The standby voltage is generated by a power supply circuit within the main power converter. This became widely used in 1995 when the ATX specification was published to allow a desktop computer to be put into a sleep-mode to save energy. The standby voltage supplies a small amount of power to the motherboard enabling the computer to quickly restart, rather than performing a full, lengthy, boot cycle. The term “standby” is often confused with an auxiliary output, which has a different function.
A standby voltage is generated by a
separate switching circuit and is not affected by the use of the remote on/off
signal or even an overload condition on the main output of the power
supply. A typical block diagram is shown
below.
Figure
1 Block diagram of a typical power supply with a standby output
The main and the standby switching
converters share the high voltage output voltage (typically around 380Vdc) from
the rectifier & PFC circuit. This
saves cost by not duplicating the rectification and filtering components. It can be seen that they are independent of
each other and the remote on/off control is only applied to the main converter.
The auxiliary output is supplied from an
additional winding on the main converter transformer. If the main output is turned off by the
remote on/off, the auxiliary output will also turn off. An auxiliary output is often used to power an
external cooling fan if the power supply has a forced air cooling rating. In this case if the auxiliary output is not
present when the power supply output is inhibited, it does not matter as the
main converter will not be providing any load and will not require additional
cooling.
Figure 2 demonstrates how the various
outputs and function interact with each other.
If AC power is removed for any significant length of time (10-50ms),
then of course all the outputs on the power supply stop functioning.
Figure
2 – Timing diagram
Many power supply designers also use the
standby converter to power any “housekeeping” circuitry on the output of the
main converter. This allows an “enable”
type remote on/off to be offered, where the signal is pulled low to activate
the main converter. Without a standby
circuit, an external voltage has to be applied to the remote on/off to inhibit
the power supply.
Manufacturers of mid to high power
converters with a standby voltage will often state the off-load power draw, or
off-load power consumption, with the remote on/off activated from the standby
voltage.
Power Guy
