UC2845 power supply problem

shaorc

MOS tube driver IC in single-ended flyback switching power supply

UC2845 is powered by a set of outputs from the secondary side of the transformer.

However, many schematic diagrams draw the output of the secondary side and the primary side on one side.

why is that?

maychang

"But many schematics draw the output of the secondary side and the primary side on one side."

The side of the core symbol on which the winding is drawn has nothing to do with the operation of the transformer. No matter which side the winding is drawn on, it means that it is the winding on this core, that is, it has a common magnetic flux with all other windings.

maychang

If a transformer winding does not have a common magnetic flux with other windings (some transformers do this), it cannot be represented in this way and must be represented in a more complex way.

shaorc

maychang posted on 2019-7-22 14:50 "But many schematics draw the output of the secondary side and the primary side on the same side." The side on which the winding is drawn in the core symbol has something to do with the transformer...

So how do we determine whether the winding is primary or secondary?

Is it through the ground of the winding? Or the same terminal?

qwqwqw2088

Flyback switching power supply chip control ICs like UC2845 and 3845 can be placed on the primary side of the transformer, that is, the primary side, and can be directly started using the DC bus voltage output by the rectifier. This is the power supply problem of UC2845.

As for how to draw the transformer in the schematic diagram, as mentioned above

maychang

shaorc posted on 2019-7-22 16:39 So how do we determine whether the winding is the primary or secondary? Is it through the grounding of the winding? Or the same-named end?

"Then how do you determine whether the winding is primary or secondary?"

In the direction of energy transfer.

伟林电源

The auxiliary winding provides the freewheeling voltage of VCC, and this group shares the same ground as the primary side.

shaorc

maychang posted on 2019-7-22 17:23 "Then how do we judge whether the winding is the primary or secondary?" According to the direction of energy transfer.

“In the direction of energy transfer”

For ICs like UC2845

The transformer has a dedicated auxiliary winding to supply power to it

Then the direction of energy transmission of this winding is to supply power to the load (UC2845)

So it's the secondary side?

maychang

shaorc posted on 2019-7-22 22:12 "According to the direction of energy transmission" For ICs such as UC2845, the transformer has a special auxiliary winding to power it...

"The direction of energy transfer of this winding is to supply power to the load (UC2845), so it is on the secondary side?"

Yes. In the transformer, this winding receives energy and then supplies power to UC2845, so it is the secondary side.