Datasheet HVLED007 (STMicroelectronics) - 9

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HVLED007 Application information 4 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: DS12866 Rev 1 9/33 33 Document Outline Table 1. Device summary 1 Block diagram Figure 1. Block diagram Table 2. Absolute maximum ratings 2 Pin connections Figure 2. Pin connection (top view) Table 3. Thermal data Table 4. Pin functions (continued) 3 Electrical characteristics Table 5. Electrical characteristics (continued) 4 Application information 4.1 Introduction 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 4.2 Input current shaping function - operating principle Figure 4. Hi-PF QR flyback converter with the traditional TM control: current waveforms Figure 5. Input current shaper (ICS) block and its interconnection with HVLED007 control Figure 6. Key waveforms of the ICS circuit in figure 5 Figure 7. Shape of the current reference Vcsref(θ) (5) at different input voltages (i.e. Kv values) 4.3 Operation of a Hi-PF QR flyback converter based on the HVLED007 Table 6. Timing quantities in a HVLED007-based Hi-PF QR flyback converter Table 7. Control quantities in a HVLED007-based Hi-PF QR flyback converter Table 8. Electrical quantities in a HVLED007-based Hi-PF QR flyback converter 4.4 Shaping capacitor (Ct) selection (pin CT) 4.5 Control input for isolated feedback and optocoupler driving (pin COMP) Figure 8. Output characteristic of pin COMP and significant levels 4.6 Multiplier input for input voltage sensing (pin MULT) 4.7 Current sensing input (pin CS). Sense resistor (Rs) selection Figure 9. Effect of ripple on Ct on current sense signal: a) within linear dynamics, close to clamp level; b) signal slightly exceeding clamp level; c) signal exceeding clamp level, with OCP activation 4.8 Zero current detection and triggering block (pin ZCD); starter 4.9 Overload and short-circuit protection (OCP function) Figure 10. Functional schematic of the overload and short-circuit protection function 4.10 Overvoltage protection (OVP function) Figure 11. Functional schematic of the OVP function 4.11 Soft-restart function Figure 12. Functional schematic of the soft restart function 4.12 Suggested step-by-step design procedure of a Hi-PF QR flyback converter based on the HVLED0007 Table 9. Basic electrical specification and key parameters of a Hi-PF QR flyback Figure 13. Typical application schematic (reference for suggested design procedure) 5 Referenced documents 6 Package information Table 10. SO-8 mechanical data Figure 14. Package dimensions 7 Revision history Table 11. Document history