Magnetising current in secondary of full-bridge smps?

[grizedale]

Hello,

In a full-bridge smps, does magnetising current flow in the secondary at all?......either in ccm or dcm

 

[digi001]

what do you mean by magnetizing current?

 

[grizedale]

i mean when a primary fet turns off, the magnetising current is flowing and cannot just go to zero..............but if it keeps flowing in the primary, then there will be a resultant current flow in the secondary.....................but i thought magnetising current only flows in one of the windings, ie not primary and secondary at the same time.....so does it all revert to the secondary?

 

[FvM]

The term magnetizing current seems ambiguous to me. On one hand, a voltage across an inductor of finite inductance involves a magnetizing current. It won't stop if you switch off the bridge transistors, it's possibly commutated to the diodes for the unloaded transformer. Or to the secondary.

Another, more unwanted kind of magnetizing current would be a DC current caused by an asymmetric load or primary duty cycle.

 

[mtwieg]

i mean when a primary fet turns off, the magnetising current is flowing and cannot just go to zero..............but if it keeps flowing in the primary, then there will be a resultant current flow in the secondary.....................but i thought magnetising current only flows in one of the windings, ie not primary and secondary at the same time.....so does it all revert to the secondary?

In a flyback converter, all current is magnetizing current, so it will flow in both windings.

But as FVM implies the concept of magnetizing inductance/flux/current is sort of incoherent without context. It all depends on how you draw your transformer model. In a transformer model with magnetizing inductance, you have the magnetizing inductance on either the primary or secondary of your ideal transformer, not both. Let's say you draw it on the primary side. If you drive the primary, you'll observe that you see some magnetizing current along with whatever current is transformed to the secondary. You could say that the magnetizing current is all on the primary side and isn't seen on the secondary. Now if you model the transformer as having the magnetizing inductance on the secondary side and drive the primary, you will observe the exact same current flowing into the primary. You can say that all the magnetizing current is on the secondary side, along with the normal secondary load current. However, that magnetizing current can't actually be measured because the magnetizing inductance isn't a discrete component; it's built into the transformer.

The two models give identical results, and they're mathematically equivalent. From a modeling perspective you can say the magnetizing current flows on any winding (so long as you claim that your magnetizing inductance has no resistance, I think).

 

[Orson Cart]

The energy stored in the transformer as magnetising current will only flow in the secondary if there is a path for it that offers a lower counter emf than the equivalent for the primary side (i.e.taking the turns ratio into account).

So in a flyback it is desire-able that all stored transformer energy goes to the sec when the primary is turned off, in buck derived converters (forward etc...) the energy in the transfomer will flow in one or possibly both sides depending on the exact conditions at turn off - usually because there are other currents flowing as well (i.e. pri side load current) most of the transformer stored energy (usually quite low for a buck derived) is returned to the pri side.