OVP,OIP and OLIP in SMPS?

[aunglay]

In SMPS design of attachment file,
I found that Overvoltage and Overcurrent are sensed from secondary of Flyback trasformer.Overload current sensing is from primary.I would like to know some questions about the circuit like this above.
1.Can current and voltage sensing circuit be connected to either side of transformer?
2.More explanation on Overcurrent and Overloadcurrent terms.
3.Can Overvoltage and Overcurrent occur the same time ?

 

[VVV]

Voltage sensing is best done in the secondary. You want to sense the actual voltage that can do any damage if it goes out of spec.

Current sensing should also be done in the secondary, but in this case, since you have a single output, you can do it in the primary. The primary will reflect the secondary current well and it's much easier to do, since you do not need isolation.
But if you have multiple outputs, then the primary current only tells you that the sum of the output currents is within limits. Yet one of those secondary currents could be outside the range, while others are at zero. Thus, the primary sensing would be good to indicate general overload, meaning that the TOTAL output power is being exceeded. But the individual output currents should be monitored separately.

Primary sensing can also detect a failed transformer of rectifier. Any of these failures will cause increased primary current, although the output currents are zero or within range.

Overvoltage and overcurrent generally cannot occur simultaneously, but I think it is conceivable that in a certain configuration, given a certain fault in the circuitry, you might encounter them at the same time. For example, on overvoltage condition is present, but the circuitry does not respond, you apply too high a voltage to the load and the load draws excessive current.

 

[aunglay]

I still confuse about following
overcurrent due to load or line ? or both?

 

[VVV]

Overcurrent refers to output (load).

 

[aunglay]

Please answer my misunderstanding.Thanks a lot everyone for solving.


1.Can overcurrent in the secondary(load) cause primary loading current?
2.Does Overload in the primary mean current due to output<Load> or other facts?
3.Why two current sensings is used in this circuit (overcurrent and overload current?

 

[VVV]

1. Yes. After all, the output power comes from the primary, so anything affecting the secondary will be reflected to the primary

2. Overload in the primary can be caused by defective components, too. For instance, a transformer with some shorted turns or a faulty rectifier.
These will cause primary current to increase, but wiill not have an effect on the secondary (unless the fault is serious enough to prevent operation altogether).

3. On the primary side, it is generally done to protect the MOSFET. See above: not all primary overloads have a root in a high secondary current.

On the secondary, it was probably done because an accurate current limit was desired. Besides, there is another winding on the transformer, which supplies power to something. Thus, the input-side protection would not have offered too much protection in case that winding was unloaded. (The primary-side sees the sum of the powers).

In addition, I think the current shunt in the secondary is also used to MEASURE the value of the current, not only to trip the protection (note the opamp after the optocoupler, feeding one of the analog inputs on the PIC).