circuit description for 2 switch forward converter.

[Orson Cart]

If L1 is coupled to L2 and L3, as I suspect, then depending on the control used, there could be an effect to improve the power factor. It appears not to be obvious to those commentators above that L1,2,3 are coupled.
Regards, Orson Cart.

 

[arr_baobao]

If L1 is coupled to L2 and L3, as I suspect, then depending on the control used, there could be an effect to improve the power factor. It appears not to be obvious to those commentators above that L1,2,3 are coupled.
Regards, Orson Cart.

ya. L1,2,3 is transformer winding. L1 is primary, L2 is secondary and L3 is the reset winding.
yes. the red box circuitry is used to improve power factor
bt for my concern i neeed to know what kind of circuit topology is that and how they work

thanks

 

[FvM]

If L1 is coupled to L2 and L3, as I suspect, then depending on the control used, there could be an effect to improve the power factor. It appears not to be obvious to those commentators above that L1,2,3 are coupled.

In fact I didn't notice the coupling of L3. Thanks for pointing to it.

I already mentioned that I can imagine a PFC function with a respective control. But it seems like the present circuit provides a sine shaped average input current with constant duty cycle control. It is working as a PFC as is.

On the other hand, it has a high input ripple that can't be easily removed. In so far it doesn't look like a usable PFC circuit as shown.

 

[mtwieg]

Okay I (and probably most others here) somehow didn't realize that L3 was coupled into the transformer. I've recreated the simulation myself and can confirm that it actually is a PFC circuit.

When the forward switches are on, the anode of D1 will be pulled to below the rectified AC voltage at that time, and current will ramp up in L4. This occurs even when the AC voltage is well below its peak. When the switches turn off, the energy that built up in L4 will be delivered to the DC bus capacitor. So in this way, current is drawn from the AC line throughout it's entire cycle. It's not an excellent PFC, since it doesn't use any feedback for current shaping, I'd estimate it's in the ~0.9 range. Still good enough for many low-medium power applications.

Anyways I don't know any name for this type of circuit, but I believe I've seen it before in app notes on PFC for LED ballasts (probably a national semiconductor app note). I'll definitely check around.

Anyways arr_baobao thanks for bringing this interesting circuit to our attention.

- - - Updated - - -

Also it should be noted that the behavior of the PFC will change when the converter is operated closed loop, assuming the loop bandwidth is higher than the line frequency. My hunch is that this will improve PF somewhat, since it will force duty cycle to be lower while the AC line peaks.

 

[AMSA84]

mtwieg,

Which software you used to simulate the circuit?

 

[FvM]

You can run LTSpice with the simulation circuit and parameters shown in the initial post.

 

[arr_baobao]

thanks a lot folks for the help.
ya. i think that is a PFC...it's looks similiar to boost converter.
from frm i have study ,typical pwr supply configuration is just the bridge rectifier followed by a filter capacitor(c1 in my case).
for this configuration the current only draw from the line when the line voltage exceed the voltage in the capacitor. and the raise and fall of the current are faster than the voltage thus this will induce odd harmonics content in the system.
to remove the harmonics content, we add in the pfc , which looks like a boost pfc converter. this pfc will booost the voltage to program the current to follow the voltage waveform. when the voltage and current are in phase, harmonics content eliminated power factor increased and enable maximum energy transfer to the load!

 

[FvM]

To allow PFC operation, the input current of the switch mode converter must be made proportional to the sine shaped input voltage. This can be achieved by a special control or by the "natural" behaviour of specific converter topologies. A boost converter in discontinuous mode with constant on time will e.g. work in the intended way.

In my understanding, DCM boost operation with constant on-time is also the basic principle of the present circuit. But the design details bring some unpleasant properties, thus I doubt if it's a usable PC circuit, as said. The 100 kHz current injected into the power grid is much more problematic than the harmonic current of a standard non-PFC rectifier. The circuit needs modifications to comply with EMI rules.