Analog and mixed signal design: The role of the heat sink in thermal management of active devices

As power circuit designs and devices proliferate in products such as LED drivers, HID lighting, motor control and electric vehicles it is becoming important to understand themal effects in active devices. All active devices dissipate power, and power active devices dissipate lots of power. This power dissipation creates heat which must be removed by some means to prevent excessive heat buildup inside a package or module which ultimately would lead to destruction of the appliance, circuit or device. One of the ways devices can be made safer, thermally that is, is the use of a passve heat sink. The role of the heat sink in active device thermal management is explored in a recent report released by Signal Processing Group Inc.’s technical staff and may be found in: http://www.signalpro.biz>engineer’s corner>heatsink.pdf.

RF and wireless design: Substrates and laminates

Yesterday we spent an absolutely intense two hours in discussions of substrates for RF and high frequency design with a couple of experts. Frequencies from about 1 Ghz to 77 Ghz were in play. The amazing part of the discussions was the level of parameters to be considered, not only in the manufacture of the laminates but also the layout of the interconnect, filters, transmission lines, and heat sinking.For high speed digital the control of the impedance/constant line width was more of a factor, unlike in RF where multiple line widths and shapes are in common use. A multitude of transmission lines are used in a bewildering array of combinations. Other parameters such as the glass weave and its impact on impedance was a discussion worth having. Three laminates emerged as winners for the a large number of applications in design. The venerable FR4 was buried under the the new requirements at 77 Ghz and even at 24 Ghz.The impact of DF and DK ( buzz words of course to be treated in some detail in subsequent posts). The use of materials and their trade-offs were fascinating. The size of the material sold has also gone through revisions and large sizes are now common. Gone are the limits of 18 X 24. The other very interesting issue that surfaced was the role of, and difficulty in, testing of not only devices but also the substrates themselves. The relationships between the thickness and the width of lines changes from the simple expressions we all knew. The difficulty of modeling has increased and very few CAD tools appear to have the capability to do what is needed. Only one CAD tool was mentioned several times as a recommended one for design and modeling at the high performance levels. Some very interesting numbers for insertion loss and actual measured values of permittivity and loss tangents were presented and argued over. Very interesting empirical design equations and data was presented as well. In this discussion the effect of the roughness factor was presented and emphasized. Finally a detailed discussion on the materials of construction such as resins,fillers and reinforcements ended the presentations. In short a very interesting couple of hours. Interested parties may contact us about these subjects through our website at www.signalpro.biz>>contact.

Analog and mixed signal design: Thermal modeling

After a number of mishaps in the design of power ICs and modules with respect to devices blowing up it was decided that we would go back to first principles and understand thermal effects and furthermore use thermal modeling to design better, safer and robust ( with respect to thermal operation) devices and modules. In our attempt to do this we came across many different types of information in the literature concerning thermal design. From the very simplistic thermal resistance and power relationships to fairly complicated thermal models. We also came across thermal modeling software information. This was in the year 2008. We took this information and wrote a brief thermal modeling technical note in the hope that we would have less incidents of thermally caused destructive events. Another interesting result of this was that we were able to set up thermal models of MCMs and devices that were not yet in existence and study the effects of thermals on these to – be devices. These models were built up in MATLAB/SIMULINK and were still fairly simple. We used commercially available thermal modeling software for more complex models. All this effort did help and in the end we were able to meet our thermal design goals in a large number of projects. The initial note was released for publication and now resides in: www/signalro.biz >> engineer’s corner for those interested in thermal modeling or thermal effects. We acknowledge the contribution made to thermal modeling by a number of authors both on and off the web.

USB3 interface design: the K28.5 sequence as a test pattern

In NRZ ( non return to zero ) signaling, a series of 1’s and 0’s are used. The probablity of occurence of a digit is 50%. As a result of this there is a relatively high probability of getting a long series of 0’s or 1’s in the signal. The spectrum of such a sequence contains low frequency content. Consequently high frequency transmission design can become difficult. In order to alleviate this problem data encoding or scrambling is used. A typical technique ( used in USB3 for example) uses 8b/10b encoding. In this case, an 8 bit word is encoded into a 10 bit word. The extra bits are added to make the number of 0’s equal to the number of 1’s in a given bit interval. Additionally this encoding can also be used to improve BER. ( But that is another posting!). For different applications, different types of encoding may be used as well as test patterns. One of the test patterns ( an ubiquitous one) is the
K28.5 pattern. This pattern is a composite of a K28.5+ and a K28.5- bit word and can be described as follows: K28+ = 1100000101 and the K28.5-: ( The inverse of K28.5+)=0011111010. The complete pattern is thus: 11000001010011111010. In USB 3 circuit design, this pattern is encountered often. Please visit our website at www.signalpro.biz and the engineer’s corner for other interesting articles on wireline communications.

RF Amp design 2: Load pull analysis

In the design of RF power amplifiers it is useful ( and important) to know how the output power of the amplifier gets influenced by changes of the the load impedance under varying conditions. In order to get an understanding of this, a useful technique is “load pull analysis”. It is a graphical ( usually) technique that uses the Smith Chart to plot the contours of the load impedance for fixed constant powers. It provides valuable information to the engineer/user about the performance of the amplifier for reasons of assessment of the quality of the amplifier, conditions of operation, design fit or various other parameters. A technical article on the technique has been released by Signal Processing Group Technical staff and is available for perusal by interested parties in www.signalpro.biz>engineer’s corner.