Other Parts Discussed in Thread: LM5180, LM25184, LM25183
Hi team,
Can you help my customer with the following request?
The board runs perfectly, as described in the data sheet. The output voltage is at a constant 5V with a load current of 10mA-1A.
But now I want to change the output voltage to 15V. I have oriented myself according to the formulas given in the data sheet and adapted the corresponding elements on the board (winding ratio of the transformer, resistance values as well as diodes and capacitors with sufficient dielectric strength). If the board is supplied with 24VDC at the input, with the appropriate re-dimensioning I get an output voltage of only 12V in unloaded condition. Additionally the board only allows a maximum load of 10mA. Then the voltage has already dropped to nearly 0V. According to my calculations I may load the board however with a maximum current of 660mA.
My calculations:
The selected transformer has a transformation ratio of 2:1 and the corresponding output voltage is to be calculated using the given equation. A maximum duty cycle of 60% at minimum input voltage is selected. It may be rounded up or down as required.
The output voltage is set to 15V.
The inductance is calculated based on the minimum switch-off time limit. That of the transformer used is 250μH and thus fulfils the requirement of equation C.2. The flyback diode D2 is dimensioned using the following equation:
Thus the dimensioning of diode D2 with a dielectric strength of 40V is sufficient and does not require any change.
The input and output capacitance should limit the ripple of the input or output voltage and are sufficiently dimmed with the given values.
By adjusting the feedback resistor, the control error of the output voltage can be reduced by minimize. Then the thermal compensation must be adjusted.
Finally, the circuit may be loaded with the corresponding maximum load current:
Thank you,
Franz
