Hello,
We are doing a FAN6300 based offline flyback converter which operates open loop and repeatedly charges a capacitor (to 300V). The flyback is simply disabled when the capacitor is charged, then re-enabled so as to re-charge the capacitor after a discharge. (The capacitor is 600uF)
We are setting the flyback’s peak current by simply connecting a 10k resistor to the FAN6300’s ‘FB’ pin. The problem with this method is that the internal pull-up resistor is poorly toleranced (3k to 7k). Also, the internal rail to which the internal pull-up resistor is connected is not actually defined in the datasheet (ie, neither its voltage , nor the tolerance on this voltage, is described in the datasheet).
Can you help us to find out what value of internal pull-up resistor we are likely to end up with in any particular batch of FAN6300 chips? (ie statistical probability of receiving any particular value of pullup resistance within that tolerance range)? Also, do you know what is the value of the internal rail to which the internal pull-up resistor connects?
Not only does the poor tolerance of the internal pull up resistor mean that the flyback’s peak current is then not well defined, but it also means that the ‘FB’ pin could end up reaching the 3.9V overload level which would result in the flyback wrongly shutting down.
Another point is that the two series diodes that connect to the FB pin will also have their own Vf tolerance, as well as variation with temperature. Again, the datasheet does not give Vf values for these diodes.
FAN6300 datasheet:
https://www.fairchildsemi.com/datasheets/FA/FAN6300.pdf
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(By the way the external resistor connected to the FB pin goes down to ground)