Diode burning in the diode bridge circuit

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azadfalah

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hi friends

In the diode bridge circuit below, the input voltage is 220 volts, from a modified sine wave UPS (Iron Core)
I used this circuit to Externaly measure the output voltage of the UPS



But there is a problem that M7 diode or 1n4007 SMD burns sometimes, usually it burns when no load is connected to the system.

Can UPS output harmonics cause this problem?

In my opinion, this problem is caused by the fact that the diodes are fake, what is your opinion ?
*The complete circuit cannot be displayed


thanks
 
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tepalia02 said:
Sorry, can't understand which one is M7 diode. I can find only diode D1,D2,D4,D5 here.
Click to expand...
Thank you for your attention. M7 is the SMD code written on the Diode
It cannot be repeated for a specific diode D1,D2,D4,D5, random diode burns every time
 

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M7 is an SMD code for a 1N4007 equivalent.

Excessive voltage from the UPS could be the problem, it doesn't seem to be a current overload with resistors that large but when you say "no load", where exactly is the load connected to, before the bridge or after it?

Brian.
 

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Thanks Brian

It was meant that there is a UPS that we did not connect any laod to its output, we just connected this measuring circuit to its output

I am completely sure that we do not have a problem with the increase or spike of the output voltage
Can harmonics cause this problem? I want to conclude whether these diodes are fake or not
 

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betwixt said:
M7 is an SMD code for a 1N4007 equivalent
Click to expand...
I didn't yet see it, which package, manufacturer?

If he diode is o.k.and we don't have excessive voltage surges, may be the unspecified "external circuit" is the problem?
Can you confirm that the rectifier bridge isn't loaded by other components than Mohm resistors, e.g. capacitors?
 
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If you have a DSO set it for single shot, say trigger at 500 V, use two channels
differential mode, and monitor the output of the SMPS. Be aware you do not use
ground leads on probes. See if you can capture any damaging spike. Do it with SMPS
loaded and unloaded, power up/down.

Probing cautions so you protect your scope, read carefully to make sure you dont
damage your DSO.


https://download.tek.com/document/ABCs of Probes 60W-6053-15.pdf pages 22-23

A differential probe suitably rated on common mode is the safest practice.
Take time to read the ap notes to save your scope when working in these
non isolated systems, so common today.


Regards, Dana.
 
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Yes, no other load is connected to the output of the diode bridge, and for this reason we are also surprised by the random burning of these diodes.

Of course, it burns about 10% of the time
So it doesn't seem like the circuit has a problem, right?
 

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Hi,

So if the is no problem with too low distance causing sparks.... then we may exclude overcurrent.
Overtemperature (by power dissipation) may be excluded, too.

So the only way to kill these diodes should be overvoltage.

The overvoltage may be generated by the (unloaded = undamped) filters, sudden remove of load, or sudden remove of input power.
It does not matter where it comes from, but it should be suppressed.
I recommend to add a MOV at the inverter output.

azadfalah said:
I want to conclude whether these diodes are fake or not
Click to expand...
Yes, you. We can not do thus for you. We have less informations than you.
We would have to guess... but you don't need us to "guess".

Klaus
 
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Put the 1.2 Meghom resistors in series with the input to the bridge instead of the output.
That way, even if there is a short voltage spike, it is much less likely to short a diode, since the current and thus the energy in the spike dissipated by the diode will be much less.
 
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Thanks Crutschow,

Basically, there is a schematic of this circuit, but I added a diode bridge at the input of the circuit, if I put the diode bridge after the Mega ohm resistors, won't there be a problem?


--- Updated ---

KlausST said:
Yes, you. We can not do thus for you. We have less informations than you.
We would have to guess... but you don't need us to "guess".
Click to expand...
Thanks Klaus
More info added
 

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Hi,

what´s the idea behind using a bridge rectifier at all? What´s the benefit?
I mean it is cheaper, more accurate and more flexible if you do this in software.

You just cut away information that could be useful (later). Like DC value, zero cross .. and other informations. But you add the diodes non linearity, the voltage drop, the recovery effects..

Klaus
 
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azadfalah said:
if I put the diode bridge after the Mega ohm resistors, won't there be a problem?
Click to expand...
Yes, after seeing your circuit, that would add the diode offsets.

If you put the bridge between the two 1M resistors on each side, that would still help protect the diodes while minimizing the diode offset (LTspice simulation below):
Note that the voltage across the bridge (red trace) is now 1/2 the supply peaks, while being protected by the 1M resistors.

 
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crutschow said:
Yes, after seeing your circuit, that would add the diode offsets.
Click to expand...
In my opinion it makes no difference for the offset caused by the diodes whether
* the bridge comes first
* the bridge comes last
* or the bridge is inbetween

Klaus
 
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If the resistors are rated for 250V then two 1Meg in series makes sense. The differential current and PIV stress on the diodes will be the same in the middle or after both R's unless there is a diode fault.

Have you tested the diodes to confirm PIV rating with 1M+1M or HV rated resistors?

Anecdotal
I recall when I had qualified a major brand PSU then they moved production to Mexico and started failing with high failure rates in our final test for Hipot with catastrophic results. So I modified our Hipot tester to add same series resistors before the output the probe to limit the current on arc failures. Then I disqualified the vendor until they fixed all problems.
 
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KlausST said:
In my opinion it makes no difference for the offset caused by the diodes whether
Click to expand...
You are correct.
The bridge is being driven by a near current-source due to the high value resistors in series which minimizes the effect of the diode drops.
So placing the bridge at the output of the two sets of series megohm resistors should work fine.
And you can then use low voltage diodes in the bridge since the reverse diode voltage will now be very low.
--- Updated ---

Below is a sim with the left circuit having the bridge connected at the middle of the two 1M resistors, and the right circuit with the bridge connected at the output of the two 1M resistors.
The two outputs (yellow and red traces) are essentially identical, while the two voltages across the bridges (blue and purple traces) are drastically different, verifying that the output is not significantly affected by where the bridge is placed.

 
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