AN827 Datasheet, PDF (8/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

AN827 APPLICATION NOTE
Core type ETD4916A
Effective core volume = 24.0 cm³.
Effective magnetic path length = 114 mm
Effective core area = 211 mm²
Ferrite material is 3C85 or equivalent
Np = 59T Ns = 5T
The ETD geometry has the following advantages:
1) Round center post for ease of winding
2) Commercially available from Philips, Siemens, Thomson, Magnetics, etc..
3) Increased winding area
4) The center leg area is equal to the sum of the areas of the two external legs. The legs are working with the
same flux density
The wire size is determined by the maximum copper losses allowed and available winding area. For this design
the wire size selected was 30AWG, 30 strand Litz.
An auxiliary winding is used to supply power to the controller. The number of turns was determined experimen-
tally to be 5. The worst case conditions for the auxiliary winding power supply voltage are at low-line input volt-
age (88V) and full load (500Watts) and at high-line input voltage (264V) and light-load.
The auxiliary winding must supply sufficient voltage to prevent turn-off (UVLO) during normal operation and also
must not supply excessive voltage causing burn-out of the controller.
CoilCraft Part Number R4849-A meets the above specifications and is available.
IC BIASING AND CONTROL COMPONENTS SELECTION
The IC biasing and control component values are derived and selected in the next section. Please refer to
Figure2, 500 Watt Demoboard Schematic.
Pin 1 P-GND (Power stage ground)
This pin should be connected to the source of the power mosfet, Q1, with a short length and wide copper trace
on the printed circuit board to minimize the copper trace resistance and inductance. Refer to Figure 3, 500 Watt
Demoboard printed circuit board layout.
Pin 2 IPK (Overcurrent protection input)
In order to obtain a very precise overcurrent protection trip level, R12 and R13 are calculated as follows:
I_aux = _R-^V----^r₁--^e--₃-^f = ₅--⁵--_.-^.1--¹--_k-- = 1mA
R12 = ^R-----^s---^e---ⁿ-_I--^s_a--^e--_u---^â‹-_x-^I--^p---^e---^a---^k- = ⁰----^.-⁰-₀---^3.--₀-³--₀---^â‹-₁--¹---⁷-- = 561Ω
Use R12 = 562 ohms, R13 = 5.1k
The peak current threshold is set at 17A and R_sense is chosen as 0.033 ohms.
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