In the late 1990s, Molex recognized that the trend for increasing power in microprocessors was becoming more than just a DC bulk power delivery problem. DC power for high performance processors was taking on AC characteristics. This was confirmed in discussions with a major microprocessor manufacturer. The combination of increasing processor clock speed and decreasing core power voltage was driving up the peak current and the slew rate.
These trends pointed to a future problem for designers as they developed a current delivery path from VR to processor that stayed within the ever smaller allowable fluctuation in voltage during processor power ramp. Servers using voltage regulator modules (VRM) were of particular interest for Molex.
It was clear that the industry needed a new VRM connector that was not only capable of effectively handling the increasing DC current, but also supported the increased slew rates, i.e., the AC characteristic of DC power.
The attributes needed for a successful new connector came from various sources. The initial list of requirements included 130 amps of output current, 20 signal lines, input power with integral latching, with voltage drop less than 20mV at 100amp per microsecond slew rate in less than a 4.00” connector.
Additional requirements were added as the project progressed including, making the vertical height of the VRM and connector less than 1.250”.
All of these variables were considered when Molex began the process of developing the iCool VRM connector.
Digital electrical solutionsMolex engineering tackled the slew rate problem first by applying aspects of high-speed digital electrical solutions. Voltage fluctuation is a function of L (di/dt).
Since slew rate (di/dt) is a function of the processor, the new VRM connector needed to have low loop inductance. It was known that multiple parallel current paths would reduce loop inductance (L), as well as connector resistance (R) but there was no known method to demonstrate the reliability of multiple pin current sharing at endof- life. To further keep inductance low, contacts were designed as short as possible from the motherboard to the VRM output contacts. The concept was modeled using contacts on 1.0mm pitch. Simulations showed that the L(di/dt) drop was well within target numbers.
Once Molex solved the slew rate problem, calculations were used to estimate the number of contacts needed for DC current. However, Molex could not accurately estimate the mutual heating of the contacts.
To understand the effects of mutual heating, Molex converted a SIMM product from 0.050” contact pitch to 1.0mm pitch and began temperature rise testing. The SIMM was selected because the contact was short and provided two contacts to the VRM— one on each side of the module PCB but with a single PC solder tail.
Testing of the connector to understand temperature rise as a function of current proved that the concept met the design goals. To better balance the current from module to motherboard, a second PC tail was added to each contact.
By doing this Molex was able to increase the parallel paths from the VRM to the motherboard, further reducing R and L.
Early test data of this new VRM connector concept gained high interest with a few potential customers, but they also pointed out two problems. The prototype had retained the “insert and rotate” latching approach, which is typical of SIMM connectors. The additional space that this required around the designers from locating the VRM in the best location on the motherboard. Either the chassis walls or the processor heat sink prevented module insertion. A second issue found that VRM designers required a module PCB of 0.062”. The SIMM prototype connector was designed for module PCBs of 0.050” thick.
With this feedback, Molex set out to make the needed changes and answer the multi-pin current share question.
Molex decided to develop a methodology to demonstrate the reliability of multi-pin current sharing first since this could be a showstopper if it could not be proven. The comprehensive solution has been well documented in a paper written by Dr. Bob Malucci titled “Current Rating for Multi-Path Power Module Connectors.”
Basically, Molex solved the problem by taking the approach of combining actual end-of-life environmental contact resistance data with actual current temperature rise data in a random, yet statistically, significant predictive methodology.
Once the current sharing methodology had been proven, the product was redesigned for the 0.062 module PCB and direct insertion in the manner of an edge card connector, eliminating the insert and rotate latching. The new connector retained the SIMM dual contact and dual PC tail design. The one-piece contact assured that the contacts would be self-supporting at high operating temperatures and would not depend on the plastic housing for normal force. Customers advised that high ambient temperatures would be mitigated by forced airflow to keep the VRM cool. Realizing that airflow would be used to cool the VRM, the design engineer suggested that the sides of the connector housing be open to allow the circulating air to also cool the VRM contacts. Surfaces were then added to the contact to aid in heat transfer and which also had the additional benefit of increasing capacitance and reducing loop inductance. While the open sided housing suggestion was initially viewed as creating potential molding and handling issues, Molex decided to include the idea in the test vehicle to determine its merits. Molex developed temperature rise vs. current curves at various ambient temperatures and airflow rates. The results proved better than expected. The T-rise curves developed now serve as the basis for accurately developing new VRM connectors perfectly sized for the specific application.
A VRM connector without a VRM to plug into is not of much value to an OEM. To overcome this challenge, Molex worked with a power silicon supplier who was developing a low profile VRM for a 1U server. The VRM designer was developing a VRM using the VRM 10 connector that was a standard height edgecard connector with contacts on 0.100” pitch. Total vertical height available for the connector and module was 1.250”. The original standard VRM 10 connector was so tall that there was not enough vertical area available above the connector for a low cost VRM to be developed for the 1U application that used standard components.
Molex advised the VRM designer that it was developing a low profile VRM connector that would provide almost double the PCB area for the VRM. By making an additional design change to lower the height of the connector by 1.0mm, Molex’s new connector design permitted the VRM designer to utilize the space savings in order to provide a lower cost module for their customers.
At this point, the VRM designer advised that two major server OEMs had requested new 1U VRM designs and needed to be introduced to the Molex low profile VRM connector.
Before taking the new VRM design to the sample stage, the VRM designer needed to know that the OEMs would accept the new Molex connector. As a result, Molex made presentations to both of the OEMs.
Separately, both OEMs agreed that the new Molex product was exactly what they needed but also asked for a few small design changes. One of the OEMs wanted the connector to be SMT, so the design was modified to include an SMT version. The other OEM wanted through hole solder, but needed to be able to route pairs of parallel traces though the VRM pin field. The diameter of the PTH was reduced to provide this capability. Both OEMs approved the product that is now known by the iCool trade name.
In the final design that was first used for VRM 10.0 applications, the effective connector resistance was 0.057 milliohms, loop inductance was 53 picoHenries, and connector capacitance was 37 picoFarads. Voltage drop from VRM to motherboard at 100 amp per microsecond slew rate was a low 5.3 millivolts.
Features addedAdditional features were added to the product family to retain modules as heavy as 125 grams and 2U tall. Also, a right angle (horizontal) option was added so that OEMs could use iCool connectors in blade servers using the same VRMs used in vertical applications by simply replacing the heatsink appropriate for local airflow.
iCool connectors have been in volume production for over a year. The T-rise curves at various airf low rates show that iCool connectors are capable of delivering up to 160 amps of processor current (320 amps output and return) with 24 signal contacts in a 100.00mm connector at 400 LFM air flow. Use of iCool connectors has expanded beyond VRM applications to also find use as a low profile power edgecard connector. Moving forward, Molex continues to innovate and improve on the original patented iCool connector design.
For example, additional signal contacts are being added to support multi-core processors without significantly growing the connector length. Plus, two signal contacts are being added per 1.0mm of connector length.
Listening to the voice of the customer takes time and can be an evolutionary process. Having input from several sources, which includes customers and partner suppliers, has resulted in a superior VRM connector product meeting the varied and evolving needs of those implementing pluggable VRMs.
