Buck-Boost LED Driver (34063)

Hi all! Earlier this year I've made improved version of 34063 LED driver. This is combined buck-boost converter witout output voltage inversion. Now I have some time to describe it =)

  Here is main circuit of a driver. Input voltage is up to 32V, max output voltage is limited to 46V (depends on D5 voltage + threshold of 34063 feedback pin). Current is set by R4 value.

L7812 is needed for MOSFET gate driver. If input voltage will not exceed 16-18V there is no need in it. Subcircuits will be described below.

Please note:

• Maximum output power depends on primary power supply maximum load capability, current sense resistor R1, MOSFET's, inductor coil and flyback rectifier D2 (see switch subcircuit).
• Output short-circuit was not tested!
• At last photo you can see 0.5W resistor at output (not shown at schematic). See notes above last photo.
• For high power output add ferrite rod inductor before C4 and take power for 34063 and 7812 before this inductor.
  

Click at any picture for full-size image.

Main schematic

Power section

  Power stage is a combination of Buck and Boost topology. The main reason for me to build this driver was this schematic testing =) I've seen it once as abstract solution for use with PWM chips.       Since 34063 is not true PWM, it was interesting for me to make this circuit in hardware. Description said that this topology works as buck for PWM fill <50% and as boost for >50%.
And yes, this works with 34063 =)

Switch subcircuit

Current Sensing

Current sensing is similar to my previous boost LED driver

Current sensing subcircuit

And some photos of device (assembled prototype v1):

  First there were no film cap and resistor at output. Output diode was VS-30BQ100 (SMD, 3A 100V). This diode was too hot for me, so later I've replaced it with VS-8TQ100 (TO-220AC, 8A 100V) with small heatsink. Maybe SMD diode temperature was in work range (datasheet says operation temperature up to 175*C), but I don't like such hot parts in my projects =).
  Additional film capacitor smooths output ripple and reduce noise.
  7.5K resistor discharges output capacitor while power is off (if you don't care about some voltage remains at output after power off, resistor is not necessary part).