Hi,
I'm trying to make a circuit using an MC34063 switching regulator and an external N-channel mosfet. The input is 36V regulated DC and my requirement for the output is 4A at 12V with <200mV ripple.
The datasheet has an example circuit using an NPN transistor, but no examples using mosfets, so I based my design on the external NPN example. I have added a resistor (R1) between pin 2 of the MC34063 and gate on the mosfet to keep Vgs below 20V. Other than I have only changed some component values.
Does this look like workable design? Any problems or suggestions for improvements?
Emil
You'll lose way too much power in the FET because you're not saturating it. It'll drop V
GS when "switched on", translating to huge power loss at 4A.
Try a PMOS, and with an active driver:
You'll have to adapt it to clamp V
GS.
Better yet, just don't use MC34063. It's horrid enough for low-power applications...
You'll lose way too much power in the FET because you're not saturating it. It'll drop VGS when "switched on", translating to huge power loss at 4A.
Try a PMOS, and with an active driver:
Better yet, just don't use MC34063. It's horrid enough for low-power applications...
I used an N-MOSFET on the high side.. Obvious when you point it out, but I missed that. Thank you.
I'm curious what makes the MC34063 horrid?
Emil
34063 is my favorite switchmode regulator - but i wouldn't use it with external transistor (not even thinking about external mosfet
)
it's a simple and CHEAP - therefore ideal for small converters in hobby designs... but it's OLD and inefficient compared to modern chips.
for 12V 4A i would look after a more modern chip capable of synchronous operation (driving 2 mosftes - top one is the switcher and the bottom one is replacing the shottky diode)
It's slow. The hysteretic control makes it unpredictable in terms of frequency, hard to filter, and in some cases quite noisy. The current-limiting behavior is overly simplistic for many applications and wasteful with its huge 1V threshold. Current-limiting is slow, meaning it doesn't do much against things like core saturation, and the output transistors are fragile.
The cheapness is a benefit, of course, and I'm not above using one where its many drawbacks aren't a detriment.
I might go with an LM2678 instead. Anything horrid about that one?
I might go with an LM2678 instead. Anything horrid about that one?
those should be OK. - simple circuit and high enough frequency (smaller inductor and caps)