LM3671MF-3.3 Output Short Circuit

Hi Mark, as you mentioned connect the en pin to VIn is not recommended in the datasheet, but I don't believe it can cause the SW node short to GND, which means the internal FET is damaged, normally this happens due to EOS event, you can submit these parts through TI quality return for more detail information.

Thank you for your reply.

I have been reading up on failure modes of smps mosfets. One thing I need to check is the switch node ringing level to make sure the low side  synchronous fet is not being over stressed.

I may need to add a snubber circuit to dampen this. The layout is very tight with wide traces and all components  close to the IC so little more I can do to reduce parasitics.

I have added a voltage detector to set enable high only when vin is greater than 3.0V. I was worried that enabling the part below its minimum voltage may cause shoot through damaging the low side fed.

The maximum voltage on the switch node is listed as vin+0.2V. I assume the body diode of the high side fet would effectively clamp any ringing?

Could you let me know what the switch node voltage should be limited to? I.e. What is the maximum safe voltage of the low side mosfet to prevent EOS damage?

Thanks, mark

VIN+0.2V max at according to datasheet.

Hi Sheng,

To me that sounds more like the fact that exceeding VIN+0.2V would forward bias the diode in the high-side switch?

Surely being a synchronous converter there will inevitably be some ringing at the switch node in any design.

Does the internal synchronous N-FET have an absolute voltage breakdown to know what to limit that ringing to in order to be safe?

For reference the switch node waveforms are shown below. The ringing does exceed VIN+0.2V - I am sure this won't be the EOS level of the FET (why only design in 0.2V margin to breakdown?!). But I do need to know if this needs suppressing with a snubber.

Note this was measured with short ground clip but only a 200MHz bandwidth scope and 16pF probe. Always difficult to know the true ringing level when themeasurement setup could be affecting it.

 

Overshoot to 5.92V, undershoot down to -2.84V.

Could you let me know if this is a design risk and any recommendations for mitigation?

Mark

Any news on the maximum switch pin voltage? I can't believe the low side fet is damaged at vin+0.2V, it must have a higher breakdown.

synchronous converters are likely to have some switch node ringing caused by layout parasitics. My current layout is tight around the IC with short wide copper pour traces

I could possibly add a snubber but I would have to move the inductor and output cap further from the switch node which seems counter productive.

Thanks, mark

Mark,
sorry for late reply, I agreed the breakdown voltage should be much higher, and the duration is such short in your scope plot. But I am checking with our design for more informaiton.

Just FYI...

Usually the dc voltage abs max comes directly from the process specification. Real life switch mode power supplies will always have over shoot and undershoot like you observe. Some datasheets have additional transient short term specification added. Very typical is -3V for 10 or 20 nsec.

Hi Guys,Here is input from our design:
Duration of the spike is ok ~10ns so that is not the problem. I don’t also see a problem with the positive excursion as the spike duration is so short. There is however a problem with the undershoot. -2.5V negative spike is way too big and can damage the NMOS body diode. It is, however, impossible to say which spike is the reason for the damage without FA.
One thing to note. If this is a bonded version of the LM3671, internal spikes can be much higher than what can be measured at the pin.

Ok. So what is the likely cause of the large negative spike? I appreciate I could add a snubber circuit but this would involve pushing my inductor further away from the IC to allow room for the R and C. This would add further parasitics to the layout.

I have also read about adding a low side schottky - this kind of negates the purpose of a synchronous converter if I need an additional external schottky but I am not too sure at the moment how this would help with the negative spike.

My other thoughts for review are the inductor specification to see if this is compounding the spikes seen. Of course with my 16pF scope probe attached could this be creating the spikes that are not really there with the probe removed?!

My layout is as shown below (apologies for the colour scheme!):

The grey trace is 5V coming into the converter. The green is 0V, pinky colour 3.3V. The inductor layout is quite tight.

The circuit to match above is below:

I would appreciate any advice on typical 'spike removal' techniques while I look in more detail at the design.

The PCB this is used on is currently waiting release based on this last detail so I am keen to get this resolved if possible.

Thanks, Mark

Mark,
I would suggest to check the Iq of the device to make sure this still a good part, do you see this kind of negative spike on all three return boards? how about these good ones? Normally the low side FET body diodle should prevent SW goes to more than -1V, if not maybe it's been damaged. adding a schottky between SW and GND should help. couple suggestion on the layout, 1) it's better to connect GND pin direct to the main GND layer; 2) it's better to connec the FB pin to the Cout using a seperate wire,not connectting to the L.

Sheng,

I do not want to change this part at this stage. We have several thousand boards in use with this part at the moment but just three recorded failures due to this IC output short circuit. I have measured the same spike on other boards and with new replacement parts.

I have recently modified the design because enable was tied to Vin and the parts appeared to have failed after powering down the board and then they would not power up again. I was concerned the enable control may have caused the failures and now have a voltage monitor to enable when VIN >2.7V.

During this investigation I have revisited the switching waveforms and noticed the spikes. There is a nice application note from Infineon called "Buck Converter: Negative Spike at Phase Node" which talks about this. A lot of application notes concentrate on the positive ringing and do not give any explanation for the negative spike.

To be sure the issue is not enhanced by my scope probe I have 'double probed' the inductor and see the same waveform on both probes with no noticeable difference in shape adding a second probe. I am therefore fairly confident that this is not created by the addition of the probe.

I am not sure whether the spike is a real risk to the internals of the converter. By adding additional components I will push out the loop to add room for the parts potentially making the spike worse. I would be really interested if you can measure this on an LM3671 development board at TI.

My layout is quite tight and I am not sure how I can improve things other than reducing the short trace inductance from the ground pin. This trace is 2mm long and 0.4mm wide so could be a fraction of a nH.

I thought about connecting the 0V pin directly to the 0V plane as you suggested but then the switching loop would have to traverse two vias to complete the circuit. The sum of the via inductance would be greater than this short trace inductance so it seemed better to go for a short trace on the top layer as all input/output caps are referenced to a mini-flood on the top layer.

As discussed an external schottky or snubber could be added at the expense of adding parasitic inductance to the loop to make room (catch22 situation?).

What would TI recommendation be?

Mark

Adding a PMEG3015EH schottky as an external rectifier reduced the -ve spike to -1.48V. Not sure if this the diode actively clamping or just its capacitance shifting the waveform.

I guess the issue is knowing what is a safe value and what is likely to damage the mosfet and I can only really get that reassurance from TI. Is this spike really present at the mosfet drain?

This issue must affect other synchronous converter designs (internal/external switching).

Mark

With the additional diode the -ve excursion is clamped at a lower voltage. Presumably the step between the two vertical cursors is caused by the dead-time of the synchronous converter. I.e. primary fet turned off, signal tries to go -ve but clamped by the synchronous fet body diode until the dead time ends and the synchronous fet is enabled and the diode is 'shorted'.

Surely then the leading -ve edge is caused due to the reverse recovery of the body diode / fet capacitance unable to react ideally and preventing the signal being instantaneously clamped?

Mark

With regards to the comment:

"-2.5V negative spike is way too big and can damage the NMOS body diode"

I believe the low side mosfet (synchronous) is the NMOS part (going by the block diagram in the DS this uses a PMOS on the high side).

I am wondering how the -2.5V spike can damage the NMOS? Technically if the switch node is -2.5V relative to 0V then the forward voltage across this diode is 2.5V briefly during switching. When the diode is active it would clamp to its forward voltage so I guess this additional voltage is caused by the inductances either side of the NMOS and the resulting voltage generated by the rapid changing current. Therefore would the NMOS actually see less of a voltage (the rest being generated across the bond wire inductance for the 0V and switch node pin)?

Could you help by explaining the mechanism whereby the low side MOSFET would be damaged by this?

It is not particularly easy to model this behaviour as I do not know enough about the internal structure of the part to get a representative model. The TI-TINA model seems to be a bit 'ideal' and even with external parasitics modelled does not really create any over/undershoot.

Mark

Hi Mark,
Large negative spikes at switch node can cause Electrical Overstress (EOS) failures which show themselves as melted and flushed metal lines and gross silicon damage due to too high power, current (body diode damage), voltage (NMOS snapback) or temperature across the damaged device. You are correct that part of this negative spike is due to diode reverse recovery loss.
For the external diode to help to clamp the negative spikes, diode switching speed is critical, so instead of adding the diode my recommendations to fix the issue is to limit the negative spikes by: 1) Improving layout 2) making sure that spikes do not grow during load transients or when device is powered off (both power switches are turned off, so before turn shutdown inductor/load current should be zero). Because the current in the NMOS switch is discontinous, loop inductance of this path should be minimized to avoid interaction of different parasitic LC-resonance circuits.
Mikko