Forward without a coil. Simple SMPS out + or -

Thread Starter

Motanache

Joined Mar 2, 2015
540
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This Forward without coil marked in blue.
I think it is possible only if the secondary consumption is maximum. Is it right or wrong?

The pulse given in the primary leads to the increase of the magnetic flux in the transformer. Ton.
At Toff, the current in the primaryr disappears.
And the magnetic flux is forced to drop very suddenly, which generates very high voltages in the coils. Faraday e=-df/dt.

But if there is secondary coil consumption on the Ton, then the secondary coil current will generate an opposite magnetic flux.
Thus the magnetic flux will no longer fall from a high value to 0, so there is no need for the coil marked in blue.

It is about 12v and comparable voltages.

I will probably put this into practice and if I miscalculated I will have an unwanted high noise..
 

DickCappels

Joined Aug 21, 2008
7,746
I think the body diode in the MOSFET will take over from the removed winding and diode, though putting a high speed diode rated to hand the average current through the MOSFET in parallel with the drain and source would be a good idea.

You need D2 and L on the output to produce a voltage at the output that is proportional to the duty cycle of the pulse from the drain.
 

AlbertHall

Joined Jun 4, 2014
11,506
I think the body diode in the MOSFET will take over from the removed winding and diode,
When the MOSFET switches off, I believe the drain voltage will go positive of the supply voltage and so the body diode is not going to help and as this is a forward converter the secondary diode will be reverse biased at this time too.
 

ronsimpson

Joined Oct 7, 2019
1,476
The point of the circuit is to get power through the transformer to the load. But some of the power does not exit the transformer when and where you want. (leakage inductance) The "ketch winding" with D3 throughs that power back into Vsupply where it can be used in the next cycle. It D3 and winding are not there the voltage on Switch gets too high when it is off. I added a diode and RC that takes the extra power and burns it in R. Usually one or two watts, not much.
1618878230608.png
 

DickCappels

Joined Aug 21, 2008
7,746
When the MOSFET switches off, I believe the drain voltage will go positive of the supply voltage and so the body diode is not going to help and as this is a forward converter the secondary diode will be reverse biased at this time too.
You are right, I did not pay attention to the phase dots.
 

Ian0

Joined Aug 7, 2020
3,281
The "ketch winding" with D3 throughs that power back into Vsupply where it can be used in the next cycle.
The purpose of the winding connected to D3 is to reset the flux in the transformer. The voltage time constant must be the same in both directions.
It can also be reset by the DCR circuit drawn in green, the same as it is done in a flyback design, but instead of returning the energy to the smoothing capacitor it is dissipated in R.
In a forward converter there is no gap, so that the magnetising inductance is large, so the magnetising current is small.
In a flyback converter, the magnetising current is large, and the flux is reset by the secondary.
Because of the opposite phase of the secondary in the forward converter, the secondary cannot reset the flux, so it needs the RCD circuit (lossy) or the extra winding and diode to reset the flux.

There is no explanation as to why D2 and L are crossed out in red, but they are absolutely necessary in a forward converter.
L is the only energy storage in the circuit.
 
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