We have been using the MC33063 as a buck converter for many years. 21-30V input to 5V / 0.5A output
But now we are facing a problem with overheating.
Compared to the successful earlier application, we increased the output current by 20..30mA.
The MC33063A works in discontinuous mode.
Cin=820 uF, Rsense=0.22 Ohm, Ct=470 pF, L=220 uH, Cout=330uF and 100nF
After a cold startup, the MC33063 works fine, but with increasing time or external heating,
the output signals get "distorted", and the MC33063 heats up until it would burn out.
The failed parts (10 pce so far) show the marking OAM, AX9C, G4.
By the way: the MC33063 from ON-Semiconductor does the job without any failure, even when heating it up with an external heater.
Any ideas what we are doing wrong ?
Measured signales on the scope:
channel 1, yello, switch voltage on IC pin 2
channel 2, red, timing voltage on IC pin 3
channel 3, blue, sense voltage on IC pins 1+7+8, AC-coupled
channel 4, green, +5V output, AC coupled
the problems I can see:
the upper threshold of the timing signal is far to low.
sense-voltage should go to 24V instantly after switching OFF.
but here it goes up slowly with big noise.
the switching voltage does not go to -0.7V as usual, but it is around 0 V.
After that the inductor current is zero in the OFF phase,
the switch voltgae goes to 5V, but the ringing is much faster than usual.
A picture after a cold startup:
A picture with a MC33063 from ON-Semiconductor: