No V ir I sensing in lamp driver

[butterworth]

Dear All,

In the following App Note...

http://www.st.com/stonline/products/literature/an/5656.htm

....a schematic is given for an electronic lamp ballast using switch mode PFC and PWM stages.

I am wondering why the PWM stage (which supplies the lamp) has no output voltage feedback sensing and no output current sensing? This seems to go against the norm for SMPS's and i was wondering if readers knew how they get away with it.

 

[ScottyUK]

From what I remember about fluorescent tubes they have a self-regulating tendency when in series with a impedance: as the tube current increases the voltage across the tube decreases giving a negative resistance change.

The ignition method using a frequency shift to excite the L-C resonance and thus develop striking voltage is something I used over ten years ago in a little demonstrator for contactless energy transfer. Works beautifully on the compact lamps.


----------------------------------

If we learn from our mistakes I'm getting a great education!