Buck-Boost Converter Design

[thetrueman]

Hi all,

I need some buck-boost converter and I reached to attached document (Page17 circuit). I prototype it and got results. It works OK from 5V to 24V it gives 12V which is required.

But the problem arise that PNP transistor heats up even with no load. I just higher the PNP transistor related resistors and it got better (Heat reduced) but output disturbed voltage regulation. Can any expert show a better way to achieve this target. Thanks & regards.

[hate]

What value did u use for the capacitor 'Ct' connected to 'Pin3' and 'GND'? Seems the system is oscillating with a very low frequency and for this the transistor doesn't switch off in time after the inductor is in saturation. U might want to lower that capacitor.

Regards...

[solutions]

I am assuming it gets hot below 7.5V input.  If that's the case, looks to me like you may not be fully saturating the PNP switch.  Not surprising, given you are saying 5V and the app note says 7.5V as a MINIMUM.  With a low VCC you are not gong to drive Q2 hard enough with that latch, no matter what you do with those external resistors. The timing cap will be out of whack as well.  if you have a scope, you'll see what's going on vs guessing.

[carbontracks]

If it's dissipating power with absolutely no load, then something is very wrong.  I've never used that specific topology before, but it's likely that at no load the duty cycle is not going to zero (which it should with a decent controller).  Probe pins 1 and 8 with no load and see what the duty cycle is.

I am assuming it gets hot below 7.5V input.  If that's the case, looks to me like you may not be fully saturating the PNP switch.  Not surprising, given you are saying 5V and the app note says 7.5V as a MINIMUM.  With a low VCC you are not gong to drive Q2 hard enough with that latch, no matter what you do with those external resistors. The timing cap will be out of whack as well.  if you have a scope, you'll see what's going on vs guessing.

The lower operating voltage will affect efficiency, but at no load it shouldn't be dissipating significant power regardless,  so there's something else up too.  Unless the chip simply can't oscillate correctly without a higher voltage (which is possible).

Also I'm surprised you can still find chips that recommend BJTs in there app notes...

[thetrueman]

What value did u use for the capacitor 'Ct' connected to 'Pin3' and 'GND'? Seems the system is oscillating with a very low frequency and for this the transistor doesn't switch off in time after the inductor is in saturation. U might want to lower that capacitor.

Regards...

I'm using 470pf which is mostly used. I've new ideas from above posts and will try them but on the output my capacitor was 470uf 16V and when voltage reached above this limit after some time capacitor burst with bullet sound and hit my finger badly but Thanks God no injury because components were on down side. IC is also shorted so have to get one new. Thanks for your suggestions. see you...

[romi007]

I'm using 470pf which is mostly used. I've new ideas from above posts and will try them but on the output my capacitor was 470uf 16V and when voltage reached above this limit after some time capacitor burst with bullet sound and hit my finger badly but Thanks God no injury because components were on down side. IC is also shorted so have to get one new. Thanks for your suggestions. see you...

Output capacitor should have low ESR (page 18). Use 4x100uF/25V 105 degres C capacitors in parallel to reduce resistance.

[hate]

12V is not enough to burst a 16V rating capacitor check if u've reversed its polarity by mistake. What did u calculate for the value of Ct, are u using 470pf because u calculated 470pf or because everybody uses it? That adjusts the frequency of the design so it should be calculated regarding the design values. Also check if the small current sensing resistor is working, it's usually .22ohm and shouldn't be omitted.

Regards...