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Advantages of power blocks for high-current POLs

Technology News |
By eeNews Europe


The power block design approach is an ideal choice for today’s power-hungry FPGAs, ASICs, computing and IBA architectures. A power block is essentially a non-isolated buck converter without the PWM controller – it includes power FETs, gate drive circuitry, input and output capacitors, output inductor, temperature sensor and current sense network. In order to form a complete POL converter, a PWM controller, gate drive voltage and some additional input and output capacitors are typically required.

For point of load (POL) and voltage regulator module (VRM) applications, there are several benefits that a modular or power block design approach provides, compared to both discrete solutions and integrated POL converters.
 
Compared to a discrete solution, power blocks can make the design of the power system much easier. As a single ready-made component, they can reduce development time and cost. Layout challenges, thermal considerations and EMI performance have already been resolved in the Power Block packaging. Power blocks are also designed to meet the high quality and reliability standards created for the computing and telecommunications industry. These power block modules have been designed to meet or exceed IPC-9592B Design for Reliability requirements and have passed all of the environmental and mechanical compliance requirements.

Compared to a complete POL module, power blocks have the advantage of flexibility. Different controllers may be used with the same power block, so the optimum balance of cost, size, features (such as a PMBus interface) and performance may be found.  There is the option to use an analogue or a digital controller, offering a wide range of features and performance. The operating frequency may be chosen, or the converter may be synchronised with existing clock sources, if required. Power blocks also achieve higher efficiency than low-profile monolithic IC-type solutions that must operate at higher frequencies (at the expense of efficiency) to achieve their small size.

Power density
Believe it or not, power blocks can save board space compared to both discrete designs and integrated POL modules. In a power block’s optimised packaging, the inductors are elevated above the PCB with the FETs and drive circuitry mounted below. The whole package for the power block therefore occupies no more board area than the inductors themselves. Since the control circuitry uses only low-profile components, this can be placed on the underside of the application PCB to save even more space. The result is a very compact layout, with higher power and current density than either integrated modules or discrete solutions.

Figure 1 shows a demo board for one of the two phase digital PWM controllers used in qualification testing for a 45A power block. The design uses 26.0 by 25.6 mm on the top of the PCB, equivalent to 665 mm2 or 1.0 square inch, including the I/O capacitors. On the underside of the board, the control circuitry takes around half that. This represents a power density around 30% higher than the best integrated module available today. This is set to improve further, as Murata Power Solutions has 60A and 80A & 100A+ versions of this product with the goal of increasing power/current density on each new design. 

 
Figure 1. A two-phase PWM controller using a 45A power block takes less than a square inch of board space. The layout for the top of the application PCB is shown on the left, the underside is on the right.

Thermal Performance
A good power converter design ensures that no components exceed their rated temperature, and that the whole system is kept as cool as possible to maximize reliability. The main culprits for generating heat are the switching FETs and the power inductor. Meanwhile, some other components (PWMs, high ESR capacitors and integrated FET drivers) may not contribute significantly to the total heat loss, but due to their packaging, they may still exhibit high internal temperature rises.  

Design for good thermal management may employ techniques such as strategic component placement on the PCB, the weight of copper and number of layers in the host PCB and the strategic use of vias to conduct heat away from the devices. Compact packaging is generally the enemy of thermal management; in fact, thermal considerations currently dictate the limit for the size of today’s power converters.

These design challenges have been fully resolved in the power block: its packaging has been carefully designed, tested and verified over the full range of operating conditions, in order to meet IPC-9592 component derating guidelines (see Figure 2). Compared to using discrete components, this saves a great deal of development time and cost.

 
Figure 2. The Power Block has been validated for thermal performance over the full range of operating conditions (top). Thermal testing was also carried out with multiple units mounted next to one another to simulate a situation where multiple power blocks are needed to deliver higher current or to provide multiple voltages to a single device (bottom).

As an example, let’s compare the thermal performance of an industry standard 50A SIP with a Murata Power Solutions 45A power block. Power density for the power block is higher, since components can be mounted on the reverse of the application PCB as discussed earlier – since the SIP is through-hole, nothing can be mounted behind it. Also, the SIP requires an integral heat sink to enhance cooling, while this particular power block doesn’t.

The SIP’s derating curves (Figure 3) show that even at 500 lfm of air flow, it still derates to about 45A at 70°C. Meanwhile, the power block shows no derating over the full temperature range with just 200 lfm air flow, for output voltages from 800mV to 1.8V. The 45A Power Block does de-rate to 35A at output voltages from 2.5-3.3V, which is comparable to the de-rating of the 50A SIP at 1.8V at the same airflow rate.  
 

Figure 3. Temperature derating curves for a standard 50A SIP (top) show it derates at 70°C, even with 500 lfm air flow. The power block with 200 lfm airflow (bottom graph) doesn’t derate at all.

New Concepts
While the 45A power block is available now, Murata Power Solutions is developing higher current power block products. These new concepts are shown in Figure 4. On the top left is a concept for a 60A power block design, a two-phase solution that uses a dual inductor wound on a single core.  This saves both space and cost, but also allows a heatsink to interface directly with the FETs. The 60A realization shares its footprint (1.0” by 0.5”) and pin configuration with the 45A power block, representing a 33% increase in power and current density. On the top right of the image is a two-phase 80A concept, in the same footprint as the 45 and 60A versions, which represents an increase in power and current density of 78% over that of the 45A product.

 
Figure 4. Murata Power Solutions’ concepts for new, higher current power block products. Clockwise from top left: 60A single phase power block with heat sink, 80A two phase power block, 120A four phase power block.

VRM Building Blocks
The 120A unit shown at the bottom of figure 4 is a four phase solution with a slightly larger footprint at 0.5” by 1.2”. This concept is particularly interesting for multi-phase VRM (voltage regulator module) applications.

Power blocks are currently available in single and two phase configurations, with the two-phase model delivering two independent outputs that can be operated individually. Alternatively, the two phases can be interleaved, doubling the current and power levels and providing lower output ripple and better transient response. Multiple power block units can be operated in parallel using multiphase controllers, to achieve currents of 90A, 135A, 180A, or even more.  Such an arrangement would provide for a scalable VRM solution in a more flexible package than industry-standard VRMs.  Use of the four-phase module delivering 80A in a footprint of just 0.6 square inches would challenge conventional, discrete VRM solutions.  Two of the four-phase 80A Power Blocks could deliver 160A in a footprint of just 1.2 square inches.  This technique is just a concept at this stage, but with a few customizations to the power blocks, it is perfectly possible.

Conclusion
The power block approach offers many benefits over both discrete designs and integrated POL modules for high current applications, particularly board space. Products up to 45A are available, with 60A, 80A and 120A under development. Customized power blocks are particularly attractive for VRM applications, as they can achieve current density high enough to rival discrete solutions, while reducing development time, cost and risk. This approach also provides for scalable solutions where common motherboards may be used for a range of performance levels with different processor options.


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