Switching Regulator; High Efficiency Power Supply
With the growing importance of power consumption in commercial devices, DC-to-DC converters primarily strive for efficiency, with new technologies playing a role to achieve that goal. A DC power supply provides a voltage of fixed polarity (either positive or negative) to a load. Depending on its design, a DC power supply may be powered from a DC source, such as battery, or from an AC source, such as line power (e.g., household electricity). Today major players in the DC-to-DC converter semiconductor industry are Texas Instruments, Fairchild Semiconductor, Maxim Integrated, Power Integration, Linear Technology and NXP. With nearly $7 billion in gross analog revenue in 2012 and an estimated 25% attributed to the power management semiconductors, Texas Instruments is one of the top companies in power solutions. Fairchild and Power Integration are involved in on-going patent battle which relates to DC power supply patents, with a $105M award to Power Integration in dispute.
The switching regulator, in particular, has been used widely since it offers the advantages of higher power conversion efficiency and increased design flexibility (multiple output voltages of different polarities can be generated from a single input voltage). There are four most commonly used switching regulator types:
- Step-up (boost): output voltage is higher than input voltage
- Step-down (buck): output voltage is lower than input voltage
- Buck-boost (or inverting regulator): output voltage is opposite in polarity to input voltage
- Flyback: output voltage can be less than or greater than input voltage
Improving efficiency for a switching regulator at light load conditions is one of challenges in designing of a switching regulator. In a switching regulator, when there is a large load at the DC output, the switching regulator responds by increasing the duty cycle and thereby delivering more power to the load. If the load becomes lighter, then the switching regulator senses this change through the feedback system and reduces the duty cycle. If the load is further reduced and the power delivered to the DC output cannot be reduced, then the switching regulator tends to raise the DC output voltage, resulting in poor output regulation. This situation becomes worse if the load is completely removed. A simple way to solve this problem is to connect a constant load internally to the power supply. However, because the internal load is always connected, when there is no load at the DC output, the power supply efficiency is decreased.
Another technique that is used to improve efficiency is cycle skipping. Cycle skipping involves reducing the duty cycle as the load decreases, and when the duty cycle is reduced down to a predefined minimum duty cycle, it alternatively switches for some duration of time and stays idle for another duration of time, depending on the load. During this mode, if the load increases very slightly, the output transistor will switch at minimum duty cycle for a short time until the power demanded by the load is delivered, and then stop switching again. In theory, the cycle skipping mode decreases the switching losses at light loads since switching occurs as intermittent groups of pulses. Also, cycle skipping eliminates the need for the constant internal load. However, if the groups of pulses occur at audio frequencies (20 Hz to 20 kHz), the power supply may create undesirable audio noise.
One of solutions to solve this problem is a switching regulator that operates at a frequency for a first range of feedback signal values and at a variable frequency without skipping cycles for a second range of feedback signal values. Furthermore, the minimum operating frequency of the switching regulator is chosen to be above audible frequencies. Unlike known switching regulators, in which cycles are skipped under light load conditions, the solution provides a switching regulator that generates reduced frequencies to vary the timing of the power switch without performing skipped cycles during light load conditions. Advantageously, undesirable audio noise produced by known switching regulators is apparently avoided.
With a wide range of DC-to-DC converters for many applications, switching regulators are an energy-efficient solution, greatly reducing power consumption near the load.
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