temp compansation * thermistor*

hai every one

i am using this ckt for o/p over current protection in smps.

this ckt is working effectively at normal operating condition but when temp rises

(@ 50°c) it is gonig gracy.example ,,,..... if nominal load current is 3A ,,, i set over

current protection at 3.2A it is working fine at normal temp if temp rises around

50°c ambient , q8 (bc547)(β 200) due to temp drift starts conducting earlier so at

normal load current it self over current protection is accuring (fold back) and o/p

voltage starts decreasing, so i am planning to do temp compansation using

thermistor,,, i want the over current protection to be same @50°c,,,, so if any one

familiar with temp compansation using thermistor,,,, give me your suggestions,,i

am running out of time ,,,,, good suggestions will be appreciated.

kamal
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One of possibilities is to replace R22 or R76 with NTC or combination of NTC with parallel and series resistor to make compensation more accurate. But another posibillity can be lowering resistance of collector resistor or adding pullup resistor. More can be said if you post all details of current limiting circuit.

q8 collector is connected to xxxxxxof the above ckt.....opto tiansistor collector is

connected to feed back pin of the top switch in the primary side ,i am using tl431

for loop control.at normal mode q8 will be off and loop will be closed through

tl431,when o/p current is out of range q8 will dominate.your suggection is correct

but give me more details and specfic answer possible explin with the ckt,,,,,,,

kamal

You can solve temperature problem by increasing initial current to transistor collector. I think 1mA current will be OK. Place appropriate resistor from Vcc2 to transistor collector.

r39 value is 100e and vcc2 is 12v,,if i increase the collector current how it will help i did not understand ,,,,,,

You are driving transistor with OPAMP and the transistor is used as comparator and switch. Small collector current when the transistor is starting to conduct is temperature dependent and it is higher at higher temperatures. Because of that current trigger level is lowered. The idea is to increase the collector current at which triggering occurs where this temperature dependance is smaller.

ok,, but 12v,,, 100e it is already enough current ,,,, any i cant reduce the collector resistor further becaust it is a loop component ,,, reducing this will make the loop un stable,,,,,,but adding 10e resistor Emitter to gnd helped me

thanks & regards

kamal