H7 power dissipation and lifetime, how to manage?

[regjoe]

Hello,

I came across this document here https://www.st.com/resource/en/application_note/dm00622045-stm32h7-series-lifetime-estimates-stmicroelectronics.pdf

We have a H75x custom board running the MCU in VOS1 at 400MHz with external 1.2V core power supply.

According to this description here

According to Figure 2, when VOS1, VDD = 3.3 V, VCORE = 1.2 V and operation ratio of 100%. Some examples are
illustrated such as:
• Tj = 105°C the lifetime estimation is > 10 years
• Tj = 125°C the lifetime estimation is 4 years
• Tj = 140°C the lifetime estimation is 2 years
In the same conditions and for an operation ratio of 20%, the lifetime estimation is as following:
• Tj = 125°C the lifetime estimation is 20 years
• Tj = 140°C the lifetime estimation is 10 years

the power consumption, heat dissipation and lifetime is influenced by "operation ratio". What does operation ratio mean and how can I influence/control it? Does this mean that only a part of chip silicon is powered/used? Or time spent in the idle loop, assuming the MCU is running in lowest VOS mode and/or clock frequency during idle time?

Thanks,

Jochen

[AScha.3]

>Never heard about failing systems due to running at CPU maximum clock speed. Is this only STM business?  

No, i think its just the chosen chip technology , tiny structures are more sensible to high temperature and migration.

>If all depends on junction temperature, how can we measure it?

Good indication is to check on center/top of package (with a thermal cam you can "see" the chip).

I have my H743 at Vos2 with 200MHz (dont need more speed), but 2x SDMMC + USB , is about +4° over ambient.

>what must be done to make the chip junction temperature rise to a temperature > 100°C.

First question is : whats your ambient temp.? 

At work, we have a customer in Indonesia, summer is warm there and big extruder in hall with sheet metal roof,

you cannot touch the metal of the control cabinet, inside the 10kW inverter with the control cpu;

here the ambient might be 90° C , so if the cpu runs on high current (= highest speed), the life time might be a problem. If cpu only heats up 20° , it might get critical. But at 200MHz, no problem (until now, 4 Y or so).

Just see ds , what this kind of chip process doing at high temp:

At 105° you get 550 mA !!! THIS it cannot survive many years.

In this table you see the strong effect of the ambient, so having +40° makes a big difference.

But if ambient not so hot and speed not at max , no problem:

my H743 with all peripherals i need runs at 83 mA , about +4° over ambient.

So the real current consumption and the resulting heat on chip is the indicator, will it get a problem - or not.

Reducing the on chip loss by external power or (some have) SMPS can help also, to keep the chip cool.

[regjoe]

---

@AScha.3 wrote:

Just see ds , what this kind of chip process doing at high temp:

In this table you see the strong effect of the ambient, so having +40° makes a big difference.

This table shows the effect of ambient? Do you mean that current @ Tj=25°C is, for example, measured at Ta=0°C and Tj=105°C is measured at Ta=60°C? The higher the ambient temperature, the higher Tj and the more current is drawn by the chip? How do you explain this effect?

If so, I misunderstood this table, I thought that Tj rises due to higher MCU activities (which rises the question, which activities drew most current).

[AScha.3]

>How do you explain this effect?

Just look at ds of a single mosfet, or any transistor...junction temperature changes almost all parameters.

And -right- its own activity produces loss = heat, so its parameters change also with activity or switching.

hot -> bad on-resistance

hot -> resistance + switching-level -> more bad

switching loss 25° -> 175°  about 300% increased  !

So same speed switching produces up to 300% more heat at same switching speed, if chip hot !

This depends a lot on the used chip process, maybe you look at arm web pages, to make a chip:

(i did) lets say, we want make a new M33 core : arm gives you 3 tested options, made by TSMC :

- a low loss process, small chip area, but 100MHz max. speed

- or a fast process, high idle loss, high power loss, but up to 500MHz

- or a medium...in between.

So if you want the fastest cpu, it will produce high loss and be more sensitive to high temperature also.

See the new U5xx series by STM, very low power loss, but 150M max or so.

Its all a sum of many decisions and compromises, what you prefer and whats the bad side you get with it.

So >This table shows the effect of ambient?  -- and the effects of self heating , that adds to ambient.

At xx MHz the core will always have same loss, running math or wait loop changes (almost) nothing,

but if chip/process is more sensitive to ambient, it will heat up much more , than at cool temp.