Digital control for PoL converters
01 July 2007
The idea of replacing traditional analogue control techniques with digitally controlled alternatives offering numerous advantages provides an attractive proposition, particularly to designers, who have to master multiple supply voltages on one board
Digital supply control, sometimes referred to as supervision and monitoring of traditional analogue- controlled power supplies, is defined as closing the feedback loop and PWM generation for switches in the digital domain. This also includes monitoring and supervision of the power supply.
The benefits of digital power supply control are flexibility and easier board development. Digital power supply control allows the designer to implement last-minute configuration changes without hardware changes. Controllers with a GUI (graphical user interface) allow quick programming with a few mouse clicks.
Other benefits include accuracy and long- term stability. Switching frequencies and error voltages are derived from accurate oscillators and voltage references. Since the digital compensation loop lacks high tolerance, ageing passive components, the digital feedback loop bandwidth can be extended closer to the theoretical limit yielding to better transient response and relaxed output capacitor requirements.
The PMBus
Digital controllers have a PMBus interface allowing them to share information with the host controller. PMBus is standard for communicating with power converters using a serial communications bus. Communication enhances system flexibility and reliability as early indicators of failure such as increased power stage temperature can be communicated and relevant actions can be applied before system failure. It also allows remote (e.g. via internet) system health monitoring and parameters updates.
Analogue control loops are designed to be stable under worst case conditions. Provisions, in terms of load transient response, must be made to satisfy the worse-case condition. If a buck converter changes from continuous mode operation to discontinuous mode operation due to a load decrease, its transfer function changes as well. The analogue control loop must satisfy both transfer functions sacrificing the optimum compensation for each mode. A digital controller can instantly switch between different sets of compensation parameters and apply the optimum compensation values for various operating modes. This improves load transient response under any given load and can save cost and board space on output capacitors.
Digital power supply controllers implement functions like communication, supply voltage supervision, fan control and sequencing into a single device. The integration into one device enhances reliability due to lower component count and reduces overall system cost. On-Chip or external flash memory allows recording of system status. The information from the error memory helps system designers to detect critical system states and implement changes. This information can also determine the reasons for failure. Applications like servers, basestations and media gateways require multiple supply voltages to power DSPs, FPGAs, microprocessors and other multi-supply voltage rail devices. Usually the power supply architecture consists of an intermediate bus converter (IBC) that brings the 48V or 24V input voltage to an intermediate bus voltage in the 3.3V to12V range. This bus voltage is distributed via a power plane across the PCB. At the PoL (point of load), e.g. a DSP, a PoL converter transforms the IBC voltage into the required PoL voltages, e.g. 1.2V core, 1.8V DDR memory and 3.3V I/O voltages.
Such a multi-rail supply voltage system challenges the power supply in terms of output voltage accuracy (the latest high- performance DSPs require tight output voltage regulation of ±3 per cent including transients); dynamic voltage scaling (to reduce power consumption, modern processors use dynamic voltage scaling depending on processing load. This means that the power supply voltage changes according to the actual computation power required by the application).
Sequencing
Sequencing conditions need to be followed to avoid bus contention. These conditions might change during the design cycle. DSPs and other processors have the supply voltage reset released after the power supply has reached a stable condition after start-up. In addition, expensive processors should be protected from over- and under-voltage conditions. Processors also introduce fast load current changes to the power supply system, depending on activity. The power supply has to react quickly to these changes to maintain the output voltage into regulation.
Some processing boards are tested for system reliability and performance when the supply voltage reaches the upper and the lower limits of the output voltage regulation window. Besides voltage and current monitoring, many systems require reporting of other critical system parameters such as temperature and load current. A digitally controlled IBC distributes its 12V output to the digital PoL blocks. They consist of a digital buck controller and a smart MOSFET driver circuit. The controllers communicate via the PMBus with a host controller to control and monitor the whole system.
The digital controllers can have the Fusion Power Peripheral (FPP), a hardware block that allows closing the feedback loop in the digital domain. It consists of a fast, 50nsec ADC, a digital PID compensator with programmable look up tables, a high resolution digital PWM with up to 175psec duty cycle resolution and a fault-counting fast protection. The compensation values are programmable via the PMBus and stored in the data flash of the controller. The UCD91xx controllers feature a single, simple PID compensator with two zeros and the UCD92xx controllers contain up to four, more complex compensators with three zeroes and three poles each.
The UCD9125 is a digital full bridge controller with adaptive dead time. The dead time is mapped as a function of output load to reduce power losses. The controller is interfaced to the power stage via the UCD7201 digital control compatible dual ±4A MOSFET drivers. The dual phase PoL converter is based on the UCD9112. This digital dual-phase synchronous buck controller supports switching frequencies of up to 2MHz per phase. It is configured through the use of a GUI. In addition to closing the control loop digitally, it monitors and manages power supply operating conditions and reports the status to the host system through the PMBus. It also includes a configurable fan controller and a current balancing scheme.
The connection to the power stage is established via the UCD7230 synchronous buck driver. In addition to 4A output drive capability, the driver integrates current limit, short circuit protection as well as under- voltage lockout protection. The UCD7230 also has a 3.3V, 10mA linear regulator that provides the supply current for the controller. The single-phase PoL converter uses the UCD9111 digital synchronous buck controller in conjunction with the UCD7230 digital compatible synchronous buck driver. A typical configuration for a discrete solution with UCD9111 and UCD7230 is shown in figure 2. The UCD9240 is a four-output, multi-phase power system controller that supports 250psec PWM resolution, and is configurable for monitoring, control and management. GUI configuration allows a designer to intelligently manage the power supply’s voltage and current thresholds and response, soft start, margining, sequencing, tracking, phase management, loop response and fan control.
The controller supports up to 100 PMBus interface commands for control, configuration and management of the power supply. The UCD9240 controller’s phase management enables the power supply to operate at high efficiency over the range of the load. Phase management allows turning on or off phases of the power supply so that only the phases required to power the load are enabled. In addition, the controller allows the user to optimise the loop response for the operating condition and meet acceptable transient response over the load range.
Configurability
The UCD9240 operates with PowerTrain plug-in modules that provide configurability when used with the UCD9K or C2000 family of controllers. The PTD08A010W and PTD08A020W 10A and 20A modules integrate the inductor, FETs and UCD7230 driver with current sense capability and integrated short circuit protection.
The Digital Power Developer is a GUI that guides the user through power supply design steps, starting with the design of the power stage and its compensation. The PowerTrain plug-in modules can be used, for fast development time, optimum power stage layout and ease of use. A second option is to design the power stage discretely and using formulae and tools from the analogue world. Next, the power stage parameters are entered into the GUI to configure the control loop. A designer can enter the output capacitors used and the gain and location of the compensation zeroes for the power stage. The result of changes is immediately displayed in the bode plot.
Fusion Digital Power Supplies support output current sensing using the DC resistance (DCR) of the output inductor. That saves additional shunts in series and yields higher efficiency and lower cost of the overall system. A ‘Current Sense’ tab opens a dialogue to calculate the resistor and capacitor value for proper DCR current sensing. Nevertheless, shunt resistors are supported as well if a highly accurate current limit is required. To configure the supply, different thresholds for over- and under-voltage and other parameters are set as well as the response to such an event. The turn on and off delay and the rise and fall times during start and stop can also be set.
MARCUS ZIMNIK is field application manager, high performance analog, central east Europe, Texas Instruments
Contact Details and Archive...
Related Articles...
Most Viewed Articles...