Digital Power Control Enables System Identification

Nov 1, 2006 12:00 PM
By Brett Etter, Marketing Manager, and Ross Fosler, Senior Applications Engineer, Silicon Laboratori

A simplified block diagram of the Si8250 (Fig. 2) will be used as the basis for demonstrating system identification without the use of a network analyzer. The voltage feedback is immediately converted to digital through an ADC connected to a compensation processing engine, such as a proportional, integral and differential (PID), before passing to the pulse width modulator (PWM). Each of these circuit components of the controller is usually either part of or interfaced to a microcontroller.

The microcontroller acts as a management element in the system. The low-bandwidth signals such as input voltage and current used for monitoring are connected to the microcontroller. There are numerous signals and, with all of these, there are multiple access points for peeking at and injecting data or signals.

System identification tests the system to monitor the real-time response of the control loop. However, not all digital signals are useful for system identification, which is primarily a function of power control and not power management. Therefore, the signals associated with digital control are more desirable than the low-bandwidth signals for digital power management. Ideal points are before and after the compensator as shown in Fig. 2.

One option is to inject data into the compensator and read back from the ADC, which offers visibility into the entire loop. Injecting the signal into the system after the compensator enables the digital controller to analyze the power train without including any problems or signal modification by the compensator.

In an analog system, injecting into the compensator is problematic because of the integral operation that occurs in the system. The integrator tends to drive the output to saturation due to noise and small offsets within the amplifiers. However, this is not normally the case in a digitally compensated system because it is completely deterministic.

The only requirement to keep the system from drifting and saturating is maintaining integral balance on the injected signal. The sum of the positive error must equal the sum of the negative error data injected; in a digital system, integral balance is completely controllable.

Another option is to inject into the PWM and sample at the output of the compensator, which enables insight into the entire loop. However, unlike the digital compensator, the analog power stage is not absolutely deterministic. Even if integral balance is maintained, small offsets due to noise could potentially yield slightly higher or lower ADC results; therefore, the integrator could drift away from the quiescent operating point and eventually saturate.

The best option for a power-supply designer is to inject data into the PWM and then sample data from the ADC output. This leaves the compensation out of the analysis; however, a digital compensator is deterministic, thus the compensator frequency response is known and can be calculated. Once calculated, the data is added to the captured system identification data to get the complete loop response.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.