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UCC24624: Using UCC24624 Synchronous Rectifier Controller, the output waveforms are uncleared to me. LLC Converter output Efficiency goes down to 70percent

Muhammad Arsalan Athar
Intellectual 410 points
Part Number: UCC24624

Tool/software:

Hi TI members,

I am following the circuit CKT 1 from the TI manual 1 "Configurations of Secondary Rectifier Circuit using UCC24624" for the efficiency improvement for my LLC Converter.

CKT 1:

 If the Vout can go higher and also to avoid DCM rings, some resistors(see RED Circle below) are used as mentioned in the TI manual: 2 of UCC24624, as given below,

"UCC24624 Dual-Channel Synchronous Rectifier Controller for LLC Resonant Converters"

Considering all the effects, I designed the schematic shown below, based on TI manual 1 above circuit CKT 1 using  4 Resistors R1,R2,R3 & R4 are included to avoid DCM rings. The Vout can go higher; therefore, another R5 is used from the Vout to VDD of both ICs (UCC24624). All resistors are set to 1K ohm as recommended by the TI datasheet of UCC24624 for approaching the requirement.

The schematic with all Node Points is shown below. Here, I designed the transformer with a turns ratio (2:1) along with center-tapped windings. The MOSFETs are used having low RDS (5m Ohms) and low charge recovery (Qrr = low). During operation, I see the results on the Oscilloscope with waveforms at different nodes from the UCC24624. Please see the Schematic below

Schematic:

Primary Side Node 1 & Node 2 Resonant Tank Outputs followed by Full Bridge Inverter (Square Wave Signals):

Node 1 & Node 2:

Secondary 1: Node 3 & Node 4(Inverted) Outputs having both RMS values 22Volt at 70Khz (near Resonance pk 60Khz):

Node 3

Node 4

Secondary 2: Node 5 and Node 6(Inverted) Outputs having both RMS values 22Volt at 70Khz (near Resonance pk 60Khz):

Node 5


Node 6

Now, the Drain voltage sensing (VD1,VD2,VD3,VD4) of Synch Rect Controller IC1 & IC2 Just after the Resistors R1,R2,R3,R4 (Same Outputs having both RMS values 22Volt at 70Khz (near Resonance pk 60Khz) :

Since no VG2 connection in the schematic, only the Gate voltage VG1 of the MOSFETS of Synch Rect Controller IC1 & IC2 are the Pulse signals shown below. Here, both gate voltages of IC1 and IC2 are VG1 = 3.7Vpk-pk, VG1 = 4Vpk-pk, respectively. (not completely synchronized or matched pulses).

VDD (Pin 7) of Synch Rect Controller IC1 & IC2 are at DC voltage = 4.7V each as shown below:

The Regulator REG (Pin 7) of Synch Rect Controller IC1 & IC2 stays at DC voltage = 4.5V each as shown below:

The final output we get at Vout at Node 7 (DC Volatge) Vout = 17V at the LLC Output at 70Khz (near resonance peak 60Khz).

With all these above measurements images, I would like you to guide me in identifying the correct operation of Synchronous controllers in the above designed Schematic and explain to me why I am losing EFFICIENCY after some calculations of Pin and Pout instead of improving it by using Synchronous Rectifier Controller.

Please suggest accordingly.

Thanks

Regards

Arsalan

  • +1
    TI__Intellectual 2480 points

    HI Arsalan,

    Your request is acknowledged and added to our queue.

    We will get back to you in two days.


    Regards
    Hemanth

  • +1 in reply to Hemanth Sreeperumbudur
    Intellectual 410 points

    Hi Hemanth,

    Thanks for your response.

    I look forward to your assistance and reply.

    Thanks

    Regards

    Arsalan

  • +1 in reply to Muhammad Arsalan Athar
    TI__Intellectual 940 points

    Hi Arsalan,

    could you let me know the specifications of your design, and which LLC controller you are using, and also please share your schematic for better understanding.

    Regards,

    Yamini.

  • 0 in reply to SIRIPURAM YAMINI
    Intellectual 410 points

    Hi Siripuram,

    I use an external controller like F28335 DSP for controlling switches Q1, Q2,Q3 &Q4 through gate driver signals.

    The spec values are:

    Input Voltage: 60V

    Output Voltage: 30V

    Turns ratio = 2 : 1 (Center tapped)

    Switching Freq range: 0-160Khz

    Resonant Frequency: 55 / 60Khz LLC Converter

    Output Power:  150Watt (30V output x Max Cuurent 5A).

    Muhammad Arsalan Athar said:
    The schematic with all Node Points is shown below. Here, I designed the transformer with a turns ratio (2:1) along with center-tapped windings. The MOSFETs are used having low RDS (5m Ohms) and low charge recovery (Qrr = low). During operation, I see the results on the Oscilloscope with waveforms at different nodes from the UCC24624. Please see the Schematic below

    Muhammad Arsalan Athar said:
    With all these above measurements images, I would like you to guide me in identifying the correct operation of Synchronous controllers in the above designed Schematic and explain to me why I am losing EFFICIENCY after some calculations of Pin and Pout instead of improving it by using Synchronous Rectifier Controller.

    Please suggest the operations of the SR controller using MOSFETs by looking at the oscilloscope results in my above Post.

    Thanks 

    Regards

    Arsalan

  • +1 in reply to Muhammad Arsalan Athar
    TI__Intellectual 1080 points

    Working on it. Will get back to you soon.

    Regards,

    Sougata

  • 0 in reply to Sougata Nayak
    Intellectual 410 points

    HI Sougata Nayal,

    Thanks for your reply.

    I am wondering if you have any suggestions about the SR controller with its operation by looking at the above waveforms in my previous post?

    Thanks

    Regards

    Arsalan

  • +1 in reply to Muhammad Arsalan Athar
    TI__Intellectual 1080 points

    Hi,

    Thanks for your patience!!!!

    Had you tried the same LLC operation without UCC24626 controller, using diodes? and measured the efficiency?

    Regards,

    Sougata

  • 0 in reply to Sougata Nayak
    Intellectual 410 points

    Hi Sougata,

    I have achieved the efficiency of more than 90% with the SR Controller. It was the problem of the right number of turns to choose on the secondary side.Now, the issue I see here is in the waveforms. The waveforms are similar, as shown above in the post, with several distortions. Like Pin 4, Pin 6 of VD1 & VD2 (the drain of MOSFETS). It seems it has a full-wave or half-wave sinusoidal signal at the Drain side. Please see the image under this quote:

    Muhammad Arsalan Athar said:

    Now, the Drain voltage sensing (VD1,VD2,VD3,VD4) of Synch Rect Controller IC1 & IC2 Just after the Resistors R1,R2,R3,R4 (Same Outputs having both RMS values 22Volt at 70Khz (near Resonance pk 60Khz) :

    My Question: Should I expect the same distortion after the SR Controller, like in the form of Full or Half wave shape at VD1, VD2 of both IC1 & IC2 ??? All the waveforms are having same shape as you can see in above Post at all pins of SR Controller.

    Please suggest me accordingly.

    Thanks

    Regards

    Arsalan

  • +1 in reply to Muhammad Arsalan Athar
    TI__Intellectual 1080 points

    Hi, Can you please share waveforms where measurement values are not there. Waveforms are not clearly visible due to measurement values.

    Regards,

    Sougata 

  • 0 in reply to Sougata Nayak
    Intellectual 410 points

    Hi Sougata Nayak...

    I am closing this thread because I see the issues are coming from the transformer. I am trying to resolve this.  From the synchrounous rectifier, I do not find a major drop in signal voltages before the rectifier. The signal you get from the transformer is already a distorted version of it. Let me try to resolve this. If it doesn't work, then I will post a new thread. 

    Thanks for your support

    Regards

    Arsalan