ALTAIR04-900 Datasheet, PDF (11/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

ALTAIR04-900
5
Application information
Application information
The device is an all-primary sensing switching regulator, based on quasi-resonant flyback
topology.
Depending on converter’s load condition, the device is able to work in different modes (see
Figure 9):
1. QR mode at heavy load. Quasi-resonant operation lies in synchronizing MOSFET's
turn-on to the transformer’s demagnetization by detecting the resulting negative-going
edge of the voltage across any winding of the transformer. Then the system works
close to the boundary between discontinuous (DCM) and continuous conduction
(CCM) of the transformer. As a result, the switching frequency is different for different
line/load conditions (see the hyperbolic-like portion of the curves in Figure 9). Minimum
turn-on losses, low EMI emission and safe behavior in short circuit are the main
benefits of this kind of operation.
2. Valley-skipping mode at medium/ light load. Depending on voltage on COMP pin, the
device defines the maximum operating frequency of the converter. As the load is
reduced MOSFET’s turn-on does not any more occur on the first valley but on the
second one, the third one and so on. In this way the switching frequency is no longer
increased (piecewise linear portion in Figure 9).
3. Burst-mode with no or very light load. When the load is extremely light or disconnected,
the converter enters a controlled on/off operation with constant peak current.
Decreasing the load results in frequency reduction, which can go down even to few
hundred hertz, thus minimizing all frequency-related losses and making it easier to
comply with energy saving regulations or recommendations. Being the peak current
very low, no issue of audible noise arises.
Figure 9. Multi-mode operation of ALTAIR04-900
f _{os c}
Input voltage
f_sw
0
Val ley- skip p ing
mode
Bur st- mo d e
Quasi-res onant mode
P_{i n}
Pinmax
Doc ID 18211 Rev 2
11/29