Multiphase DC/DC converters provide high efficiency over the entire load range

Overview
As demand for Internet services has grown significantly in recent years, electricity consumption in data centers around the world has become a significant issue. Data centers host web pages, enable social networking and streaming services, provide music and video downloads, provide Internet access, and run simulations. In addition, they provide computing power for traditional and private users of banking and other financial services. Data centers often occupy multiple rooms, multiple floors, or even entire buildings, and contain computers, storage, and network equipment. Between 2000 and 2005, total electricity use in data centers doubled -- from 70 billion kWh per year to 140 billion kWh per year, and continues to grow at an average annual rate of 16.7%, with Asia Pacific (excluding Japan) being the only major region in the world to far exceed that average growth rate [Source: "Worldwide electricity used in data centers," by Jonathan Koomey, Lawrence Berkeley National Laboratory, 2008].

The computers used in data centers (often called servers) are similar in architecture to PCs, with a CPU, ASIC, FPGA, and memory. Unlike PCs, however, servers in data centers are packed as closely together as possible and consume a lot of power, which generates heat that must be dissipated. Power is delivered to these servers via an uninterruptible power system (UPS), which is typically followed by a distributed power system and step-down DC/DC converters for point-of-load (POL) power. This method of power delivery is less than 100% efficient and generates a lot of heat. This heat must be carefully and continuously managed to keep the system operating within its specified operating temperature range. Regardless of the type and efficiency of the cooling system, some method must be used to remove heat from the data center. To do this, additional energy must be used to operate the cooling facilities.

It is estimated that the additional power consumption in the data center due to inefficiencies and cooling systems is roughly equal to the amount of power consumed by servers, storage and network equipment. Users of individual PCs, workstations or laptops do not consider system heat to be a problem, but for data centers, managing this thermal overhead is just as important as the servers themselves. If system power is reduced, the available overhead can handle a larger IT load and do more useful work, while the power consumption level remains the same.

As power demands in data centers continue to increase, more efficient power conversion is necessary to reduce the power wasted as heat. Intelligent multiphase controller technology is an excellent solution for high current POL applications. This architecture enables high current regulators to achieve efficiencies well over 90% under full load conditions. However, most of these designs do not meet the need for higher efficiency at light to medium loads. Saving power wasted at light to medium loads is just as important as saving power wasted at heavy loads. Most embedded

systems are powered by a 48V backplane. This voltage is normally stepped down to a lower 24V, 12V or 5V intermediate bus voltage to power the circuit board chassis within the system. However, most sub-circuits or ICs on these boards are required to operate at voltages ranging from less than 1V to 3.3V and currents ranging from tens of mA to hundreds of A. Therefore, POL DC/DC converters are essential to step down from the 24V, 12V or 5V voltage rails to the voltage and current values ​​required by the sub-circuits or ICs.

Clearly, the desire to increase current at decreasing voltages continues to drive the development of power products. Many of the advances in this area can be traced back to advances in power conversion technology, specifically improvements in power ICs and power semiconductor devices. Overall, these components have contributed greatly to improved power performance because they allow for increased switching efficiency with minimal impact on power conversion efficiency. This is achieved by reducing switching and on-state losses, which improves efficiency while allowing for efficient heat removal. However, the move to lower output voltages places greater stress on these factors, which in turn leads to significant design challenges.

Multiphase topologies
Multiphase operation is a general term for conversion topologies where a single input is processed by two or more converters that are synchronized but run at different and locked phases. This approach reduces input ripple current, output ripple voltage, and overall RFI (radio frequency interference) characteristics while providing a single high current output or multiple lower current outputs with fully regulated output voltages. This approach also allows the use of smaller external components, resulting in a higher efficiency converter, and provides the added benefit of improved thermal management with less cooling.

Although buck converters are generally the more common application, multiphase topologies can be configured as buck, boost, or even forward converters. Today, conversion efficiencies as high as 95% from 12V _IN to 1.xV _OUT are common. At higher power levels, scalable multiphase controllers use input and output ripple current cancellation (achieved by interleaving the clock signals of multiple parallel power stages) to reduce the size and cost of capacitors and inductors. Multiphase converters help minimize the number of external components and simplify the overall power supply design by integrating PWM (pulse width modulation) current mode controllers, true remote sensing, optional phasing control, inherent current sharing capability, high current MOSFET drivers, and overvoltage and overcurrent protection. This simplifies the manufacturing process, which not only helps improve the reliability of the power supply, but also makes the power supply scalable. Such systems can be expanded to up to 12 phases to provide high current outputs up to 300A. Linear Technology has several multiphase DC/DC controllers, including the LTC3856 and LTC3829 single output synchronous buck controllers for high current POL conversion. These devices not only improve full load efficiency, but also have an optional "stage shedding ^TM " function that reduces power losses at light to medium loads. The circuit in Figure 1 shows a typical application schematic of the LTC3856, which is used to generate a 1.5V/50A output from a 4.5V~14V input voltage using two phases.

Figure 1: Schematic diagram of high output current 1.5V/50A application circuit

The circuit in Figure 2 shows a typical LTC3829 application schematic, which uses 3 phases to generate a 1.2V/75A output from a 6V~28V input voltage.

Figure 2: Schematic diagram of high output current 1.2V/75A application circuit

The LTC3856 has two channels and can achieve up to 12 phases using multiple ICs. The LTC3829 has three channels and can operate with up to 6 phases when two ICs are used. The built-in differential amplifier provides true remote output voltage sensing of the positive and negative terminals, enabling high accuracy regulation that is not affected by IR losses in traces, vias and interconnects.

Additional Benefits
These controllers use all N-channel MOSFETs, operate over an input voltage range of 4.5V to 38V, and produce an output voltage of 0.6V to 5V with an accuracy of ±0.75%. The voltage drop across the output inductor (DCR) is monitored by sensing the output current or by using a sense resistor to achieve the highest efficiency. Programmable DCR temperature compensation maintains an accurate overcurrent limit set point over a wide temperature range. Powerful built-in gate drivers minimize MOSFET switching losses and allow the use of multiple MOSFETs connected in parallel. Fixed operating frequency can be set from 250kHz to 770kHz or synchronized to an external clock using its internal PLL. The minimum on-time of only 90ns makes the LTC3729 and LTC3856 ideal for high step-down ratio/high frequency applications.

Step-down Operation
At light loads, power losses associated with switching often dominate the total losses of a switching regulator. Eliminating gate charge and switching losses in one or more output stages at light loads greatly improves efficiency.

Step-down operation allows one or more phases to be shut down at light loads to reduce switching-related losses, and is typically used when load currents drop to less than 15A. Overall efficiency can be improved by as much as 13%, as shown in Figure 3. This graph also shows the efficiency of the older and comparable LTC3729 two-phase controller. Due to stronger gate drive and shorter dead time, the LTC3856 is able to achieve approximately 3% to 4% higher efficiency than the LTC3729 over the entire load range.

Figure 3: Efficiency curve of the LTC3856 with step-down phase (compared with an earlier controller)

When the output voltage of the built-in feedback error amplifier reaches a user-programmable voltage, the step-down operation mode is triggered. At this programmed voltage, the controller shuts down one or more of its phases and prevents the power MOSFET from turning on and off. The ability to set when the step-down operation mode is triggered provides flexibility in deciding when to enter this operation mode. Figure 4 shows the SW waveform and how the LTC3829 enters and exits the step-down operation mode.

Figure 4: LTC3829 step-down phase waveforms: (a) entering step-down operation mode, and (b) exiting step-down operation mode

The LTC3856 and LTC3829 can operate in any of three modes: Burst Mode® operation, forced continuous mode or step-down mode, all of which are user selectable. Under heavy load conditions exceeding 15A, the devices operate in constant frequency PWM mode. Under very light load conditions, Burst Mode operation can be selected and will produce the highest efficiency at load currents less than 0.5A. Burst Mode operation switches between bursts of one cycle to several cycles, with the output capacitor providing energy during internal sleep periods.

Active Voltage Positioning
The LTC3856 and LTC3829 also feature active voltage positioning (AVP), which further improves their efficiency by reducing the maximum voltage deviation during step loads and reducing power dissipation at heavier loads. Figure 5 shows the difference in operating characteristics of the circuit in Figure 1 with and without AVP. Without AVP, the maximum voltage deviation for a 25A step load is 108mV. With AVP, the maximum voltage deviation for the same 25A step load is 54mV. In addition, when the output current increases from 25A to 50A, the output voltage drops by 54mV, resulting in a lower 2.7W of power dissipated by the load.

Figure 5: Load step characteristics (a) without active voltage positioning, and (b) with active voltage positioning

Conclusion
The need to reduce power consumption in data centers will be a major focus in the coming years. As with almost any kind of system, designers of POL DC/DC converters face many challenges due to constraints such as limited space and cooling in a given cabinet and the need for high efficiency over the entire load range. Despite the numerous constraints that must be overcome, many recently introduced multiphase regulators offer simple, compact and efficient solutions. By moving toward a diverse multiphase topology, designers can effectively save space, simplify layout, reduce capacitor ripple current, improve reliability and reduce the power wasted as heat.