MC34063 boost drooping

throbscottle said:

Yes, I finally discovered that the losses you mention make a big difference at low input voltages. I just hadn't realised how much. Given the PSU I'm using is also a little low, it's no wonder I'm having problems.

Well, I intended a learning exercise here, it's certainly educational!

Now I am re-designing it for more current, I can include the range-extender at the same time.

Click to expand...

Hi,

Yes the inductor is tricky sometimes. It's hard to nail down the effectiveness of a random inductor until it is tried in the circuit. There are a lot of things to consider here.

It is a curious thing to note, designing for higher output currents produces a lower minimum inductance value. In practice, with my light load, I tried various values from 56uH to 680uH and the result was pretty much the same.

So, if the core becomes saturated, what happens? And given an inductor I pulled from something, so the core material is unknown, how can I test what the saturation current is?

I have a couple of old linearity coils from monitors, which are magnetic. I will give one of them a try, since someone on here once told me they can be used to give an extra boost for this type of converter.

Hi,

The test for saturation is easiest to do right in the application itself, where everything is already set up. To make a test jig is almost the same as making a power supply and then using the inductor as the main inductor and looking for the same things.

When we apply a pulse of voltage to an inductor in say a buck circuit, a pulse of voltage appears across the inductor before much of a change in current through the inductor occurs or the output voltage begins to rise much. The level of the voltage and the time it is applied to the inductor can be measured in volt seconds. If we apply too high of a voltage or leave the voltage there too long (volt seconds too great) the inductor saturates. If during normal operation the control circuit does the same thing, then the inductor is not a good choice. This makes it a little easier to test.

During the time the pulse is applied if the inductor is not saturated it will respond more or less in a linear fashion, so that the current rises as a straight line ramp, starting from some smaller value and rising to some higher value, and then falling back down again after the pulse goes away.
When the inductor saturates however, the rising ramp suddenly rises up sharply, and appears like a big spike near the end of the ramp or even before that if the inductor is a really bad choice.
For example if the current was rising as a ramp of say 0.2 amps per microsecond for some time and then started to rise at 0.4 amps per microsecond and then 0.8 amps per microsecond, that probably means that it started to saturate.

When the inductor saturates it means that it can not take the current required to provide linear inductor action for the circuit it is installed in, so it must be changed or the current must be lowered.

The waveform of the inductor current is easy to understand if it is viewed on a scope. If it ramps up in a straight line or curves up just a little, it's ok, but if it ramps as a straight line and then curves up sharply that means it is going into saturation.

The point where it saturates is usually with a higher load and certain input voltage level, but you can test it at full load and over the entire range of input voltages.

Helpful as always Mr Al - Thanks

throbscottle said:

Helpful as always Mr Al - Thanks

Click to expand...

Hi again,

You're welcome, and i might add that when this is tested usually either the load is increased gradually or the input voltage is increased gradually, so that we might see the point where it goes into saturation without blowing any transistors. If we just snap on the power at full load and the inductor saturates, it presents a much much lower impedance to the transistor(s) so the transistors could see too much current and burn out very very quickly, before we get a chance to shut down. Some circuits have built in protection while others do not.

I wish you good luck with it