FPGA temperature measurement

Cute graph.

Cheers

Phil Hobbs

I had a minion photograph my whiteboard data and type it into Excel. I don't do Excel.

Being now calibrated, I stuck a short pin-fin heat sink on top of the FPGA with some grease, and the chip temp dropped 4C. A tall pin-fin dropped it 4C. A 0.7" square of 0.062 thick aluminum, greasy-stuck to the top, dropped the chip temp 4C.

Neat!

Suggesting that the main mechanism is helping transport heat to the leads (or the more distant solder balls), rather than to the air.

Pin fin heatsinks are a crock. You don't get any more surface area than a parallel-fin design, and all the discontinuities interfere with the airflow very badly.

Cheers

Phil Hobbs

Right. It spreads the heat laterally from the hot spot in the center of the package. So, why didn't Altera do that for me?

But I can buy one with a thick flat base and peel-off acrylic sticky on the bottom. The pin-fins are just for show.

It would be amusing to calculate how thick the aluminum has to be before the sticky stuff dominates the thermal conduction. My guess is about 10 mils.

Cheers

Phil Hobbs

In natural convection environments, the discontinuities break up the laminar air flow and improve heat transfer to the air, while having negligible impact on overall airflow.

Different horses for different courses...

Andy

Neato!

Say, how's the voltage coefficient on that, with respect to Vcore I suppose? Would be good for calculating how much ripple it can tolerate.

Tim

-- Deep Friar: a very philosophical monk. Website:

Vcore comes from an LM1117 with the ADJ pin grounded, so it's not really easy to twiddle. I might if I'm ever feeling energetic.

--

John Larkin Highland Technology Inc

Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators

Did you try different kinds of heat sink grease? Apparently toothpaste and Vegemite work pretty well:

Matt Roberds

The plot has increasing temperature with decreasing frequency which doesn't make sense. Increasing the frequency should be increasing the current consumption which results in increasing the temperature.

Are you doing this to confirm Altera's die/pkg thermal coefficient data or do they not publish that information?

Ed McGettigan

I don't think you understand what he is doing. The concept is to construct a ring oscillator that consists of elements within the silicon of the FPGA. This circuit will have a natural oscillation frequency related to the delay of the circuit. The delay in the transistors will be related to the temperature which will make the oscillation frequency inversely related to temperature. Measure the frequency and you can calculate the temperature. The heat dissipation of the ring oscillator has negligible effect on the temperature.

You are confusing the cause and the effect. The temperature is the cause and the ring oscillator frequency is the effect.

He is doing this to measure the temperature of the chip with different heat sink designs rather than to actually design a cooling solution by calculating results based on the thermal parameters of the various components. Thermal design can be rather complicated if there are more than the one heat source involved. Experimental methods can yield quick results especially if some aspects of the design are done and fixed and you don't really have a specific goal.

You're right I misunderstood the use of the ring oscillator with respect to the temperature measurement. I should have read the original post more thoroughly.

Ed McGettigan

Here are comparable experiments done with Xilinx Virtex 5:

regards, Bart Fox