I'm using the MC34063 for my 35 VDC -> 4VDC SMPS. Here is the circuit (modelled in LTSpice):
Edit: However, using a MC34063 calculator and setting Ipeak = 0.5A,
- L1 = 37 uH
- C3 = 12.5 uF
Which, plugging those values into LTSpice, results in a 40V transient.
Here is the output waveform with a C3 MLCC and no load (I know, probably bad idea but I'll explain a little more later):
And here is the output waveform with a C3 electrolytic capacitor and no load:
So, the Vpp values with the MLCC is 376 mV and the electrolytic is 120 mV. Is this because the electrolytic capacitor is serving as a load (since it has a higher resistance than an MLCC)? Or is there something else at play?
Regarding why these experiments are done with no load, when adding a 10 ohm high wattage resistor, the MC34063 burns out. This does not make sense in the steady state since the MC34063 can handle up to 1.5A. In the transient state, LTSpice shows this:
Which is about 1.3A (for <1 ms) but still less than the maximum current. Any other ideas why the chip is being fried?