Device description

HVLED807PF

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Doc ID 023464 Rev 5

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 11).

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 occur any more 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 11).

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 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.

Figure 11.

Multi-mode operation of HVLED807PF (constant voltage operation)

4.2

Power section and gate driver

The power section guarantees safe avalanche operation within the specified energy rating

The internal gate driver of the power MOSFET is designed to supply a controlled gate

current during both turn-on and turn-off in order to minimize common mode EMI. Under

UVLO conditions an internal pull-down circuit holds the gate low in order to ensure that the

power MOSFET cannot be turned on accidentally.

AM13561v1

0

f

sw

Pinmax

Input voltage

Burst-mode

Valley-skipping

mode

Quasi-resonant mode