For many years, semiconductor manufacturers involved in the power management field have been striving to meet the evolving demands of end-system users. With the growing popularity of portable electronics, these devices are being equipped with enhanced functionalities. However, these advancements come at a cost—higher power demands. Designers now face the challenge of achieving maximum efficiency within the physical constraints of the devices. Meanwhile, while the battery industry is making strides in developing alternative battery technologies that surpass the capabilities of traditional nickel-cadmium (NiCd) batteries, these innovations still fall short of meeting the energy requirements of future generations of portable gadgets. Consequently, portable applications must explore innovative solutions in low-power circuit design to empower engineers to optimize the efficiency of end-use systems.
In portable devices, components play a crucial role in the overall power budget. It's evident that semiconductor device manufacturers must continue to innovate in order to help reduce the power consumption of portable products. For instance, taking mobile phones as an example, one of the ways to lower power consumption is by reducing the operating voltage of critical components like analog and digital baseband chips. When the DSP or microprocessor isn't required to operate at its maximum performance, the core supply voltage can be decreased, along with the clock frequency. Newer low-power applications increasingly employ this strategy to maximize system energy savings. The formula PC~(VC)².F explains the power consumption of a DSP core, where PC represents core power consumption, VC is the core voltage, and F is the core clock frequency. Reducing the clock frequency decreases power usage, and lowering the core supply voltage further cuts down on power consumption.
What can advanced silicon and packaging technologies contribute?
There are numerous design factors influencing the performance of emerging high-power portable devices. This article will focus on the most prevalent power switching power MOSFETs used in low-voltage applications, highlighting how recent breakthroughs in silicon technology have impacted the need for increased power handling. To fully appreciate these technological advancements, it’s essential to grasp some key parameters of power MOSFETs.
The on-resistance (rDS(on)) of the channel is influenced by the lateral and longitudinal electric fields of the channel. The channel resistance is primarily determined by the gate-to-source voltage difference. Once VGS exceeds the threshold voltage (VGS(th)), the FET starts conducting. Many operations necessitate a switch ground point. The resistance of the power MOSFET channel is linked to the physical size, described by the formula R = L / A, where Ï is the resistivity, L is the channel length, and A is W x T, representing the channel's cross-sectional area.
In a typical FET structure, L and W are defined by the device's geometry, while the channel thickness T is the distance between the two depletion layers. The position of the depletion layer shifts depending on the gate-source bias voltage or the drain-source voltage. When T is reduced to zero due to the influence of VGS and VDS, the two opposing depletion layers connect, and the channel resistance (rDS(on)) increases dramatically, nearing infinity.
Figure 1 shows a graph of rDS(on) versus VGS characteristics. Region 1 corresponds to cases where the accumulated charge is insufficient to cause reversal. Region 2 indicates conditions where there is adequate charge to partially reverse the P region and create a channel, though the "space charge" effect is also significant. Region 3 relates to situations where the charge is limited, and raising the gate potential does not significantly affect rDS(on).
Figure 1: Relationship between rDS(on) and VGS characteristics
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