This is a schematic of TI's reference design for a 65-W USB Type-C high density active clamp flyback with GaN (PMP21639), which can be found from this reference design page.
The page claims that the design has a maximum power rating of 65W at 20V output, but is "adjustable for 20V/15V/9V/5V" output voltage and 3A.
How does this circuit adjust the output voltage? For instance, what if I need a fixed 15V/3A output from this design?
Thanks in advance.
In normal operation, an external circuit would be connected to the CTL pin to handle negotiation. It will add a resistance to ground to increase the voltage.
An example of such a device is the TPS25740.
You can just add a resistance where R43 (DNP) is shown in order to statically change the output voltage.
The ATL431 will have 2.5V from pin 2 to pin 3 when it is in regulation, so with CTL open you get 5.0V out.
With CTL shorted to ground you get 20.06V out, because of the internal 24.9K resistor. For intermediate voltages just calculate the required shunt resistance and subtract 24.9K to get the required external resistor.
If you want 15.0V then the current through the top 150K resistor R41 is (15-2.5)/150K= 83.33uA. So the bottom resistors must combine R45||(R42+R43) to represent 30.0K.
Which means that R42 + R43 must add up to 37.5K, so you would install a 12.6K resistor for R43.
simulate this circuit – Schematic created using CircuitLab
There's a control voltage input you may use to modify the output voltage. It's the CTL label that is connected to R42.
R41 and R45 fixes the output voltage at a certain level. Then, you may increase or decrease that output voltage using the CTL input signal.
