Unusual current trafo connection probably doesnt work ? - Page 1
Forum section: electronics / projects

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 05, 2018, 07:00:00 pm
content: Hi I am looking at robust overcurrent protection for a half bridge converter, I want to be able to trip on shoot-through as well as output overcurrent.I conceived an idea of using a current transformer with TWO primary wires, one for each mosfet to the centre tap in anti-phase. This results in a unipolar secondary signal (rather handy for sensing without rectifier) and a nice fat signal for shoot-through that would not normally be detectable by a single CT in the output.However I have a horrible feeling the core will simply saturate especially when the duty cycle approaches 95% as there will be insufficient time for a core reset ?To add some specifics the peak current is about 25A, Fo 50Khz and dead time ~400nS, trafo could behttp://uk.farnell.com/coilcraft/cs1750l/transformer-current-sense-1-750/dp/2457940Does anybody have any experience/suggestions ?

author: capt bullshot
author_rank: Super Contributor
timestamp: January 05, 2018, 07:12:32 pm
content: That's an interesting idea. I have no idea if such a CT is able to reset in this circuit, but my guts say: Don't do it, since it's basically DC that you measure. Have you though about an active CT (LEM or whatever) in this configuration, these should work fine here? I've seen such (more than one primary wire) stuff done with LEM etc. current sensors in all kind of professional equipment, that's an easy way to add / subtract current. Using pure AC, this also works with ordinary CTs.Anyway, shoot through isn't supposed to happen (except while debugging the circuit) ...

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 05, 2018, 07:22:39 pm
content: Thank you for your LEM suggestionI have had a few inexplicable mosfet failures recently one of which blew the tracks clean off the pcb! Just could be deadtime not always sufficient under all conditions so I thought I would try and integrate protection for it too. I have tried the ACS712 style sensor but apart from not liking external magnetic fields it is to slow for this application so I will look at the LEM with interest

author: ogden
author_rank: Super Contributor
timestamp: January 05, 2018, 08:39:09 pm
content: Instead of building shoot-through protection, you shall drive (half-)bridge correctlyBetter sense overcurrent at supply input (ground terminal), using low resistance shunt resistor + amplifier.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2018, 09:24:13 pm
content: Use a per-transistor shunt resistor or CT.The proposed CT connection works as long as total duty cycle (D_HS + D_LS) stays low enough for the CT's flux to reset.The CT secondary circuit has a diode into the burden resistor, so that only forward current goes into it, and the reverse (flux resetting) voltage can be much larger.I would be worried about total duty over 90%.For a forward converter application, duty over 90% is indicative of a lack of headroom. You might adjust the turns ratio so it can operate at a lower minimum input voltage, or adjust the output range spec, so it operates at a lower maximum output voltage (same idea). You'd need to clamp the duty cycle at the controller somehow (usually by setting dead time).Also for a forward converter application, you need to sense output current, so you need two CTs or shunts anyway; using normal mode rather than CM+diff arrangement (i.e., one independent CT per transistor, rather than one cross-wired for both transistors, and one for output alone) would seem simpler, and more robust (you can operate at full 50% duty per transistor).If this is for motor control, or any sort of synchronous buck/boost converter, you may still be able to limit duty, but probably one or the other transistor will need to deliver 100% duty somehow, and for that, you must use a shunt resistor. On the upside, it's probably not an isolated application, so a ground side shunt resistor is perfectly acceptable. Use a current sense amplifier if necessary, to avoid burning huge watts in the shunt.Tim

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 06, 2018, 07:05:29 am
content: Good morning allI had a look at the LEM's but they are hall effect like the ACS712 though toroid based, in any case I fear they are to slow for this application.Yes Tim I had read about the use of a diode to allow the resetting voltage to be much larger but I was having trouble with calculating the result.Two CT's would indeed allow more time for reset at double the cost and space.Shunts with amplifiers are difficult to employ in such a noisy environment and one of them ends up on the switching node or high side.I was probably over-egging the max duty cycle a bit but it is difficult to control as the deadtime setting of the UC3825 is notoriously inaccurate and the supposedly improved UC3825B extortionately overpriced! The load is a transformer coupled LCL filter with attendant resonance so the DC is often pushed to the max.I think this is going to be very CT characteristic dependent so I will trial one with the reset diode and if that fails resort to two, thank you everybody for your helpful replies

author: capt bullshot
author_rank: Super Contributor
timestamp: January 06, 2018, 08:29:10 am
content: Quote from: fourtytwo42 on January 06, 2018, 07:05:29 amNo, look at the compensation type ones, with a secondary winding and a hall element in combination. The hall does the DC and LF AC, the secondary winding does the higher frequency AC. They have typical rise times of 0.5 to 1us and BW of some 100kHz, should be sufficient for shoot through detection.

author: jbb
author_rank: Super Contributor
timestamp: January 06, 2018, 09:55:04 am
content: A whole lot of products don’t do shoot through protection.If you need it, I have had very good results with desaturation detecting gate drivers. They sense excessive voltage drop across a device that’s meant to be on, and turn off quickly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2018, 11:10:53 am
content: Quote from: jbb on January 06, 2018, 09:55:04 amI don't normally bother with this for smaller converters, where it's not worth the added expense -- but I have never built a converter of multiple kW without it, and I can also hold, in one hand, all the transistors I have blown in those designs!Tim

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 06, 2018, 01:54:18 pm
content: I am kinda in the middle for power, 2Kw peak, 1Kw average on an ETD54 trafo.I am kicking myself thinking I should not really need shoot through protection were it not for the fact I am operating a wide temperature range and the deadtime control of the UC3825 is a dog. The UC3825A is better (seehttp://www.ti.com/lit/an/slua125/slua125.pdffor details) but neither of these chips really meet my needs which is an analogue input PWM generator with accurate deadtime control and LEB (BTW I am aware UC28xx is industrial temp range).I am trying to avoid a DSP as that simply bangs up the cost, complexity and the dreaded spectre of an SMT component (I am trying to stick to through hole for cheap prototyping). I have some ideas leftJust found out how slow LTspice runs when it includes a behavioral voltage source including the delay function so may be several days hahahaYes I did find the other LEM's, there site is very hard to navigate and doesn't even have data nor mention for all there series as far as I can see, but I agree the pulse response can be down to 400nS (on a good day) but by the time you add all the other delays can still be enough to destroy a mosfetTalking of hard to navigate sites TI's is getting pretty crazy if you just want to find some chips, OMG it's all about SOLUTIONS these days!

author: capt bullshot
author_rank: Super Contributor
timestamp: January 06, 2018, 03:13:38 pm
content: Quote from: fourtytwo42 on January 06, 2018, 01:54:18 pmMwahaha! All the semiconductor representatives that visit you on a more or less regular basis (at least at work) will try to sell you a shiny new solution for your solved problem ... The main drawback is: Their new solution, of course using only their (and maybe partners) parts cost more than your established and working solution made of simple components that you already stock in huge quantities (at least at work)Same for many of their websites ...

author: C
author_rank: Super Contributor
timestamp: January 06, 2018, 06:07:32 pm
content: fourtytwo42Using a center tapped inductor is not a new idea and it can work. I know it was used in military equipment.Back in the 70's or earlier this idea was used with SCR's.Can not get much simpler, turning on one SCR caused the current in other SCR to drop below holding current and turn off.Very simple three phase square wave motor drive can be built out of 6 SCR's and 3 center tapped inductors.So for your half bridge if not using SCR's you would need to add a turn off function like the SCR's.Where for the SCR's you need to work with SCR current, here you just need to get a signal for control circuit and inductor should give you some more time.

author: David Hess
author_rank: Super Contributor
timestamp: January 07, 2018, 02:59:24 am
content: Quote from: fourtytwo42 on January 06, 2018, 01:54:18 pmI have had problems with TI's site for years now not showing older inexpensive parts which are still fully in production in favor of newer expensive ones in the selection tables. If I did not remember that they exist and have old databooks and notes to look them up in, I would not be able to find them.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 07, 2018, 09:01:18 am
content: Quote from: David Hess on January 07, 2018, 02:59:24 amExactly and pointers from other sites or component distributors, you would have thought they would be doing there best to promote the use of there chips since they have swallowed up lots of other respected players, that makes it particularly annoying as they have eliminated much of the competition Grrr

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 07, 2018, 09:08:16 am
content: Quote from: C on January 06, 2018, 06:07:32 pmI was thinking of it as a new idea to meAnyway it's a current sense transformer and although the primary is centre-tapped the currents flow is in opposite directions so the core magnetization is in the same direction irrespective of what half of the winding is used hence the potential saturation problem.Interesting you mention SCR's, I remember before the advent of serious power mosfets there were quite a few fancy SCR's around vying for converter switches, GTO's & asymmetric ones come to mindI don't know where there place is now as most people seem to use IGBT's for higher voltage work including traction converters when mosfets run out of steam!

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 09, 2018, 01:28:17 pm
content: To continue the thread I did think the controller was really the nub of the problem and set about to replace it, one thing led to another and I started thinking about embedded MPU PWM's, so that led me to finding out how much processing delay a typical average current mode converter can manage in the control loop without breaking into sub-harmonic oscillation. The answer is not much and is directly proportional to switching frequency. So you either have to use a very fast and expensive MPU or a very large and expensive inductor, just love engineering

author: capt bullshot
author_rank: Super Contributor
timestamp: January 09, 2018, 01:36:00 pm
content: There are some MCUs (like the STM32F3 series) that provide you with an analog internal comparator and it's output routeable to a PWM unit just for this application scenario (current mode regulation).So the controller is halfway analog and halfway digital then, but still within the MCU.

author: C
author_rank: Super Contributor
timestamp: January 09, 2018, 07:58:56 pm
content: I stated I saw working equipment that used a center tapped inductor to turm off one scr when the other turned on in a half-bridge.Just to be clear, an SCR will turn off when current through device drops below holding current.Quote from: fourtytwo42 on January 07, 2018, 09:08:16 amThink about what you said aboveNow what is a current sense transformer.The primary is as close as you can get to no voltage, no resistance primary.Simple to see that this type transformer was not used to turn off the SCR above.You might use your current sense transformer to turn off the gate, but it will have little effect on load currents or shoot through currents directly.So if you were to copy the SCR idea of center tapped inductor or center tapped transformer while adapting it for a different device would be a good idea I think to keep the reduction of current in the currently on device to limit possible shoot through currents in device turning on.In simple words, start from a working circuit with SCR's. before seeing if it can be modified to your device.Saturation is part of transformer & inductor design. You design a bad inductor or transformer or pick the wrong part then you have saturation problem.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 10, 2018, 06:54:48 am
content: Quote from: capt bullshot on January 09, 2018, 01:36:00 pmThese comparitors are intended to terminate the cycle in case of fault and act asynchronously to the PWM, in average or indead peak current mode the duty cycle is controlled synchronously (e.g. by comparison with a ramp). I have been looking at sampling the analogue error signal using the ADC and loading the result into the PWM dc register but this is a reletevly slow process that consumes much of the MPU's MIPS.Actually I am sorry i started a thread about one thing and drifted to anotherI realise in peak current mode the current signal is synchronous with the pwm and so may be used directly in this form of comparitor but in average current mode things are considerably more complex, also my above description of controlling the dc register from the error signal would not work either!I think the summary of this thread is the current transformer is very liable to suffer saturation as the duty cycle aproaches maximium due to inadiquite reset time even with a secondary diode.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 10, 2018, 06:57:20 am
content: Quote from: C on January 09, 2018, 07:58:56 pmI am sorry we seem to be at cross purposes here, my problem does not involve SCR's at all, rather the switching devices are mosfets so no reverse bias is required for turn-off. The problem was with current transformer core saturation when the current transformer is used in a particular way, again nothing to do with SCR's.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 10, 2018, 09:26:01 am
content: Quote from: fourtytwo42 on January 10, 2018, 06:54:48 amFor a current mode regulator you'd use this comparator to end each PWM cycle. The comparator compares a setpoint (by the MCUs internal DAC) to the current sense. For the next level control loop(s), you'd use MCU code running each PWM cycle to set a new DAC value. The STM32F3 series can do these control loops at 30kHz to 70kHz PWM in an interrupt routine, using C code.Doing this comparator with an ADC and SW alone would require quite high sample rates, your common MCU can't handle this.

author: C
author_rank: Super Contributor
timestamp: January 10, 2018, 01:32:08 pm
content: fourtytwo42Think I am looking at problem a different way and could be seeing something you are not.Will state what is see in simple terms so you might see what I am.Half bridge: To reverse current direction in load.Guess you haven now,power <=> switch <=> switch <=> power with wire from between switches to load.When both switches are fully on, load current has changed fast to 50%.Change one, add resistance between switches.power <=> switch <=>wire <=> load <=> wire <=> switch <=> powerYou now have 2x wire resistance between switches for only cost of wire. Max Shoot through current dropped some.If load has some inductance, it will make some change across wire resistance. Guess is greater then 1/2 current change of resistor divider wire.Change two, wrap some wire around core and add two secondary windings.Switch drive <=> secondary winging <=> switch.Now could be full of it here.Normal: secondary winging slows turn-on of switch & off switch turns-off more.When turning on second switch with one already on.Shoot-through mode: secondary winging slows turn-on of off switch & turns-off second switch some.Could be full of it or could have something to build on,you decide.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 11, 2018, 09:25:43 am
content: I imagine this describes reducing shoot through by inductance and I agree it wouldDue to other issues I am about to change the entire topoligy anyway but thank you for your help.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 11, 2018, 10:56:13 am
content: Quote from: capt bullshot on January 10, 2018, 09:26:01 amI agree this would work for peak current mode control however I am using average current mode control just to be awkward and different hahaha well it kinda makes life harder to impliment but thanks anywayAs the control is so damm dificult I have decided to fix that first and bin the UC3825, maybe the shoot through problem will just resolve itself in the process

author: C
author_rank: Super Contributor
timestamp: January 11, 2018, 09:05:07 pm
content: Way back,Some of the equipment was built in the days where a special transformer was no big thing.Today you see,Current sense transformers that I think fall and one end.Voltage sense transformers at other.Normal transformersBack then it was not that uncommon to see a transformer that used a bigger gage wire then needed.Too large or small amount of iron for field.DC on windings.Transformers stored data ( core memory ).Transformers regulated powerand many othersKeep in mind that if you can use two wires from your half-bridge it is an almost free change with some benefits.Your response to my last post is not bad.I would think of it as delaying shoot through as much as possible to allow something more time to prevent shoot through.Most times I see a good reason to have all that extra crap in a newer chip.Today you sometimes see a three phase driver with 4 half-bridges. The forth there to protect PS from over-volt on fast stops or direction changes.In your new design, you might want to keep an eye open for problems created or fixed by adding distance between your half-bridge and the load.Good luck

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 11, 2018, 09:37:55 pm
content: Quote from: C on January 11, 2018, 09:05:07 pmHi C many thanks for your interesting reply, I actually used to write software for identifying the faulty diodes or drivers in core stores as I had so many of the dam things stacked up on me for repair (I was the fall guy) but omg that was a long time agoGuess I have always been interested in statistics one way or another. Sometimes your stuck between a rock and a hard place, is it better to modify the power electronics to work with some chip or other, or is it better to try find a better chip and/or if that fails make your own control circuit. Well in my case having tried the former I am now doing the latter because in my gut I know its the crap chip at fault!Your obviosly a servo man, I spent a large part of my life in big computers and had quite an afinity for anything electromechanical, early 14inch disk drives with voice coil actuators, big tape drives and daisy wheel printers with servo carriage and wheel drives were fun for me although I also did my time sorting out processor and memory timing problems long before they became single chips. I once got involved through the family with industrial electro-hydraulic bending machines, now that was real funAs for distance to the load in my case the distance is an LCL filter and the load is the electric grid, pretty imovable! Thats why the control electronics has to be adroit at protecting the power electronics and a class better than that found in off the shelf chips like the UC3825, frankly the manufacturing tolerances of such devices are not anywhere near good enough. Anyway I had one of those brainwaves today and feel confident in setting a new course that will bring results though the initial complexity is mind bending, but that's what I enjoy

author: C
author_rank: Super Contributor
timestamp: January 12, 2018, 01:38:02 am
content: Thought you would be fast with you online.My first post with the center tapped inductor and SCR's.It could be stated this way,"Try to create a shoot-through and get normal square wave driver function."Very backwards thinking for today, not normal even back then.Servo man is close but not correct.lFirst work location 43m in 74, Some old crap from late 50's or 60's up to stuff most could not buy then.Worked on all you listed and more, But you left out the insane stuff.One was Graphics terminals + multi-user mini-computer + mini-computer +z80 + core memory, + + +Get ready for it, was a RADIO.Left out the very power hungry flat-pack logic on copper boards & extreme analog stuff.Any way, a lot of stuff I worked with had many feed back or feed forward paths. Some were very bad when looked at standalone, but when put all together became supper reliable and very safe and often simple.Where normal down to a price, keep it simple thinking lets you overlook things you would see if money and simple were not a factor, it is often good idea to start on paper with that fancy cost no object thinking and reduce it in final design. For computers this often gives you option of just adding a extra board.That radio used three types of memory & drum & 14 " disk storage to get job done. Simple was address = speed or device = speed.So starting from big cost is no object to electric grid.simple gas generator does not need much to connect to grid.Freq a little less than grid, generator becomes a motor.Freq a little to high generator trips on over output.Two Freq meters, two voltage meters and phase lights used to sense.Two controls Freq & voltage.When correct, output on.Power To grid, you keep trying to run a little higher freq. If this freq change happens you have grid drop.Power From grid, you keep trying to run a little lower freq. If this freq change happens you have grid drop.Voltage or current is how much power.Not quite that simple as you can have big grid generator and a lot of local ones but it was and is done.Do you keep this in your design?Working from small simple you might not see this. Freq control can go back aways in circuit.When you toss cost is no object you might think ofFreq difference stacka modified 50 Hz with 0, positive, 0, negative, 0with a second stacked at 100 hzand upwith the top/bottom doing voltage adjustment with just time for others.voltage difference stackMatched voltage stackParallel stack with options of matched or different.For places to look at or copyYou have grid tied ups backup to grid.di-directional motor drive with breaking.Soon or now car battery as backup to house and/or grid.Japan is part 50hz and part 60hz last I heard.The converter I read about uses stacked modules to get past semiconductor voltage limits, Parallel stacks for current.Over voltage protection for module is limit max voltage across it ( think zener diode) and stack more modules then needed for system max voltage.So in simple termsIf you stack stages it could only take one stage to prevent shoot-through.Simple and cheap could be separate circuit for shoot-through with second for current sense.Using current sense transformers that could be two, three or four.with 25 A normal, you might use 30a sensors for current and 100s sensor for shoot-through.for shoot-through you might start turning off both switches to gain time for actual protection to function.Some power supplies use a scr to blow fuse for protection. You might use one to kill gate voltage.Whole lot of options if you think outside of current box.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 12, 2018, 07:46:29 pm
content: haha yes I like to think out of the box but as I get older it just takes longer