AN4129 datasheet(32/39 Pages) STMICROELECTRONICS

32 / 39 page

Circuit description and design guidance

AN4129

32/39

Doc ID 023314 Rev 1

Figure 41.

Voltage and current stress

●

Yellow = FET drain voltage

●

Blue = FET current, 0.5 A per division.

Note the headroom available. The 800 V rating of the HVLED815PF is unnecessary for 120

V applications.

4.17

Extensions and modifications

4.17.1

Lower output voltage, higher current

The unit was designed for 18 LEDs, but it can easily be modified to drive 9.

The transformer has two secondary windings that are connected in series on the

demonstration board. If the foil is cut between the secondary side center pins, and the

outside pin pairs connected, the secondary voltage is halved. The current regulating

circuitry now sees a different turns ratio, and correctly regulates at twice the load current.

Some other changes must also be made if this is done:

●

Change snubber capacitor C10 to a high-quality part of four times the value, such as a

1200 pF COG ceramic rated for 200 V.

●

Change diode D3 to a 150 V Schottky type such as ST's STPS1150 to reduce voltage

drop. Efficiency is reduced it this change is not made.

●

Change the output capacitor to one having 4 times the capacitance and half the voltage

rating, 1200 µF at 35 V. This maintains the ripple current near the same percentage as

the original.

●

If a preload resistor is used, change it to one having ¼ the original resistance.

4.17.2

EMI filter alternatives

A second pattern was included on the PC board to allow experiments with a different filter

configuration. A common mode filter may be needed in some applications, but it requires the

damping network to be re-tuned. A small CM choke (such as Wurth, part number

750311897) does a better job of suppressing common mode noise. This inductor type has

another feature - the differential mode inductance is relatively high and stable, simplifying

the damper design.

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