comment on this article

Power supply standby voltage and off-load power draw

The 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 ‘Standby Voltage’ is always present even if the main outputs are inhibited using the remote on/off.

Figure 1 shows a block diagram for a power supply like TDK-Lambda’s 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. Figure 1: CUS350M power supply block diagram

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 turn 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.

Figure 2: Timing diagram

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.

Off-load power draw

With low power products (typically 150W or less), the off-load power draw is measured by removing the load and measuring the input power.

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 ‘Standby input power draw’ or ‘remote off power draw’. Users who want a low off-load power draw must turn off the main converter using the remote on/off function.

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!


Comment on this article

This material is protected by MA Business copyright See Terms and Conditions. One-off usage is permitted but bulk copying is not. For multiple copies contact the sales team.

What you think about this article:

Add your comments


Your comments/feedback may be edited prior to publishing. Not all entries will be published.
Please view our Terms and Conditions before leaving a comment.

Related Articles

Eel inspired battery

The electric eel has inspired researchers at the University of Fribourg, the ...

Peeling the onion

The Internet of Things is developing from being a concept just a few years ago ...

The Sonic Internet

Whether in the smart home or in in-car entertainment the audio market is ...

Get to market faster

A quick look at using Vicor's PFM and AIM in VIA packaging for your AC to Point ...

Shaping the future

Alexander Everke, the CEO of ams, started his career in the semiconductor ...