Intelligent Power Shield

Introduction

During solid state drive (SSD) operation, data is temporarily stored in the DRAM cache memory. This is done to take advantage of the quick response time of the DRAM to boost the performance of the SSD. As the DRAM cache memory is volatile, an external power supply is required to hold and to flush data.

In a proper power shutdown process, commands are given by the host to the SSD controller that power may be immediately cut. The controller sends an acknowledgement back to the host and then flushes all data that is temporarily stored in the DRAM cache to the NAND flash. See Figure 1.

However, during an unexpected power loss, the in-transit data can neither be stored nor flushed to the flash without prior notification and power. In this case, data stored in the SSD is not updated, which can lead into a problem with data incompatibility, data corruption, or even total device failure.

Data at risk in a power failure

In the event of an unexpected power failure, data loss may take place in any of the following flows shown in Figure 2:

Transcend’s patented Intelligent Power Shield is designed to minimize the risk of data corruption and ensure data integrity.

What is Intelligent Power Shield?

Intelligent Power Shield (IPS) is a technology for DRAM SSDs operating at 5V to ensure the integrity of data in the event of a sudden power outage. By adding capacitors, IPS can prolong the time to complete the flushing process after a sudden power loss. When the power is turned on, these capacitors act as batteries and are charged with electricity from the computer. During an unexpected power loss, the charged capacitors can continue offering power to the SSD, allowing the data to complete the writing process.

How does IPS work?

The SSD has a voltage detector that is constantly monitoring the voltage level coming from the host. In cases of sudden power loss, the voltage detector will trigger IPS as soon as the voltage drops from 5V to 4V. At this stage, IPS ensures the data stored in NAND are undamaged. The built-in capacitors continue to offer enough power for data to flush from the DRAM cache into the NAND flash. When the voltage drops below 2.3V, the NAND flash enters a write protection mode and no more data can be written. See Figure 3

Power failure with and without IPS

By prolonging the time between sudden power loss and the write protection mode, the SSD can complete more writes from the DRAM cache to the NAND flash. See Figure 4.

As shown in Figure 4, with IPS, the SSD has more than 75 milliseconds to complete the data flushing process before NAND flash enters write protection mode at 2.3V. Without IPS, the SSD has 2 milliseconds.

Advantages of Transcend IPS

Transcend has exclusively obtained the IPS patent. For better performance, Transcend uses superior components such as an advanced voltage detector to trigger IPS. Additionally, Transcend utilizes polymer tantalum capacitors – heavy-duty and low profile capacitors that are able to operate stably in harsh environments. See the table below:

  IPS Without IPS
Hardware With built-in voltage detector (VDT) and additional polymer tantalum capacitors (PTCs), IPS creates more time for more data to be written from DRAM to NAND flash memory when the voltage drops from 4V to 2.3V. When the voltage drops to 4V, the capacitors start to discharge and the SSD controller stops accepting new write commands.
Firmware Once IPS is triggered, the firmware automatically flushes the data from DRAM cache to NAND flash memory. SSD controller stops accepting new write commands from the host.

To ensure high credibility, Transcend’s IPS technology utilizes Ulink DriveMaster power hub to test our power off protection mechanism, each cycle includes the following steps:

  1. Host writes commands to the DRAM.
  2. Ulink powers off the host. When the VDT detects that the voltage drops to 4V, the data flushes from the DRAM cache to NAND flash.
  3. Ulink powers on the host and compares the data in host with that in NAND flash. If there is inconsistency in the data, the report shows “miscompare”.

Transcend’s IPS has gone through 3,000 testing cycles. As a result, zero inconsistent messages were shown.

Summary

Transcend’s SSD products provide an integrated power cut protection mechanism to ensure data transfer integrity and to minimize the possibilities of device failure during unexpected power outage.

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