Power supply for TFS7702H.

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Hello!
I am developing a power supply using the TFS7702H.
I do not want to install an additional transformer to power the controller.
Can I make a bias winding on the main transformer?
The input voltage range is 150-240VAC.
Thanks for the answer.

It's not quite as simple as that. You will need some sort of high voltage start-up circuit to provide voltage to the TFS bypass pin in order for it to boot up.

Please understand also that a bias winding derived from the main forward transformer will be essentially unregulated, and will track your input voltage, unlike a winding on a standby transformer, which would be regulated. You will need some extra parts to make sure you don't shut down the TFS at line extremes via excessive current into the bypass pin.

A primary-only bias supply using the standby controller can be very simple and small, with a tiny transformer (an EE10 or thereabouts) and minimal circuitry (no TL431!) to connect to the TFS enable pin for bias regulation.. The costs involved may be a wash or even more for the no-standby solution, as there would be a fair number of parts to get the job done..

Thanks for the answer!
Can I use a non-isolated power source?
https://ac-dc.power.com/sites/default/files/PDFFiles/rdr506.pdf
Inductance is cheaper than a transformer :-)

You can use a separate buck converter for bias, but then you will need to bring in an extra controller. (more money). You already have a controller in the TFS. Also, you will need to specify an inductor that is rated to handle the 300+ volts that will appear across it during normal operation, a spec that missing for almost all common inductors. The inductor purchasing spec will need to be tightly controlled to avoid an inappropriate but less expensive part being substituted to save money once the supply is released to production. An inexpensive "barrel" type inductor, even if it is rated for the operating voltage, will be a source of potential EMI due to its lack of shielding.
A bias supply using the TFS internal standby controller will need 1) a tiny (EE10 or EF12.6) transformer. Since the transformer does not bridge the primary - secondary barrier, the construction can be very simple. 2) A diode and resistor from the standby drain to the snubber that is already in place for the main converter 3) a rectifier and filter cap for the bias winding, and 4) two NPN SOT-23 transistors, a zener and a small resistor - that's it. The two transistors serve to provide a pull-down to the TFS enable pin to control the bias voltage similar to the optocoupler used in a standard application. You could also use a small diode and a SOT-23 transistor if that is cheaper. An optocoupler could also be used, but that would likely be more expensive.

In conclusion, you have 3 possible approaches for bias 1) a design using a separate HV startup circuit and bias regulator 2) a small primary-side flyback bias supply using a transformer and some circuitry to provide a pull-down for the TFS enable pin, and 3) a buck converter using a separate controller.

You will need to do the numbers to see which approach is most suitable from a cost standpoint

I will provide a rough schematic for a design using a HV startup circuit in a later post., so you can use it as a comparison guide.

Attached are a couple of schematics showing a fairly generic TFS primary side circuit with HV startup circuit and with primary side non-isolated flyback using the TFS standby controller.

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TFS_BIAS_OPTIONS.pdf41.66 KB