Application information HVLED805
12/29 Doc ID 18077 Rev 1
5 Application information
The HVLED805 is an off-line all-primary sensing switching regulator, specific for offline LED
drivers based on quasi-resonant ZVS (zero voltage switching at switch turn-on) flyback
topology.
Depending on converter’s load condition, the device is able to work in different modes
(Figure 9 for constant voltage operation):
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 will be 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. The resulting constant current mode fixes the
average current also in case of a short-circuit failure of one or more LEDs.
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 will 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 will no longer increase
(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 will enter a controlled on/off operation with constant peak current.
Decreasing the load will then result 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. Thanks to this feature, the application is able
to safely manage the open circuit caused by an LED failure.
Figure 9. Multi-mode operation of HVLED805 (Constant voltage operation)
0
f
sw
Pinmax
Input voltage
P
in
f
osc
Burst-mode
Valley-skipping
mode
Quasi-resonant mode
0
f
sw
Pinmax
Input voltage
P
in
f
osc
Burst-mode
Valley-skipping
mode
Quasi-resonant mode
0
f
sw
Pinmax
Input voltage
P
in
f
osc
Burst-mode
Valley-skipping
mode
Quasi-resonant mode
