{"id":3511,"date":"2016-05-18T06:59:41","date_gmt":"2016-05-18T06:59:41","guid":{"rendered":"http:\/\/blog.uk.tdk-lambda.com\/uk\/?p=3511"},"modified":"2016-05-12T12:07:24","modified_gmt":"2016-05-12T12:07:24","slug":"how-to-differentiate-between-power-supply-standby-voltage-and-off-load-power-draw","status":"publish","type":"post","link":"https:\/\/blog.uk.tdk-lambda.com\/uk\/2016\/05\/18\/how-to-differentiate-between-power-supply-standby-voltage-and-off-load-power-draw\/","title":{"rendered":"How to differentiate between power supply standby voltage and off-load power draw"},"content":{"rendered":"

Power supply standby voltage and off-load power draw are two terms that seem similar but are really quite different.<\/p>\n

Standby Voltage<\/strong>
\nThe use of a Standby Voltage on computer power supplies became widespread when the ATX specification was published in 1995. It enables a computer to be put into a low power consumption mode without fully powering down the processor, allowing a quick response when activated again. This was achieved by using a remote on\/off signal to turn off the outputs, but still supplying a small amount of power to the processor from an independent voltage. This \u201cStandby Voltage\u201d is always present even if the main outputs are inhibited using the remote on\/off.<\/p>\n

Figure 1 shows a block diagram for a power supply like TDK-Lambda\u2019s CUS350M, which features a main output, an auxiliary output (typically used for driving fans) and a standby voltage. A remote on\/off is also provided.<\/p>\n

\"Fig<\/a>
\nFigure 1: CUS350M power supply block diagram<\/em><\/p>\n

The main and the auxiliary outputs use a common transformer and switching circuit. If the remote on\/off is activated, both the main output and the auxiliary will turned off. As the V Standby output has an independent transformer and switching circuit, it will continue to operate. Figure 2 shows the timing diagram under the different conditions.<\/p>\n

\"Fig<\/a>
\nFigure 2: Timing diagram<\/em><\/p>\n

It can be seen that when the remote on\/off (dark blue line) is pulled high, the 5V Standby (purple) continues to operate, but the main DC Output (green) drops until the remote on\/off is pulled low again. Of course if the AC Input voltage (red) is removed for an extended period, then the 5V Standby will eventually fall.<\/p>\n

Off-load power draw<\/strong>
\nWith low power products (typically 150W or less), the off-load power draw is measured by removing the load and measuring the input power.<\/p>\n

With products greater than 150W that have a remote on\/off feature, often the off-load power draw is measured when the remote on\/off is activated. This may be written in the specification as \u201cStandby input power draw\u201d or \u201cremote off power draw\u201d. Users who want a low off-load power draw must turn off the main converter using the remote on\/off function. <\/p>\n

It must be noted that if the power supply has a standby voltage, then that load must also be at zero when minimum off-load power draw is needed. If for example the standby is supplying 5V 0.25A (1.25W) and the remote on\/off is applied, the standby output will continue to delivery 1.25W, making it impossible to achieve <0.5W input power draw!<\/p>\n

For more information about the CUS350M power supply, please visit: www.emea.lambda.tdk.com\/uk\/cus350m<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

Power supply standby voltage and off-load power draw are two terms that seem similar but are really quite different. Standby Voltage The use of a Standby Voltage on computer power supplies became widespread when the ATX specification was published in 1995. 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