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Benefits of power blocks for high-current POLs

Posted: 25 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:power block? point of load? voltage regulator module? EMI? digital PWM controllers?

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 C 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 maximise 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.

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