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are they snubber circuit?

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micro_man

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Hi
I am troubleshooting a pcb. One section i have drawn on pcb software as follows
snubber.png

i wanna ask, Are red circle sections acting as snubber circuit or not?
if yes how they are working?
 

I would guess yes thats what they are for. It's a bridge rectifier with no reservoir capacitance after it and it appears to feed a H-Bridge circuit so the voltage and current through the diodes is likely to full of spikes. Some diodes produce switching transients under those conditions so I would assume they are there to limit them.

Brian.
 

Thanks betwixt
But normally we have snubber connected across drain and source, right? How they are working in the attached image? Are they enough for these 4 mosfet's?
Actually the transformer primary has 3 terminals, One terminal is connected to the +ve side of battery,the other two terminals are connected to the mosfet which are switching according to the PWM they get.
 

Hi,

they are snubbers just for the diodes, not for the MOSFETs.
And the energy stored in the diodes and the leakage inductance and capacitance is very small.

I think this is just a snippet of the circuit. On the power bus of the MOSFETs there is definitely something missing: Either capacitors, snubbers, MOV or zeners....

So our answers can only be as good as the informations we get. It leaves much room for guessing.
How are the MOSFETs driven.. for example...

Klaus
 

Hi everyone
Thanks for your replies.
The complete scheme is as follows
snubber_2.png
There are two NTC and a capacitor before Load Connector (J5).
 

But normally we have snubber connected across drain and source, right? How they are working in the attached image? Are they enough for these 4 mosfet's?
Click to expand...
My original post still seems correct, they are for suppressing transients from the diodes and have little to do with the MOSFETS.

Actually the transformer primary has 3 terminals, One terminal is connected to the +ve side of battery,the other two terminals are connected to the mosfet which are switching according to the PWM they get.
Click to expand...
That isn't what the schematic shows and I can't see a sensible way of configuring it that way.

Is this a proven working design or something new? You said you were troubleshooting a PCB so I assume it is beyond prototyping stages. As it is, it might work under certain circumstances but it's difficult to see what it is supposed to do. With a suitable high and low side MOSFET driver it can at best work as an inefficient AC power controller if the PWM is synchronous to the incoming AC.

Brian.
 

Actually its an inverter.Some company has made it.I am troubleshooting it because whenever i change the MOSFET's, they get damaged after somedays.I don't know the reason.
That's why i am reverse engineering the PCB to understand the issue.
 

Hi,
I don't know the reason.
Click to expand...
--> overvoltage spikes.

the answer is already given in post#4

Klaus
 

OK. Kindly suggest me some solutions
 

The transformer must be center trapped else there is no return path for the current. What are the current and voltages involved?
 

Hi,

I´ve already given 4 solutions. What else do you expect?

You ask for solutions but refuse to give enough informations.

Klaus
 

What information you required?
Actually this is the complete circuit diagram of transformer's secondary section.
If i have to add a snubber circuit, should i place snubber circuit with each MOSFET?
 

Hi,

Why don´t you read our posts?
What are the current and voltages involved?
Click to expand...

On the power bus of the MOSFETs
Click to expand...
Alternatively you may connect the snubbers across each MOSFET
The capacitor at the load may push up to twice the bus voltage back to the bus. You need to say to what value to limit the bus voltage.
We don´t have any information about switching frequency and load. How much energy is stored in the load ... that can be expected to come back to the bus voltage.
Switching OFF the primary side of the transformer may additionally cause a hig voltage peak at the power bus. We don´t have any information about that.
If switched with an SCR--> the energy is low. If switched with a mechanical switch --> the energy may be high.

Klaus
 

At the output side the voltage is 220Vac & current is 4A.
The primary side of the transformer side is switching using MOSFET (HRF3205).

Presently the bridge is switching against 0.47 uF capacitor C5 which is an incredible bad idea. If this is the real circuit, it's not surprising that the MOSFETs fail frequently.
Click to expand...
Should i use larger value of Capacitor maybe 1uF?
 

Presently the bridge is switching against 0.47 uF capacitor C5 which is an incredible bad idea. If this is the real circuit, it's not surprising that the MOSFETs fail frequently.

The bridge can under circumstances work without bus a capacitor, but this operation would cause large current transients in the in put transformer and the inverter feeding it.

The transformer must be center trapped else there is no return path for the current.
Click to expand...
Look sharp, it's just a regular bridge rectifier.

- - - Updated - - -

There should be no capacitor directly across an inverter output because it causes huge switching losses.
 

Actally some company made this inverter and i have bought some pieces form them.
Problem is of MOSFET Damaging is there.I don't have proper details to give you but i want to solve this problem.
A snubber circuit is required as you guys suggested. But without having much knowledge how i calculate the snubber component values?
 

A MOSFET bridge with state-of-the-art PCB layout doesn't require snubbers, they may at best reduce RF interferences. Rectifier can be necessary for some rectifier diodes.

It's not clear how your actual circuit looks like, as said there should be bus capacitors, but no filter capacitor directly across the bridge output.
 

here is the complete diagram of primary and secondary side of transformer
snubber_3.png
 

So I have to keep the final comment in my previous post.
 

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