Solving Power System Design Challenges With Point of Load DC/DC Converters

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

At the most basic level, the job of a power supply is to convert an incoming AC voltage to one or more lower voltage and higher current DC voltages in the most efficient way. A typical design begins with a bulk AC/DC conversion block, including power factor correction (PFC), to maximize the power available from the AC mains and meet the various regulatory power factor requirements.

There are numerous ways to design the DC/DC section, but in the quest for improved power supply efficiency, size, speed, and cost, the power supply architecture has evolved from bulky, inefficient centralized power architecture (CPA) to compact, high-efficiency distributed power architecture (DPA) and intermediate bus architecture (IBA).

 

Figure 1. RECOM’s point of load DC/DC converters. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 2 shows the three types of design. The CPA architecture generates all the system voltages at a central location. Then, it uses distribution buses to route the required DC voltages, such as 12 V, 5 V, and 3.3 V, to the individual PCBs and devices.

 

Figure 2. Centralized and distributed power architectures. Image used courtesy of Bodo’s Power Systems [PDF]

 

In contrast, the DPA and IBA rely on a higher DC system voltage of 24 V or 48 V. A point of load (PoL) DC/DC converter then converts the voltage to the required value. As its name implies, the PoL DC/DC converter is as close as possible to the load.

Both architectures aim to minimize power loss. Transmitting power from a power source to a load over a resistive connection, whether a wire, bus bar, or PCB trace, incurs power losses in the form of heat, known as distribution losses. The distribution loss for a current is proportional to the square of the current and is given by P = I2R, where R is the resistance of the wire or bus bar. Reducing it requires either reducing the current or the resistance of the wire.

Reducing the current while delivering the same total power to the load requires increasing the voltage (P = VI). Doubling the voltage from 24 V to 48 V, for example, reduces the current by 50%, reducing the distribution loss by 75%. Attempting to achieve the same result by lowering the wire resistance requires quadrupling the conductor cross-section area, which adds weight and cost.

Improved thermal management is another reason to switch to a distributed power architecture. A distributed design spreads the heat-generating elements across the surface area of the equipment to minimize hotspots. Heat is the enemy of electronic components.

It has been well established that increased operating temperatures lead to increased failure rates. In some applications, such as the large-scale LED displays used in sports stadiums or Las Vegas, the effects of excessive heat are noticeable—the light output of an LED declines with increasing junction temperature. Local hotspots can result in LEDs failing faster and appearing dimmer throughout their operating life.

 

DPA or IBA—How to Decide?

Many industrial and factory automation applications use 5 V or 3.3 V as the system voltage for digital devices. These applications are well suited to a DPA with a single PoL DC/DC converter at each load to step down the 48 V rail to the lower voltage.

However, many other applications, including 4G and 5G telecom installations, high-performance blade server racks in data centers, cloud computing, and enterprise IT systems, rely primarily on digital devices such as MPUs, GPUs, and ASICs with small-geometry designs that run on voltages of 1.8 V or even lower. These devices require hundreds of amps and extremely fast transient load response from the power source. It is inefficient to step down 48 V to 1.8 V in a single step since the duty cycle of the DC/DC converter switching transistors would be very low, leading to degraded transient response.

In such cases, an IBA is preferred. An intermediate DC/DC converter stage converts 48 V down to voltages such as 3.3 V or 5 V, and a final PoL DC/DC converter performs the 3.3 V to 1.8 V step. In addition, the smaller ratio between input and output allows for much faster response times.

 

RECOM DC/DC Converters for PoL Applications

RECOM’s RPMGQ-20 and RPMGS-20 DC/DC regulators are well-suited for 24 V, 28 V, and 48 V power rails as a PoL solution in a DPA.

The RPMGQ-20 and RPMGS-20 are open-frame through-hole and nonisolated DC/DC buck converters with a 20 A output rating. The RPMGQ-20 is in an industry-standard quarter-brick format, while the RPMGS-20 is in the emerging standard package size of 36.83 mm x 34.04 mm (1.45” x 1.34”) with a standard sixteenth brick pin-out. Both products have a maximum height of 15 mm from their mounting surface. The parts operate from an 18 V–75 V input, and optional nominal outputs are 5 V or 12 V, trimmable over a wide range, 3.3 V to 8 V and 8 V to 24 V, respectively.

The efficiency of the RPMGQ-20 and RPMGS-20 parts is very high, peaking at 98% for the 12 V output versions and 94% for the 5 V output versions, with a nearly flat efficiency curve down to around 10% load. Due to the low losses and advanced thermal design, full load is available with airflow to an ambient temperature of more than 90°C for all variants derating to 120°C.

The products feature comprehensive protection against input undervoltage, output over-current, short circuits, and over-temperature. Remote sense and control input are also provided.

 

Improvements in Power Supply Architecture Design

Increasingly stringent requirements for higher efficiency, better thermal management, and higher transient performance have driven improvements in power supply architecture design. PoL and intermediate bus DC/DC converters with high efficiency are key components in these new approaches, and RECOM’s new RPMGQ open-frame designs are ideal for distributed power applications.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.