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Push Pull SMPS operation

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david.cano90

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Hello everyone!

I have a question and maybe some of you could help me.

I would like to know the reason why the voltage of any of the switches is Vin when both of them are off. I understand why it is 2Vin when the switch of off and the other one is on, but I do not get it when both are off.

Thank you for your help.

Greeting from Colombia.

David.
 

Because the transformer primary center tap is VIN and
nobody is arguing with it.
 

At switch off you do indeed get 2 x Vin, when both switches are off, after the energy in the leakage goes way you are left with Vin, on the schematic you can draw a line from Vin to both of the drains, so even before a single switch has occurred ( at power up for instance ) Vin is on both fets until one switches on ...
 
dick_freebird said:
Because the transformer primary center tap is VIN and
nobody is arguing with it.
Click to expand...

If that is so, then when Q1 is on and Q2 is off, V_{Q2} would be Vin as well but not 2Vin.

When Q1 is on and Q2 is off, V_{Q2} is the induced voltage on the winding (Vin) plus Vin.

Now... what happens when both switches are off?
 

Would it be the leakage or the magnetizing current the one that causes that?

What roll do they both play during the transition?

Thank you for your help.

- - - Updated - - -

david.cano90 said:
Would it be the leakage or the magnetizing current the one that causes that?

What roll do they both play during the transition?

Thank you for your help.
Click to expand...

Easy peasy said:
At switch off you do indeed get 2 x Vin, when both switches are off, after the energy in the leakage goes way you are left with Vin, on the schematic you can draw a line from Vin to both of the drains, so even before a single switch has occurred ( at power up for instance ) Vin is on both fets until one switches on ...
Click to expand...

Would it be the leakage or the magnetizing current the one that causes that?

What roll do they both play during the transition?

Thank you for your help.
 

Hi,

Would it be the leakage or the magnetizing current the one that causes that?
Click to expand...

To get (measure) voltage, there is no need for current.

It's sufficient if the series resistance (or impedance) is not infinite.
And the coil's impedance is not infinite at any time.

Thus - when both switches are OFF - you see the voltage applied at the center tap (Vin) at the other taps of the primary coil.

Klaus
 

David;
do you have access to an osciloscope?
If so, I would suggest that you connect one channel to the Mosfet gate diriver and the other to the drain. And observe the waveforms both during startup and in steady state operation.

The explanations will immediately become crystal clear.

Of course, if you have a 4 channel scope, you can do the same with the opposite Mosfet and it will be clearer still.
 

Hi,

The leakage inductance causes overshoot above 2xVin ...
Click to expand...
No it's not the leakage inductance.
It is 2x Vin, because both coils are coupled. The active coil voltage is (V_in - 0V) = V_in.
Because the other is coupled the voltage is also V_in.
V_in is in the center tap plus another V_in across the not_active coil gives 2V_in

Klaus
 

please read my posts more carefully, the transformer effect gets you to 2x Vin, the leakage effect produces a spike above 2 x Vin ...

- - - Updated - - -

When both are OFF, XFMR windings have 0 V across them.
Click to expand...
this is correct, but all 3 points of the Tx sit at Vin, this can be confusing for a newbie ...
 

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