Re: QCX-mini critical modification advisory


Jim Mcilroy
 

Hi Hans

Here is a web link that you may be aware of:

https://electronics.stackexchange.com/questions/200565/capacitors-for-ams1117-for-5v-to-3-3v-regulation

73

Jim

On 15/12/2020 13:09, Hans Summers wrote:

Hi Steve
 
I notice the data sheet for the AMS1117-5.0 is different from the data sheet for the LM1117-5.0.  The manual calls out an LM1117-5,  IC11 on page 8 in the parts list but the board may be using an AMS1117-5.0 which are not the same chip design and have different requirements.

It seemed to me likely that AMS1117 and LM1117 were supposed to be functionally equivalent, in much the same way as many other parts are, which are produced by different manufacturers who prefix their own letters... 

But to make things even more confusing, there are at least two different versions (refer to Google) of AMS1117 datasheet, which also contradict each other in certain areas...  
 
The actual problem is the quality of the OUTPUT capacitor.  The regulator chip is likely unstable and going into oscillation causing that big spike seen on the input.  The AMS1117-5 calls for a 22uF solid tantalum and the LM1117-0S calls for a minimum 10uF but low ESR type with a specified ESR between 0.3 and 22 ohms for the OUTPUT capacitor.

I'm not totally convinced yet. The AMS1117 datasheet says in the paragraph on Stability, that the output capacitor 22uF covers all operating conditions, including the use of bypass capacitors across the potential divider which feeds the Adjust input. It goes on to say that if there are no bypass capacitors then smaller capacitors can be used with equally good results. The AMS1117-5.0 has no bypass capacitors because it has no Adjust input, it is a fixed regulator. Therefore it follows that smaller capacitors ought to be fine. 

Additionally note that there is a 470uF capacitor (C47) hanging on the 5V rail, admittedly via 47uH L6 which will change things of course. 

I did try additional capacitors at the output, and I tried removing the 470uF capacitor completely, and nothing made any difference to the spikes that were seen at power-up. A 10uF electrolytic or 10uF tantalum (both were tried) solved it extremely comprehensively. I tested both with extreme intermittent power connections, lots of connecting/disconnecting over extended time, and nothing I could do could fry the system. When I removed the input capacitor, my 20m QCX-mini got smoked within seconds. 

I did also remove a 10uF SMD capacitor and measure it; it was a bit on the low side but these things have quite wide tolerances anyway and my measurement method (time decay across a resistance) wasn't carried out particularly carefully - so I felt that within reason the capacitor actually IS a 10uF capacitor. But the quality, ESR, type etc is a different question. 

Anyway that was my night time's work from midnight until 4:30am this morning - except for a few nappy-changing (a.k.a. diaper) interruptions - and in the end, I had managed to convince myself that spikes were seen at the input, with the stock capacitor; I could kill a QCX-mini by application of power intermittently; and by adding a 10uF electrolytic or tantalum, I could comprehensively cure the issue such that on both my 5A and 20A bench supplies, no amount of messing about would kill the QCX-mini. 

It goes without saying that in all the prototype QCX-mini units I built, and the three production units from the first batch, and in all the other times I have used the identical AMS1117-5.0 chip in an identical way from the exact same supplier etc., I had never seen this failure mode. 

73 Hans G0UPL
http://qrp-labs.com

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