TPS92691: flybuck topology

Tuan,

Thank you for your reply.  "flybuck" is a trademark that TI came up with the represents the isolated flyback topology.  You have several drivers and application notes that reference it.  

http://www.ti.com/lit/an/snva674b/snva674b.pdf

https://e2e.ti.com/blogs_/b/powerhouse/archive/2014/04/01/flybuck-pcb-layout-tips

As I understand it, current mode control will stop the gate signal on a cycle-by-cycle basis if the inductor reaches a peak current.  The level of the current is set by an appropriately selected current sense resistor.  But because the flybuck converter includes reflected current from the secondary, I believe that TI states that you cannot use a peak current controller.  However, there is some discrepancy concerning this.  I understand that you would lose the advantage of peak current mode control but wouldn't the PWM signal to the switch(s) still be controlled by the voltage feedback to the controller IC?

Hi David

Let me understand your application first and what you are trying to do.  Are you driving an LED's?  The TPS92691 is a current controller so it controls the current that will be driven into an LED load.  What are you driving at the output along with the load voltage and current?  What is the input voltage?

Thanks Tuan

Tuan,

I am driving a single LED with a programmable current from 250mA to 4A.  The input voltage is 24V ±5%.  The design is intended to work with several different wavelengths of LEDs that can range in a forward voltage from 1.5V to 6V at 4A.  I have the TPS92691 configured for 300kHz switching frequency.

Thank you,
Dave

I should point out that the reason for the need of an isolated output is that I need it to be able to drive an LED with the anode or the cathode connected to ground.  

Tuan,

I am not using the non-isolated output of the flybuck (i.e. 0A).  Simulations show that this is just fine.  The single isolated output is required so that I can drive either an anode grounded or cathode grounded LED (i.e. + or - output based on the LED connection).  Since the common mode voltage on the isolated side is at most +-6V (non-isolated ground referenced) , I am using the scaled output of a differential amplifier to measure and feedback the LED current to the TPS92691 CSP/CSN input.  I do this in another design that uses an LT3761 controller that is nearly identical to the TPS92691 controller.  

What I would like to do is to disable the peak current detection mode of operation on the TPS92681.  I understood that it was possible to disable peak current mode of operation, essentially turning the controller into a voltage (or in this case a current).  

Maybe the best thing for me to do at this point is to simulate it.  I see that TI has a spice model of the TPS92691.

I'll let you know what I discover.

Thanks,

David

Tuan,

The TPS92691 model that I downloaded from TI (TPS92691-Q1_TRANS.lib) does not seem to work with LTspice.  LTspice is reporting "Too few nodes:"

This is an encrypted model so I don't have any more details about the error.

Do you have a model that will work with LTspice?

Thank you,

David

David,

In general we do not provide models for LTSPICE.  We just have the TPS92691 model in PSpice.  We do have evaluation board that you can get and modify to do some testing if that works for you.

Thanks Tuan

Tuan,

All I need is the unencrypted version of the .lib file.  From a closed thread, you sent an individual that unencrypted .lib to solve his LTspice issue.  I am very experienced with spice and will not need any support to get that version working with LTspice if there are any compatible issues.

My client plans on using many 1000's of the TPS92691-Q1s and there is a tight deadline that does not afford me the time to make decisions based on the evaluation board that is not the same topology that I will be using.  My solution is not typical.

Is it possible I could receive the unencrypted model of the TPS92691-Q1?

Thank you,

David

Hi Issac,

Currently, I do not have an NDA with TI.  I would be glad to sign an NDA however.  

I am an independent consultant operating as corporation under the name of Micro Dynamics Engineering, Inc.  Officially, I am the president.

Let me know what information you need from me and I will be glad to provide it.

Regards,

David Naviaux