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[SOLVED] Boost converter using MC34063 - problem when Iout is lower than designed

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pete16

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Hello,

I have used the MC34063 to build a circuit to output 18V 100ma from 12 V input and used an electronic load to test in constant current mode of 100ma.
It is fine when my load draws 100ma it works and the output is 18V as designed.
However, when I lower the current draw to below 100ma the output voltage starts to go up to 21V and if the load draws below 40ma the output Voltage oscillates between 18V and 22V.

Any ideas what is going on? How come it voltage oscillates when the output load draws less than 100ma?
I though this was supposed to keep voltage constant at 18V for anything UP TO 100ma.

Thanks

BoostCkt.jpg
 

Even without looking more closely at controller details, the first thing that comes to mind is to increase the response time of the output, which could be achieved by either increasing the capacitor value (which seems already high) or increasing the value of the inductor.
 

I already tried those options. I have doubled inductance to 200uH and also tried increasing capacitance. Same results.
I'm starting to think the batch of MC34063 chips I have are bad.
 

Hi,

Do you have a proper PCB layout?

If not: we don't have to discuss test results, because they won't be reliable.

Klaus
 

The circuit is currently on a Breadboard. I don't see an issue with making this work on a breadboard. I have done this on a breadboard successfully before.
I suspect the chips are bad. I got these chips on ebay 10 for 99 cents shipped from China. I think they might be bad.

I have ordered new chips from a seller that has original NOS chips.
I will try the new ones when they arrive and update tis thread.

Thanks
 

I suspect the chips are bad
I would not bet on this possibility, though not so implausible nowardays. The MC34063 it's a very simple device, essentially composed of a switching transistor driven by a comparator. Furthermore, unloaded SMPS outputs are typically unstable, particularly in cases like the above where there is no damping device in the feedback control. Maybe if you added a ceramic capacitor at the output close to the IC terminals, this could reduce the effect of the ESR resistance of the electrolytic capacitor. Also, another ceramic capacitor close to Vcc is mandatory.
 

Thanks You Andre!

You Nailed it! I put a ceramic cap to in 1 fixed it. The voltage now remains at 18 regardless of load current.
 

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