EEVblog #909 - Defibrillator Teardown

[EEVblog]

Defibrillator Teardown!
Inside the Heartsine Samaritan Pad AED heart defibrillator.

Datasheets:
http://www.mouser.com/ds/2/205/CS20-22moF1-475252.pdf
http://www.vishay.com/docs/94386/vs-30tpspbf.pdf
http://www.mouser.com/ds/2/196/irg4ph40upbf-937889.pdf
https://www.fairchildsemi.com/datasheets/FQ/FQP45N15V2.pdf
http://www.promelec.ru/pdf/ISD4004.pdf

[Andy Watson]

No DSP? I was expecting some DSP goodness in there along with a well protected ECG amplifier to determine the presence (or lack) of a pulse or fibrillation.

[Solberg]

@3:17 lol. That's not a European Conformity Mark, it's a China Export mark.

https://en.wikipedia.org/wiki/CE_marking

https://www.google.dk/search?q=china+export+differences+european+conformance&biw=1525&bih=746&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiS2KXN9LbOAhWMIJoKHas7CeoQ_AUIBigB&dpr=0.9#imgrc=o5IB3xidF06snM%3A

[bktemp]

Interesting teardown.
The current switching during the biphase pulse can not work as shown: When the SCR is triggered, it can't switch of until the current goes to almost zero. So the IGBTs must be used for the first half of the biphase pulse to interrupt the current before the SCRs dump the remaining energy at reverse polarity into the PUT. That's probably why they had to use IGBTs in one leg of the H-bridge. The relays can't be used to interrupt the current (switching 1kV DC at >10A  off requires a really large switch).
But the reverse engineered schematic looks right, so I can't figure out how it works.
Is is probably impossible to fool the device by putting the electrodes into a bucket of water to make it work to measue the output waveform.

[elliottveares]

Dave, you don't Defibrillate someone having a heart attack (Myocardial Infarction)  ; you defibrillate someone having certain forms of cardiac arrest.

[Goodwill Hunting]

Dave, you don't Defibrillate someone having a heart attack (Myocardial Infarction)  ; you defibrillate someone having certain forms of cardiac arrest.

Actually, you don't defibrillate someone having cardiac arrest.  Cardiac arrest means literally no cardiac electrical activity.  In medical terms, we call it asystole.  In lay terms, it would be "flat-lining".  Defbrillating someone in asystole will accomplish nothing.

The point of defibrillation is to override a lethal rhythm, allowing the heart's natural pacemaker to restart.  The rhythm you would defibrillate would be ventricular fibrillation or v-fib.  That's the big one everyone is worried about, and it's the one that kills the most people by far.  There are other rhythms that get defibirillated, such as pulseless ventricular tachycardia, torsades de pointes, and even some atrial dysrhythmias, although the atrial dysrhythmia shocks are syncronized to occur on a specific point of the cardiac electrical cycle so we don't make the patient go into an even worse rhythm. 


As a point to the video, Dave made note of the Joules going from 100 to 120 to 150 (or something like that).  The concept here is that the first shock is at the lowest Joule.  Following that is 120 seconds of CPR, followed by the AED analyzing the rhythm again.  If the AED determines a shock is warranted, it will step up to the middle Joule because the lower Joule did not work.  Then, 120 seconds of CPR and the AED will analyze and, if warranted, shock using the highest Joule.  Any further shocks will be at the highest Joule.   Each manufacturer of biphasic defibirillators uses or can use different Joule setting depending on what they get certified with.  It is not important for a user to remember the exact Joule setting, because each machine may be different.

Monophasic defibrillators were all the same and used 200J, 300J, and 360J.  These have been mostly phased out, although you will still find monophasic defibirillators in older and less advanced healthcare settings.  These monophasic defibrillators used more Joules because they were less effective.


For anyone worried that they need to understand any of this stuff to use an AED:  Your concerns are unwarranted.  The AED talks to you.  You open it and it will tell you to apply the pads and how/where to do it.  It will tell you to press the anaylyze button.  It will tell you to start chest compressions.  You don't need to know anything to use an AED, and you might save a life.

[filssavi]

the reverse engineered schematic looks right, so I can't figure out how it works.

My guess is that the series inductor is part of a resonant tank that lets the 2 series Tyristors turn off, then the igbts/big SCR is triggered and the negative pulse is delivered, that said I haven't done any circuit analysis and i might be talking out of my a**  so don't quote me on this

[max666]

That turning off of the SCR's is in deed a big crux in Dave's explanation. Is it possible the polarity of sternum and apex is wrong?

[alxpo]

Very disappointing. I waited when Dave explain main magic of the circuit - closing of SCRs in proper time, but it was futile.
The version with wrong polarity and first pulse through IXYS and IGBTs looks plausible.

[LazyJack]

Regarding the clock drift of the RTC. It's not really an issue. If you need the timestamped log of what happened, you can read the current time from the device and correlate the log to that. I assume that you want the log shortly after it was activated.
Provided this is really the purpose of the RTC in this device.

EDIT: Oh, just saw that Mike had already commented just about the same on Youtube, before I wrote this here.

[mtdoc]

Nice teardown Dave.  It would be interesting to know more about how it does the rhythm analysis to determine if there is a shockable rhythm - but I suppose most of that magic happens in software.

Dave, you don't Defibrillate someone having a heart attack (Myocardial Infarction)  ; you defibrillate someone having certain forms of cardiac arrest.

While technically correct, since most cases (about 70%) of cardiac arrest are due to myocardial infarction - it does turn out that most uses of defibrillators occur as the immediate result of an MI. 

So, I think it is understandable that for the general public - the 2 terms are used synonymously - even if it is not quite correct.  "Heart attack" is really a layman's term in any case.

And FWIW, the vast majority of MI's do not result in cardiac arrest.

[elliottveares]

Dave, you don't Defibrillate someone having a heart attack (Myocardial Infarction)  ; you defibrillate someone having certain forms of cardiac arrest.

Actually, you don't defibrillate someone having cardiac arrest.  Cardiac arrest means literally no cardiac electrical activity.  In medical terms, we call it asystole.  In lay terms, it would be "flat-lining".  Defbrillating someone in asystole will accomplish nothing.

Ventricle Fibrillation and Pulse-less Ventricle Tachycardia are both forms of Shockable Cardiac arrest as cardiac arrest means to have no heart output (to have no pulse).  Asytole as you said is also a form of non-shockable cardiac arrest what can not be treated with a defibrillator.

[max_torque]

I did a limited teardown on a Pico Differential probe a while ago:


https://www.eevblog.com/forum/projects/what-am-i-mystery-ic-competition!/


Links in that thread to more info on the circuit architecture used etc.

[marshaul]

Yes, please, Dave! Please do a teardown/reverse engineering of your differential probe.

[mikeselectricstuff]

No DSP? I was expecting some DSP goodness in there along with a well protected ECG amplifier to determine the presence (or lack) of a pulse or fibrillation.

It's a very low bandwidth signal, and doesn't need to be analysed in real time, so shouldn't need a DSP to process.

[Andy Watson]

No DSP? I was expecting some DSP goodness in there along with a well protected ECG amplifier to determine the presence (or lack) of a pulse or fibrillation.

It's a very low bandwidth signal, and doesn't need to be analysed in real time, so shouldn't need a DSP to process.

It is low bandwidth but I would expect it to be analysed in real-time. You don't want to shock a person with a normal pulse.

[Maxlor]

No DSP? I was expecting some DSP goodness in there along with a well protected ECG amplifier to determine the presence (or lack) of a pulse or fibrillation.

It's a very low bandwidth signal, and doesn't need to be analysed in real time, so shouldn't need a DSP to process.

It is low bandwidth but I would expect it to be analysed in real-time. You don't want to shock a person with a normal pulse.

That LPC2210 is a 60MHz ARM7 part; it should be fast enough for the signal analysis of a single channel ECG. Data recording happens at 100-500Hz typically, so after a few seconds of recording, there's not a big amount of number crunching that has to happen. Mind you, the first ICDs (implantable defibrillators) came out in the 1980s, and they managed to figure out whether and when to shock

[Maxlor]

So what I don't get with Dave's diagram (aside from the part that he says Thyristors can't turn off, but then they're supposed to?) why do the current path parts use so many different topologies? SCRs of various sizes, IGBTs, one path with an inductor one without; why not use only IGBTs? Or why not use more of those big SCRs to replace those pairs of SCRs in series?

Btw, I work for a company that builds med tech products, although being in R&D, I'm not directly involved with the production stuff, so I'm not very familiar with the regulations; My impression though is that they focus more on process than on parts. What I do know is that we're not required to have certification for every single component; it would be infeasible if not impossible. What we are doing however is machine-inspect every single part down to small passives like ceramic caps, and this data including photos from all sides is kept in storage. This is so that in the event that there is a defect in a part that went undetected, we can go back and check whether maybe the whole series might have been affected, and find out which products exactly those parts have gone into. So yep, full traceability for every single 0201 passive. Just in case you wondered why health costs are so high

[bktemp]

So what I don't get with Dave's diagram (aside from the part that he says Thyristors can't turn off, but then they're supposed to?) why do the current path parts use so many different topologies? SCRs of various sizes, IGBTs, one path with an inductor one without; why not use only IGBTs? Or why not use more of those big SCRs to replace those pairs of SCRs in series?

SCRs are probably easier, cheaper and more robust, because you give them a short pulse (can be easily coupled using a small transformer) and they keep conducting. For IGBTs you need to keep the gate for the full on time at 15V otherwise they will go into linear operation and fail.
The IGBTs are probably necessary to switch the current off before reversing the polarity. That can't be done using SCRs.
The inductor makes sense, because SCRs don't like a rapid current rise at turn on. You often see inductors in series with SCRs when they switch a resistive load.

[filssavi]

now that others have brought it up, wrong polarity on the electrodes is much more plausible than a resonant switch...

Still as maxlor said the whole switching topology is strangely a mix of SCR and transistors, maybe SCR having a much simpler structure is more reliable, and they used them in as many places as they could get away with, so the only transistors (IGBT) are the ones that must be also turned off not just turned on, what i dont really get is why the 2 SCR's in series in one place and a single bigger one in the other, i would be tempted to say cost but i don't know it doesn't seem to make much sense to me

[VK3DRB]

The medical device industry is a very tough environment for good reason - safety. The Cornell Dubilier caps were used because they are deemed as critical components and of course would need to be of the highest quality and reliability. Everything about them would need to be reported for IEC-60601 compliance, along with other devices.

There is a massive amount of paperwork, processes, testing and time to get a product to market. Hardware development is one thing (IEC-60601) but software development is another major task (IEC-62304). Writing code or creating a circuit that works is only one small part of the huge bureaucratic processes to get a product developed. Testing and documentation is very extensive. As are the QA processes. And IEC 61000 compliance for EMI/ESD. It is not the easiest career.

At the heart of everything is risk analyses - identifying and mitigating risks. The manufacturer can be audited at any time by the FDA, TGA or whoever else is relevant to the geography you are selling product. The manufacturer owns the product risk. If a manufacturer has falsified any documents, he can end up being shut down or even doing time (esp if someone died as a result of a dodgy product).

Dave had issues opening the pads. They would have been hermetically sealed and sterilised. And that is another complex controlled process. The defrib would have had multiple feedback mechanisms to control and detect that everything was working correctly. I would suspect even with the power "off" there would be battery monitoring going on in the background. You would not want to need to use one of these to discover the batteries are flat.

[Paul Moir]

The medical device industry is a very tough environment for good reason - safety. The Cornell Dubilier caps were used because they are deemed as critical components and of course would need to be of the highest quality and reliability.

Couple other things about them: a +10%/-10% spec is a little unusual for an electrolytic, but obviously required for this thing since you don't want too much energy available & can't overload that smaller SCR.  Also there doesn't appear to be balancing resistors so perhaps they're matched for leakage.  Maybe that's what the dots are indicating.  Throw in a form factor requirement and you might be married to one manufacturer.

The diodes appear to be straight up 1000v PIV so they're not pulling anything fancy there.
 

[zl2wrw]

A quick word of warning if you ever have to use an AED on a casualty for real - many models "chip" the electrodes, so that they can only be used for one patient. Usually they do this by checking the state of the electrodes (new or used) when you wake the AED up (typically by opening the lid).

The catch is that after putting the electrodes on your casualty, and finding that they do not require a shock, or after a shock successfully restores a useful heart rhythm, you definitely should NOT "tidy up" and unplug the electrodes or close the lid on the AED (leave that the ambo crew). This is because unplugging the electrodes or closing the lid can put the AED back into deep sleep mode, and should your casualties condition worsen, requiring a shock, a prematurely shut-down AED will likely refuse to work because on wake up, it will detect that its electrodes are "used" 

[DrSchweizer]

Hi Dave

A defibrillator isn't used "to restart a heart that had a heart attack", its used to resynchronize the electrical activity of all cells when that was lost. So when someone has by example a heart attack and in regions with insufficient oxygen supply the cells start to act chaotically and do not spread the electricity uniformly over the heart but every cell does whatever it wants the heart stops to pump (called ventricular fibrillation) and now a defibrillator will cause all cells to "discharge" at once and restart a normal rhythm.

I hope that was explained well enough ...

Regards from Switzerland


-Schlomo (cardiologist)

[Halcyon]

This particular (as most AEDs) will actually detect CPR being performed and provide visual/audible feedback. Every compression on the chest is detected and measured by the device. The AED will only allow a shock when it detects a certain irregular heartbeat. It won't help anyone who has no cardiac output.

[mcinque]

Maybe it's a noob question but... why 3 diodes in series after the transformer? 1 diode doesn't do the job for a half wave rectifier as intended?

[b_force]

Maybe it's a noob question but... why 3 diodes in series after the transformer? 1 diode doesn't do the job for a half wave rectifier as intended?

Probably because the huge voltage spikes from the switching.
Bear in mind that we are talking about 1500-2000V, which means that switching spikes will be even higher.

[mikeselectricstuff]

Maybe it's a noob question but... why 3 diodes in series after the transformer? 1 diode doesn't do the job for a half wave rectifier as intended?

Probably because the huge voltage spikes from the switching.
Bear in mind that we are talking about 1500-2000V, which means that switching spikes will be even higher.

could also be  a safety thing in case a diode fails short - if it dumped all the cap energy into the transformer secondary it would probably explode.

[chris_leyson]

This is somewhat off topic but about 20 years ago the company I was working for had an inverter designed by a third party to charge up a bank of capacitors fairly quickly. You could hear the magnetics in the converter changing frequency slightly as the capacitors charged up and a colleauge of mine who had worked on defibrillators in the past said that when they had self oscillating converters running at tens of kHz you could "hear" the converter running and knew when the capacitors were charged. As the electronics improved and the switching frequencies got higher you couldn't hear the converter anymore, some customers complained and they had to add additional electronics just to generate the classic defibrillator sound.

[AF6LJ]

This is going to be Good.....
I have to wait until my work is done...

[max666]

Couple other things about them: a +10%/-10% spec is a little unusual for an electrolytic, but obviously required for this thing since you don't want too much energy available & can't overload that smaller SCR.  Also there doesn't appear to be balancing resistors so perhaps they're matched for leakage.  Maybe that's what the dots are indicating.  Throw in a form factor requirement and you might be married to one manufacturer.

The diodes appear to be straight up 1000v PIV so they're not pulling anything fancy there.

Huh, that's somewhat disappoint, I was hoping those diodes could be TVS (transient-voltage-suppression) diodes, that way you would have reverse polarity and over voltage protection. Or are TVS diodes a bad substitute for a balancing circuit?

[ChunkyPastaSauce]

The RTC is for the built in event data recorder, which records not only time stamp data of events but also records an ECG during each stage intervention. The software can also calculate CPR information from the data.

The data recorder can be read out using their software, see http://heartsinelive.s3.amazonaws.com/uploads/2015/06/H013-001-400-2-US-ENGLISH-SAVER-EVO-1.4.0-MANUAL.pdf

[Fenichel]

This particular (as most AEDs) will actually detect CPR being performed and provide visual/audible feedback. Every compression on the chest is detected and measured by the device. The AED will only allow a shock when it detects a certain irregular heartbeat. It won't help anyone who has no cardiac output.

  This is almost right, but "cardiac output" is a plumbing term, not an electric one.  All patients on whom defibrillators are used have no cardiac output.  What they must have are signs of cardiac electrical output.

  When the plumbing and wiring are all working well, the heart pushes out a few liters of blood every minute.  That is cardiac output.  Sometimes all the wiring is OK, but there are leaks, obstructions, or nonstandard paths for blood to take, other than the proper one out to the body.  After heart attacks, parts of the cardiac muscle are dead, so the pump can't properly respond to electrical signals, however well organized those signals are.  In any of these situations, the electrocardiogram may be almost normal, but the cardiac output may be very low.  Defibrillation wouldn't be relevant, and an AED would refuse to fire.

  Sometimes the plumbing is fine, and the muscle is healthy, but the wiring is shot.  Instead of getting organized signals, the muscle gets useless chaotic instructions, or none.  Cardiac output may be low or zero. 

• Looking at the electrocardiogram, one may see a pattern of electrical activity that has a fair chance of responding to defibrillation.  If the AED thinks that's what's happening, it will fire.  It's analogous (very loosely) to rebooting the computer. 
• Other electrical activity seen on the ECG may be responsible for poor, even life-threatening, pump performance, but not likely to benefit from defibrillation.  There may be other appropriate interventions, depending on what is seen.
• The wiring may be intact, but the pulse generator may be working only intermittently or too slowly.  One can see that on the electrocardiogram, too.  That's when one wants a pacemaker.

[Cerebus]

This is somewhat off topic but about 20 years ago the company I was working for had an inverter designed by a third party to charge up a bank of capacitors fairly quickly. You could hear the magnetics in the converter changing frequency slightly as the capacitors charged up and a colleauge of mine who had worked on defibrillators in the past said that when they had self oscillating converters running at tens of kHz you could "hear" the converter running and knew when the capacitors were charged. As the electronics improved and the switching frequencies got higher you couldn't hear the converter anymore, some customers complained and they had to add additional electronics just to generate the classic defibrillator sound.

Older professional photoflash packs are the same. After a while working with a particular pack you learned to judge the state of charge of the pack from the pitch and volume of the charging whine.

What I felt was missing from this teardown was an analysis of what had been done to combine circuitry that whacks out several hundred joules at 2kV with circuitry that amplifies around 60 mV peak nerve pulses - some interesting input protection challenges there.

[Brumby]

The RTC is for the built in event data recorder, which records not only time stamp data of events but also records an ECG during each stage intervention. The software can also calculate CPR information from the data.

The data recorder can be read out using their software, see http://heartsinelive.s3.amazonaws.com/uploads/2015/06/H013-001-400-2-US-ENGLISH-SAVER-EVO-1.4.0-MANUAL.pdf

I was just thinking .... the RTC clock doesn't need to be second perfect.

A unit can lay idle for years - and even if the clock is out by hours in absolute terms, it will still record fairly accurate timestamps, relatively speaking.  If you want accurate absolute time, simply connect the device to a computer with an accurate clock, determine the difference and apply that as an offset to get sub-second accuracy.

[trophosphere]

Generally speaking, efibrillation protection is a lot more straight-forward than the rest of the circuitry given that a couple things are addressed.

• The first is that the protection circuitry must not shunt an excessive amount of energy through itself and therefore "taking away" energy (energy reduction) that can otherwise used to depolarize the heart muscles: This can be fulfilled by placing current limiting resistors before the protection circuitry.
• The protection circuitry needs to have sufficient spacing/insulin/power rating to withstand multiple pulses.
• The high-pass filter used to remove DC offset/baseline wander needs to recover from saturation (after being subjected to defibrillation) within a specified amount of time.

An example pathway would be current limiting resistor->GDT/MOV->resistor->TVS/Zener/Capacitor*->resistor->diode (need to specify adequate dv/dt rating)->resistor->Input to In-Amp
*Capacitor acts as low pass filter and can be used for common mode/differential mode filtering

Note that with this protection circuitry in place, there is a trade-off between protection and SNR.

[Fungus]

Maybe it's a noob question but... why 3 diodes in series after the transformer? 1 diode doesn't do the job for a half wave rectifier as intended?

Safety.

(belt + braces + a piece of string)

[AF6LJ]

The USB to Serial cable is most likely for firmware updates.
I did a firmware update on an American Made AED for a client about eight years ago. A cable was supplied with the unit, Firmware was supplied from the manufacturer's website and was required to maintain certification. The firmware update app required Internet access in order to update the AED, not only for downloading the software but to confirm that AED had been updated.

[tatus1969]

I am wondering about one thing: they use two SCRs in series, where one of them would not be able to widthstand the working voltage. I know that this would be safe when using MOSFETs, because their overvoltage breakdown mechanism is of the Avalanche type and acts like a zener diode, so we only would need to care about thermal limits. Of course one would choose a particular part that is actually specified for going to that limit.

Does anyone know more about SCRs and how they behave during overvoltage? I looked in the datasheets but couldn't find anything. Also, voltage distribution between the two devices will depend on their leakage currents, and these aren't defined as well, and they will certainly spread widely. Unless you select devices, you have to expect that one of the two may see most of the bus voltage, and the other, more leaky one, will see only a little. Will the first one then break down because of overvoltage?

[SeanB]

SCR overvoltage behaviour is very simple, they turn on. Either the avalanche leakage current as it approaches the breakdown voltage turns the junctions on, or the rate of rise of voltage induces a capacitive current that does the same thing. Using 2 SCR devices in series means they will be able to still function even if one is leaky or failed short, as the other is a back up for the off state guarantee.

Turn off is done with the relays, which are energised to hold the contacts closed during the pulse, but they are released when the pulse is at a lower level, using the breaking ability of the relay to interrupt the current pulse. Then, after a short interval, they close again, so the reverse polarity pulse can be applied, with the massive high voltage SCR using it's turn on immunity to not trigger during the positive pulse and short the power capacitor bank, with the snubber assisting there as well. The reverse pulse is lower energy, and needs a better control of turn off, thus the IGBT devices to ensure a fster turn off than the relay can provide.

Then the relays are released, and the monitoring circuit can then read the heart again with minimal interference.  Relay life, even for those Matsushita units, will be very poor, around 500 cycles, but for a device that in it's entire life will only have 50 cycles of test firing in production, and perhaps another 20 cycles over the entire standby time, that is still a very reliable lifetime.

Failure mode of a SCR depends entirely on the energy available, it will always turn on as a protective mechanism, and then fail as a short if the energy is moderately past the maximum it can handle, which overheats the die and destroys it, or it will blow open if the switched energy is too large and at the level of fusing the bond wires.

[tatus1969]

SCR overvoltage behaviour is very simple, they turn on. Either the avalanche leakage current as it approaches the breakdown voltage turns the junctions on, or the rate of rise of voltage induces a capacitive current that does the same thing.

Sounds good. So they will effectively balance out when sharing the voltage, and cannot be destroyed when excessively unbalanced.

[SeanB]

SCR overvoltage behaviour is very simple, they turn on. Either the avalanche leakage current as it approaches the breakdown voltage turns the junctions on, or the rate of rise of voltage induces a capacitive current that does the same thing.

Sounds good. So they will effectively balance out when sharing the voltage, and cannot be destroyed when excessively unbalanced.

No, they are still effectively limited to the current of a single device, and the voltage of a single one as well. All you get is redundancy, unless you add balancing resistors and can live with this extra off state current.

In this application it is so that there is a much lower chance of a false triggering on when the other side is fired, as the turning on of a single device is not going to turn the other on easily, and if both turn on at once there will be a very big bang as the capacitors all discharge through this and blow them apart.

[bktemp]

Turn off is done with the relays, which are energised to hold the contacts closed during the pulse, but they are released when the pulse is at a lower level, using the breaking ability of the relay to interrupt the current pulse.

They look much too small to me to be able to interrupt something like 1kV @ >10A DC safely. At that power levels you typically need much larger relays with blowout horns or other arc quenching devices.
The relays used are RTE24012, rated for 250VAC 10A or 250VDC 0.2A. It looks like they have connected both contacts in series on each relay, but it is still way out of specs if it is used for interrupting any current at 1kV.

[SeanB]

The relays only have to interrupt current long enough for the SCR's to turn off, thus they will not have high voltage applied to them, and only have to break a current for a short time.

[bktemp]

That does not work:
If you disconnect 1kV DC at several amps, it will form an arc between the contacts shorting the interruption and dropping only some 10 volts. So the SCRs will not turn off.

[max666]

SeanB, I know you are a very smart cookie and I won't pretend to know what is going on, but I can not believe those relays are used to break the current. Like others have mentioned, I don't think that relay (TE Connectivity/Schrack RTE24012) is up to the task of breaking up that discharge. And even if it is, it would be right at the absolute maximum ratings and I don't think a medical device would be designed that way, especially since others keep throwing around the term "belt + brace" as reasons why things are done the way they are, in this unit.
To me the function of those relays is to isolate the high voltage capacitor bank from the helper handling the pads (SCR's could break down, or leak, or how about ESD from touching the pads).

But even ignoring that, the relay has a max. release time of 6ms, the positive pulse is 4ms after which you should break it. How could you possibly operate a relay reliably in the ms range?

[tatus1969]

SeanB, I know you are a very smart cookie and I won't pretend to know what is going on, but I can not believe those relays are used to break the current. Like others have mentioned, I don't think that relay (TE Connectivity/Schrack RTE24012) is up to the task of breaking up that discharge. And even if it is, it would be right at the absolute maximum ratings and I don't think a medical device would be designed that way, especially since others keep throwing around the term "belt + brace" as reasons why things are done the way they are, in this unit.
To me the function of those relays is to isolate the high voltage capacitor bank from the helper handling the pads (SCR's could break down, or leak, or how about ESD from touching the pads).

But even ignoring that, the relay has a max. release time of 6ms, the positive pulse is 4ms after which you should break it. How could you possibly operate a relay reliably in the ms range?

When talking about this being a safety appliance, my first guess would be that they let the SCRs do all the high v/i switching work, and put the relays in series with them in order to have a reliable air gap between the dangerous energies and the "PUT".

[bktemp]

When talking about this being a safety appliance, my first guess would be that they let the SCRs do all the high v/i switching work, and put the relays in series with them in order to have a reliable air gap between the dangerous energies and the "PUT".

Yes, the relays are most likely only an additional safety feature in case one of the SCRs or IGBTs fails. They may help when sensing the heartbeat because otherwise the leakage current from the SCRs or their parasitic capacitance could disturb the very low level signals.

[Brumby]

Yes, the relays are most likely only an additional safety feature in case one of the SCRs or IGBTs fails.

Indeed.

I don't think the relay operating speed has any real impact on the timing of the shock pulses - other than to be closed before the pulses are delivered and open after they are finished.  100ms either side would do the job and this is easily within the capabilities of a relay.

They may help when sensing the heartbeat because otherwise the leakage current from the SCRs or their parasitic capacitance could disturb the very low level signals.

This sounds like a far more likely explanation.

[mal_uk]

I'm trying to work out the thyristor commutation on this thing
I can see how it might work by momentarily using the igbts to divert the current from the bottom scr and turn it off
but...
going back to the description of the pcb, Dave you suggested there are three igbts and 2 1200v SCRs so is the bottom SCR on the Davecad diagram actually an IGBT?
If thats the case it would be a deal less clunky and you could actually redraw the circuit as a bridge with SCRs as the top legs, IGBTs as the bottom and the PUT (load?) across the middle

[Brumby]

Ahhhhh. ..
You want to see the optimised DaveCad.

[filssavi]

The relays only have to interrupt current long enough for the SCR's to turn off, thus they will not have high voltage applied to them, and only have to break a current for a short time.

Problem with that is that the SCR will not turn off, if you open such a small relay it will arc, and without relay internaly designed to quench the arc  at that power levels(which those are not) the voltage drop across the arc will not be enough to turn the SCR off

I think you are underestimating how much an scr loves, to be turned on (or more properly speaking you underestimate the power of positive feedback) to swtch a normal SCR off it must be starved off current, so the voltage has to drop at least to zero (or there-about)




This is the structure of an SCR, the "important" region is the big n- in the middle, the junction associated with her are the only one in the whole device able to substain a significant voltage without breaking down (due to high doping the others will break down at 10V or even less), this is important because to bring one of that two junctions in reverse you will need to deplete the space charge region, form the carriers that are in there due to diffusion current in the active region, to do so you need either a negative anode-cathode voltage for a relatively short time or a very low voltage (lower than a threshold voltage roughly) and a lot of time the n region is quite big and has a very low doping (for high breakdown strenght) and so there is a lot of charge to take out, carrier densities when the device is on are much much higher than the doping atom's concentration, to get low losses the carrier lifetime is quite long and so relying on recombinations woks but is slow.

[Cerebus]

I think you are underestimating how much an scr loves, to be turned on

Dirty, slutty little SCRs. 

[NivagSwerdna]

It would be nice to get the audio samples out of the audio chip... looks like simple... isd4004... spi from a friendly microcontroller?

[rs20]

I wonder if the inductor is part of a clever dance to turn off the SCRs, rather than a PUT protection device. Consider that the circuit is in the first phase (red diagram in the video), and we want to turn the SCRs off and switch into the second phase (green diagram in the video).

1. Now, we turn on the bottom right IGBTs. This is not shoot-through, purely because the inductor is there -- it keeps the current steady, and allows the IGBT to succeed at pulling the Sternum point to ground.
2. With Sternum at ground, the Apex will fall to ground, and with zero voltage across them, the bottom left SCRs will turn off. Victory #1.
3. Now we turn the IGBTs back off. With the bottom left SCRs now off, there's nowhere for the inductor current to go, inductor current collapses to zero, and the top SCRs current consequently drops to zero and they will also turn off. Victory #2.
4. Now we trigger the IXYS and turn the IGBTs back on, and we've successfully transitioned into Phase 2 (green).

The only missing piece I see in my story is the inductive voltage spike that the inductor would produce in step 3, but maybe there's some sort of clamp/snubber/flyback diode somewhere.

[Pras]

I teach equipment for doctors and would be very very grateful if the role of the inductor in this circuit is clarified for me. In monophasic defibs, the inductor was hugely important in protecting the patient from a initial high current. However, I was told by an expert that inductors do not do this in modern biphasic defibs. So I am very surprised to see an inductor here. So, is this inductor there to protect components or patient. Grateful for any clarification. Thank you.


http://www.google.co.uk/patents/US5824017

[Pras]

Please forgive my ignorance, I am a medical doctor and not a trained engineer. I am trying to make sense of the diagram. Is it possible that in the diagram, the IGBT and the lower left TPS have got interchanged ?
 If you change it, then the circuit will make sense. Phase one, TPS after inductor and IGBT in new position come on. Then the IGBT is put off, and since there is now no voltage, TPS after inductor also shuts off. Phase 2: TPS (where current IGBT is located) gets switched on and IXYS gets switched on. Then sequence is terminated with IXYS remaining on, and IGBT (where lower left scr is in diagram) going on . This shorts capacitor terminating sequence. The inductor is there to protect the SCR from inrush of current from capacitor. Does this make sense ?                         

[Andy Watson]

At 22:18 in the video a diagram of the electrode placement is presented. This diagram reappears at 25:10 but it now has the Sternum electrode labelled as positive. I might have missed it, but I did not see where this information came from - I believe this may have been a wrong assumption in Dave's explanation.

This paper says that electrode polarity is not important: https://www.ncbi.nlm.nih.gov/pubmed/11719177 however these people have taken the sternum electrode to be the "cathode" (which I am assuming is relative to the first, main portion of the bi-phasic pulse): http://circ.ahajournals.org/content/91/6/1768.full and http://circ.ahajournals.org/content/91/6/1768?cited-by=yesl91%2F6%2F1768r91%2F6%2F1768

If we take the sternum as the negative then the order of operation that Dave gives must be reversed and the schematic (to my mind) makes much more sense.
I think the correct operation is as follows:
The "shock" cycle begins with the capacitors being charged to the required voltage. The relays will be closed and remain closed until the cycle is complete. The IGBTs will be turned on and the first positive pulse to the apex will be delivered by triggering the large IXYS device. This pulse is terminated by turning off the IGBTs - note these are the easiest devices in the bridge to reliably turn-off - the IXYS device will also turn-off. The negative portion of the cycle is achieved by triggering all the smaller thyristors - the ones labelled TPS. The negative pulse is terminated by either using up all the charge in the main capacitors, or by turning on the IGBTs to by-pass the patient. I think the inductor is there to limit the current under these latter conditions. It may also be possible that the two thyristors in series with the IGBTs can be used to deliberately discharge the main capacitors - again, the inductor would limit the initial current.

I don't not believe that the inductor in this circuit plays a significant part in limiting the current through the patient. I think the patient current is limited simply by the resistance of the body between the electrodes!  In the case of the older, mono-phasic defibrillators I am guessing that the inductor is present to shape the waveform, i.e. it is intended to provide "adequate" current for a longer period of time. To achieve the same adequate current for the same time with an R-C circuit would necessitate a much higher initial current - I think this is the sense in which the inductor is protecting the patient.

[rs20]

Does anyone have any problem with my explanation for the inductor presented in my previous message?

[bktemp]

Does anyone have any problem with my explanation for the inductor presented in my previous message?

Look at the link to the patent. It explains the operation in detail:
It works the way Dave showed in the video, but with both phases swapped: The IGBTs are used to interrupt the current in the first phase.

The patent isn't very clear, but it says the inductor is both inductive and resistive. It probably avoids exploding semiconductors if the electrodes get shorted and it is also used to dump the remaining part of the energy if the second phase is stopped before all the energy is used.