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UCC29002: UC39002 for HV application( 28V output) - Rbias1,Rbias2

Part Number: UCC29002
Other Parts Discussed in Thread: UCC39002


i am using UCc39002 for load sharing between multiple power supplies, output voltage 30VDC,

The schematic is as per below , how to setup Rbias1 and Rbias2 for 30V application,

i am using 2.8K for Rbias1 and 2K for Rbias2

  • i get only around 2.8V at VDD pin of UCC39002, i think the IC gets damaged, can anyone let know how to correctly setup this 

  • Hi HKUL,

    thanks for your interest in UCC39002. In this case, it looks like the IC already damage.

    with Rbias1=2.8k, Rbias2=2kohm, the VCC of UCC39002 is 12.5Vdc that is safe for the controller.

    can you please check whether during start up, you have a overshoot that cause the VCC for UCC39002 over 15Vdc and damage the controller. You can also add a 12V zenar diode on the VCC of UCC39002,

  • Hi thanks for reply,

    ok, i will very soon check this and connect a 12V zener for safety,

    i see when i connect the controller to 12V VDD without bias resistors, it does not fetch much current, so controller looks to be safe,

    the bias resistors Rbias1 also driving transistor, which may be taking some current,and that curretn results in drop across Rbias1 and so voltage gets reduced to very little on Rbias2,

    i just randomly picked these values of Rbias1 and Rbias just keep in mind to divide the voltage, but current has to be taken card also,

     can you let know to choose Rbias1 and Rbias2 

  • should'nt there be a third resistor on transistor base to limit base current

  • Hi Hkul,

    the supply current for UCC39002 is 10mA according to datasheet, so 2.8k/2k with 12V output can not handle 10mA, I suggest change to 700ohm/500ohm.


  • i) i tried with 1K and 470 ohm, and get around 9.05V when ic is powered , out of 4 board two boards i see sharing approx same current, but other two boards totally different results, there is very little output on those modules LS output when powered standalone without sharing, 

    ii)in the high voltaeg applicaton with bias resistors, Rbias2 is connected to SNS-, and UC39002 is sensing current below its GND, in datahseet it says upto -0.3V it tolerates, 

    my question is that why Rbias2 cannot be connected to VOUT-, so voltage sensed on CS+ and CS- pins is always above its GND terminal

  • helo sir,

    one board working with Rbias1 = 2k8, Rbias2 = 2K

    other board is working with Rbias1 = 1K ,Rbias2 = 470E,

    i tried to divide 30V by 510E and 510E divider for Rbias1 and Rbias 2, the voltage i see is around 9V, when IC is connected and 

    15V when ic is not connected, so 11mA is consumption in this case, i doubt if ucc39002 ic is ok in this case,

    the board which is working is working with higher value divider also, 

     i) C37 = 1uF 

    i) is it could be beacuse of error amplier compensation also 

    ii) do i need to measure crossover frequency as below, set set error amplifier compensation, is there a way to measure this on oscilloscope,spectrum analyzer 

  • Hi hkul.

    I suggest you add the system parameter in the calculator, then we can further discuss this.

    there are also excel version on if you cannot open the mathcad.


  • HI thanks for reply, 

    the values which i do not knw are cross over frequencies of load share loop and power supplies 

    i have excel file with me and filled with parameters that  i know , how i can share excel file with you, is there option to share here 

  • 2086.UCC29002_Calc.xlsx

    hi attached file with parameters filled 

  • Hi Hkul,

    it may be not possible for the load share controller works normally if we do not know the cross frequency of the module.

    because we need the cross over frequency to determine the loop response of load share. usually load share loop should be 1/10 of module cross over frequency.

    you need a loop analysis for the test and oscilloscope can not do it.

    the RCSA1&2 in the calculator right now do not match your schematic although I can not see clearly the schematic.

    in the calculator, only 2 units need to do the load share, but you are saying that there are 4 units, can you clarify this?

    so I have following steps need your help:

    1. test the module loop to get cross over frequency by loop analysis equipment.

    2.fill in the system parameter in the UCC39002 EXCEL calculator to check the external circuit value.

    3.change the value in the schematic.