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5V buck Vreg Overload!

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Mosaic

Well-Known Member
Hi all:
I built the suggested buck v-reg for a PFM package 5V regulator as per figure 16 in this:
**broken link removed**

Note that C1 is actually 0.68uf, I replaced with a 1.0 uF 50V X7R ceramic.
The FB is connected directly to the 5V out as I have the 5.0V version of the part.

My D1 is a 3A 40V Schottky and C4 is a 470uf Elec. Cap.
I have a 20K IADJ resistor to limit max current to 2A.

What may be significant is I am using a 22uH inductor rated as per this:
https://www.newark.com/multicomp/mc...shielded-22uh-4-7a-smd/dp/82R4330?ost=82r4330

In testing all seems fine with a 12V supply, including a 1A load (resistive). Temp rise is a max 12°C delta.

With 30VDC applied....all seems fine with around a 50mA draw, I see a 8°C delta.
BUT when I apply the 1A load...the Buck Vreg burns up within about 1 second in a puff of smoke and a new hole in the case! This happened twice.

I need some advice on this....I don't see what is wrong.
 
Last edited:
well i checked #3 and the current limit would be 3.2A using the min 3.4A peak spec.
So it's not supposed to overload at all..
 
First thing maybe the diode is not Schottky.

Or the core is saturating. Really sounds like the core.
See the data sheet attached.
I think the discretion is wrong. Please look at the data sheet and see. I think the core is saturating at 1.5 amps.
 

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  • 1359652.pdf
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Well, it is a 5 ohm load on a 5V rail...so 1.5A isn't possible and the diode is a 3A 40V schottky.

However after killing 3 bucks and a 16F886 and 74HC595 as collateral damage :arghh: when the buck fused the problem seems to be the USB connector used to connect the load.
It's a PL2303 cable I had with the 4 wires. I loaded the red and black with the 5 ohm 10Watt resistor load and plugged it into the USB A receptacle in my circuit. While it would 'work' with 12VDC as the intermittent short wasn't enough to kill the buck. It's another story at 30VDC. The buck would fuse by the time I had inserted the PL2303 fully.:banghead:

I soldered wires directly to the PCB and applied the load and it was ok. Cost me 4 hours of review and detailed calcs as it was the first time I used this particular buck.
BTW I ONLY had 4 of the bucks and I have to deliver this PCB by Friday...lol. I said prayers over the last one.:nailbiting:
 
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