Startup Issues in Flyback converter based on UC3842

[sabu31]

Dear All,

I have designed a Flyback converter 85-245 Vac to 12V ,4A using UC 3842 controller. As per design, the startup resistor is supposed to be 100kOhms. However, the converter didnt start. But when I reduced the startup resistor to 24k 4Watt(using a paralle combination of 68k 2Watt and 39k 2 Watt), the converter is working. However, the startup resistor is getting heated after about running for 1min. What could be the reason for this startup resistor heating and how can i reduce this

 

[KlausST]

Hi,

most probably your circuit is wrong.
-> Show your schematic with all necessary informations. (not the datasheet schematic). Also please post a photo of your circuit.

Klaus

 

[Easy peasy]

there is no simple way to reduce the heating - you need to have a high voltage mosfet providing the start current via the resistors and then turn this mosfet off when the aux supply off the transformer comes up to 12V ... else you will have to re-examine your circuit to draw as little current as possible when running so you can use the highest value start up resistors ...

--- Updated Aug 13, 2020 ---

what is your min input volts?

 

[std_match]

100k should be enough.
The resistor only charges a capacitor that will power the device during startup.
The device is in standby with max 1 mA current consumption until the capacitor voltage is at least 14.5 V. This means that the current in the resistor must be at least 1 mA. When the device is activated, it will be powered by the capacitor until the startup is finished so the capacitor can be charged from the transformer. The capacitor voltage must not drop below 11.5V during the startup, because that can put the UC3842 in standby again until the capacitor is charged to at least 14.5V. When your startup fails, you should see the capacitor voltage going up and down.

Is your capacitor big enough?
Is the capacitor charged from the transformer via a diode when the power supply is running?

 

[Easy peasy]

I am assuming there is no aux supply ... off the main Tx ...

 

[zenerbjt]

If you really want a "nice" solution to this...and have the space and a bit of money..,...add in a LNK302 high voltage buck converter, and power the uc3842 from the output of this.......its probz a smaller solution size than a startup cap, startup res , and bias winding components.