I would like to use MC34063 in step up mode, but when I change the resistor value it stopped to work

I would like to use MC34063 in step up mode, to step up 19V to 24V and 1Amps current.

I made this circuit: (the values not correct its only an image from the internet) The correct values: input 19V, 470uF capacitor, output 24V 1000uF capacitor, inductor 470uH 5.8Amps, timer capacitor 200pF

I recalculated the capacitors and the inductor value with a calculator.

First I made it without transistor and the R1 was 0.27R. It worked fine but the current was only ~400mA.

I added the transistor and changed R1 to 0.1 ohm to be able to serve 1Amps.

When I modified the MC34063 stopped to work, the output voltage equal with the input voltage.

The transistor is fine, because I tried with another MC34063 and the R1 resistor value was 0.27R. When I changed to 0.1R it stopped to work again...

What is the problem? Why doesn't work with 1A output current?

• $V_L=L\ di / dt$. So you shouldnt use the same inductor for different max. output current Commented Jun 2, 2017 at 1:24

Here's your diagram: -

If you compare your diagram with the diagram proposed in the data sheet there are some differences: -

Firstly, some general thoughts: -

• You don't have a resistor from base to ground and this will significantly increase the turn off time for the external transistor and
• A TIP31 is a rather slow device anyway so it's going to add to the inefficiences.
• You don't specify the inductor and this could be crucial if you have chosen badly. If it is going into saturation then you begin to significantly lose energy that would otherwise be transferred to the output capacitor.

Regards the 1 ohm resistor - are you sure it wasn't 10 ohms - can you measure it and confirm. Also, what load are you using - your schematic shows an output of 12 volts and you are going for 24 volts - it's very, very important that you present the correct schematic if you want proper help.

• The image from the internet. I think that the left linked picture equal with my picture. Input voltage 19V, output voltage 24v. The max current 1A (out). When I tested nothing connected to the output, only a multimeter, i measured the output voltage only. The R1 resistor is 0.1R, not 10R. The inductor 470uH 5.8Amps.
– Lobi
Commented Jun 1, 2017 at 21:00
• And what is the self resonant frequency of the inductor compared to the switching frequency? Commented Jun 1, 2017 at 21:53
• I dont know it exactly, no information about this. I have this inductor: electronicplus.com/images/products/387161.jpg
– Lobi
Commented Jun 2, 2017 at 4:18
• What transistor do you recommend? It ca be good? files.ruselectronic.com/200000381-12a1e139b7/…
– Lobi
Commented Jun 2, 2017 at 4:34
• A picture of a toroidal inductor doesn't help much. A data sheet is required - let's decide on the inductor first as that is likely more important at the moment. Commented Jun 2, 2017 at 9:15