Author Topic: MC34063 with external N-mosfet  (Read 16936 times)

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Offline EmilTopic starter

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MC34063 with external N-mosfet
« on: May 05, 2015, 01:16:33 pm »
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
 

Offline c4757p

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Re: MC34063 with external N-mosfet
« Reply #1 on: May 05, 2015, 01:26:56 pm »
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:



You'll have to adapt it to clamp VGS.

Better yet, just don't use MC34063. It's horrid enough for low-power applications...
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Offline EmilTopic starter

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Re: MC34063 with external N-mosfet
« Reply #2 on: May 05, 2015, 03:36:31 pm »
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...

 :palm: 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
 

Offline rob77

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Re: MC34063 with external N-mosfet
« Reply #3 on: May 05, 2015, 03:54:39 pm »
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)
 

Offline c4757p

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Re: MC34063 with external N-mosfet
« Reply #4 on: May 05, 2015, 03:57:55 pm »
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.
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Offline EmilTopic starter

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Re: MC34063 with external N-mosfet
« Reply #5 on: May 06, 2015, 03:17:41 pm »
I might go with an LM2678 instead. Anything horrid about that one? :)
 

Offline rob77

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Re: MC34063 with external N-mosfet
« Reply #6 on: May 06, 2015, 05:35:53 pm »
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)
 


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