Why is saving energy so important? Saving power reduces carbon dioxide emissions, produces a better environment, promotes a greener lifestyle, and reduces the cost of applications where the electric bill is significant, especially if air conditioning is required to remove dissipated heat.

This article introduces power saving methodologies in switch-mode power supplies using digital power technology.

Switching frequency control along the input line voltage
The main sources of power loss in a switch-mode power supply include: switching loss, magnetic core loss, copper loss, gate drive loss, and ripple current flowing through capacitor ESR. The switching frequency directly affects these losses. This section tries to optimize the switching frequency to achieve lower power loss while maintaining the overall performance.

Taking full-bridge topology as an example, the output inductor’s peak-to-peak current ripple is:

Figure 1 shows an example of output inductor current ripple vs. input voltage. We see that the output inductor current changes nonlinearly with input voltage. To meet the output ripple specifications, the switching frequency should be high enough so that ?I at maximum input voltage is within the limit, but efficiency will not be optimal in most input voltage situations.

If we allow the switching frequency to change with an algorithm, the switching frequency can be reduced at low line voltage. In this case, the power supply can achieve both high efficiency and acceptable output current ripple. This algorithm can be easily implemented by a digital power controller.

Example: Vin=36V~72 V, Vout=12 V, n=5:2, Lo=10 uH, fsw=100 kHz, ideal model

Suitable dead-time setting is important for improving efficiency. A long dead time increases power loss due to hard switching and high conduction loss in the body diode. A short dead time also increases power loss due to cross conduction. Optimized dead time is necessary to achieve high efficiency. However, the optimized dead time values are different under different operational situations, such as full load or light load, and high line voltage or low line voltage.

To solve this issue, adaptive dead-time control is introduced. One simple solution is to have several dead time settings based on different output current thresholds. With the programming of these settings, the dead time can be optimized under different load conditions. Figure 2 provides an example to set the dead time vs. the load current.

To achieve power savings along the load range, the switching power supply can be set in different operational modes, including normal, light-load, and deep-light-load modes. Under different operational modes, the synchronous rectifiers have different operational schemes.

Normal mode is enabled when the power supply operates at medium and heavy loads. The synchronous rectifiers are complementary with the full-bridge PWM (pulse-width modulation) channels. Light-load mode is enabled when the load typically drops to 20 percent to 30 percent of the full load. In this mode, synchronous rectifiers are still enabled, but they are in phase with the full-bridge PWM channels. Deep-light-load mode can be enabled when the load is very light. In this mode, the synchronous rectifiers are disabled.

Using the load current information, the digital power controller can be programmed to have different light-load and deep-light-load thresholds. Figure 3 shows the operation of normal, light-load, and deep-light-load modes.

Phase-shedding control
The interleaving technology improves circuit efficiency, decreases output-current ripple, increases the effective ripple frequency, and reduces the output filter capacitor requirement. The interleaving approach can also significantly reduce the input filter inductor and capacitor requirements. Parallel operation of two phases reduces the conduction loss at full load, but increases the switching loss at light loads. With one phase turned off, the conduction loss is increased, but the switching loss is reduced, leading to higher efficiency at light load. By monitoring the output current, real-time optimization of phase number can be realized. The user can change the load-current threshold for phase shedding.

In a dual-phase system, the controller should be able to operate using interleaving phases, balance currents, and add or shed phases. With digital-control technology, these functions can be easily implemented in the controller. Figure 4 shows the experimental test results on the efficiency improvement at light loads using phase-shedding control.

Cold redundancy
To improve system energy efficiency and achieve power savings in idle mode and other low power conditions, cold redundancy is introduced [2]. In cold-redundancy mode, the control circuitry activates only the power supply modules needed to achieve power savings. Other power modules are turned off in standby state. The redundant power supply can be activated once the load becomes heavier or a fault condition occurs in the active power supply.

To achieve cold redundancy, the switching power controller should be able to monitor the system and control the power supply under different situations. For example, the digital controller is able to detect the load and fault conditions and then activate the standby power supply with different soft-start timing. Compared to analog solutions, digital power technology provides flexible ways for intelligent control of cold redundancy.

Conclusion
Digital power technology is providing significant technical and economic benefits in power savings. Due to its high flexibility, advanced real-time control algorithm, and operation intelligence as compared to analog control schemes, digital power technology can easily save power under both heavy and light load conditions. Based on the fast developing digital control techniques discussed in this article, digital power solutions will have an increasingly important role in power savings in the future.

References:
[1]. Viktor Vogman, “Cold redundancy – a new power supply technology for reducing system energy,” Intel Development Forum, IDF2009
[2]. Analog Devices, Inc., “ADP1043A datasheet rev 0,” October 2009
[3]. Analog Devices, Inc., “ADP1053 prelim datasheet,” August 2010

Jerry Zhai is director of Power Management Products for the greater China region at Analog Devices, Inc. He is responsible for power IC development, regional marketing and business development. He also leads the power management application team to support field sales and customers in Asia. He can be reached via email at Jerry.Zhai@analog.com.

James Xie
is an application manager, Power Management Products, Analog Devices, Inc. He is responsible for digital power applications and customer support. He can be reached via email at James.Xie@analog.com.

Jason Duan
is an application engineer, Power Management Products, Analog Devices, Inc. He is responsible for digital power techniques. He has a Master’s degree from Xi’an Jiaotong University. He can be reached via email at Jason.Duan@analog.com