[Design View / Design Solution]
Customize Power Supplies Freely With A Digital Feedback Loop
Digital signal controllers plus power-supply-friendly on-chip peripherals are the building blocks for an easy and cost-effective method of digital power conversion.
PID ALGORITHM IN SMPS DESIGNS Using the PID algorithm, the proportional, integral, and derivative errors of the actual versus the desired output voltage are combined to control the PWM duty cycle. There are three basic forms of the algorithm:
Series, or interacting
Parallel, or non-interacting
Ideal parallel
The PID algorithm can be deployed in both voltage- and current-mode control loops. Also, complex DSP programming skills aren’t required with DSCs, because they offer DSP functions as peripherals within the familiar MCU environment.
Duty cycles greater than 50% may present current-mode stability problems. However, you can easily handle this through the PID software, which sets the required current level. As a result, it’s trivial to scale the DAC value. This makes implementing slope compensation digitally easier than using the analog technique, which requires a ramp generator synchronized to the PWM pulse and a summing junction in which the ramp adds to the current feedback.
The result of this technique is a current-mode SMPS design that’s based on economical, lower-MIPS DSCs, as opposed to a fast controller running at 1 to 2 BIPS. For example, the dsPIC30F202X DSC from Microchip features high-resolution digital PWM generators, an ADC rated at 2 million samples per second, high-speed analog comparators with associated 10-bit reference DACs, and a 30-MIPS, DSP-capable controller (Fig. 4).
The PID control loop is the core of the control software (Fig. 5), which runs under an ADC interrupt on a fixedtime basis. System functions such as voltage ramp-up/ down, error detection, feed-forward calculations, and communication support routines should be executed in the “Idle Loop,” in order to reduce unnecessary work within the PID control software.
The PID loop is the most timecritical portion of the software. So, to make sure the DSC’s resources are used efficiently, the loop should use no more than approximately 66% of the available processor bandwidth. This should leave the design with sufficient horsepower to handle idle-loop functions like communications, or support functions like softstart and sequencing.
In a 30-MIPS, DSC-based SMPS application, this translates to a PID loop comprising 30 instructions, with an execution time of approximately 1 µs. Keeping to an iteration rate of 500 kHz (or 2 µs), the PID-control loop uses one-half of the available processor bandwidth, or 15 MIPS.
FREE TO INNOVATE There are several advantages to power supplies that utilize digital feedback control. Mostly, they involve flexibility and giving a designer the freedom to innovate. As noted above, a frequent concern in a design is the availability of the appropriate technology to implement the design. The advantage of the DSC is its configurability, which lets the designer create the appropriate technology that’s specific to the required design.
For example, a power supply may need to coordinate multiple output voltages during startup and shutdown, or perform load or current sharing among a group of independent powerconversion modules. In these cases, digital feedback control can provide such functionality at no additional cost. Customizing power supplies in these ways using analog components can only be very expensive.
Another advantage is the ability to change a system on the fly, or “hot-swap” capability. For example, if a power module in a telecom or other mission-critical application fails, a service technician can replace the defective power module with a new one while the system continues to operate. This “hot-swap” capability can be very expensive using analog parts, but quite cost-effective if the power supply is digitally controlled by a DSC.
Moreover, if a power supply must be able to adapt to changing requirements, a DSC can be easily reprogrammed. If it’s an analog-based power-supply design, you must start over with a new module. In addition, because of on-chip flash memory, DSCs can enable a simplified power-supply production assembly line. That means a single hardware design can be configured for multiple customer voltage and/or current requirements.
Further, power-supply trimming and calibration can be performed by programming the DSC’s flash memory. This eliminates trim pots or laser trimming of resistors. Digital power supplies can also load test-friendly software for board test, or make multiple custom products based on the same DSC hardware platform.
CONCLUSION The bottom line is that the benefits of digital power conversion are numerous, and designers can now enjoy them in an easy and cost-effective way by using DSCs with power-supplyfriendly on-chip peripherals. Digital power frees designers to innovate and design power supplies with increased reliability, flexibility, and transient response that can also be easily customized at the end of production through firmware, rather than hardware.
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