User Benefits of Programmable Power Supplies with Digital Cores

Author : Alan Hill, XP Power

27 November 2023

Figure 1: An example of an analogue power supply control loop, showing compensation components around the error amplifier
Figure 1: An example of an analogue power supply control loop, showing compensation components around the error amplifier

The ‘digital’ tag is applied to many modern products. Often it just signifies that there's a microcontroller doing some housekeeping work, perhaps setting the brew time for a coffee maker or controlling a microwave oven's clock display. The same is sometimes true of power electronics, with terms ‘digital power’ and ‘software-defined power’ now seen in the marketing. However, if true digital control is implemented, dramatically improved performance plus various knock-on benefits can result.

Although some power conversion products have digital interfaces only for simple control and monitoring, this can still be advantageous. For example, in critical systems - where knowledge of degradation or change can warn of impending failure, or perhaps where remote adjustment of the output voltage by a small amount can give some dynamic energy savings. However, extra benefits come when the compensation of the converter control loop is implemented digitally, along with the control and monitoring functions. This would be a power supply with a true ‘digital core’.

Figure 2: Digital loop compensation
Figure 2: Digital loop compensation

Defining control loop compensation
Power conversion will always basically be an analogue function, with significant energy storage necessary in linear components (such as capacitors and inductors). However, to provide a controlled output voltage or current with changing conditions, like input, load, temperature and age, a feedback control loop must be implemented. This must react without instability across the operating range of the power supply with fast response time after system changes, such as load steps, with minimum under- or over-shoot.

An example of a classical analogue control method is shown in Figure 1, where the target output is compared with a reference and the resulting error signal is used to generate pulse width modulation (PWM) of the drive to a power switch - in this case a buck, or forward converter arrangement. The error signal must be amplified to achieve accurate control over the output. However, this gain increase combines with the 180° phase shift of negative feedback and inevitable delays and phase shifts around the control loop, risking overall positive feedback and oscillation at some frequency, or at least poor response characteristics. To counter this, ‘loop compensation’ is applied around the error amplifier - shaping its frequency response in amplitude and phase to provide best performance. Good results are achieved when the phase margin at the unity gain frequency is around 50° away from positive feedback, occurring at 360°, and when the gain margin at the frequency where the phase does reach 360° is around -10dB. The rate of change in gain and phase across the bandwidth of the control loop is also relevant to stability. Playing safe and forcing large gain and phase margins under all conditions is not ideal, as this produces slower response to system changes.

Figure 3: Phase margin in a buck converter changes from high to low line
Figure 3: Phase margin in a buck converter changes from high to low line

A power supply with full digital loop control replaces the error amplifier and its compensation with a processor. The target output is fed to an analogue-to-digital converter, a digital representation of the error is generated and this is manipulated to give the desired gain and phase response. The methodology is that the input data, representing discrete measurements in the time domain, is mathematically transformed into the complex frequency domain by the ‘z’ transform method, similar to a Laplace transform. In the frequency domain, any filter characteristic can be achieved by simple arithmetic operations, multiplication and addition, standard instructions for the processor. A further digital-to- analogue operation generates the control signal for an analogue pulse width modulator, or alternatively the PWM function can be within the processor. Of course, a major difference between analogue and digital compensation is that the former is fixed by discrete component values, but digital can be changed at will, during operation if required.

Benefits of digital control
While digital control can give almost arbitrarily precise compensation characteristics, it is true that analogue control can give perfectly adequate performance with less complexity - but only under one fixed condition. Depending on the circuit topology, if the circuit conditions change, such as load current, or output capacitor equivalent series resistance (ESR) shifting with temperature, the optimum compensation component values are different, so a compromise solution must be found - which would then be sub-optimal for most conditions. Equally, discrete compensation components have tolerances and can drift with age, so a professional design will need to increase gain and phase margins to allow for these worst cases. To illustrate this, Figure 3 illustrates the gain and phase response of a buck converter control loop with a 4:1 or 12dB variation in input voltage. The effect with this particular converter is a low-line phase margin of 50°, as gain crosses unity at Fc low, which is good, and 30° at high line which is marginally stable. Significantly, at high line the gain is dropping at 40dB/decade contributing to a very underdamped response. Improving this would typically degrade performance at low line with severe underdamping, showing the compromise necessary with discrete component loop compensation. In analogue compensation designs, techniques can be used such as ‘slope compensation’ to improve stability under particular conditions - such as at high duty cycles. Again however, optimum values for slope compensation components depend on output voltage in our example buck converter, so ideal values cannot be found for a power supply required to adjust its output voltage over a wide range.

Figure 4: UV curing application
Figure 4: UV curing application

An advantage of digital control is that it allows optimum compensation under all conditions, with scaling factors adjusted ‘on-the-fly’ if necessary. However, major functional changes such as swapping between voltage and current control can also be implemented for battery charging, or non-standard output voltages and monitoring thresholds can simply be selected. This leads to opportunities for field configurability for different applications and use of the power supply as part of a larger process control loop, such as those found in factory automation/robotics. Characteristics and loop compensation can be controlled over a digital bus, and on modern power supplies this might be optionally PMBus, CANBUS or Modbus with RS 485 or I2C physical layers.

Suppliers of power supplies with digital cores will provide software that communicates to the product using a graphical user interface (GUI), where parameters can be selected and optimised and then saved in non-volatile processor memory. After system development and optimisation, changes to the factory defaults can be hard-coded into the product on request at their factory.

Figure 5: The HPT5K0 series digital power supplies from XP Power
Figure 5: The HPT5K0 series digital power supplies from XP Power

Example application - UV curing
Curing of inks and coatings is a global market worth over $5 billion and is typically implemented using ultraviolet (UV) LEDs at power levels of 5kW to 20kW. The LEDs are driven in strings at a precise current, ramped up in a controlled fashion at start up. Dedicated high-power, constant current power supplies are not common and a constant voltage supply with an additional current limit module is expensive. However, general purpose digital power supplies from XP Power’s HPT series or its new single-phase input HPF series can be configured in parallel and series combinations to operate this way. Actual current can be programmed over an RS485 digital interface or through an analogue input. The supplied GUI and close technical support from XP Power make this a quick and economical solution.
 
Example application - micromachining laser 
Laser micromachining is used for the cutting, milling and marking of a wide variety of materials - with applications ranging from trimming semiconductor wafers to text marking at the micron scale and diamond cutting and drilling. The laser requires typically 100V at 1kW to 20kW with precise control of current and voltage, with remote control and monitoring - an ideal match for the capabilities of a digitally controlled power supply. Again, XP Power offers the HPT5K0TS100, a solution with 5kW rating and programmable output from 0 to 105VDC (see Figure 5) or the HPLK50 which is a low line input version. Control protocols include PMBus, CANopen, Modbus and SCPI.

Conclusion
Digital control of a power supply comes into its own when optimum static and dynamic performance is required over a wide range of operating conditions, improving end product capabilities. Even in less critical applications, the versatility of a digital power supply can allow one variant to cover multiple uses - thereby lowering purchase and stocking costs. At the same time, the ability to monitor and control the power supply remotely can bestow significant system benefits, especially when integrated into a larger process control loop.


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