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Flyback SMPS with poorly defined peak current

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Flyback

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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
 

alec_t

Well-Known Member
Most Helpful Member
We are setting the flyback’s peak current by simply connecting a 10k resistor to the FAN6300’s ‘FB’ pin.
Where does the other end of the resistor go?
Aren't you using a current-sense resistor connected to the CS pin as per the datasheet?
 

crutschow

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Most Helpful Member
Why aren't you using the CS pin for peak current control, as suggested in the data sheet?
 

spec

Well-Known Member
Most Helpful Member
Hi FB,

In addition to Alec and Crutschow's comment about the use of the current sense input to the FAN6300, I am wondering how you are stopping the pulse width modulation when the output reservoir capacitor is charged.

For an off-line flyback PSU you may need an isolator in the feedback loop. The comparator to decide if the output voltage is at the desired level is normally done by circuitry on the isolated side. I am pretty certain that the FB pin just functions as an inhibit rather than an analog reference input.Thus the analogue characteristics of the FB pin are unimportant.

spec
 

Flyback

Well-Known Member
Where does the other end of the resistor go?
..To ground
Aren't you using a current-sense resistor connected to the CS pin as per the datasheet?
Yes we are using a CS pin resistor, but we are setting the peak voltage across that resistor by the FB pin as discussed above.
 

Flyback

Well-Known Member
I am wondering how you are stopping the pulse width modulation when the output reservoir capacitor is charged.
...by simply turning off the Vcc supply to the FAN6300
 

Flyback

Well-Known Member
For an off-line flyback PSU you may need an isolator in the feedback loop. The comparator to decide if the output voltage is at the desired level is normally done by circuitry on the isolated side. I am pretty certain that the FB pin just functions as an inhibit rather than an analog reference input.Thus the analogue characteristics of the FB pin are unimportant.
Thanks spec but you are referring to the "normal" way of operating this chip, ie with an feedback opto connected in common emitter and connected to the FB pin. That is not the way we are doing our feedback......

We sense the capacitor voltage with a divider into an ADC of a micro at the isolated secondary side..then the micro powers the FAN6300 down via an optocoupler which is set up so that the FAN6300 supply gets cut off when the capacitor has gotten up to the required voltage.
 

spec

Well-Known Member
Most Helpful Member
...by simply turning off the Vcc supply to the FAN6300
Thanks spec but you are referring to the "normal" way of operating this chip, ie with an feedback opto connected in common emitter and connected to the FB pin. That is not the way we are doing our feedback......

We sense the capacitor voltage with a divider into an ADC of a micro at the isolated secondary side..then the micro powers the FAN6300 down via an optocoupler which is set up so that the FAN6300 supply gets cut off when the capacitor has gotten up to the required voltage.
What you are doing is more of less what I suggested: the measurement of the 300V is done externally (however sophisticated) as is the decision to shut the inverter off, and hence my question about the feedback pin.

spec
 

Flyback

Well-Known Member
Thanks, it was a good question, and our way of doing this is a bit off, but its cheap...the thing is we dont know what the statistical probability is on the internal FB pin pullup resistor value.....nominal its 5k but whats the probability of getting a chip with say a 3k pullup resistor in it?, or 7k? Also, we dont know what is the voltage internal to the FAN6300 that the internal pullup resistor pulls up to? (its an unmarked rail on the block diagram on page 3 of the datasheet).
Do you know any of these points?
 

alec_t

Well-Known Member
Most Helpful Member
If you buffered the FB voltage to make it a low impedance source you wouldn't have to worry about the FB pin pull-up resistance value or voltage . But then, if you're doing it on the cheap ..... :rolleyes:.
 
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