Intersil – Powering High End CPUs and FPGAs with Digital Multiphase

The explosive growth of cloud services has driven major advancements in data center, networking and telecom equipment. The next wave underway is the Internet of Things (IoT) with its billions of devices connected to the cloud. All of this growth greatly impacts the servers, storage, and networking switches that process an ever-increasing amount of data and video. It’s pushing infrastructure equipment to the limit in terms of processing power and bandwidth. For power designers, the main challenge is how to efficiently power and cool this equipment, while ensuring minimal electricity usage. Designers also must balance board power footprints with thermals when using today’s advanced processors, ASICs and FPGAs.

This white paper examines the evolution of the multiphase converter architecture from analog to digital implementations, and compares the various control mode schemes. We will look at a new class of multiphase controllers that leverage digital control techniques to provide synthetic cur- rent control. This evolution in control technology allows the power solution to provide cycle-by-cycle current balancing and faster transient response, while tracking each phase current with zero latency.

The Multiphase Evolution
With the increasing functionality of end systems, greater capabilities and IoT interconnectivity, there is a corresponding increase in processing power to address these requirements. This processing capability is centralized in data centers where high end CPUs, digital ASICs, and network processors run servers, storage, and networking equipment. They are distributed across the network through telecom equipment, and occurring at the point of transaction with point of sales machines, desktops or embedded computing systems using CPUs or FPGAs.

What all of these devices have in common is that their digital processing needs have a similar power profile. With shrinking processor geometries and increased transistor count, processors are now requiring higher output currents that can range anywhere from 100A to 400A or more, depending on their complexity. While this trend has persisted for years, the industry has been able to adapt by integrating lower power states into the digital loads. This allows them to idle at lower currents, and then peak to full power when demanded. While beneficial to the overall system power budget, it adds another challenge to the power designer. The full load current in excess of 200A still needs to be delivered and thermally managed, but now the supply has to react to the demand of a large load step of over 100A in less than a microsecond while keeping the output in a narrow regulation window.

In end systems, the common solution has been to use a multiphase DC/ DC buck converter to provide the required power conversion, typically ~1V output from a 12V input. To provide the large load currents, it’s easier to design a multiphase solution splitting the load across smaller stages (called phases) rather than trying to deliver it via one stage. Attempting to handle too much current in one phase presents challenges in designing the magnetics and FETs, as well as managing thermals, from a (I^2)*R perspective. A multiphase solution offers high efficiency, smaller size and lower cost than a single stage for high currents. This approach is analogous to the technology direction taken by the end loads where multicore CPUs divide the workload. Figure 1 illustrates a multiphase solution that utilizes four phases to provide 150A to the CPU.


Figure 1. Multiphase solution using four phases

Voltage Control Schemes
While multiphase solutions provide the best power architecture, the implementation needs to be carefully evaluated to match up with the latest generation of processors. The trend with end systems has always been enhanced features, smaller size, and improved power management. This is reflected in power designs increasing their switching frequencies to minimize size and manage lower output voltages with higher current in full load and transient conditions. These trends have presented problems in how power supplies are regulated, requiring control loops to evolve over time to keep pace. The key challenge in a multiphase controller is managing the current in each phase, which requires considering these key points:

• Each phase current must equally share the load. If N number of phases exist, the current for each phase should be Iphase = Iout / N at all times.
• Phase currents must be balanced during steady state and transients.

It is important to maintain these conditions; otherwise, you’ll be stuck overdesigning your power supply. It’s important that the control loop has full knowledge of phase currents and the output voltage at all times, without latency or delay in sampling. Historically, this was achieved by using analog current mode control schemes that maintain phase balance cycle-by-cycle. This approach, however, disappeared in the market years ago as output voltages dropped and frequencies increased presenting a challenge in obtaining an accurate signal.

As a result, the market moved towards analog voltage based control schemes including Constant On Time (COT) or traditional voltage mode with type 3 compensation. This was done in an effort to obtain a fast transient response, and minimize the output capacitance needed to manage large transients while avoiding the signal integrity issues in current sensing.

The downside with voltage loops is that you lose the phase current information that is critical for phase balancing and current positioning, either from keeping inductor size down or for positioning with load lines – large digital processors typically implement a load line where the output voltage is moved in relation to the output load. Figure 2 illustrates the problem by showing the current response to a load transient.


Figure 2. Idealized waveform showing phases responding to transient

Today’s newer digital ASICs actively manage their power footprint by continuously scaling their power requirements. This is done by turning
on and off sections of the ASIC as needed by their processing demands. Instead of a constant full load current, their power needs continuously change based on operating conditions. This poses a unique challenge for the power supply. Figure 3 illustrates a common situation with a 6-phase power solution that has to respond to a continuous load transient slewing exactly at the power supply switching frequency and high slew rates.


Figure 3. Idealized waveform showing phases responding to transient

A Breakthrough with Synthetic Current Control
A new approach was to directly solve the problem of current sensing, as opposed to avoiding the issue using workarounds in voltage control. Intersil’s breakthrough was made possible by utilizing state of the art, digital control technology. Advanced control methodologies could be applied by moving the entire control, monitoring and compensation into the digital domain. The result is a synthetic current control loop that provides cycle-by-cycle phase current balancing with fast transient response.

The genesis of the new control scheme was the realization that while the high side current signal is critical in the loop, it is not possible to measure directly due to the short on-time and high noise environment. Instead, the Intersil controller uses a synthetic current signal that is artificially generated, giving it the benefit of being noise-free and accurate and with zero latency. The basic principle is that all the parameters involved in determining the phase current can be measured directly each cycle, allowing the controller to derive the current, as shown in the Figure 4 current waveform.


Figure 4. Inductor current waveform

The Synthetic Control Advantage
The benefits of synthetic current control are that a multiphase power sup- ply can now be designed with cycle-by-cycle current balancing and fast transient response. The current in each phase is known precisely, allowing the device to maintain stable operation under continuous load transients, where all phases equally share current. Combined with zero latency in cur- rent feedback path, synthetic control enables the device to respond faster to load conditions, minimizing output capacitance. Even with high current CPUs, it is possible to utilize an “all ceramic” output capacitor solution. With zero latency, full bandwidth, digital current waveforms, the control loop can position the output voltage exactly according to the load line, mimicking the exact response of the load profile. This avoids the traditional analog RC decay that is seen in output voltages as they settle to the new target voltage. Figure 5 shows that in situations where a load line is not required, the device is still able to meet any load transient, while keeping the device in regulation.


Figure 5. Transient response of 90A load step

As shown, a synthetic current control loop allows multiphase controllers to power any modern high current loads, whether it’s a CPU, FPGA or ASIC. The accurate control and positioning of the phase currents allow the controller to meet any transient with minimal output capacitance without oversizing the inductors.

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