Designing for Power Density and High Reliability

April 20, 2016 // By EDN Europe
Bob Cantrell, Senior Application Engineer, Ericsson Power Modules
The quest for the highest efficiency, reliability and lowest footprint are at the top of the power designer’s priority list. Optimising the efficiency of power converters not only saves energy and minimises the customer’s environmental footprint, but in conjunction with enhanced thermal performance, improves reliability and delivers lower cost of ownership. As a generally accepted rule, for every 10°C rise in operating temperature, life expectancy decreases by around 50%. On the other hand, reducing operating temperature by 10°C can double life expectancy.

The major contributor to excessive operating temperature is internal heat dissipation due to inefficiency in the conversion process. In effect, the operator pays twice for inefficient power conversion : every watt dissipated is another watt that must be cooled to keep the ambient temperature within specified limits. Clearly, improving energy efficiency can improve reliability and also reduce operating costs by reducing system-cooling demand.

Power/ Current density

A vitally important figure of merit (FoM) for power converters is current density. Higher current density means smaller devices for a given power rating, which ultimately allows system architects to utilise more of the valuable board real estate for revenue-generating devices such as processors, ASICs or FPGAs that add commercially attractive functionality.

As a result, designers need a converter to be simultaneously smaller, more energy efficient, with excellent heat-dissipation properties. Dealing effectively with heat is critical to maximising current density and power-handling capability. It’s just not acceptable to pack in the power components into their designated (and usually severely limited) space on the board without paying attention to these factors, otherwise the spectre of field failures will certainly ensue.

Solving the issues - digitally

Digital technology helps considerably to overcome the challenges facing power conversion designs today. Digital converters can be smaller since they require fewer components than a conventional analog converter topology which also helps to boost reliability. In a digital converter the output voltage is sensed in the same way as in an analog design, but there is no error amplifier. Instead, the sensed voltage is digitised by an analog-to-digital converter, and the digitised values are input to a control algorithm hosted on a microcontroller. A variety of algorithms are available to optimise performance as operating conditions change. Figure 1 illustrates the main functional blocks of a typical digital converter.

Figure 1. Digital power conversion simplifies circuit design and reduces component count

Figure 2. Digital power converters can deliver significantly higher efficiency at light loads

Ericsson’s 3E single-phase PoL (Point-of-Load) converters feature advanced energy-optimisation algorithms to maximise efficiency across the whole range of loads. Also, with a specific input voltage, output voltage, and capacitive load, these converters permit the control loop to be optimised for robust and stable operation. This minimises the amount of output decoupling capacitance required to achieve a given load-transient response, thus delivering optimised cost and minimised board space. In effect, this simplifies hardware design, reduces overall module size, and helps to boost reliability. Figure 2 shows just how digital converter technology enables designers to maintain high efficiency at light loads, where traditional analog converters are often less efficient. 

Importantly, these 3E PoLs utilise the latest-generation power MOSFETs, featuring low internal capacitance and optimal on-resistance x gate-charge FoM (RDS(ON) x Qg) which minimises conduction and switching losses under all operating conditions.

The latest converter in the family, the BMR466, is capable of delivering up to 60A and has been demonstrated to achieve efficiency of 94.4% with 5V input and 1.8V output at half load. The MTBF of the BMR466 is calculated at 50 million hours - based on the industry standard ‘Telcordia’