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UCD3138A: Halfbridge LLC Converter Firmware Output Voltage Limit

Part Number: UCD3138A


I am using UCD3138ACCEVM149 (Control Card) with UCD3138ALLCEVM150 (Daughter Card).

I changed  capasitances,transformers and  secondary side FET's on EVM150( Daughter card) in order to achieve my desired output levels.

After the hardware changes that I made  , I tested the board (EVM150) with two external PWM signals.

I reached my desired output levels so Im certain that there is nothing wrong with my hardware setup.

The problem is when Im using "Fusion Digital Power Studıo" to tune UCD3138A chip on EVM149( Control Card) , I am not able to reach any output level which is higher than 18Vdc.

EVM150(Daughter Card) comes with components which have 16Vdc ratings. So I suspect there is an extra protection which supressing the EVM.

I changed several parameters  both in Firmware ( with using CCS) and Fusion Digital Power Studıo but nothing solved my problem.

What changes should  I make in firmware to have higher voltage outputs than 18 V ?

Thank you,


  • Hello,

    You may reach the maximum output voltage limit by the transformer ratio. Did you change the transformer?



  • Hi,

    Yes I changed the transformer. As I mentioned before I tested the hardware setup (EVM150) with two external PWM signals which applied to the primary side MOSFET driver .

    After this test I saw that I am able to reach my desired output voltage and power levels.

    So there is nothing wrong or miscalculated on hardware setup.

    Thank you,


  • Hi, Efe,

    I missed you changed the transformer. I think output voltage is 18V, and more likely, OVP is triggered.  One is hardware OVP protection shown below; the other one is firmware  OVP protection. You need increase the OVP threshold in order to output 18V.

  • Hi Sean,

    Thank you for your reply.

    I think I understated myself.

    I have already changed the OVP level on firmware. 

    On my current setup , output voltage command is set to (VOUT_COMMAND) 23Vdc. As I mentioned before I cannot read a higher voltage output than 18V .

    While using my current hardware and firmware setup (diffrent switching frequencies, deadtimes, vout_command )  "over voltage protection" is not triggered

    and  LLC circuit performs normal operation with a stable 18V output . The problem that im trying to explain is that when I change VOUT_COMMAND to a

    higher value from 18Vdc , I suspect that firmware doesnt let hardware to reach output levels higher than 18V.

    For  VOUT_COMMANDs which are lower  than 18Vdc, the firmware and hardware works fine.

    I cannot find any extra protection in firmware which prevents the hardware to go higher than 18 Vdc.

    What could be the reason for this "18Vdc" limit ?

    Beside VOUT_COMMAND , switching frequency and deadband timing ; what changes should I make in order to achieve 23Vdc as output ?

    Thank you,


  • Hi, Efe,

    Each pin has max voltage limit. 

    EAPx pins are not more than 1.6V;

    ADC pins are not more than 2.5V.

    With VOUT is high, make sure the sensing voltage in a good range.