DC to AC converter

[Simon]

well if what you say about military certification is true I'm pretty sure it is not happening now for all systems. We supply a company with air con units who build the bare military vehicle (I'll let you guess where they are made) the vehicles are then shipped to the UK where proper military speced companies install all of the real electronics and another company (or maybe the same not sure) puts all the amour on. So by the time the army get these wonderful machines of destruction and defence it does beg the question as to if they even know half of what is in any system and many other parts of the original vehicle baring in mind that they will not inspect or test anything but just want to see a humungus paper trail that suposedly ties all parts back to the original suppplyer, and this will not be questioned until a problem occurs.

I had to carry out tests on our aircon system to prove that we were not generating the back EMF that was destroying our electronic thermostat (that is commercial grade), ok by the skin of our teeth's i was able to demonstrate that the thermostat was not blowing up in just our system but i had some stiff recommendations for our technical department. having concluded that although our design was not too clever it was surviving it remained to listen to the rumours that the asian vehicle maker was indeed having issues on their alternator system, this supposedly a fully fledged top of the line company, a company that want's everything supplied in lego sets because they are not clever enough we have to supply the bolts that will bolt the aircon system into the vehicle (and we change then more for the service than it's worth for a few standard bolts in packets).

of course if your working on weaponry you are going to be designing to the top specs and are being checked for it but there is so much going into military machinery as there is into any complete vehicle that there will be things in it you will never know about. it would be simpler if the whole thing was made in the UK but really I'm pretty sure we would still cock it up, at the end of the day although quality in military stuff is higher spec than commercial it's everything is never as dandy as it should be in an ideal world and cost plays a high role in many decisions

[Zero999]

Induction leakage spike, not reverse  recovery. When you switch on an inductor it can cause large spikes:

Surely you mean high current spikes not high voltage spikes?

I would recommend you check with Vishay and ask if the caps you've choose have are mil-spec certified and what applications they are certified for use (ie weapons, ground, naval, aerospace, etc) . I believe usually you need to document all of the components used and provide testing data the shows MTBF, enviromental tests (ie corrosion, temperature swings, radiation, etc) to prove that the components used are reliable. I believe your design also needs to be tested under OV/UV high EMI noise, to obtain certification. To my knowledge you can't just slap together a circuit and sell it to the Miltary without any certifications. There should be a specification for the requested system provided by the DoD, that lists all of the requirements they require. The engineers must follow those requirements and provide documentation that proves that the design meets all minimum requirements, which is than certified by the DoD.

There are different levels of radiation hardening, EMC noise and over voltage. .

All manufacturers state that their components shouldn't be used in critical life support situations but it's nonsense really. What one has to do it look at the component, its MTBF and what the different failure modes are, their consequences and formulate a design which contains no single point of failure and if it does fail the person signing off the design must take responsibility, they can't just blame Vishay or whoever.

Just saying that an engineer should not use a certain type component for military/aerospace use full stop is wrong. Really it depends on how mission critical the design is and what the consequences of it failing are: will it endanger life or will it just be a nuisance and can easily be fixed back at base?

The same actually applies in a commercial setting to some degree too, for example mains RF suppression capacitors have to meet specific criteria depending on whether they bridge functional, basic or reinforced insulation because it's safety critical.

To be honest, I didn't get too much involved in calculating the MTBF of the entire design, we had a reliability engineer for that who could be a pain because he would keep asking me the MTBF for different components used and ask me to change them if it wasn't could enough. All I remember is that plain old aluminium electrolytics are used liberally in military designs and they get past the approvals system just fine.

[Simon]

the circuitry that I'm replacing was an absolute joke, enough of a joke to warrant a six page report ! not that they listened very hard. a 25v electrolytic cap on a 28V supply, this is what gets into military stuff if it's not mission critical, then again a fire in your aircon system is not a joke

[alm]

a 25v electrolytic cap on a 28V supply, this is what gets into military stuff if it's not mission critical, then again a fire in your aircon system is not a joke

With some luck, that (aluminium?) electrolytic will be replaced by tantalum in the future. Even more fun/fireworks .

[Simon]

you would probably have to trust to luck, I'm still trying to figure if the ditsy company that we got them from made them or had them made in china in which case: good luck and bang!

[Zero999]

of course if your working on weaponry you are going to be designing to the top specs and are being checked for it but there is so much going into military machinery as there is into any complete vehicle that there will be things in it you will never know about. it would be simpler if the whole thing was made in the UK but really I'm pretty sure we would still cock it up,

In theory it would be better if the whole thing was designed by one company, although that can't possibly happen as no company will have experts in everything from semiconductors to air conditioning.

at the end of the day although quality in military stuff is higher spec than commercial it's everything is never as dandy as it should be in an ideal world and cost plays a high role in many decisions

There's a lot of commercial grade equipment which goes into military systems. One of the reasons why vehicles are designed to have such robust environmental control systems is because some of the components used in the cabin won't operate over the full temperature range, for example try finding an LCD monitor which will work from -50°C to 50°C? You can't, even a ruggedised military spec' one wont' do that, so it's better to make sure the temperature range in the cabin ranges from 15°C to 25°C. Of course there are other options such as a CRT or plasma but then there are other things to consider such as weight, space and reliability. Even if you then found a monitor which will work down to -50°C, human factors would tell you to find a keyboard with large enough keys to be operated with someone wearing thick gloves. It's better to just buy a monitor and keyboard with a standard working temperature range of 5°C to 40°C, make sure it can survive being stored at -50°C and ensure the ambient temperature is suitable before using it.

the circuitry that I'm replacing was an absolute joke, enough of a joke to warrant a six page report ! not that they listened very hard. a 25v electrolytic cap on a 28V supply, this is what gets into military stuff if it's not mission critical, then again a fire in your aircon system is not a joke

Yes, if it's not mission critical then commercial grade is often used to save cost.

What's the point of it costing ten times the amount when nothing bad is going to happen when it fails and it can be replaced easily?

Did you have the schematic for the 28V PSU or did you look closely at the PCB to see how the 25V capacitor was being used? Could it be that it was on a part for the circuit running at a lower voltage? For example a microcontroller running from a 5V supply.

[Simon]

the circuitry that I'm replacing was an absolute joke, enough of a joke to warrant a six page report ! not that they listened very hard. a 25v electrolytic cap on a 28V supply, this is what gets into military stuff if it's not mission critical, then again a fire in your aircon system is not a joke

Yes, if it's not mission critical then commercial grade is often used to save cost.

What's the point of it costing ten times the amount when nothing bad is going to happen when it fails and it can be replaced easily?

Did you have the schematic for the 28V PSU or did you look closely at the PCB to see how the 25V capacitor was being used? Could it be that it was on a part for the circuit running at a lower voltage? For example a microcontroller running from a 5V supply.

no it was the input cap for the 78L05 reg coming off a 24V automotive supply, everything we supply for 24v is speced up to 28 (battery under charge)
I'll email you the circuit let you have a giggle too. In my view this was a very dangerous setup: an under spected cap which if blown (lots of spikes on power line) would cause the 5V reg to fail (which was already unstable and had 500mVpp output spikes due to lack of output cap) this could destroy the MCU which would could them leave the 50Hz driven solenoid pump frozen with 28VDC through it. The pumps must function in order to keep cool as the water they pump keeps them from over heating. so you have a very hot pump in a plastic case..... fire !

We regularly receive units back with slightly melted cases from commissioning attempts. The jerk that made these even tried to dupe us into believing that he was upgrading the software through the sensor port which was impossible because it was a 2 pin line and not the minimum of 4 i'm not sure they have understood this at work as I'm the only person thatunderstands. Really to think that such a heap of junk designed by such an incompetent person ended up in a military machine is hair raising

[Zero999]

Agreed, it was designed by an idiot, either that or the unit is only rated to 25V maximum and was really designed for a 12V to 18V supply which still wouldn't surprise me. It also might not be the design engineer's fault, purchasing could've ordered the wrong capacitor and the production engineer didn't notice, still someone's dropped a bollock somewhere and the lack of a 100nF capacitor across the LM7805's output is inexcusable.

More detail for everyone else: it's just an LM78L05 with a 1N4007 diode and 10R resistor in series with the input and a 10µF 25V 85°C capacitor from the input to 0V, I'll redraw the relevant part of the circuit if it's all right with you Simon?

I don't see who failure of the capacitor could likely cause the MCU to fail other than by some shrapnel breaking off ,when it explodes, an shorting between some of the pins which isn't likely. I think there's more chance of the 10R resistor burning up and the circuit just shutting down. I also imagine the PIC may have kept resetting itself thanks to the unstable power supply due to the lack of a decoupling capacitor.

The person you spoke to may have been marketing and was obviously trying to fob you off with some bullshit , don't stand for it!

[Simon]

yea sure post a portion of the diagram should not be a problem, I just don't want to upset anyone by revealing all but it don't really matter, the guys a twerp anyhow. I made the mistake once of not using an input cap and it kept blowing the 78l05, so my guess is that if that cap goes the 78L05 will be next and then it all goes real wonky.

As far as i know BLE (the manufacturer) was possibly bough 4 years ago (their website stoped updates in 2006) and the electronics side of things is a minor part of what they do. the guy supposedly designed those himself and he has been coming out with all sorts of bullshit, I think the boards were made and assembled in china as i have seen a prototype that he must of made it was that bad and again he used a 25v cap, the unit is meant for 24V as that is the voltage of the pump. my lot were non the wiser until things went quite wrong and i was very quickly unimpressed with him when i first started talking to him about failures. He tried to tell us these are used in military boats, nobody believes that any more.

I think the hope is that the pumps are all replaced and the old ones all sent back but i doubt we will be getting all of our money back which is why I took it upon myself to provide my report for ammunition in a payments row.

[Simon]

attached is my design, probably crude and I've omitted the back EMF diodes for the minute I'll probably go with an output TVS diode at 40v if need be. There may yet be changes after i test it tomorrow, if anyone wants the software (in mikroe basic) or layout I'll post that too.

[Zero999]

I think a few mods are in order.

I've added some protection against all the MOSFETs from turning on simultaneously.

I think 3k3 for R1 was far too high, you probably forgot the quiescent current drawn by the LM7805, which could be as high as 5.5mA.

Putting C1 after D5 will improve ripple by preventing the capacitor from discharging back into the power supply and other circuits being powered by it.

I think there should be a fuse, unless you're certain it's being run from a current limited supply: a PTC resistor (polyswitch) would be ideal and should even protect against reverse polarity, ideally I'd recommend 2A but you may want to go low to avoid meltdown but note the tripping current at higher temperatures will be much lower so beware.

[Simon]

yea protection against both sets going on at once is a good idea, the idea of C1 was to provide decoupling to the H-bridge load, the diode will stop ripple on the bridge affecting the pic's supply.

I'm playing around with the input resistor values and may just go with an input resistor and 18V zenner to protect the reg properly and be able to raise the voltage of input TVS to say 40V

[Simon]

I've just tried swapping the 3.3K for a 100R on the breadboard and the output ripple of the reg is the same (and the diode is not in the circuit as this was just to prove the principle but the bridge only has a 10K load), bear in mind i'm dropping 25-30V down to 5V with 7V on the regs input. i probably need to look at the caps on the output of the reg

[Simon]

the pic is using 4.15 mA so even 3.3K gives a wide margin while forming a robust filter from spikes

[Zero999]

the pic is using 4.15 mA so even 3.3K gives a wide margin while forming a robust filter from spikes

That's not right, you can't test it unloaded, if the LM78L05 is only drawing 3mA the voltage across R1 will only be 9.9V so of course it will be fine, until you connect a microcontroller to it.

The LM78L05 could be using as much as 5.5mA, which is 9.65mA in total so the voltage drop could be as high as 9.65*3.3 = 31.845V, even with the typical figure of 3mA for the LM78L05, the voltage drop will be 23.595V, which leaves <1V left for the LM78L05.

You need to budget for a voltage drop of 13V maximum, meaning it'll be guaranteed to work down to 20V (flat batteries or vehicle starting).

If your maximum current (including the regulator's quiescent current) is 9.65mA, call it 10mA, then the maximum value for R1 is 1k3 but you might as well to round to 1k.

[Simon]

I measured the 78L05 with the pic connected ie: as it will run in the circuit, it's running fine right now (whole circuit on a breadboard)

[batee]

Here's what I learned with my inverter project:

0)  I initially used the IR High/Lowside FET drivers, but blew up some FETs before I read the datasheet fine print that said they're not usable around 0% or 100% (they're switched capacitor devices and rely on output transitions to charge the caps).  So I found the optoisolated Agilent part and went with that.  It probably wasn't necessary on the bottomside FETs, but I used them anyway for symmetry purposes.  The DC-DC converters are cheap ($5) and are isolated (necessary for the topside FETs, as the gate voltage needs to be 15V above the DC link voltage when the FET is on).  The converters output 15V, and the higher voltage helps the FETs turn on faster.  So each FET driver has its own isolated power supply.

1)  Initially I blew up a lot of FETs.  I went to 600V FETs for a 170V DC Link design (the peak value of 120VAC).  The diodes in FETs aren't enough to protect them, even from resistive loads.  I used some On Semi Ultrafast diodes with 200A surge capability as flyback diodes.  I added MOVs at the input (the HV DC Link voltage) and the output.  I replaced the TVSes shown in my schematic with discrete 15V zeners back-to-back.  This stopped the FET damage.

2)  I used all N channel FETs for the bridge.  More dev has been done here, and N ch FETs usually have lower on resistance.  Also:  The idea was to design something that I could use with the larger FET modules by Powerex.  I added gate resistors to limit the gate current to a value less than that specified by the FET datasheet, and added an "antiparallel diode" to reduce the time it took to turn the FET off.

3)  I connected this to a PIC running at 40MHz.  I tried Don Lancaster's Magic Sine Waves, but couldn't make it work immediately.  I calculated the duty cycle at each of the 360 degrees for a sine wave with 100% modulation (duty cycle is 1 at the peaks and 0 at the zero crossings), then wrote a script to output PWM of that duty cycle to only the bottomside FET to minimize the switching losses.  The topside FET only needs to switch at the time when the polarity changes.  I can alter the firmware to output higher or lower frequencies and different wave shapes.

4)  There isn't any hardware protection against shoot-thru.  There is a programmable delay for dead time (all FETs off around the zero crossing).  I haven't had any problems, but the possibility of a programming error destroying FETs isn't something I like, so I might add some logic to the next design to do this.  That would add gate propagation delays that would reduce the overall speed.  

5)  This has been tested to about 15A, and no problems so far.  The traces on the board need to be reinforced for higher currents.  This design totally works to power high current AC motors (I used it on a 1HP drill press, fan, and some other things).  None of the MSW inverters, even the commercial ones, would do this at all.

http://batee.com/projects/electronics/fet_high_side_gate_drive/fet_high_side_gate_drive_v_3.5.html

[Zero999]

I measured the 78L05 with the pic connected ie: as it will run in the circuit, it's running fine right now (whole circuit on a breadboard)

It's basic Ohm's law, V*I, do the calculations yourself, if it's working, something is certainly wrong with your measurements, it must be drawing less current than you think or you've put the wrong resistor value in there.

What's the supply voltage, have you actually measured it?

How much current is really drawn by the whole thing?

What's the input voltage to the LM78L05?

Have you actually measured the resistor value?

Try using a different meter, are the results the same? Try measuring the current using a 'scope or an RMS meter.

Is the output from the LM78L05 stable?

My guess is that the power supply voltage is somewhere around 30V and the total current is about 7mA so the voltage across the LM78L05 is 6.9V which will still give 5V out, as long as the current is low.

[Simon]

Thats a beefy project Batee, of course for 120V creation you needed a much more complex design, for 24V 300mA I think it's overkill 

Hero,

I'm running off 23 volts at the moment (my 13.8V power pack with a tampered feedback trimmer - must get around rebuilding into that case a variable votlage reg  ) and the 5V part of the circuit uses 4.15 mA so thats 4.15mA * 3.3 K = 13.7V  23-13.7 = 9.3 and the regs minimum input requirement is 7V so thats 2.3V clearance

[Zero999]

Hero,
I'm running off 23 volts at the moment (my 13.8V power pack with a tampered feedback trimmer - must get around rebuilding into that case a variable votlage reg  ) and the 5V part of the circuit uses 4.15 mA so thats 4.15mA * 3.3 K = 13.7V  23-13.7 = 9.3 and the regs minimum input requirement is 7V so thats 2.3V clearance

You've not understood what I've being saying, all along.

The LM78L05 will also consume some current, look at the datasheet.

Measure the current going into the LM78L05 not the output current, unless this is what you've been doing all the time, in which case you haven't made it clear.

[Simon]

yep i measured the total in current, that's the safest way, I was actually surprised that the pic used so much, I think the reg is supposed to be using like 100uA

[Zero999]

I still don't think you understand, the PIC isn't drawing 4.15mA it's the LM78L05.

Now disconnect the PIC and you'll see that the current drops a bit and the current consumption of the PIC depends on what you're doing with it.

Look at the datasheet for the LM78L05.
http://www.national.com/ds/LM/LM78L05.pdf

The LM78L05 can draw as much as 5.5mA so you need to design for that in addition to what the circuit connected to it uses, which is what I've being trying to tell you.

[Simon]

ok would that be quiescent current on the datasheet ? in the case of the reg I'm using its 3-6 mA so yea i may have to make some careful calculations there.

looks like worst case for 20v in is 1.8 K

[Zero999]

ok would that be quiescent current on the datasheet ? in the case of the reg I'm using its 3-6 mA so yea i may have to make some careful calculations there.

Yes that's what I've being saying: look at the maximum quiescent current and add the maximum possible current consumption of your circuit and design for the minimum power supply voltage you need it to work from.

looks like worst case for 20v in is 1.8 K

That seems more sensible.

[scrat]

I think a few mods are in order.

I've added some protection against all the MOSFETs from turning on simultaneously.

Since I'm a beginner at hardware tricks, I'd ask a few questions:
-why a 10k resistor in series with only one of the two PIC's output to gate? Then why such a high value? I'd have used the highest current possible, since slower gate turn-on means higher switching losses, isn't it? If PIC's outputs are typically 25mA capable, 5/25m = 200 Ohm (PIC consumed power is not so much different, since you only have to charge/discharge a gate at each cycle). Even R4/R5 partition values seem swapped (and again, I'd have put lower resistor values).
-is the added diode a sufficient solution to avoid shoot-thru? In the case GP4 is high and GP2 low, MOSFET Vgs will go to Vbus-Vlogic = 24-(5 - Vdiode) = 19.5 V (about), which I suspect means turn-on. However, I guess it is needed on the two low-sides of both legs. I think that the red-linked diode in the attached drawing could solve this issue.