DC to AC converter

[Simon]

it needs to be as small as possible so unless the driver chip can also generate 50Hz it's a nogo, I'm hoping to show him something (on a PCB) when he comes back from holiday so have to use what I have.

With regards to punch through as I'm manually programing the timings I was going to leave a few mS of dead time, the original controller I beleive has quite a large dead time which i assume is something to do with the physical spec of the pump it drives. I was going to introduce a couple of uS of dead time so that it would also help with back EMF as the pump would soon be switched on round the other way so back EMF is out of the equation again, at 50Hz I should think the diodes in the mosfets can cope ?

[Zero999]

Well using a microcontroller to do the switching, rather than a self-oscillating circuit will totally eliminate punch-through, although it's probably still a good idea to provide some hardware protection in case the software fails, causing both the high and low side MOSFETs to turn on simultaneously.

The question is what's most important?

If space is at a premium and cost is critical then the self-oscillating circuits I posted earlier are best. If you opt for an MCU, then you've got to have a voltage regulator and at least two other transistors to level shift or bootstrap to drive the high side. The self-oscillating circuits require no other semiconductors than the transistors which double as the oscillator.

If the current is only 300mA you could remove the 100µH inductor and replace it with a 2R2 resistor which will limit punch through to <11A. Alternatively you can use BJTs which will have a much smaller punch-through current, especially if you reduce the value of the base resistors.

Looking at the BJT circuit again, according to LTSpice if R1 to R4 are 1k5 and C2 and C2 are 10µF the frequency is 46.7Hz and the punch-through current will only be 6A and will linearly decay to 3A in just 65µs.


If frequency stability and minimising punch-through are more important than space and cost, then the MCU based circuit offers a better solution.

[Simon]

well it's for a military thing and is already a replacement for another cockup we made in supplier choice, punch through must be non existent I'd say as they're talking potting this thing or at least putting it in a box (that does into another larger box) so the dissipation that punch through would cause is a bit no no

[Zero999]

How do you know that?

Surely it depends on what the dissipation due to punch-through is?

I know that with the MOSFET circuit it would've been unacceptable, mainly because the oscillator takes 200ms which could kill the transistors if it isn't limited by a suitable inductor and resistor.

I've done another quick simulation on the BJT circuit with 1k5 for the resistors and 10µF for the capacitors with a 48R load. The losses in the entire circuit are 2.57W: 0.38W is in the resistors, 2.2W in all the transistors: 1.04W due to punch through and 1.16W due to saturation and switching losses.

The question you have to ask is whether the transistors can dissipate that amount of power whilst embedded in resin? Thermally conductive resins are available but will probably be more expensive.

If you go for the MCU option, you could probably cut the losses by a factor of 100 or more but it'll take up more space and be more expensive but that will be ofset against the cheaper resin.

[Simon]

well I'm trying to do it fool proof and simple and with bits I have already so that I can do it quickly without ordering parts. The guy i'm doing it for is clueless with electronics so I'm trying to steer around potential problems particularly those caused in the practical implementation. The "instructions" he showed me show a square waveform that has a lot of dead time and I don't know why and if it is needed, I may end up asking to speak to the pump manufacturer myself to find out exactly what waveform i am to produce, I don't suppose the oscillator method will allow programmable dead times, like i said I'm going the MCU way around it so that there are plenty of back doors open for later when he says: yea but..... and then if he insists in burying it in resin (I have to hope the manufacturer he chooses gets it right) it's not a problem, I've already raised the question of SMD parts and he has no idea of his makers abilities so the first prototype will be all through hole parts and then we'll see where he wants to take it.

[Zero999]

It looks like the MCU idea seems best then, especially as like you say, it leaves plenty of other doors open. Yes I'd get onto the pump manufacturers if I were you, it's possible that it's designed to work from a sine wave and that the controller used a square wave in a bid to save time and money.

[Simon]

The original demo board clearly used an MCU as there are jumper settings, All i'm concerned about is that the pump can withstand an almost 100% duty cycle, the leaflet I looked at was very simplistic and may have been exaggerating the dead time on the graph to m,ake a point, basically I have to get the question asked: what is the maximum duty cycle allowed, if the thing was originally used for a sine wave 24V is a standard voltage so no doubt the pump will take an RMS of 24V (100% duty of sqr waves) with a peak of 1.414 X 24V, I'm guessing the MCU and mosfets total rise and fall time will be a matter of a uS so it's like 9.999mS positive 0.001 dead 9.999 mS negative, 0.001 dead and so forth

[NiHaoMike]

It's possible to have jumpers with a generic oscillator chip. But microcontrollers are cheap nowadays...

[Simon]

well I'm at work now and glancing at the bit of paper i was given it say's Microprocessor board, so there you have it.

[TechGuy]

The frequency does not have to be spot on but cannot vary that much, I suspect problems with similar pumps in the past were due to them being driven at 130% the speced frequency (67 Hz instead of 50).

Mostly likely the problem is driving an AC motor using a squarewave AC signal. AC motors require Sine wave AC to run properly. Other wise they over heat caused by core saturation, or they will blow your bridge Mosfet when the current demand soars due to core saturation. It should be powered using a sine wave. Since the current is very low, you might be able to drive using a BTL (Bridge tied load) circuit.

The original controller uses 4 N channel mosfets in a bridge formation. the circuit drives a reciprocal pump, presumably the original one uses a switched capacitor boost circuit to produce the drive for the N channel mosfets on the positive side.

The best option is to use a pair of half-bridge Mosfet drivers. Mosfet drivers will protect your MCU from voltage spikes that are generated by the motor. Ideally you connect the the MCU to the half-bridge drivers using a 10k resistors and a small TVS diodes to ground so that the MCU does get fried, in the event that the voltage spike exceeds the half-bridge drivers voltage rating. Most half-bridge drivers can operate up to 600V, (some up to 1200V), I have seen instances where a 36V H-Bridge driving an induction load exceeded 600V, blowing the half-bridge driver and taking out the controller when it fried.

well it's for a military thing and is already a replacement for another cockup we made in supplier choice, punch through must be non existent I'd say as they're talking potting this thing or at least putting it in a box (that does into another larger box) so the dissipation that punch through would cause is a bit no no

Wow! No EMP\EMF protection, no RAD protection, using commerical components! Hope this isn't for use in NATO!

Shoot-through can be avoided using half-bridge drivers that have shoot-through protection (ie will not permit the high-side and low-side from being turned on at the same time). That said its still possible to have a shoot through issue if you don't discharge the MOSFET gate fast enough. Its possible that you turn off one transistor and start turning on the other transistor before it full turns off. Dead-time is added to protect this from happening. Gate resistors should also have a parallel diode so that the MOSFET turns off faster than it turns on all four mosfets should have a 10K resistor connected between source and gate to prevent the MOSFET from automagicially turning on when you don't expect it. If the gate is not held low, a MOSFET will slowly turn itself on, usually with  disappointing results!

it needs to be as small as possible so unless the driver chip can also generate 50Hz it's a nogo, I'm hoping to show him something (on a PCB) when he comes back from holiday so have to use what I have.

Your not going to be able to drive a MOSFET directly from an MCU, especially if its a 3.3V MCU. You might be able to get away with a logic level MOSFET, but they are more prone to failure, especially when switching inductance loads. Your much better off using a pair of H-bridge drivers. If size is a problem, there are drivers in DFN packaging. You can also use dual sided component boards to reduce PCB size. You can also use the smaller DirectFET packaging now available. If size is very restricted then you have to implement a PWM Sine wave logic drive to power the motor:



BTL requires a pair of inductors and caps, which take up lots of space. Plus Electrolytic caps aren't milspec unless you get an exemption.

Here is an half-bridge MOSFET driver in a LLP-8 (4 mm) package
http://cache.national.com/ds/LM/LM5109A.pdf

and then if he insists in burying it in resin

Perhaps to hide the design, since its not milspec. FWIW: I would not touch this project. If its for the miltary and not Milspec and quoted as Milspec, you could end up doing some serious jailtime. If he gets in trouble, than passes the buck onto you blaming you for the poor design. I think this project is over your skillset in my opinion. Hope it works out though.

[Simon]

Thanks for your insight tecguy, it's only an air conditioning condensation pump, nothing life critical and you would be surprised to see what gets supplied in contracts that have the military as the end user (who is not necessarily the direct customer), our whole air con system had some interesting EMC issues due to a lack of back EMF diodes that i had to point out to them (they were clued up on diodes on relays but didn't think 300 W fan motors needed them  ). We do seem to have a habit of using commercial parts for military orientated units as far as i know there has never been any validation of anything like this, it's just a case of: does it work ? does it fit in with the provided specs ?, I took it upon myself to dismantle one of the old pumps, reverse engineer the driver board and point out all the fuckups (not that they listened seems stuff designed by a 10 yr old is good enough)

The original driver board is supplying a square wave drive, the pump is not of the motor type but a solenoid driving a diaphragm. And yes there seems to be precious little protection on the original board but then this is just a 300 mA load and I'll be having a more thorough look at it later, i don't know what sort of back EMF I can expect although I'm sure it could be plenty. I think worse case i'm looking at a back EMF diode on each mosfet. perhaps a bidirectional TVS diode on the pump output would be beneficial ? I've put in a TVS on the supply but that's more for incomming protection from the vehicles supply that is non too smooth to start with

The idea of burying it is resin was for water protection something he picked up off the supplier of the older pump design (that is going tits up all over the place), I think a bit over the top but that's his call.

[Simon]

By the way I don't quite understand how the back EMF will get back to the gate ?

[Zero999]

The best option is to use a pair of half-bridge Mosfet drivers. Mosfet drivers will protect your MCU from voltage spikes that are generated by the motor. Ideally you connect the the MCU to the half-bridge drivers using a 10k resistors and a small TVS diodes to ground so that the MCU does get fried, in the event that the voltage spike exceeds the half-bridge drivers voltage rating. Most half-bridge drivers can operate up to 600V, (some up to 1200V), I have seen instances where a 36V H-Bridge driving an induction load exceeded 600V, blowing the half-bridge driver and taking out the controller when it fried.

Surely that shouldn't happen if correct back-EMF suppression is used?

Plus Electrolytic caps aren't milspec unless you get an exemption.

Not true, you can buy special millspec electrolytic capacitors - I know because I know because I've encountered them before on a timer which deployed a retarder attached to the back of a bomb and I know there was no exemption because the capacitors were mission critical, if they failed the retarder wouldn't operate because they stored a charge used to operate the actuator.

The only problem I can see with your PWM solution is that the pump is designed for 24VAC operation and the circuit is run from 24VDC the peak output voltage will be lower than it would be from AC, giving an RMS voltage of just under 17VAC. In order to get 24VAC a boost converter or a transformer is required, the former is probably the best option because it'll be lightweight, a transformer will be too big and heavy and a non-standard winding will be required to boost 17VAC to 24VAC. Maybe the whole circuit could be powered from an off the shelf 24V to 36VDC converter? That will also isolate the AC from DC which will reduce the risk of a short circuit.

[scrat]

Hero pointed out a heavy concern. From 24 Vdc you can't get a 24 Vac rms sine wave directly (as a PWM sine). That' probably the reason why they ran the pump with a square-like wave: the first harmonic of a square wave is 4/pi * amplitude ( > sq. wave amplitude), so in this case you can get an rms value of about 4/pi*24 / sqrt(2) = 21.6 V, which perhaps was enough. I suspect that the "exaggerated" dead time was a way to get the correct amplitude for the first harmonic, or the desired effect (since this is a solenoid, even higher order harmonics could give their contribute).
However, in the case a fixed-frequency sine wave was required, you could also use something different than a table to generate its values: could implement a digital "resonant" filter, and thus achieve a sine wave with only one coefficient stored and a little bit of calculation for each sample (3 sums + 2 mult).

I think that using an H-bridge with p-channel MOSFETs at the high-side will simplify your circuit, avoiding the use of a bootstrap, or you can easily find integrated bridges or half-bridges with p-n couples, and even integrated driver+bridge. Some drivers, AFAIK, are secure against punch-through because they start the rise of a gate only when the opposite (high/low) has gone below a certain threshold.

[Simon]

As far as i know the pump is driven from a 24V square wave, I'll have to enquire as to the exact drive requirements but as far as i know thats it, no higher voltage required no sinusoid simulation required.

[Zero999]

To be honest, I think that a pure sine wave would be overkill and don't see any reason why it shouldn't work off a square wave.

It's probably just a solenoid with a permanent magnet core so will work from either a square or a sinusoidal waveform, a square wave may even be better.

I agree that P-channel MOSFETs should be used for the high side and that punch-through can be eliminated using software, although there should be some safety feature in the hardware which stops it from blowing up if the software or PIC fails and decides to turn both the high and low transistors on but this could be as simple as a fast blow 1A fuse which should trip before the MOSFETs blow up. Another consideration is what happens if the PIC freezes and keeps the coil connected in one direction causing 24VDC to flow through it continuously: can it withstand such abuse? If not will the current be high enough to blow the fuse in time?

If the answer to both of the above questions is no, the pump won't withstand 24VDC continuously and the current won't be high enough to blow the fuse in time, you have a problem. If this is true then you need another solution. Perhaps you could use a logic gate IC to make a couple of monostable so it's not possible for either side of the bridge to be turned on for longer than a certain length of time? But then you might consider using logic gates for the whole circuit.

[Simon]

I suppose it's possible to do a two comparator astable circuit ? with a delay for turn on and each triggering the other so it's like a dog chasing it's own tail ? would also eliminate the need for level shifting mosfets

The previous pump and drive circuit could have suffered the same fate you describe for this one, I suppose any software is a liablility

[Zero999]

I'm not sure if that would work. I can think of a ring oscillator but that will need some gates on the output to get the desired waveform.

[TechGuy]

By the way I don't quite understand how the back EMF will get back to the gate ?

It will come from the Half-bridge common connection that is connected to the load (V+ - Hi Mosfet - Bridge Common - Low Mosfet - Gnd). The Solenoid will create a spike. The Bridge Common is connected to the MOSFET driver so that it can turn on the high side mosfet. Remember that a voltage beween the Source and the Gate pins (Vgs) is requrired to turn on or off the high side mosfet. There needs to be a voltage difference ~10 Volts between the Gate and Source. In a bootstrap mosfet driver, the driver uses the switching output to to charge the boostrap cap for highside switching.

You can squelch the spikes with a pair of snubbers (RC filter), one on each side of the half bridge. to cull the spike. How big will depend on how large the spikes are. You have to run some tests to see how big your snubber will be. As a safety measure use a low value resistor (3 to 4 ohms) between the gate driver and the half-bridge common connection, On the driver side of the resistor, connect a SMB (600W) TVS avalanche diode. The Resistor will limit the current inrush to prevent the TVS diode getting blown. The TVS will insure the mosfet driver doesn't get blown if the Snubber fails to squelch the solenoid spike. You can also try using a PTC fuse in substitute for the resistor, A PTC fuse will have a low resistance, unless the current level is excessive, which causes the PTC to heat up and causing the PTC resistance to soar.

I would also recommend including some dead time when you drive the solenoid This will prevent flux walking that could result in unexpected core saturation, blowing out one of the Mosfets caused by a current spike.

Not true, you can buy special millspec electrolytic capacitors - I know because I know because I've encountered them before on a timer which deployed a retarder attached to the back of a bomb and I know there was no exemption because the capacitors were mission critical, if they failed the retarder wouldn't operate because they stored a charge used to operate the actuator.

Most likely excempted a very long time ago, or a grandfathered design prior to new cap designs. Most of the miltary designs require non-electrolytic caps, film, or solid electrolyte. It might also be exempt because it for a one-time-use device (a bomb), and it not used in a plane or vehicle that is carrying people. The problem with Electrolytic caps is that excessive temperature swings cause rapid loss of the electrolytic fluid.

I suppose it's possible to do a two comparator astable circuit ? with a delay for turn on and each triggering the other so it's like a dog chasing it's own tail ? would also eliminate the need for level shifting mosfets

The previous pump and drive circuit could have suffered the same fate you describe for this one, I suppose any software is a liablility

 
This will do what you need. A Self-Oscillating Full Bridge driver:

http://www.irf.com/product-info/datasheets/data/irs2453d.pdf
http://www.nxp.com/documents/data_sheet/UBA2032.pdf
http://www.irf.com/product-info/datasheets/data/ir2086s.pdf

I would recommend including overcurrent detection and UV/OV lockout if you want to make sure its rock solid

[scrat]

TechGuy preceded me while I was thinking and writing. Surely he has given better explanation, but since I've already wrote this post, here's it

By the way I don't quite understand how the back EMF will get back to the gate ?

I'm not an expert, but...
One way can be through capacitance. Since gate is "connected" by means of parasitic capacitors to source (and drain), voltage changes on those terminals will reflect instantly on the gate, since (like a teacher of mine always said at school) "capacitor doesn't have voltage bounces". Just think of the effect of a commutation from OFF to ON of the high-side source for an n-channel MOSFET: even if there isn't any B-EMF, gate passes from nearly zero to nearly power bus voltage (at least some volts over this, and that's why bootstrap is needed). If there is B-EMF and a proper diode does not activate, at commutation the load inductance can make the MOSFET's source rise much above the power rail (and gate to follow it).
Also, for high current transients (like those on the transistors and on their gates) large voltages are produced, and typically there is resonance on the gate voltage...
Is it right or are these only bad guesses? Someone more experienced may confirm or deny what I tried to explain  

[Mechatrommer]

i'm looking forward to see a design (preferably simple) for a pure sine wave inverter. even more interesting is the possibility to generate a pure sine from mcu pwm as TechGuy has presented. Pure Sine Wave Inverter or UPS cost thousands in our currency.

[Simon]

that is relatively simple for someone good on software, just generate a PWM model of a sine wave, this will require preferably something like 20 times the frequency of the signal you are replicating, not sure how you do it in software but bet it can be done.

[scrat]

Unfortunately, I can only speak for the MCU side of the thing...
Well, at each switching period (interrupt) you generate a numerical value for the sine [0, sin(2*pi*fgrid*Tsw* 1), sin(2*pi*fgrid*Tsw* 2), ... , sin(2*pi*fgrid*Tsw* n) ] properly scaled according to your PWM register length. Here fgrid is the frequency of your grid AC voltage (50/60 Hz) and Tsw is the desired switching period.
To make things better, at each switching period you normalize the value (without offset) taking into account the measured value of the bus voltage (preferably low-pass filtered). This isn't needed if you're sure that the bus voltage is well regulated to a known voltage.
If you want to use 3-level PWM, you will need generate a half period of the sine and then reverse polarity of the switching (making only one switch to commutate, alternatively on the right or left side of the bridge). With Enhanced PWM in PIC18F, for example, this only costs changing a bit in a register (forward/reverse).

Generating those sine values is usually done by means of a look-up table containing pre-calculated values for the sine. The way I posted above (a resonant digital filter) also works (implemented on an ATmega48), but could be more useful if different frequency values are required, or the MCU has very little EEPROM.

[Zero999]

It will come from the Half-bridge common connection that is connected to the load (V+ - Hi Mosfet - Bridge Common - Low Mosfet - Gnd). The Solenoid will create a spike. The Bridge Common is connected to the MOSFET driver so that it can turn on the high side mosfet. Remember that a voltage beween the Source and the Gate pins (Vgs) is requrired to turn on or off the high side mosfet. There needs to be a voltage difference ~10 Volts between the Gate and Source. In a bootstrap mosfet driver, the driver uses the switching output to to charge the boostrap cap for highside switching.

You can squelch the spikes with a pair of snubbers (RC filter), one on each side of the half bridge. to cull the spike. How big will depend on how large the spikes are. You have to run some tests to see how big your snubber will be. As a safety measure use a low value resistor (3 to 4 ohms) between the gate driver and the half-bridge common connection, On the driver side of the resistor, connect a SMB (600W) TVS avalanche diode. The Resistor will limit the current inrush to prevent the TVS diode getting blown. The TVS will insure the mosfet driver doesn't get blown if the Snubber fails to squelch the solenoid spike. You can also try using a PTC fuse in substitute for the resistor, A PTC fuse will have a low resistance, unless the current level is excessive, which causes the PTC to heat up and causing the PTC resistance to soar.

Sorry, what I meant to say is that if the correct clamping diodes are used (they always should be) how could that happen?

Most likely excempted a very long time ago, or a grandfathered design prior to new cap designs. Most of the miltary designs require non-electrolytic caps, film, or solid electrolyte. It might also be exempt because it for a one-time-use device (a bomb), and it not used in a plane or vehicle that is carrying people. The problem with Electrolytic caps is that excessive temperature swings cause rapid loss of the electrolytic fluid.

I think size was the issue, there were two 40V 470µF capacitors and film/ceramic would've been too large.

Can you point me to a defence standard that states electrolytic capacitors are unsuitable for military applications?

I don't believe this to be the case after working for a defence contractor for nearly 10 years and used electrolytic capacitors in numerous designs which have been approved by other engineers with over 40 years of experience in field.

Maybe some old electrolytic couldn't hack it and a crappy commercial grade Chinese capacitor would obviously fail fast but there are capacitors with a temperature range of -55°C to +125°C and rated to 2000h at the top end of the temperature range available in Farnel. I can't see why such a capacitor would be unsuitable.
http://uk.farnell.com/vishay-bc-components/2222-118-18102/capacitor-electrolytic-63v-1000uf/dp/1692350

This will do what you need. A Self-Oscillating Full Bridge driver:

http://www.irf.com/product-info/datasheets/data/irs2453d.pdf
http://www.nxp.com/documents/data_sheet/UBA2032.pdf
http://www.irf.com/product-info/datasheets/data/ir2086s.pdf

I would recommend including overcurrent detection and UV/OV lockout if you want to make sure its rock solid

Those ICs look good but there doesn't seem to be an easy way to set a long dead time, for example, if he requires 5ms what could he do?

[TechGuy]

Sorry, what I meant to say is that if the correct clamping diodes are used (they always should be) how could that happen?

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



How large depends on the inductor, and how much parasitic capacitance it has. Usually low frequency AC solenoids have a lot of turns, which increases the parasitic capacitance, leading to large spikes.

Can you point me to a defence standard that states electrolytic capacitors are unsuitable for military applications?

I don't believe this to be the case after working for a defence contractor for nearly 10 years and used electrolytic capacitors in numerous designs which have been approved by other engineers with over 40 years of experience in field.

No, I have read many app-notes and published articles about issues with electrolyte caps in milspec designed.
Here are some links about mil-spec caps:
http://www.dscc.dla.mil/Programs/MilSpec/listdocs.asp?BasicDoc=MIL-PRF-39003
http://www.dscc.dla.mil/Downloads/MilSpec/Docs/MIL-PRF-39018/prf39018ss3.pdf
http://snebulos.mit.edu/projects/reference/MIL-STD/MIL-STD-202G.pdf

Maybe some old electrolytic couldn't hack it and a crappy commercial grade Chinese capacitor would obviously fail fast but there are capacitors with a temperature range of -55°C to +125°C and rated to 2000h at the top end of the temperature range available in Farnel. I can't see why such a capacitor would be unsuitable.
http://uk.farnell.com/vishay-bc-components/2222-118-18102/capacitor-electrolytic-63v-1000uf/dp/1692350

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.

Those ICs look good but there doesn't seem to be an easy way to set a long dead time, for example, if he requires 5ms what could he do?

He will have to use an oversized timing cap. If you read the datasheet there should be an explaination, or formula to calcuate deadtime. Perhaps one of them has programmable deadtime using a deadtime resistor. There might be other self-oscillating full bridge drivers available too. I just did a quick search in Digikey to find a few examples.