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MC34063 with Very Bad eff. ?? !!


i am trying to make a dc-dc converter  and saw a video here in this nice blog about using MC34063 and i though life is easy

it is from 9-12 Volt to 18 volt 4 amp

my first trial was using a discerect comp. using 300 Khz but without feedback just constant load ( i was about to begin to make the feedback cir. )

the mosfet peak curr was about 10A and coil peak curr was about :10A

now i said to use MC34063 since desing the feedback maybe it will so diff. , i faced a problem that the IC could only support switch curr less than 1.5A and using MC34063 datasheet design equ. my peak will reach 16 Amp. so i have to use an ext. transistor and on freq, 50Khz

but i shocked i found
the mosfet peak curr became 48A and coil peak curr was about :48A !!!!!!!!!

i used the same mosfet & the same diode in both design so i guess changing them will not solve the problem

so any tips to improve the eff.

please check the attachment images


Use a better controller.


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