Gary Bocock, Technical Director, XP Power
Digital control in power supplies can mean different things ranging from simple digital signalling of status/alarms and on/off control of a traditional analogue controller, through addition of more sophisticated functions with a simple microcontroller, all the way through to full implementation of feedback loop compensation with a Digital Signal Processor (DSP). Latest techniques allow flexibility in configuring a power supply’s characteristics and performance during development and commissioning, and dynamically in the end application.
Although a DSP chip is higher cost than a dedicated analogue controller, prices are reducing and performance improving, so the benefits become increasingly compelling. This is especially true at higher power with higher value products, where the system performance improvement is more evident and component cost is less of an issue.
DSP in a power supply design does require knowledge of control loop stabilisation in the frequency domain and how to implement it in efficient code in the time domain, but the techniques are understood and mainstream power supply manufacturers are embracing the technology. Investment is required though in skills and time to develop, verify and document firmware to achieve a reliable and robust solution but the code is re-usable and common hardware platforms can be designed that can be quickly software-configured for multiple applications – a real benefit in production, passed on to users in time-to-market and unit cost economies.
Digital Signal Processing – the features and benefits
The main advantages of a fully digital implementation of the control and monitoring of a power supply are flexibility and ability to adjust performance and characteristics for different applications –something that would require hardware changes with a traditional analogue controller. Digital control loops also have the advantage of being immune to environmental changes and variation of component values with tolerance and age. This enables more exact definition of performance. The DSP chip can also respond to system performance variables, for example detecting load characteristics and optimising the control loop for best load transient response or dynamically lowering output voltage to reduce load power dissipation in standby modes.
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Figure 2 - DSP controlled digital power supply
Adjustment of output voltage can be extended to a wide range, even 0 – 105/110% for more general applications or when the final optimum system voltage is simply not known. Current limit is similarly adjustable, not just in ‘knee’ value but also for its characteristic in overload or short circuit, perhaps ‘constant current’ latch-off, or ‘hiccup’ mode. This is important in modern high-efficiency resonant topologies where the multiple switching and loop compensation schemes required are impractically complex to achieve with a traditional analogue controller.
Apart from controlling the feedback loop, in a digital power supply, it is easy to include a host of other programmable features with system benefits that would entail hardware changes in analogue implementations. For example, start-up/shut-down ramp rate and sequencing of multiple supplies is settable with different schemes such as ‘ratiometric’ or simultaneous; polarity of enable inputs and signal outputs can be toggled at will. Fault and status conditions can be monitored and signalled such as input under-voltage, output under/over-voltage, output over-current and over-temperature. All thresholds can be adjusted, dynamically if necessary. For example, if output voltage is adjusted higher ‘on-the-fly’ the over-voltage detection threshold can also be adjusted higher.
The response to warnings can also be programmed with variable delays before signalling and the resulting action varied, perhaps just logging of the occurrence or shutdown with a specified number of re-tries at specified intervals or immediate latch-off for more serious events such as a sustained output over-voltage. Non-volatile memory in DSP chips can be utilised not just for the control algorithm and as look-up tables for response characteristics but also for recording other information such as model and build standard, serial number, run time and an event log of temperature records and faults/warnings.
Latest microcontrollers with DSP functionality allow this degree of functionality as they are fast enough to sample output voltage every switching cycle, perform A-D conversion within a few hundred nanoseconds, execute the feedback control algorithm and still have plenty of bandwidth for slower speed control and signalling. For flexibility and backwards compatibility, an in-built A-D converter can also provide for analogue control inputs such as 0-5V or 0-10V for voltage setting. Individual controls and signals are also typically available on connector pins as well as through the communications bus, which might be accessible externally for system control or just internally to the power supply for service engineer use.
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Figure 3 - Simplified example of control loop & spare bandwidth
Digital power in the wider world
Power supplies with digital control and signalling come into their own in ‘smart factory’ applications where process hardware is interconnected in an IoT configuration to maximise production capacity and efficiency on-the-fly. A combination of intelligence at ‘the edge’ and central monitoring and overall control not only adjusts parameters in real time for process efficiency but also logs data for aggregation showing faults and transient events historically. This, in turn, allows prediction of future performance and devising of preventative maintenance schemes for maximum up-time. Reporting of loading on power supplies could also be used as an indication of energy usage and at what time, giving valuable information that could be used for load levelling and cost savings. Digitally controlled power supplies could also be used to replace dedicated laboratory programmable power supplies in automated test for further cost savings or to power equipment burn-in, where automated ‘margining’ of supply voltages is easily programmed.
Although the typical communications interface to a DSP chip would be I2C using PMBus commands, digital power supplies can often also include RS232/RS485 and DeviceNet/EtherCat interfaces more suitable to the industrial environment.
Power supply vendors will often provide a hardware and Graphical User Interface (GUI) for easy setup of the power supply and monitoring of its performance. A typical example is shown below.
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Figure 4 - Graphical User Interface (GUI) for XP’s HPT series digital power supplies
Digital control of power supplies can still benefit applications where external communications are not needed and dynamic adjustment of performance is no advantage. Potentially, power supply vendors can offer a solution, digitally configured from a standard platform, that would otherwise have to be a custom product, saving the user time and extensive qualification and safety certification costs. If the end application does change, due to a product upgrade for example, it may be possible to re-configure the power supply with a simple firmware change through the communications interface, again saving considerable cost and extending the useful life of the supply.
To summarise, although analogue control will still have its place for simple and low power applications, digitally controlled power supplies, particularly at higher power, enable many benefits including increased functionality and flexibility with potential cost savings from reduced energy consumption in larger integrated systems and extended product lifetime.