and then it does it all over again.2. "The 10 dollar transistor dies to save the 10 cent fuse."Fuses cook off in tens or hundreds ofmilliseconds, transistors in tens ofmicroseconds.That's another liability in using hundred-ampere transistors, by the way -- you've got enough short-circuit current capacity to blow out the whole lot (transistors AND fuse), should nasty things happen!Not that smaller transistors are automatically better (there's less current capacity, but less die area, too; the time from short-circuit to burn-up is the same), but there are other advantages.If price and availability is a motivation, then why not something utterly abominable, like IRFZ46N's or 50N06's? Those stupid things arestill everywhere...And, about the transistors: ideally, you'd have the current sense shunt resistor on the primary side, at the bottom of the H-bridge. Then, you could close the inner current feedback loop with one of the TL494 error amps, and the opto would be transmitting the current setpoint, instead of the PWM% setpoint. (The secondary side would be pretty much unchanged.)Downside? The loop is only sensing current when the transistors are on, which isn't always when the output inductor is carrying current! (The net bridge current is zero when both top transistors, or both bottom transistors, are on -- they are circulating the load current by shorting the transformer across. But the inductor current remains constant during such time.)So you'd still want an output current loop, to keep the output current limit usefully accurate.At that point, you might also go a different route, like instead of the TL494-for-historical-accuracy's-sake, maybe a UCC3808 -- which is basically a UC3844 with the toggle flip-flop brought out to dual latches and drivers. Which is also better for the low voltage here, because then you'd use push-pull instead of full bridge, saving a bit of current capacity in the transistors (though making the transformer slightly bigger). (Which, I mean, you can do that with the TL494 right now, too, and I would recommend. But whichever.)Speaking of; UCC3808 and UC3842 are peak current mode controllers, which:QuoteHmm, yes, though it's rather technical (if you're versed in control loop theory, then by all means please read in detail!), and aimed more at peak-current-mode circuits. The peak mode controller has the added quirk of being a chaotic system. Namely, that as it goes into CCM, it's an implementation of theLogistic map, giving rise to that same beautiful histogram plot.With the inner current loop and PWM generator topology, you getaverage current mode control, which is generally much more stable (it is prone to period-doubling under some conditions, but won't descend into full chaos). The downside is it's a bit slower. But if you're going to have slow things anyway, that's just part of the design process (bigger inductor, capacitor, tweaking the error amp time constants).Oh, and I was talking about using the Hall effect sensor; that's not needed. With all the brains on the secondary side, you can simply use a shunt resistor in the diode bridge ground return.QuoteMaybe a vitreous power resistor on pigtails...That would be one motivation for choosing 100V (or more) transistors.BTW, TVS diodes (even the really big, really expensive ones) aren't capable of handlingthat muchenergy. If you need to absorb load dump, use MOVs. (You can use MOVs in parallel, to get enough energy handling. And yes, you'll have to figure out what kind of load dump capability your alternator might have.) Downside? The peak clamping voltage is 2-3x nominal voltage (whereas it's 1.5-2x for TVSs). But that means a "30V" MOV is still more than plenty of protection for 100V MOSFETs, so if you're going to the trouble of using higher voltage parts, you do get the handy bonus of them being nearly indestructible.The fuse will probably burn out during the load dump event (and maybe the MOVs too). Expect to need replacement fuses after such an event -- but as for the circuit, heck, it may even ride things out while maintaining full regulation! It might be preferable to put a pass regulator (instead of zener shunts) in there, so the controller can ride it out while the power section simply does its job.Tim

author: SaabFAN
author_rank: Frequent Contributor
timestamp: January 29, 2017, 08:21:22 pm
content: I have made the suggested changes to the feedback-circuit and the boards just arrived.I have to experiment a bit with the resistor-values (should have added a pot, but there wasn't enough space) to get the right voltage, but I'm still having some problems.At first I tried using simple isolated copper-wire for the transformer-windings, which caused the transformer to start smoking when I connected a big variable power-resistor to the output.The power that was put into the converter at that point was about 70W (14V, 5A) at that point.I then replaced the copper wire with HF-Litz Wire and used 4 Litz-Wires in parallel (twisted together) to form the primary winding and 3 Litz-Wires to make the secondary.This reduced the heating in the transformer by a HUGE amount (I can still touch the windings when I connect the power-resistor), but I still can only get about a quarter of the power to the other side of the converter than I intended.The Primary side has 5 turns, while the secondary has 11 turns. The core is an E39 Core. Switching frequency is 84kHz. The duty-cycle of the controller reaches the maximum at about 3A output-current (1 Channel of the half-bridge sees a Duty-Cycle of 44.50%).At the moment I suspect that I need more turns on the secondary side, despite all kinds of tools telling me otherwise and the entire thing producing almost 40V when I deactivate the voltage-regulation.The maximum Dutycycle of each channel is about 44% btw. So if I understand this correctly, the overall max. dutycycle is just a little over 90%, which is further reduced by the deadtime in the IR2111 Driver-chips.Am I right with my assumption that the problem and the solution is to be found in the transformer?Edit: I have tried powering the Converter with a PC Power-Supply, which delivers a solid 11.7V, regardless of the load. With that I can get at best about 49W of power at the output.Edit 2: I have added 2 Screenshots of the waveforms at the output of the transformer. I doubt that they should be looking like that. The first one was taken with the variable resistor set to about 80 Ohms. The other one was taken with the resistor set to a value that made the controller set the DutyCycle to about 43% per Half-Bridge.I now have new suspect: Too low frequency. The frequency at which the dampened oscillation sits is about 3.5MHz, while the switching-frequency is set to 84kHz.

author: james_s
author_rank: Super Contributor
timestamp: January 30, 2017, 02:29:08 am
content: Is this just a DC-DC converter to provide a floating supply isolated from the input? If so there's a very simple way to do that which doesn't require any IC at all. A Royer oscillator consisting of a pair of transistors driven by a feedback winding on the transformer driving a center tapped primary. A few years ago I put one together in free air literally out of parts that were in a pile on the floor. It worked, and lit a 12V 10W lamp that also happened to be nearby. For simple DC-DC stuff where tight regulation isn't required I like self oscillating converters because they're so simple.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2017, 06:26:59 am
content: The transformer is wound for too high an impedance (and probably too low of a cutoff frequency), and there's an even higher load on it than that (this is no load conditions??!).Waveform seems about right.What was the windup, and the expected nominal load condition?Tim

author: CM800
author_rank: Frequent Contributor
timestamp: January 30, 2017, 12:15:13 pm
content: I saw you put [ISOLATION SLOT HERE]but the isolation slot is cutting right through one of the power planes... Is this gonna be a problem?

author: SaabFAN
author_rank: Frequent Contributor
timestamp: January 31, 2017, 03:54:48 pm
content: Okay, I have given the whole transformer-thing a little thought while riding the bus today.Up to now, I have only been involved in designing DC-DC Converters that do not require transformers (Boost, Buck, SEPIC-Topology).If I understand this correctly, there is no way the transformer can transfer the power to the other side, because there isn't enough current flowing through the primary winding, because before the current can really start to flow, the Full-Bridge reverses the current flow. This is caused by a too high inductance for the set frequency.So I either have to lower the frequency (already changed the frequency to 43kHz and got a slightly better result), or reduce the turns on the primary winding to allow enough current to flow to get the required power to the secondary side.Am I right so far?

author: jbb
author_rank: Super Contributor
timestamp: January 31, 2017, 06:49:02 pm
content: Hi SaabFANQuote from: SaabFAN on January 29, 2017, 08:21:22 pmThat would be a skin-effect issuehttps://en.wikipedia.org/wiki/Skin_effect. Basically if your copper wire is too thick current only flows on the outside (skin), which leads to high AC resistance (even if the DC resistance is good). You don't necessarily need full-on Litz, but using a number of finer enamelled wires in parallel is a very useful trick.At 100 kHz the skin depth of Cu is around 0.2mm. You can make the wire 2*skin depth = 0.4mm diameter. Then use several in parallel to achieve the required total cross section. This can be easier to source and wind than Litz wire. But a good Litz wire will be better.Quote from: SaabFAN on January 29, 2017, 08:21:22 pmFrom these waveforms, it looks like you might have a leakage inductance problem. Your H bridge should generate a sharp-edged square-ish wave (it will spend some time at 0V). Your transformer should pass this through to the output with a degree of fidelity. What we're seeing in the second figure is that the output voltage is a) sagging and b) has heaps of ringing.Quote from: SaabFAN on January 31, 2017, 03:54:48 pmI think it's the leakage inductance of the transformer causing the problem. It is set by the geometric properties of the primary and secondary winding. Given that the impedance of the transformer (ignoring magnetisation) is X_L = j \omega L_lk, you can see that reducing the switching frequency or reducing the leakage inductance will help.Could you give us some more info on the transformer?Core sizeCore materialA cross-section diagram of how you wound itIt may be possible to improve matters.After you've addressed your output current capacity problem, we can look at adding a snubber to damp that 3.5MHz ringing. At present you will be kicking out lots of RF (unless in shielded box).

author: SaabFAN
author_rank: Frequent Contributor
timestamp: January 31, 2017, 07:30:07 pm
content: I figured that the problem with the heat would be due to the skin-effect.It was also much easier to wind the transformer with the litz wire. Looks a lot cleaner than beforeThe core I use is this one: TDK ETD34Datasheet:https://en.tdk.eu/inf/80/db/fer_13/etd_34_17_11.pdfCore-Material: N27You can see in the attached picture how I positioned the windings.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 31, 2017, 08:05:15 pm
content: Try winding the secondary on the top of the primary (or vice versa) (instead of doing it side by side), using the minimum thickness of insulating tape that provides the necessary insulation level you are aiming for. It appears your winding window is big enough you won't need multiple layers of wire; if you did need that, you would need to consider winding layers of primary and secondary alternatively.Then, it would make sense to utilize the whole winding window - now, in your picture, the primary takes much less space than even 1 full layer, so after you have spread out the secondary to a nice flat layer, you can go up to bigger wire diamater on the primary, creating a primary layer that utilizes the full width, too, on the top of the secondary. (Of course, do this using several paralleled strands. Don't gotoothin, or you'll end up taking more space in the wire insulation without achieving any AC resistance improvement.)

author: SaabFAN
author_rank: Frequent Contributor
timestamp: February 05, 2017, 05:46:05 pm
content: I have now rewired the transformer with both windings utilizing the whole space available and there's a massive improvement.I haven't tested the whole thing to the maximum power, but I now get the same power-output with just a fraction of the duty-cycle than before.Pretty clear actually, when I think about it now: The magnetic field immediately starts to fold back to the other side of the primary coil and thereby cannot induce that much current in the secondary winding. Not that they're positioned on top of each other, the area influenced by the fields of the two coils is more or less the same.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2017, 06:54:05 pm
content: Getting the primary and secondary in as close proximity, as much overlap, as possible, is the challenge with low voltage switching transformers.Unfortunately, this is exacerbated with the use of litz: because the magnetic field is able to penetrate the wire (indeed, that's how the current spreads out through the bundle!), it means the average distance between primary and secondary is the center-to-center distance, not the surface-to-surface distance. More distance means more space for magnetic field to fill in: leakage!This can end up trading resistive loss in the transformer, for snubber loss, handling the energy stored in the increased leakage inductance.The ideal approach, for very low impedances (ohms), is interleaved foil or sheet. The broadside faces allow primary and secondary currents to flow in close proximity. (Note that the currents are opposite directions, so they act to cancel out: the closer together they are, the less space there is for stray fields.) The main downside is, it's abitchto fabricate more than one turn of foil, and impractical to use with wide bobbins (that are better suited to high impedance converters, and inductors).The role of impedance cannot be emphasized enough.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 05, 2017, 09:53:06 pm
content: Here's a good starting point for an automotive power supply:http://sound.whsites.net/project89.htm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2017, 11:25:57 pm
content: Ugh, ughhhhhhhhhhhhh.That's not a power supply, it's a charge pump.Sadly common in automotive apps, but so are blown fuses!Tim

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 08, 2017, 11:16:39 pm
content: Okay, the thing worked until it overheated while I did a Full-Current test. It took about 1 minute for the diode-bridge to start smoking (I had no heatsink mounted on the Diode-Bridge and miscalculated the dissipated heat by a multiple of 4, also didn't really account for resistive losses in the inductor L1 - Naturally, Diodes Dbr1 and Dbr2 overheated when I drew about 16A) and another 10 seconds for them to fail short-circuit. After they failed, something happened that killed two or all transistors in the Full-Bridge. The short-circuit protection in the PC-PSU I used to power the device prevented further destruction and smoking.Apart from the diodes overheating, the remaining components remain at ok temperatures (without airflow from a Fan about 70°C for the MOSFETs, the driver-chips reach about 55°C, the Zener-Diode becomes quite hot at ca. 115°C, but within spec - although I saw the regulated voltage drifting to about 11V after about 5 minutes at that temperature and then staying there even when the diode is cold - So I'm considering a LM7810 or 7812 as a replacement - the voltage from the Alternator is 13,8V, so enough room for the regulator to work).I have therefore redesigned the schematic to include thermal shutdown, as well as reversed the control-voltages: The Controller is now by default set to turn off the driver-stage and has to be actively turned on by the controller on the secondary side. To allow startup with no power at the output, I have added Cstart and Rworkaround1 (the name stems from an earlier version, forgot to change it). They will momentarily turn on the transistor in the Opto and pull down the control-voltage to enable the Driver-Stages long enough for the secondary side to take over control.On the board I have added heatsinks that are soldered directly to the board and the Diode-Bridge will be getting a heatsink that is glued onto the packages.Updated Schematic is attached to the post.@ T3sl4co1l: Is this implementation of the regulation what you had in mind when you posted your suggestions earlier? It worked pretty well, after some tweaking of the resistor-values (until the overheat^^), but I'm not sure if this is how you envisioned itEDIT: The board is pretty full with stuff already, but I think I might be able to fit additional current-limiting functionality on the primary side to prevent another catastrophic failure on that side.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2017, 12:18:01 am
content: Yeah... that happens when it's not current mode. Momentary output short, diode failure, whatever... chain reaction, the rest of the circuit grenades, and you better have put a fuse on the input side or that baby's going up in flames.What is this feedback circuit? Diode drops in the feedback signals? Op-amps that don't pull to GND? Positive feedback on the error amp? Cap load for compensation?Tim

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 09, 2017, 01:29:31 am
content: Oops... It's obviously too late. The inputs on the error-amp are mixed up... Should have simulated it in LTSpice before uploading the schematic to catch thatFeedback-Resistor R17 and C20 are in there to dampen oscillations around the regulation-voltage. Without those I figured that the PWM-Duty Cycle would jump around between 0 and 50% all the time. I wanted to avoid that. C20 should be smaller though.The Diodes are in there because I felt more comfortable with using diodes for this task. LTSpice-Simulation suggests 10k PullDown-Resistor - Real world measurements say it works: Voltage at diode-output follows input-voltage almost immediately.The voltage-drop across the diode is very low - well within the margin of regulation I'm happy with (+/-5% are ok), but yes, diodes are probably not the best choice for this.It worked pretty good though when I tested it in my prototype. I got a stable voltage regulation and when I tested the max current, the feedback-circuit started to reduce the PWM-Cycle as well, unfortunately starving itself of power in the process (now of lesser concern, because supply-voltage for feedback-circuits is 5V instead of 10 and anything that pulls down the output-voltage down that far is not good).

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2017, 01:34:55 am
content: 115C is higher than I'd be comfortable with, even if the diode is rated to tolerate it. The PCB will likely brown and deteriorate with time and the thermal cycles will be hard on the solder joints. Also the die temperature is probably much higher than what you can measure externally.

author: mariush
author_rank: Super Contributor
timestamp: March 09, 2017, 04:01:00 am
content: If you're going to have a metal case, think about using 4 mosfets in a package like to-252 / d2pack / dpack/ h2pack / whatever , put them under the pcb and use a thermal pad (they're cheap :http://www.digikey.com/short/32hfh3) between them and the bottom of the case.No screws to loosen up due to vibration, no individual heatsinks to shake around causing weak solder joints on the to-220 leads - you can still make some through holes / vias near the tab of the surface mount mosfets in which to plug some copper/aluminum metal sheet that could help with the heat if you're not gonna have metal caseThose packages are still easy to replace just need a good soldering iron.. worst case scenario you cut the leads , flood the metal tab with solder (for heat transfer) and as you heat the solder the component just falls off the board. Easy as through hole.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 09, 2017, 11:02:21 pm
content: Okay, obviously I misunderstood some ideas, so I redesigned the circuit with maximum current-control on the primary side and voltage and temperature-control on the secondary side.I kicked out the resettable fuse (don't need that anymore, because current-limit is regulated on the primary side) and went back to a dual OpAmp, saving precious space on the secondary side.Also the Supply for the PWM-Controller is now regulated by a 7812.The Current-Control sets DutyCycle to 0% if the current through the transistors exceeds 40A. A LowPass-Filter prevents false triggering and, together with a bit of hysteresis, also prevents too wild oscillations in case of an overload. The MOSFETs can handle 75A, so there's enough room for the current-control to be a bit slow. The exact frequency response of the LPF is most likely dependent on all kinds of factors and therefore requires some tinkering, so I haven't put any values in there yet. I want to keep ringing away from the Comparator-Input, but the actual waveform of the voltage across the resistor to be represented as best as possible at said input. Using a ferrite-bead instead of a RC-Filter is also an option - I have enough parts here for extensive experimentsWould you say that this is now the right control-scheme?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2017, 12:02:10 am
content: Doing average current mode control on the secondary side is better, because the inductor current is continuous there. The H-bridge isn't carrying the (reflected) inductor current all the time, so the primary isn't continuous.You can do peak current mode control, but that's not as desirable for a high current forward converter (and needs a different controller chip, like UCC3808 -- check it out if you're interested).Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 10, 2017, 12:11:45 am
content: Just a quick note - R1 and R2 should be returned to their respective MOSFET's source terminals, NOT ground as shown in redesign 2.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 10, 2017, 06:06:28 pm
content: Okay, I'm putting the average current control on the secondary side back in. But I'm going to keep the peak current shutoff on the primary side to prevent another situation where the transistors burn out because of some sort of failure on the secondary side.I have looked at the UCC3808 and it looks like it might be suited for the task: Two output that are alternating, which could generate the control-signals for the driver-chips.I'll have to investigate further into that.But I still would need an external comparator or amp that brings the current-sense voltage high enough to trigger the Overcurrent-Comparator inside the chip without dissipating a lot of heat in the current-sense resistor.@ MagicSmoker:I think you're right about that - If I calculated it correctly, the resistors are sucking away about 20mA from the gate (Gate-Voltage: 23.3V if referenced to GND, even more if supply-voltage is 14V instead of 12V), which have to be supplied continuously by the driver-chip. It can supply that current for the required time, but with them connected to the source of the MOSFET, this current-figure can be at least cut in half. Also the resistors can probably be a bit higher in resistance to bring down that current even further.So I'll change that part of the circuit too

author: phenol
author_rank: Regular Contributor
timestamp: March 10, 2017, 07:32:58 pm
content: I would recommend LM5030 for peak current mode control as it has easy to adjust slope compensation mechanism unlike that other chip where the compensating ramp is obtained by buffering the RC oscillator pin. I used it for one of the boost converters (12-440v, 500W) i was working on earlier as a replacement for UC2825, which is basically crap imho. Every time the primary current ramped up to the pulse termination threshold, uc2825 would somehow fail to maintain proper flux balance in the core resulting in squealing, screeching and what not... LM5030 was perfectly "symmetrical ", but then i figured current-fed push-pull worked better for a boost converted and haven't used it since.Why don't you use a current sense transformer instead of sense resistors? I made my own from k2006 kaschke 10x6x4(?) ring... also why full bridge and not push-pull?..

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 10, 2017, 10:10:37 pm
content: Attached is the updated schematic. Average current-control is back on the secondary side and peak current-control is implemented on the primary side.The Supply for the Opto is also now 12V to reliably completely turn off the controller if necessary.OpAmp on secondary side is an LM324 (did a quick check on the breadboard and realized that the TL074 isn't suited for this task).To be honest, I'm not entirely sure why I went for a full-bridge instead of a push-pull configuration.I guess, because I was able to find more easily to understand information about the Half- and Full-Bridge converters. Also it seemed to me that the transformer for a full-bridge is easier to make.

author: lukaq
author_rank: Regular Contributor
timestamp: March 15, 2017, 02:54:15 pm
content: why do you torture yourself with full bridge when this is application for push-pull, more simple board and less transistors, no high side fet driver,...?