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6th June 2014, 12:49 #1
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Variable Voltage & Current SMPS
Dear All,
Required Guidance,
Design specifications are as follow;
1)Input Voltage : 90VAC  270VAC (Universal Range)
2)Power Rating : 100W
3)Output Voltage : 1VDC to 80VDC
4)Output Current : 100mA to 2A
If we go with Flyback Topology,which control IC suits best for the application as duty cycle variation is an external function.
Variation of Voltage & Current will be an external function through Pot.
Can we use voltage divider resistor & by varying lower pot then feeding to comparator we can change the duty cycle just like we do in current control loop technique.
Thanks & waiting for knowledge enhancement,

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6th June 2014, 15:23 #2
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Re: Variable Voltage & Current SMPS
if its for 100watt then 1volt should output a 100amps current and 80v should give 1.25amps by P=V*I so the ouput current must be min 1.25 to max 100amps.....
you have to design for transformer for 100W and reduce input current with duty cycle,
such that u change the input power.
P=1/2*L*I^2*f
L is constant
f is constant
I varies with duty cycle
while changing the duty cycle the auxillary supply to ic also changes so this is not possible with an ic powered by auxillary supply. like uc3844
Not a good idea. U should have something like autotransformer for the output winding
may someone well knowledged than me can give u a better info.
good luck

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6th June 2014, 21:03 #3
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Re: Variable Voltage & Current SMPS
Use a stepdown transformer. A ratio of 4:1 is in the ballpark. (This works in simulation anyway.)
Suppose we assume the supply ranges from 90 to 270 VDC. The challenge is to discover how long you must make the duty cycles in order to get 80V at 2A.
To go from 90V to 80V you'll use a high duty cycle, maybe 85%.
By necessity the secondary will need to produce pulses upwards of 14 A.
To go from 270VAC to 80V, your duty cycle will be about 6065%. The secondary will produce pulses over 6 A.

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10th July 2014, 08:31 #4
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Re: Variable Voltage & Current SMPS
Required external Pot to control Output Current & Voltage.
The rating required for Pot are as follows,
1)Maximum Voltage at output = 80Vdc Variable Rating required using Pot from (1V to 80V)
2)Maximum Current at Output = 2A Variable Rating required using Pot upto 2A
Which Control IC to be used for SMPS to have extrenal provision for Current & Voltage Variation?
Thanks & waiting for knowledge enhancement,
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