Using TIP122 to build a buck converter

C

Chuoiujk

Member
Hi guys,


I'm building a Buck Converter to step down a 12V input to a 5V output using the TIP122 darlington pair as the switch.


I'm having a problem with the output stage. The tip switches smoothly with just VCC and signal to it. Once the output stage is connected. i.e the diode, the inductor and capacitor, the output reads 12V >> seems like it isn't switching anymore.


Here is a circuit diagram:



Please note the inductor value used is actually 220uH, and capacitor 22uF. The diode is a Fast Recovery diode. Currently I'm testing at open loop, no load conditions. i.e no feedback to the micro, no voltage divider and no load.

Please Help. I've tried using a variety of different Inductor and Cap sizes and this didn't change anything.
 
iHearAll

iHearAll

Well-Known Member
SMPS-basic-2.gif

Is it hooked up like such? Remember the BE voltage is in reference from BE and not ground to B. So if your device is sharing a ground and the pulse appears to be floating in reference to the emitter you will quickly break the tip122 or it will stay high and clip off the pulse peaks that exceed the input value (like guitar distortion or really basic analog to digital conversion). What are you driving your pulses with?
 
mauricem00

mauricem00

Well-Known Member
Chuoiujk said:
Hi guys,


I'm building a Buck Converter to step down a 12V input to a 5V output using the TIP122 darlington pair as the switch.


I'm having a problem with the output stage. The tip switches smoothly with just VCC and signal to it. Once the output stage is connected. i.e the diode, the inductor and capacitor, the output reads 12V >> seems like it isn't switching anymore.


Here is a circuit diagram:



Please note the inductor value used is actually 220uH, and capacitor 22uF. The diode is a Fast Recovery diode. Currently I'm testing at open loop, no load conditions. i.e no feedback to the micro, no voltage divider and no load.

Please Help. I've tried using a variety of different Inductor and Cap sizes and this didn't change anything.
Click to expand...
storage time of your pass transistor may be to long.what are you using as a turn-off circuit for the output structure?is the inductor storing enough energy to complete the cycle without reverse biasing the base emitter junction?
 
iHearAll

iHearAll

Well-Known Member
BuckRegulator.gif

Here's a self oscillating buck converter. Notice the 5.6v zener is clipping the base voltage a that after the .6v silicon clip from the transistor, the 47uF only ever charges to 5v. And the transistor that this transistor is switching is PNP, so they are on in opposite times. The PNP is kept from getting too negative a voltage by the use of the 470ohm BE resistor to positive.
 
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