标签: cycle

buck dc-dc 或 step-down dc-dc 经常使用。在给电池充电时，有的时候希望DC-DC能像LDO一样输入输出低压差。但实际上能做到的几乎没有。 原因在于开关管的占空比不能做到100%。 假设在开关管上没有电压降，DC-DC的电压降直接取决于占空比。 分析一下，最常用的buck dc-dc 使用N沟道场效应管作为开关管。因为N管成本低，性能好。但是导通时，需要有正Vgs。N管 D极进S极 Vd, 比供电电压还要高。这个高电压一般使用升压电容 boost 来解决。 boost电 容一般是一端通过一个二极管接到供电端，另一端接到电感续流端。可以想像在处于续流状态时，续流端被接地，boost电容被充电。而 当开关管导通时，boost电容接到续流端的电压接近电源电压，而此时电容是被充电到电源电压的，所以电容另一端会被抬高到两倍电源电压 。这个抬高了的电压给栅极驱动使用。这也就说明，不能占空比100%，否则没有续流环节boost电容无法充电。 如果使用P管，则不需要这个boost电路，应该能达到100%占空比。

A long time ago, the IBM manufactured hard disks with their own brand name. The two well-known series were "Deskstar" and "Travelstar", which are still being used by HGST (of Western Digital). Deskstar was 3.5-inch HDDs for desktops while Travelstar was 2.5-inches for notebooks.   Fig. An IBM's deskstar hard drive (Image: http://www.xbitlabs.com) I found that the arm (slider) and read/write head of my IBM hard drive would 'park' outside the disk area when it is powered on, where that of other brands seemed to be not moving out. After power on, the slider would move into the disk area.   Fig. The read/write head is outside the platter when disk is powered off (Image: http://www.getprice.com.au)   Fig. Slider (with read/write head) could park outside the platter area (Image: http://www.hgst.com) As time went by, more and more hard disk manufacturers use this mechanism in their drives. The read/write head would park outside when idle during the power is on. Then the Load/Unload cycle count, which means the times that the slider move into and out of the platters, would increase significantly in a short period of time. Fig. A large load/unload cycle count of a 'quite-new' hard drive (Image: http://www.overclock.net) Since a hard drive could fail when the read/write head is crashed with the platter and damage it, many people are concerned about this issue. I even tried to increase the idle time to park the head of my WD's hard drive using wdidle3. The manufacturers claim that there are many benefits of using this mechanism, including reduced power consumption.   Fig. Said benefits of parking the head (Image: http://www.hgst.com)   As the manufactures are putting a huge amount on researches, as consumers we just can believe that this technology is meaningful.... However, do keep it mind that we should check the SMART status regularly, avoid any shock to the disk drives and keep a copy if necessary... since the data inside is usually more valuable than the drive Reference: A document from HGST about the Load/Unload technology: http://www.hgst.com/tech/techlib.nsf/techdocs/9076679E3EE4003E86256FAB005825FB/$file/LoadUnload_white_paper_FINAL.pdf