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

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With digital supplies, the number of points that can be monitored and analyzed far surpasses the capabilities of analog including viewing values and parameters not available in analog supplies.

Typically, power-supply designers can monitor the output voltage, output current and error amp voltage. Sometimes, sense current also can be gauged. To know the actual transfer characteristics of the control loop, often referred to as system identification, the loop must be measured with a network analyzer or similar capable piece of external equipment. However, a network analyzer adds significant equipment and labor costs to the typical design cycle.

With the implementation of a digital controller, the designer has complete visibility into the entire control loop to measure frequency response in real time without the use of a network analyzer. The frequency response is communicated to a host system, such as a typical PC.

Digital Control

Silicon Laboratories' Si8250 digital power controller has a unique architecture that enables a digital control loop and a host of other capabilities. Fig. 1 shows the architecture of the Si8250, which includes a high-speed control processor and a system management processor.

The high-speed control processor runs at four discrete frequencies between 1.25 MHz and 10 MHz and is usually operated at 10x to 25x the switching frequency of the supply. For example, a 50-kHz PFC, 200-kHz full-bridge, 400-kHz POL, and 750-kHz to 1-MHz POL would use the 1.25-MHz, 2.5-MHz, 5-MHz and 10-MHz settings, respectively. This control processor includes a specialized DSP filter with which all of the loop compensation is accomplished.

The system management processor runs independently of the control processor and performs a variety of tasks. It is based on a Silicon Laboratories 8051 MCU core with 32 kbytes of Flash memory and a host of typical microcontroller-like peripherals. Additionally, it includes a 200-kHz auto-sequencing analog-to-digital converter (ADC).

One set of tasks it performs includes external tasks such as monitoring a fan tachometer. Based on the internal supply temperature, it can modulate the fan speed using one of the PCA channels to regulate the temperature to a programmed level. With a UART and an I2C interface, it can manage all communications with the host system or outside world through PMBus, Z-1 or any other future power-supply communications protocol.

Its second major task is to monitor and control the control processor itself. For example, during a load transient, there is a transient detector that generates an interrupt to the system management processor, which can then modify the coefficients in the DSP filter. This helps resolve the transient much faster than normal. With this nonlinear control capability, the control loop can be modified in real time so that when a transient occurs, the control loop can be changed to have a very high bandwidth for resolving the transient. Once it is over, the loop can be returned to a low bandwidth for improved noise rejection.

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