LT4295 POE+ 66W Forward mode topology simulation

Hi Pradeep,

You have the active clamp FET directly connected to the drain of the primary FET. The active clamp FET (driven by the SG pin) should only connect to the transformer through the RC circuit. You should also connect D9 to L11, instead of the gate of Q7. Give that a try.

Best Regards,

Eric

Hi Eric,

Thank you so much for your suggestion.

We have implemented the changes that you have suggested and simulated it again. Still the output is not ramping up to 12V. It is just fluctuating around 3V. Attaching the waveforms and simulation file. PFA

Also requesting to check on the transformer inductance on both winding we have used is correct or not. We are not clear on the transformer winding inductance values. Kindly suggest us the primary secondary inductance values..4295_NEW_selva-Winding_002.asc

Thanks,

Pradeep. S

Hi Pradeep,

I'm not sure if you can use the LT1009 for the feedback reference in this design. I replaced it with a model for the TL431, similar to the ZR431 that we use on our demonstration board and the 12V rail came up and seems to be in regulation.

Best Regards,

Eric

Hi Eric,

Thanks for your comments..

I have replaced LT1009 with TL431 as you suggested, and found output is stabilized at 12V !!

But when I tried to draw more than 4A, output voltage became dropped to 10V.. I think this problem is due to the inefficiency of transformer. If it is so, could you please suggest your ideas on improving transformer used in simulation.?

Also, the transformer I used in this simulation is not the one recommended in Reference schematic. So, in order to get more realistic results we need to use actual transformer model for EPC3577G I think. Please suggest me how to get this transformer model.

Thanks,

Pradeep. S

Hi Eric,

Thanks for your comments..

I have replaced LT1009 with TL431 as you suggested, and found output is stabilized at 12V !!

But when I tried to draw more than 4A, output voltage became dropped to 10V.. I think this problem is due to the inefficiency of transformer. If it is so, could you please suggest your ideas on improving transformer used in simulation.?

Also, the transformer I used in this simulation is not the one recommended in Reference schematic. So, in order to get more realistic results we need to use actual transformer model for EPC3577G I think. Please suggest me how to get this transformer model.

Thanks,

Pradeep. S

Hi Eric,

In addition to above, I would like to give few points here..

As I continued simulation after changing transformer parameters such as primary and secondary inductance and DCR as shown in EPC3577G datasheet, simulation output goes to 19V and even drawing more current around 8.4A. But the same transformer part is used in evaluation board and output is said to be 12V and 5.5A.

I think I have missed something here.. Please let me know your suggestion.

Thanks

Pradeep. S

Hi Pradeep,

What did you change about the transformer? The 180uH primary winding inductance and 100uH secondary winding inductance is a good way to model the EPC3577G. I double checked your simulation and it seems like your active clamp PFET was only rated for 20V and appeared to be breaking down. I changed it to a 200V part in LTSpice's library and everything seems to operate fine even loaded up to 5.5A.

Best Regards,

Eric

Hello Eric,

Thanks for your suggestion. I found the design is working as expected after changing the 200V part.

Thanks a lot.

Regards,

Pradeep.S

Hi Eric,

In LT4295 datasheet, the external inductor and capacitor required are specified as two options in Table-3. One for PoE mode operation and another for auxiliary operation.

My design accepts both power modes. In this scenario, which inductor and capacitor value I need to take from the datasheet. Attaching the image of that.

Kindly provide your suggestion.

Thanks

Pradeep. S

Hi Pradeep,

PoE voltage at the PD ranges between 37V and 57V, depending on several factors. A wide input auxiliary such as 9V to 57V includes the PoE operating range of 37V to 57V. If you are just operating off PoE or with an auxiliary that has the same 37V to 57V voltage range, then use the values in the bottom row. If you are using an wide input auxiliary, use the top row.

Best Regards,

Eric

Hi Eric,

Thank you for your suggestion, we implemented it.

And as from datasheet, Vauxon voltage level must be lower than VHSOFF = 31V. But we are designing the aux supply to take over at only 37V which is violating the above rule.Attaching the images. PFA

If I have to adhere this rule, my board's primary transformer side power traces capability has to be increased which is not possible now. Requesting for your suggestion on this.

Regards,

Pradeep. S

Hi Pradeep,

I've answered this question in a separate thread.

Best Regards,

Eric

Hi Eric,

Good Day!!! It has been after a long...

With your guidance, we have successfully simulated LT4295 chip for obtaining 12V/5.5A output from either Auxiliary input 37-57V or PoE 37-57V inputs using DC2584A schematic.

Now, we have another design requirement which has two ranges of lower auxiliary input voltages 12V/24V & PoE 37-57V & output of 12V, 25 Watts.

We have modified the existing design with the below changes,

• Tested as two cases with the below changes. Case1: Aux IN =12V & case2: AUX IN= 24V
• Changing the existing forward mode transformer model (1.5:1 ratio) to the ones supporting lower primary input voltages (POE38PR-12E_ or 7491196212-1723807) having pri-sec winding ratio of about 1:2.115. 
• (This is done by changing the transformer model winding inductances as Primary inductance=128uH & Secondary inductance =578uH)
• Adjusted the Auxiliary sense resistor divider value to required AuxON/AuxOFF thresholds.

After the above changes, the simulation result is somewhat good, but having below observations, (attached the result images. PFA). 

Case 1 Observations:

• 12V Output is oscillating around 11.4V to 11.8V, but not an exact 12V.
• Output have ripples of +/-450mV .
• Also I measured that PG pin duty is 60% and more...I understood from the datasheet that recommended duty should be around maximum of 53%. Is it okay??

Case 2 Observations:

• And for case2: AUX IN=24V, Output comes only around 9V.. My design should accept both 12v as well as 24V aux inputs.. what is the problem here? Anything to do with output inductor???  Please suggest 

Case 3 _PoE powered:

Also, the same result is coming for PoE mode operation as shown below. During PoE mode, I have changed the auxiliary input voltage down to 1V.

Please help us to solve them & please provide the way of calculating the output inductor.

Also please provide your kind suggestion that the way we have modified this design is right or not? 

Thanks & Regards,

Pradeep. S