[Design View / Design Solution]
Digital PWM Controllers Augment System Reliability
Intelligent digital controllers can simplify power-supply design as well as enhance overall reliability through monitoring and statistical interpretation of key performance metrics.
Because the compensating filter for a digital controller operates numerically, minimal offset and gain tolerances are associated with its transfer function. It will also have little drift with time and temperature. This means that the only variation in the transfer function of the compensator will be due to tolerance in the clock frequency for the digital logic.
Therefore, any variation in the measured loop transfer function should be due to changes in the analog power stage and not the controller. If the compensator transfer function is divided out of the measured open-loop response, an accurate picture of the power stage, and any variation, can be observed.
In addition to monitoring the small signal ac transfer function, a digital controller has ready access to the instantaneous and average duty cycle. In a digital pulsewidth modulation (PWM) controller, a digital filter performs compensation (Fig. 3). The filter’s output is proportional to the control effort necessary to regulate the output voltage.
Since the filter is digital, the filter’s output can be easily sampled by the supervising microcontroller. In fact, the authors of the PMBus Command Standard for digital power supplies anticipated this and defined the standard command: READ_ DUTY_CYCLE.
Using Multiple Parameters For a buck regulator, it’s well known that the duty cycle must increase as losses grow in the system. This concept can be used to estimate the series resistive losses in the power stage. In a simplified buck power stage, we can see that the series resistive losses are lumped together as RS (Fig. 4). At dc, we can write the expression for the output voltage as:
Solving for the average duty cycle D and replacing RLOAD with VOUT/iL yields:
Then we can solve for RS:
This says that, if we monitor the duty cycle, VIN, and inductor current (all things that the controller already monitors), we can estimate the series resistance in the power stage. A change in this parameter would indicate that the health of the power stage has been compromised.
Any real-world power supply has some associated switching losses. In part, they will affect the value of RS measured by this method. However, when making a health assessment of the power supply in situ, the principal item of interest isn’t the absolute value of RS, but the relative change in RS. As such, this method of RS measurement also provides a figure of merit on the switching losses in the regulator.
Statistical Process Control A digital controller’s embedded processing power can be utilized to interpret measured and calculated data through statistics. Manufacturers use statistical process control (SPC) techniques to maintain control of their manufacturing process. An electronic system can use the same technique to measure critical parameters relating to the power supply.
The general approach is to first estimate the expected mean and standard deviation for a measurement. This is usually done during product development. Then periodic measurements are made, and the measured value is compared against limits based on a confidence interval.
To determine the deviation that represents a problem, define some interval [µ – k, µ + k] such that, if the averaged measurement values fall outside of this interval, we can state with some percent confidence that the mean has changed. Here, k is calculated as:
where s is the expected population standard deviation, n is the sample size, and za/2 is the double-sided probability that the sample mean is within the confidence interval. Some typical values for za/2 are 1.96 for 95%, 2.58 for 99%, and 6.0 for 2 parts per billion.3
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