Philips 7629, Service Manual

Page: 83 / 374

Operation
The supply can run in three different modes depending on the output power:
•
Quasi-Resonant mode (QR) The QR mode, described above, is used during
normal operation. This will give a high efficiency.
•
Frequency Reduction mode (FR) The FR mode (also called VCO mode) is
implemented to decrease the switching losses at low output loads. In this way the
efficiency at low output powers is increased, which enables power consumption
smaller than 3 W during stand-by. The voltage at the pin 3 (Ctrl) determines
where the frequency reduction starts. An external Ctrl voltage of 1.425 V
corresponds with an internal VCO level of 75 mV. This fixed VCO level is called
V
VCO, start
. The frequency will be reduced in relation to the VCO voltage between
75 mV and 50mV (at levels larger than 75 mV, Ctrl voltage < 1.425V, the
oscillator will run on maximum frequency f
oscH
= 175 kHz typically). At 50 mV (V
VCO, max
) the frequency is reduced to the minimum level of 6 kHz. Valley switching
is still active in this mode.
•
Minimum Frequency mode (MinF) At VCO levels below 50 mV, the minimum
frequency will remain on 6 kHz, which is called the MinF mode. Because of this
low frequency, it is possible to run at very low loads without having any output
regulation problems.
Figure: