HVLED007TR datasheet(9/33 Pages) STMICROELECTRONICS

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DS12866 Rev 1

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HVLED007

Application information

4.1

Introduction

The HVLED007 is intended to drive Hi-PF QR flyback converters, an extremely popular

topology in SSL applications because it is very cost-effective and addressable with the

same transition-mode (TM) PFC controllers used for boost PFC stages. However, the TM

control technique that in boost PFC stages theoretically provides a sinusoidal input current

and unity power factor, in Hi-PF QR flyback converters features inherent distortion of the

input current, as depicted in the diagrams of Figure 3.

Figure 3. Input current distortion in Hi-PF QR flyback converters with traditional TM

control: current shape and resulting total harmonic distortion and power factor vs.

Kv (= Vinpk/VR) ratio

Traditionally, to keep current distortion within acceptable limits, the converter is designed to

operate with low Kv (= Vinpk /VR) values, achieved using a large reflected voltage VR = (Npri

/ Nsec) Vout. This, however, requires the use of a power switch (MOSFET) with higher

breakdown voltage.

Additionally PFC controllers intended for boost topology, this being a non-isolated topology,

do not normally have on-board protection functions suitable for a flyback converter and that

therefore need to be implemented with additional external circuits. On the other hand, they

have provisions on board that are little useful in an isolated topology like a flyback (e.g. the

error amplifier).

The HVLED007 addresses these aspects specifically, providing a novel TM control

technique able to provide a sinusoidal input current and unity power factor in flyback

converters as well and protection functions typical of offline flyback controller ICs.

4.2

Input current shaping function - operating principle

In a Hi-PF QR flyback converter powered from an AC line with a sinusoidal voltage Vin (θ) =

Vinpk sin θ, (with θ = 2fline t) the input current is the average of the primary current, which

flows only during the ON-time of the power switch and is a series of triangles separated by

voids corresponding to the OFF-time of the power switch (see current waveforms in

Figure 4). This can be expressed quantitatively as follows:

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