Digital DC/DC converter benefits in MicroTCA power modules
Steve Taranovich - February 19, 2013
Thanks to Ericsson for the following white paper:
Industrial equipment intended for relatively small, low power applications where it can be more cost effective than other architectures. The key power assembly in MicroTCA is called the ‘MicroTCA power module’, which, by its own definition has to deliver and secure the power required by Advanced Mezzanine Cards and other accessories that plug-into the MicroTCA enclosure.
The MicroTCA power module, which contains the majority of the power conversion and control-circuitry, eliminates the need for the large planar carrier-boards required in the AdvancedTCA systems. The MicroTCA power module includes the functions of power filtering, DC/DC conversion, as well as power management. DC input power is plugged to the connectors on the front panel of the power modules, while 12 V and 3.3 V payload and management power is connected to the MicroTCA backplane at the rear of the MicroTCA power modules.
2. The MicroTCA Module
MicroTCA power module (figure 1), provides both payload (12 V) and management (3.3 V) power for all of the loads in the MicroTCA enclosure. These loads may include two Cooling Units (CU) and two shelf-level MicroTCA Carrier Hubs (MCH) in addition to the maximum of 12 AdvancedMC modules, resulting in a maximum of 16 ‘channels’ of output power. Over and above the power channels, the MicroTCA specification requires other functional content in the power module, resulting in a list of functional requirements not detailed here.
Figure 1: Ericsson MicroTCA power module BMR 911 483/1
In order to meet MicroTCA specifications, and to consolidate both power handling circuitry and system level control/management functionality into the relatively small centralized MicroTCA, it requires highly integrated power conversion modules, which, on top of converting the system 48 V to an intermediate voltage of 12 V with high efficiency, has the ability to communicate with the rest of the system through the enhanced Module Management Controller (EMMC) (figure 2).
Figure 2: MicroTCA power module block diagram
3. System requirement for paralleling
The MicroTCA specification includes provision for redundant power modules to increase system availability in critical applications. When needed, this capability can function quite well and achieve the system availability goals. It is important to understand however, that power modules designed for redundant operation are inherently more complex and costly than power modules intended for stand-alone operation.
The required output voltage tolerance on the power module’s 12 V payload output depends on whether power module redundancy is used in the MicroTCA system design or not. But to guarantee the highest flexibility for system architects, Ericsson decided to consider all options built-in one unit, which puts a higher demand on the internal DC/DC converter.
So, does the evidence really suggest that nothing is changing when simultaneously the number of control circuits devoted to add digital performances to power conversion and to optimize power management has never been so high?
Let’s consider the impact of redundancy on the 12V DC/DC converter. The basic MicroTCA specification defines the tolerance range for the AdvancedMC module input voltage as 10 V to 14 V. Since the load module will operate at any voltage in this range, the 12 V DC/DC converter could have a +/-10% tolerance in a non-redundant system.
As highlighted earlier, in a redundant system the situation becomes more challenging, and in order to keep the voltage budgets of both the primary and the redundant power modules within the same overall range at the AdvancedMC inputs without possibility of overlap, the tolerance ranges for the primary power module would be approximately 12.25V to 12.95V and the range for the redundant power module from 11.6V to 12.0V. These ranges include the effects of line and load regulation as well as temperature.
This means that the DC/DC converter in a power module intended for operation in a redundant system must have a +/-2% output voltage tolerance. Going from a +/-10% to a regulation tolerance of about +/-2% has a significant impact on the DC/DC converter design.
The first generation of MicroTCA power modules integrated a standard intermediate bus converter with the external complex analog circuitry that was required to meet the tight specification inherent to redundancy. When considering the challenging requirement to meet such specifications, but as well to lower energy consumption by optimizing parameters to various load conditions while reducing cost, Ericsson considered the implementation of a brand new digitally controlled DC/DC converter, the BMR453 (figure 3).
Figure 3: BMR453 top and bottom view
The BMR453 is a digitally controlled, isolated DC/DC converter operating from the -48V telecom input source and providing 12V output at up to 400 W, featuring a +/-2% output voltage tolerance and packaged in a quarter brick footprint equivalent to the previous analog DC/DC powering the first generation of MicroTCA power modules. The BMR453 is based on a digital controller, which combined with a very efficient power-train, and adaptive control confers a flat efficiency curve in the region of 95% efficiency, from low load to high load conditions.