Isolated Flyback with TPS54060?

Hi Seth,

Do you have any more details on your input and output requirements, such as voltage, current or power levels?

Depending on the power level, the TPS54060 does not work too well driving a transformer. The reason being is to do a topology similar to the TPS55010 synchronous rectification is required. The device can then sink current and always operates in CCM.

Regards,
Anthony

The LM5017 might also be of interest to you. Please see the schematic in Figure 17 in the datasheet for the isolated application.

Thanks for the replies Anthony.  I didn't notice until now - the email notification was never sent to me.  The LM5017 might be an option if operation under 9V input was  possible.  One major concern is just being able to start up with a high source resistance, but in the lab under higher source resistances and low output loading, we had luck with some complete off-the-shelf DC-DC modules (I think they were Murata modules using LTC switchers).  

Here is a breakdown of our requirements:

1.  The power source is a phone line, which provides a nominal 48V up to max of 56.5V.  

2.  The ohmic resistance of the phone line is anywhere from 400 to 1740 ohms, so the voltage at the input of the switcher varies significantly with changes in load.  Soft start inrush limiting is also necessary.

3.  The product is specified to operate on a phone line source current of 17-20mA.  

4.  The switcher output voltage is 5V.  Due to the high source impedance, its necessary to specify a very low minimum switcher input voltage, say 6V, to prevent loss of regulation at high system loads.  The maximum input is desired to extend to the peak voltage of the source for startup and low source resistance conditions, at 56.5V.

5.   Power supply efficiency is also a very big concern.  The product is specified to operate from 480mW source power, and will consume at least 300-350mW.  

6.  An auxiliary power source will also be optionally supplied to the switcher as well.  When auxiliary power is supplied, the system will consume up to 1.5W additional power.

7.  PCB real estate is extremely limited.  1 to 2 in^2  power supply area is desired.

8.  Power supply cost must be low.  My non-isolated design with the TPS54060 is under $3.00 total BOM cost.  $5.00 total cost would be a high limit for an isolated design.  

Any thoughts?

PS. I recently found these notes for using a buck in isolated topologies which looked interesting:

http://www.ti.com/lit/an/snva005a/snva005a.pdf
http://www.national.com/an/AN/AN-2292.pdf 

Also forgive my ignorance, but can you clarify a little more why the TPS54060 is not suitable for operation with a transformer?

Hi Seth,

The main reason it is not recommended is the device cannot sink current without synchronous rectification so it operates in DCM. In DCM the output regulation accuracy is highly degraded and the output voltage will drift higher. I need to confirm a few more details and will try to give you some more information later this week.

Also both application notes you linked are great references for isolated designs. Please notice SNVA005 is a flyback design using a boost, the LM2587. This is why synchronous rectification is not needed. AN-2292 is a fly-buck but it uses the LM5017 which is a device with synchronous rectification and can sink current. The application section in the TPS55010 datasheet is also a good reference for fly-buck designs.

Best Regards,
Anthony 

Hi again Seth,

A bit more information. The fly-buck topology could also work but you would need to design the inductance so it operates in CCM for good regulation of the output.

Alternatively for the isolated design another option is to design as an inverting buck-boost then use a coupled inductor for the isolated output. This is somewhat similar to what is shown in http://www.ti.com/lit/an/slva369a/slva369a.pdf. However the regulation point will only be on the negative output voltage and the positive output will be the isolated output voltage. This will also have the same issues with poor regulation while operating in DCM because the non synchronous rectification cannot sink current. To over come this it can be designed so the inductor value keeps it operating in CCM for the loads where best regulation is needed.

In creating the SLVA369 application note there was some testing of this indirectly. It was evaluated in both a buck topology to create the split rail which would be similar to the fly-buck topology. It was found given the same filter selection the inverting buck-boost with a couple inductor had better performance. However with the inverting buck-boost topology you will need to be careful with a 56.5V input. The 60V absolute maximum limits the primary side regulated voltage to 3.5V so a 1:1 transformer is not an option with this input voltage.

The other option is to design a flyback for the isolated design. In order to have primary side sensing and avoid using an optocoupler you could use a bias winding for the regulation of the output.

Best Regards,
Anthony