Author Topic: Synchronous Buck IC with Internal Power Switches - failing  (Read 1275 times)

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Offline SystemtekTopic starter

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Synchronous Buck IC with Internal Power Switches - failing
« on: November 24, 2019, 04:55:31 pm »
Hi,

So I'm working on a switching regulator using the LTC4162. It is a synchronous buck converter IC by linear/analog and it keeps failing and turning into a resistor of a few ohms after running nice and cool for a few minutes with a constant load. My guess is that ringing on the switch node is causing the primary switch to be pushed past it's Vds breakdown. I haven't rebuilt the circuit since I came up with this idea so I can't check the current through the inductor. If it turned out that the inductor did not have a current passing from the switch node I suppose the next possibility is that the low side switch has failed.

I cannot find the breakdown voltage of the internal power switched in the datasheet https://www.mouser.co.uk/datasheet/2/609/LTC4162-L-1398190.pdf. The Absolute maximum ratings listed on page 3 and the electrical characteristics starting on page 4 are pretty light on info it seems for such a highly rated manufacturer. (annoying this sheet gives lots of general switching regulator design advice and of course masses of info about the digital telemetry system but does not specify the anything more than the Rds on for the power switches).

This means I cannot make a spice model of the internal power switches because one important parameter for simulating the ringing on the switch node when the primary/upper power switch/FET turns on is the output capacitance of the lower/secondary/rectification FET.

have been running the circuit with an Vin of 35v only one volt away from the Absolute max but this is clearly how it's designed to run and I check the bench supply using a meter.

Also I have done my best to clean the flux after soldering this WQFN package with out ultrasonic cleaning. I did not manage to specify the solder mask expansion as I wanted it so there is not solder mask between the pads for the IC pins but visual inspections and R measurement said no bridges.

I appreciate there are probably a lot of factors that could go into a failure mode like this but I thought I'd try to outline some of parameters of the design and make some of my own guesses about whats happening and see if more experienced people can offer at least some suggestions of what to try. This is my first serious switching converter design, speak slowly :)

BTW these ICs are pretty expensive for a hobbyist so I'm taking it easy and trying to get some good ideas of what might be causing this before ordering more and stressing myself out trying to solder the bl*?dy things.

Many thanks,

Han
 

Offline SystemtekTopic starter

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #1 on: November 24, 2019, 05:04:29 pm »
Here is a model of my design.
 

Offline Jay_Diddy_B

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #2 on: November 24, 2019, 05:26:22 pm »
Hi Han,

Operating a 36V ABSmax part at 35V  is asking for trouble. Normal (industry) design margins are at least 25% so 24V nominal is probably the maximum input voltage that should be used.

Also read AN-88

Link: https://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdf

This applies to the input and the battery connection in the case of a battery charger.

Regards,
Jay_Diddy_B
 

Offline ConKbot

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #3 on: November 24, 2019, 05:39:44 pm »


have been running the circuit with an Vin of 35v only one volt away from the Absolute max but this is clearly how it's designed to run and I check the bench supply using a meter.

This combined with no electrolytics onboard makes me wary of the input ringing up above the max input voltage.  The wires to the power supply can have quite a few uH of inductance. This also introduces the hazard of turn on transients ringing up very high, but using a bench supply with a controlled turn-on should negate this.  Unless your bench supply just clicks on an output relay, in which case, not so much.

Electrolytic caps provide the "bulk" decoupling which reduces change in current during the switching cycle, as seen by the bench supply and test leads, and also provide damping to the input LC network though their ESR.

If you want to simulate something, stimulate the power supply output(few tens of miliohm ESR), leads (resistance and inductance), input capacitors (remember voltage coefficient of ceramics, even a 1206 body cap will have significant capacity loss at 35V)  and put a switched load on your input network.

« Last Edit: November 24, 2019, 05:41:19 pm by ConKbot »
 

Offline SystemtekTopic starter

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #4 on: November 24, 2019, 07:30:03 pm »
Thanks for your replies.

So lets say I can design an input filter that is optimized for stopping the input voltage ringing/overshooting. Is it still the case that when charging the 8 Li-ion cells it is designed for it might just keep failing?

The fully charged voltage of the stack is 33.6V it doesn't allow me to drop the input voltage much lower at all. There also isn't enough of a margin there to use a TVS diode that wouldn't have too high of a leakage current as far as I can tell. 

I can't find another 8 cell solution as good and feature rich as this one so I'd hate to have to give it up.
 

Offline SystemtekTopic starter

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #5 on: November 24, 2019, 07:39:04 pm »
It is designed to be supplied from the output of another DC/DC converter with a large output capacitance. I don't yet know what the DV/DT of the output of the supplying converter is when it starts up.
 

Offline Jeroen3

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #6 on: November 24, 2019, 08:49:04 pm »
Who said this works with lion batteries? The thing is made for lead batteries. Specifically:
Quote
The switching charger can
charge 6V, 12V, 18V or 24V lead-acid batteries.

A 24V system absolute max voltage is often 33.6V. So the 35V maximum voltage is for fault conditions.
I'd expect intended operating voltage of this chip to be 14 or 28 Vdc.
 

Offline SystemtekTopic starter

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #7 on: November 24, 2019, 11:22:31 pm »
Quote
Who said this works with lion batteries? The thing is made for lead batteries. Specifically:
Quote
The switching charger can
charge 6V, 12V, 18V or 24V lead-acid batteries.

A 24V system absolute max voltage is often 33.6V. So the 35V maximum voltage is for fault conditions.
I'd expect intended operating voltage of this chip to be 14 or 28 Vdc.

Apologies I meant to link the datasheet for the li-ion/lifepo4 model. I ordered this exact part number - LTC4162EUFD-LAD#PB. From this page and datasheet.https://www.mouser.co.uk/ProductDetail/584-4162EUFDLADPB
« Last Edit: November 24, 2019, 11:24:35 pm by Systemtek »
 

Online nctnico

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #8 on: November 25, 2019, 12:00:43 am »
Thanks for your replies.

So lets say I can design an input filter that is optimized for stopping the input voltage ringing/overshooting. Is it still the case that when charging the 8 Li-ion cells it is designed for it might just keep failing?

The fully charged voltage of the stack is 33.6V it doesn't allow me to drop the input voltage much lower at all. There also isn't enough of a margin there to use a TVS diode that wouldn't have too high of a leakage current as far as I can tell. 

I can't find another 8 cell solution as good and feature rich as this one so I'd hate to have to give it up.
I'm afraid you have to. I ran into a similar problem with one of my own designs but fortunately I could set the supply voltage lower and use a TVS diode to keep the device within limits. The absolute maximum rating doesn't have any margin on it.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline SystemtekTopic starter

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Re: Synchronous Buck IC with Internal Power Switches - failing
« Reply #9 on: November 25, 2019, 01:14:31 am »
Maybe it's worth just adding a cheapy high ESR electrolytic across the input filter ceramic and starting it up with a current limit to create a slower rising edge while monitoring Vin at the pins of the IC and switch node with a scope. I could run it at a lower voltage but no so much as to voltage coefficients of the ceramics don't throw out the filter cap tuning and see if I can work it out by experimentation.
 


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