Hi Stephen. I was probably a bit harsh in responding to you. Your idea is a good one, and there might be a great part out there that reduces parts count at low cost and is repairable by the average purchaser. A second source would be a big plus. That would be a winner.? I have been bitten every which way in the points I brought up in my previous design life. A single source component is my biggest issue.? Here's an example I experienced.? Many aerospace companies build custom processor boards using industry standard chips.? A new high performance/low power Power PC processor chip had been developed by a a particular company, who had been in business for many years.? All the aerospace companies started designing processor boards around that chip.? When the boards designs were just about complete by many companies, In a surprise move, Apple Computer bought out the entire design team to make processor chips for Apple products!? The chip's availability became unobtanium. All the aerospace companies were left holding the bag.? This caused massive amounts of wasted development costs, destroyed schedules, and beginning all over in these design efforts.
In the aerospace industry, things had to work reliably, and cost was not a big issue, within reason.? We used BGAs all the time but sometimes had sourcing issues and BGAs had to be removed from defective boards to use on new boards.? This was an expensive process, as it required re-balling the BGA with new solder balls before it could be re-used on a new board.? A few companies do this. BGAs are out of the question in the QMX type application unless you are willing to scrap the board when the BGAs fail. Even so, they would have prohibitive manufacturing costs as this is a relatively low volume endeavor.
Best of luck in your search.
-Steve K1RF?
------ Original Message ------
Date 8/29/2023 8:40:41 PM
Subject Re: [QRPLabs] QMX - smoke - another C107/Q108 failure
Those are all good reasons the LTM8078 won't be the best choice for this particular application, I really just picked it by going on the Analog Devices web site and filtering for a dual-output regulator with integrated inductors. There are tons of parts out there, some more cost effective and/or practical than others. My basic point is that switching regulators are a problem that has been solved and packaged into off-the-shelf components by companies with more engineering resources than the entire amateur radio community combined. There's very little reason to roll your own solution unless you have very unique needs.
I don't think that "it's a single source part" is a strong argument in the context of a product that contains an ARM CPU, which I can personally attest to being frustratingly unobtainable during the semiconductor shortage. I also see an ADC from Texas Instruments and a DAC from Cirrus Logic, also not generic parts. At least the power supply boards are modular. If Analog Devices' foundry burned to the ground and they stopped manufacturing switching regulators overnight, it's not a huge deal to just spin up a different power supply board that occupies the same area and produces the same voltages, without needing to touch the design of the main PCB.
As far as BOM cost, if every dollar of BOM adds two dollars to retail price (this is my formula for things I build and sell), I'd have paid an extra $20 for a QMX with that extra $10 of BOM in the power supply... And spent several fewer hours troubleshooting my dead radio. That time is worth so much more than $20 to me. I actually ended up deciding it was more worthwhile to spend $100 on another kit than gamble on the possibility that the same event that killed my IC403 didn't also damage other parts in less obvious ways, forcing me to spend even more time (and shipping costs from the local Digikey warehouse) troubleshooting. Take my $20 instead, please.
>Of all the QMX I have yet seen, other than the Q103/Q104 Drain short (manufacturing problem) I have yet to see a failure that is not attributable to shorts, damaged components or other construction errors.
I do remember reading about at least one other person who killed their QMX by toggling their power supply between 6 and 12 volts, in order to toggle their transmit power. This is similar to my own scenario, where I forgot to raise the current limit on my power supply, which went into current limit when I keyed the radio into a dummy load, then came out of limit when I released the key, putting a similar voltage transient into the power supply input. Should I have been more diligent about making sure my current limit was set correctly? Yes. Is halving and doubling your input voltage a relatively extreme condition to subject a power supply to? Also yes. However, simply switching the thing on is a power supply transient from 0 to 12 volts, so a transient between 6 and 12 should be well within what it should be expected to tolerate. I don't think there is a single DC-powered device in my house that would be harmed by subjecting it to voltage transients within its specified input range. So I would have to disagree that this phenomenon should be categorized as a "constructor error."
