Hello,
I have a Feit overhead lamp (43W) I was trying to install. It slipped out of my hands and banged hard on one side. When I tested it, it flashed on/off once every two seconds (i.e. on for a second, off for a second). I took it apart, and disconnected the LEDs and a "motion sensor" from the power board, and I still heard the on/off sound (I think via ringing of an inductor, so figured the problem was still there and I could isolate it to this PCB). I had to learn about SMPS to begin to figure out what was going on, and how a mechanical impact could damage something to cause this. I have removed various parts to confirm the high power side is fine, and the post bridge rectifier DC voltage is stable (at ~130VDC). The MOSFET seemed to be okay. Various diodes seem to be okay. In frustration, I started to map out the circuit. I do not know if I got it exactly right, but I think it is pretty close. What I don't understand is how the IC is powered from a transformer auxiliary coil when the primary coil isn't powered on until the MOSFET is turned on, which doesn't happen unless the IC is powered?! I will attach the circuit diagram, but if you could offer help on a) how is this suppose to work to produce what I believe is the 60V constant current needed to power the strip of 60 LEDs? and b) how can an impact cause the behavior I'm seeing? That is, what could be damaged to cause this? Please note that I do have a basic understanding of how the SMPS works, but many schematics show the IC powered from the DC side before depending on the MOSFET to power the transformer. This circuit is different and confuses me. Also, my suspicion is the IC is powering on, then off, and maybe that's why it cycles (perhaps no current sense, or low voltage, etc.)? But not at that cycle rate. I don't see anything in the specs to suggest 1 second on/off anywhere. Thanks for any help or insight you can provide!

 

You probably have a damaged Solder-Joint somewhere.
I would Re-Solder every Solder-Joint on the Board, and then test again.
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The 1 second on, 1 second off thing is similar to older halogen electronic power supplies when you put LEDs on them. Basically, the load was too low so the voltage would rise, exceed the maximum, shutting down the power supply, which would then reset.

So, it would seem something is happening along those lines, either excess voltage for whatever reason, or some other overload.

It could be that voltage or current sensing components have been damaged, or as LowQCab said, check solder joints. I suspect the weight of a component has either broken a solder joint, or possible broken the component itself internally. I've seen plenty of resistors that have failed open circuit/high resistance with no external signs of damage (often caused by thermal cycling, particularly fusable resistors, those things are a pain).

Really, it could be a lot of things, you are going to have to keep testing components until you find something iffy I suspect.

 

Indeed! As both have mentioned, it could be a damaged solder connection. But it might also be that the mounting of somehing to a heat sink has been dislodged.
One other possible explanation, if you have not checked that the light functioned correctly before attempting to install it, would be that it was defective as it arrived.
I see in the circuit that it has a light sensor, and if the LEDs illuminate the light sensor that will make it switch off also. So the path from the light source to the light sensor may not be as intended.

 

You probably have a damaged Solder-Joint somewhere.
I would Re-Solder every Solder-Joint on the Board, and then test again.
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Thanks, I tried this, but it made no difference. I still do not understand how the circuit even works. How does the IC get powered if it depends on the MOSFET, which is driven by the IC, to drive the auxiliary coil?

 

Indeed! As both have mentioned, it could be a damaged solder connection. But it might also be that the mounting of somehing to a heat sink has been dislodged.
One other possible explanation, if you have not checked that the light functioned correctly before attempting to install it, would be that it was defective as it arrived.
I see in the circuit that it has a light sensor, and if the LEDs illuminate the light sensor that will make it switch off also. So the path from the light source to the light sensor may not be as intended.

I did try to reflow, but it didn't help. The light functioned before I started to install it, so something happened to it after it fell. I did suspect that somehow the light sensor (motion sensor connected to dimmer input on IC) was a possible problem, but when I initially disconnected everything, the PCB made the same once a second sound it did when it was driving the LEDs on and off, so I decided it isn't being affected by the motion sensor.

 

The 1 second on, 1 second off thing is similar to older halogen electronic power supplies when you put LEDs on them. Basically, the load was too low so the voltage would rise, exceed the maximum, shutting down the power supply, which would then reset.

So, it would seem something is happening along those lines, either excess voltage for whatever reason, or some other overload.

It could be that voltage or current sensing components have been damaged, or as LowQCab said, check solder joints. I suspect the weight of a component has either broken a solder joint, or possible broken the component itself internally. I've seen plenty of resistors that have failed open circuit/high resistance with no external signs of damage (often caused by thermal cycling, particularly fusable resistors, those things are a pain).

Really, it could be a lot of things, you are going to have to keep testing components until you find something iffy I suspect.

Yes. That's very painful as many components are small SMDs, and it is very difficult to solder those on and off. I've checked just about all of them in circuit, which I know isn't always best, and at least the diodes seem to be okay, and the resistors give reasonable values. Man, I hate to just toss this in the junk bin. I just don't know enough to systematically debug it in a logical way. As I've mentioned elsewhere, I don't even get how this powers the IC, which depends I think on the aux winding, which depends on the MOSFET being turned on, which is driven by the IC! Ahh! I think the primary winding is okay since it is around 0.5 ohm, but the aux winding is like 138k ohm. That doesn't seem right to me. I can't find anything on PQ2020, which is just a description of the transformer dimensions, nothing about the windings themselves, which are probably proprietary. Pretty frustrating because I think I kind of understand it, but as I say, just don't know how to attack solving it.

 

With that high of a resistance reading it seems to me that you have located the problem, which is an open circuit in the transformer connection. So now a very close inspection of the transformer connection area should reveal a broken wire. I have repaired a few of those broken connections on small transformers that were not adequately supported on circuit boards.
So now you have a suggestion as to where to look and what sort of failure to search for.

 

Two cents worth here - inside IC's are tiny wires connecting the leads to the actual chip inside the IC. Dropping a poorly made chip can result in a broken wire.

Your symptom sounds like something charging up and turning on but then discharging and extinguishing the LEDs. It can be any number of components that have suffered high shock and vibration. Shock from an impact that is.

 

The transformer is the most fragile, at least many of them are. No support except the terminals soldered to the PCB. The fine wires have no slack, so if the steel core moves at all it will reak the wire at the solder connection.

 

The transformer is the most fragile, at least many of them are.

I can't see that sort of failure causing a 1Hz cycle. IF the core heats up and expands and thus opens a broken wire - it will power off. Then after a period of time it will cool enough to remake contact. The broken wire will weld (poorly) and conduct current again until it heats sufficiently to break the newly welded joint. Rinse and repeat. But on an order of dozens of seconds to dozens of minutes. There's a streetlamp outside my house, one house down that will come on for about 5 minutes then extinguish for somewhere around the same period of time. I've not timed it so I can only guess it's about 5 minutes. That's just a guesstimate. For a one cycle per second with a duty cycle of approximately the same time off as time on - I just don't see it being a transformer. Even a bad solder joint will not behave in a cyclic pattern. I have a TV which has a bad solder joint. Turn it on and it takes anywhere from a minute to several minutes before the picture clears up. Given that it's a 55" flat screen LED TV I just don't want to mess with pulling it apart and looking for a bad solder joint. There HAVE been times when a wrap on the side would restore the picture. But once restored, whether with time or a whack - it works normally until it's turned off for the night. The next day it's the same thing almost always. But sometimes when the weather is warm the TV doesn't do that. It's heat expansion related. And not cyclic. The TS is having an issue with a cyclic pattern of approximately one second on, one second off. Something somewhere is disconnected and some amplifier is charging up until it's charged and switches on, then some capacitor must be holding it on for about the same time it took the cap to charge. Once the cap discharges it comes back on and again charges up, then shuts off. Then drains. Then charges. Then drains (rinse and repeat).

 

Thanks everyone. So I can assume an AUX winding of 138k ohm is definitely not right?

Also, can anyone explain to me how the IC Vdd gets power from the transformer AUX, but the transformer AUX doesn't get powered up until the MOSFET is turned on, YET that doesn't happen until the gate driver output comes from the IC, which I assume requires Vdd? This doesn't make sense to me. I do see the High Voltage (HV) input, so maybe the IC is powered up enough by that to start the PWM to the MOSFET, and then it sustains itself thereafter? Is that it?

 

OK, for the flashing: No, not transformer heating, but constantly re-triggering a protective circuit because of not enough load. That is how a lot of over-voltage and over current circuits work. Fairly fast but not instant.
Now for how the circuit starts until it can support itself?? That is more complex, and sometimes there are a couple parts that only provide startup power. They are seldom efficient but for that 40 milliseconds each startup it does not mater. Recall, "Swithing regulators are complex, and every connection is also a circuit element." Besides that, there are a few REALLY SMART folks who design the best switchers. And I am talking about those who have incredible insights. Not mere genius types.

 

Thanks everyone. So I can assume an AUX winding of 138k ohm is definitely not right?

Also, can anyone explain to me how the IC Vdd gets power from the transformer AUX, but the transformer AUX doesn't get powered up until the MOSFET is turned on, YET that doesn't happen until the gate driver output comes from the IC, which I assume requires Vdd? This doesn't make sense to me. I do see the High Voltage (HV) input, so maybe the IC is powered up enough by that to start the PWM to the MOSFET, and then it sustains itself thereafter? Is that it?

I do not know if I got it exactly right, but I think it is pretty close.

Maybe you've drawn it wrong.

 

Okay, I guess I have to give up on this. I can't seem to find out what's wrong no less fix it.

So I will now try another tack. Can anyone specify a small power supply that I could use to power the LED strip I have? The only identification on the strip is E329419 PS-2 and at the center of the strip, 802M9D3H1C (see photos). I actually do not know the required voltage/amperage. I do think it is a constant-current supply.

Other than that, there are 60 LEDs and the box says it consumes 52W.

Any suggestions appreciated. Thanks!