Reverse engineering the power connections
My aim is to power this VRM up using a bench power supply and then to probe the various parts to find out what’s what. To do that I need to locate the power input and ground. I don’t need to locate the power output because I can probe for that but I think it’ll be an easy one to guess anyway.
Firstly, let’s locate ground. Here’s the pinout of the IC.
Ground is on pin 11. I set my meter to the continuity testing buzzer and probed around the golden fingers. Easy. All the fat fingers on the heatsink side are ground. No worries.
Let’s move on to VCC, the power input. It’s on pin 18 and a big obvious hint on the control circuitry side is the presence of a white SMD fuse down at the bottom right. The ‘LF’ logo is Littlefuse and it’s rated at 15A.
Yes, that was it. The fat finger on the right is the power input. I could go ahead and power it up right now and probe for the power output but let’s see if it’s where I think it is.
Switching regulators like this one have an inductor in series with the output voltage so all I need to do is probe either pin of that big inductor under the heatsink and the suspect fat fingers on the control side that are unaccounted for.
Sure enough those pins confirm to be the power output. Those metal bars are interesting aren’t they? Each one of them bridges over some vias in a large copper fill. The silk screen label prefix of ‘BB’ probably stands for ‘bus bar’ which is a common engineering term for a big fat conductor. I assume that the designers were trying to ensure that the current delivery is as even as possible across all the golden fingers. It’s certainly an interesting design decision and does hint at a belt-and-braces design.
Now I can power it up and see what it actually does.
