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Injecting voltage not producing the expected results, help please!

This applies to more than just the iPhone 7. When there is a short on a line I’ve learned that you connect a lead to ground and one to the line the short is on and apply voltage so I did. I have a Flir One Pro thermal camera to help spot the shorts but what is happening is the heat is always staring from where I am injecting the voltage, which is usually a test point on that line and spreads from there. I must be doing something wrong. This has happened on all devices that have shorts that are not on VCC. VCC shorts have been easy to find. Any suggestions, or video tutorials you can point me to would be appreciated.

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That really depends on the type of short you are dealing with and how your DCPS works. If you think about it, a very low resistance short will usually force your DCPS into current limiting mode. That means the voltage goes down and in some cases, the voltage can go down to near zero. With such low voltage, it can be hard for the failed component to generate any amount of heat (P=VxI).

I see this question every once in a while. So if you have a DCPS and you set it to 4.2V and you connect it to something that is essentially a dead short, such as a logic board with a short on VCC_MAIN and the DCPS is delivering 4A, what do you think is really happening?

The answer is…not much. Most bench or lab power supplies are current limited. That means that when the current draw reaches the limit, either the limit you set or the max limit the DPCS can provide, the voltage starts dropping to compensate and maintain the set current.

You can actually simulate this directly with your power supply. Set a voltage and a max current and then short the leads. What you’ll see on the display is that the voltage drops to near zero while the current hits the top end. You will feel some heat in the leads because there is some resistance. On my DCPS, if I set the current limit to 1A and the output voltage to 4.2V, what I see on the display when shorted is 1A and 130mV. That means the DCPS is “seeing” a resistance of 130 mOhms (R=E/I —> 0.130V/1A=130 mOhms) and the power dissipated by the leads under full short conditions is 130mW (P=V*I —> 0.130mV * 1A=0.130W), which is less than the old school 1/4 watt through-hole resistors of yesteryear. Because the leads are long, you just feel a warm touch along the lead wire.

The same applies to troubleshooting. If a main cap is a dead short, let’s say 0.1 Ohms or less and you apply 4.2V, in theory you should see 42A (!!) and the power dissipated by this component would be 176W (P=VI —> 4.2 * 42). That’s enough to get the whole board glowing red hot. But that’s not what happens…is it? No, if you’re connected to a current limited power source like a DCPS, the voltage drops and the current maxes out and at best, the DCPS is delivering a fraction of a Watt…and the leads are dissipating most of that. Now imagine a 1V8 line…To get a part glowing hot, it either needs to be slightly higher resistance, say 1 Ohm or you have to be able to pump current through without limit.

VCC_MAIN is a complex circuit with lots of caps hanging off of it. Secondary lines are simpler and come directly from an LDO or an LDO output of the PMIC and are usually limited in current capability (unlike the main lines which are switchers and can send a lot of current downstream). If the short is low resistance, you won’t see much heating at all. That’s why some times you have to resort to pulling parts.

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Aaron さん、ありがとうございました!
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