What is power loss protection?
Power loss protection (PLP) is a commonly referred to attribute of any system that functions to mitigate detrimental effects of sudden power loss. Commonly seen in volatile memory applications, such as RAM based memory cards, PLP typically contains some form of additional energy storage accompanied by firmware routines for transferring the data from volatile memory to non-volatile memory. Similar to a back-up power system for a house or building, the backup energy supply is designed to provide enough energy to safely transfer and shutdown any non-critical processes. PLP often functions throughout many layers of a system architecture from firmware to periodically write data to non-volatile memory, to distributed memory systems for risk mitigation, to redundant processing and energy supplies.
Why do you need power loss protection?
Power loss protection ultimately stems from the question: What happens if power is suddenly cut off? This question should be posed for all systems, large and small, whether electrical, pneumatic, or hydraulic. The consequences of sudden power loss can be severe and perhaps irrecoverable. Perhaps the most visible examples of power loss damage come from large and often hazardous industries such as chemical refineries, hospitals, mines, and public safety facilities. In the digital age, businesses and consumers may fall victim to power loss by other means; loss of information. It's estimated that 1.7 trillion dollars are spent every year from data center down time and data loss and that over half of this can be attributed to hardware failure . With more of our lives being transferred to memory cards, any loss of information can be significant and a major liability for businesses.
For memory cards, power loss can stem from a variety of sources and the best way to handle them depends largely on the available architecture. The PLP can only support power failures upstream from their plug in point. That is, a building backup generator support grid failures but cannot support a server rack supply from failing. Likewise, a rack lead acid battery supports building power failures but cannot support a board regulator failure. Therefore, the last line of defense PLP is typically on-board, capacitive energy storage.
What does on-board PLP typically look like?
Here we'll assume a generic memory card board with a primary high voltage bus (12V) and one or more low voltage buses (5V, 3.3V, 1.8V). Managing each voltage bus conversion are DC/DC converters. The load, represented in Figure 1 as a simple resistor, are all board components at the voltage level such as processors, memory arrays, sensors, etc.
Figure 1 Generic schematic of multi-voltage memory board
Typically power loss protection devices will either occur directly on the load power rail or in between the high voltage bus and low voltage bus converters to better support a range of applications. Of course, remember that the backup energy storage only supports up-stream failures so the closer the energy storage is to the sensitive load, the better.
Figure 1 illustrates a capacitive solution whereby the energy is stored on a series connection of capacitors. The number of capacitors in series or parallel depends on the desired voltage, power, and energy needed to support backup operations. In memory card applications, this may be as short as 50ms at ~2W to over 15W.
This example, if the 12V DC/DC converter were to suddenly fail or lose power, the energy stored in the capacitors would hold up the intermediate voltage rail. As power is drawn from the capacitor bank, the voltage level decreases. The low voltage converter provides a regulated voltage bus to all sensitive electronics until the capacitor voltage drops below its lower voltage limit. At that point, hopefully at information has been transferred and organized in non-volatile memory to prevent data loss and corruption. Once the power is turned back on, the 12V regulator re-charges the capacitor bank and operations proceed as normal.
What kind of energy storage is best?
FastCAP Systems has developed a capacitor specifically for PLP applications to outperform traditional storage such as tantalum capacitors, electrolytic capacitors, and batteries. For more information on this capacitor, please see our knowledge base section: FastCAP Reflowable Chip Ultracapacitor
 K. Vatto, Power Loss Protection in SSDs: How SSDs are Protecting Data Integrity, Samsung, 2016