TNY286KG-TL poor transient response
Posted by: Liubomyr
on
We built a 5V PSU on TNY286KG-TL following the Application Note AN-82 TinySwitch-4 Family Design Guide.
It works fine, but has very slow transient response.
The recommended combination of CFB2 and RS1 places the crossover frequency very low (even much lover for higher output voltages), which results in poor transient response.
What is the reason of using that low crossover frequency?
Please provide recommendations for compensation design for best transient response.
Files
combination of CFB2 and RS1
(47.86 KB)
Hi Liubomyr,
Thank you for reaching out to Power Integrations and for your trust in our devices.
The recommended design in AN-82 is found to be the one working with most of the designs showcasing a good phase-margin and the oscillation during transients are damped well. The reason for the recommended values is due to how the TL431 acts to maintain the output voltage. The feedback resistor divider network of TL431 uses its internal op-amp to drive the TL431 output transistors, but we want to ensure that the output will not be controlled by this op-amp so we need to introduce a path to roll off the high frequency gain by the op-amp and we can do this by placing a compensation capacitor CFB2. This CFB2 will then ensure that the feedback resistor divider network will still maintain a fixed 2.5V in the feedback of TL431 and not control the transistor output. After testing multiple designs, these initial values provide the best performance for mots designs.
If the issue you are facing is only concerning the slow transient response, I suggest increasing the value of RBIAS and not CFB2 nor RS1 while ensuring that there is still at least 1mA of biasing current flowing in that path.
Kindly play with the values starting 800ohms.