SPRADJ1A June   2024  – August 2024 TMS320F280021 , TMS320F280023 , TMS320F280023C , TMS320F280025 , TMS320F280025C , TMS320F280033 , TMS320F280034 , TMS320F280037 , TMS320F280037C , TMS320F280039 , TMS320F280039C , TMS320F280041 , TMS320F280041C , TMS320F280045 , TMS320F280049 , TMS320F280049C , TMS320F28075 , TMS320F28076 , TMS320F28374D , TMS320F28374S , TMS320F28375D , TMS320F28375S , TMS320F28376D , TMS320F28376S , TMS320F28377D , TMS320F28377S , TMS320F28378D , TMS320F28378S , TMS320F28379D , TMS320F28379S , TMS320F28P550SJ , TMS320F28P650DH , TMS320F28P650DK , TMS320F28P650SH , TMS320F28P650SK

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2HHC LLC Control Architecture and Logic Diagram
    1. 2.1 CMPSS
    2. 2.2 EPWM
    3. 2.3 Configurable Logic Block (CLB)
  6. 3C2000 Configurations in HHC LLC
    1. 3.1 CMPSS Configurations
    2. 3.2 EPWM Configurations
    3. 3.3 CLB Configurations
  7. 4System Control Method
    1. 4.1 Soft Start
    2. 4.2 Burst Mode Control
    3. 4.3 Minimum and Maximum Frequency Clamping
  8. 5Resonant Capacitor Voltage Sensing Design
  9. 6Summary
  10. 7References
  11. 8Revision History

5 Resonant Capacitor Voltage Sensing Design

In HHC LLC control, the control object is the voltage variation on the resonant capacitor in a half switching cycle, and since normally the digital controller is placed in the secondary side, the VCR signal is required to be sensed across reinforced isolation. In the reference design PMP41081, it is implemented by a current sense transformer (CST) and an amplifier, as shown in Figure 5-1

The VCR signal is regenerated by integrating the CST current on a capacitor Cs. And, only the AC part of the resonant voltage is obtained, because the resonant current only includes the AC component. In other words, any DC offset or low frequency ripple across the resonant capacitor is ignored.

Besides, the operation amplifier is used to convert the differential voltage into single end with customized gains. The recommendation is to add another current sensing resistor, which is used for the resonant current for the system protection.

Simplified VCR Sensing Circuits Figure 5-1 Simplified VCR Sensing Circuits

Before designing the VCR sensing circuits parameters, the requirement is to understand the LLC operation condition with the maximum VCR voltage amplitude, which generally refers to the minimum switching frequency with the minimum input voltage and maximum load current. This can be done either by a simulation, or an open loop testing based on the actual power stage.

We then need to select the sensing capacitor, to make sure the CST is operated far to be saturated. Based on the CST turn ratio and volt-time parameters in the data sheet, the limitation for sensing capacitor value is calculated with the following equation.

Equation 1. C s   > C r × Δ V C R m a x N c t × 1 5 × V T p r o d u c t × 4 ∙ f s . m i n
  • Cs is the sensing capacitor value
  • Cr is the resonant capacitor value
  • ΔVCRmax is the maximum amplitude of the VCR voltage
  • Nct is the turn ratio of CST
  • VTproduct is the volt-time product of CST
  • f s . m i n is the lowest operating frequency

After selecting the sensing capacitor, the differential sensing gain for the amplifier KOPA can be decided by matching the maximum VCR sensing voltage to 2.0V in this design, which is limited by the ADC range of controller. Considering the stability of normal amplifiers, the recommendation is to design the differential sensing gain larger than 1.

Equation 2. K O P A = 2 . 0 V Δ V C R m a x × N c t ∙ C s C r

In summary, the VCR sensing parameters can be designed with below process:

  1. Get the maximum peak-peak VCR voltage by simulation or testing in the worst case;
  2. Choose proper CST, and check the turn ratio N:1 and volt-time product;
  3. Select a sensing capacitor, make sure CT far to saturate;
  4. Adjust the sensing gain ratio and map the to 2.0V, recommend to use KOPA > 1;