DIGITAL POWER SOLUTIONS Going digital
By using digital signal processing techniques digital power solutions are able to offer both performance and integration enhancements, as Alexei Zernov and Yngve Wernqvist explain
Digital Power means controlling power conversion by using digital signal processing techniques and the advantages of digital power are being applied to power conversion with the benefits of efficiency, power density, reliability, robustness and ease of use. The process of regulation and control of a switching power supply includes generation of a pulse width modulated (PWM) signal which drives one or more power transistors. In a sense the PWM signal is, in all switching regulators, a digital signal. Thus, it was a natural concept to consider a digital controller for generating the PWM.
Digital signal processing technology is suited for digital PWM signal generation and allows implementation of advanced processing algorithms: filters, performance optimisation algorithms, and nonlinear control and auto- compensation. Optimised, low power voltage setting DAC and Voltage and Current monitoring ADC’s provide telemetry facilities and information far advanced compared to analogue world controllers. All of this enables digital power technologies to deliver new levels of conversion performance, functionality and integration. Thanks to digital control, the ZL6105 is
circuitry must be designed such that top and bottom MOSFETs are never in conducting state at the same time. Conversely, a long period of time when both MOSFTS are off reduces circuit efficiency by allowing current to flow in their parasitic body diodes. The ZL6105 has an algorithm that constantly adjusts dead-time non- overlap to minimise losses, thus maximising efficiency. This circuit will null out dead-time differences due to components variation, temperature and loading effects.
Figure 1. Digital Power Controller using DSP or general purpose microcontroller
Some of the earliest digital power controllers used special purpose microprocessors called digital signal processors (DSP) and general purpose microcontrollers (uC). In these controllers, the analogue signal representing the output voltage of a regulated power supply was digitised and the digital signal was then processed in the DSP. While the DSPs were quite capable in terms of processing power, to achieve the fast processing speeds needed for high frequency switching power supply control, high clock speeds were required. The high clock speed and the inherent high bias current required in those DSPs meant a significant amount of power was consumed in the power conversion process. Furthermore, the DSPs were too expensive for the switching power supply applications. See Figure 1. About 10 years ago, dedicated function state machine based digital power controllers began to appear, first in academia and then in commercial offerings. These state machines were designed specifically to be used as digital switching power supply controllers. Controllers contained specialised HW peripherals for power conversions purposes. They were optimised to the point that digital power started to be economically viable across a wide range of applications. This was that turning point in the history of digital power. A block diagram of a modern digital power converter - the ZL6105 - is shown in Figure 2. A regulation loop key element is a special state machine - PID Digital Compensator.
capable, for example, of executing algorithms to optimise the dead-time applied between the gate driver signals for the top and bottom FETs. In a synchronous buck converter the MOSFET drive
Integration and reliability Reliability is a term used to describe the relative likelihood that a power supply will not fail. In general, the reliability of any system, including a power supply, decreases with an increase in the number of components. An advantage of modern digital power controllers is that they are highly integrated and require fewer components to achieve a full featured power supply. The ZL6105 digital power controller integrates the power conversion control and power management, fault management, and telemetry. Synchronisation functions such as ramp up and down sequencing, switching phase spreading, current sharing, fault spreading and others are performed using communication via a proprietary communication bus. The system monitoring from the host is performed via I2C interface using the industry standard Power Management Bus commands (PMBus). All this eliminates dozens of components from the design. Further HW integration, and therefore reliability improvement can be found in integrated FET controllers and fully integrated power supply modules. Figure 3 shows how
Figure 2. Digital Power Controller ZL6105 using PID Digital Compensator and optimised set of HW peripherals
14 CIE Power Supplement May 2013
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