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PMP8740: NO Load Dissipation For PSFB MOSFETs

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Replies: 6

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Part Number: PMP8740

Hi Robert,

I have designed AC-DC charger by taking PMP8740 as a reference. I have a PFC stage with UCC28180 and followed by a 1KW PSFB stage with UCC28950.

My PSFB Specification as follows

Vin=400V(From UCC28180 PFC)

Vout=20V to 60V

Iout= 17A

There is No Fan in My design due to some reason. I am getting around 96% efficiency for PSFB alone. 

I have used IPP65R190CFD as my Full bridge MOSFET and 30uH as shim inductor.I am getting ZVS action more than 20% of my output Load. Now I have some questions

1. My Full bridge  MOSFETs reads around 50 degree more than 20% output load(ZVS action) But it reads around 90 degree temperature with less than 20% Load(non ZVS region) at ambient.

Since I dont have Fan this temperature is not acceptable. Can you suggest some way to solve this issue?

2. I have a 1uF capacitor series to Shim inductor just to balance the volt second. But in your design it is not present. What is the risk in removing the capacitor?.

You have not faced any volt second balance issue without capacitor?

Waiting for your valuable comments

Regards

Aneesh

  • Hi Aneesh,

    1) All MOSFETs from primary side of the full bridge should have smaller losses at light load compared to full load, even if ZVS is lost for Iout < 20% of nominal. If you experience these temperatures, you have probably shoot through or not enough dead time in these MOSFETs. Please double check by watching the switching nodes (center point of each leg) and the gat driving as well. If this happens, you might have the dead times on primary side dependent on the load (which in my PMP8740 I don't have, because they are fixed). So, please, disable the dependency from the load and/or increase the dead time. Also check out the dead time between primary and sync-FETs.

    2) The PMP8740 works in peak current mode: this is necessary to balance the flux in he transformer and avoid saturation. It is possible to add a capacitor in series to the shim inductor, but only if you change the modulation from peak current mode to voltage mode; if you add a capacitor and still running peak current mode, there might be slight unbalance in the voltages during the two half cycles (Ton), resulting into subharmonic ripple on output. In addition to that, the capacitor balances the flux in the transformer and also the peak current mode modulations wants to do that; this might result into fighting against each other.

    Best regards,

    Roberto

  • In reply to Roberto S.:

    Hi Roberto,

    Thanks for your valuable feedback. I have some more questions.

    1.1 I have set Rab=Rcd=82K(R9 and R10 in your case) to get a maximum delay time.

     Is my MOSFET heating because of more dead time added?, Because as per Figure 29 and Figure 30 in datasheet Current sense voltage will reduces the dead time right?. So if dead time is less i should get more problem at high load.

    Please correct me if my understanding is wrong.

    What is the delay resistors Rab and Rcd you recommend?

    1.2  Can you explain how you avoided the dead time dependency on primary side load ? 

    1.3 I am not using the Synchronous rectification,i am using diodes to rectify the output.

    2. I added a 1uF capacitor series to shim inductor. but i dont know how to change the modulation from peak current mode to voltage mode. Can you explain how? 

    Regards

    Aneesh

  • In reply to Aneesh TS:

    Hi Aneesh,

    You wrote:

    "Is my MOSFET heating because of more dead time added?, Because as per Figure 29 and Figure 30 in datasheet Current sense voltage will reduces the dead time right?. So if dead time is less i should get more problem at high load"

    In my board, the CS (current sense) signal is added by means of resistor R8. As you can see, this resistor is "DNP", so is not populated. This setup eliminates the dependancy from the bridge current to modify the delays AB and CD. Do you have the R8 populated?

    If you also have R8 as DNP, then the total delay will be defined by the formula (3) in the datasheet Tab_set=5*Rab/0.26 (when CS=0), where Rab is in KOhm; that means, at Rab=82KOhm, the AB delay is 1.577 microseconds. This delay is very long and, and I do not suggest to keep it. Maybe, since the delay is so high, the switchnodes are swinging up and down and the low side FET is turned on when it reaches the highest value (involving high switching loss). Please double check how the switchnodes at the bridge look like.

    BTW, as starting point, I suggest to employ the values I used in the PMP8740.

    My board works in peak current mode. The way to change from peak to voltage mode is explained in paragraph 7.3.11 of the datasheet.

    Best regards,

    Roberto

  • In reply to Roberto S.:

    Hi Roberto,

    Thank you very much for your patience for answering my questions. I have two more points

    First point is  initially i connected  R8 as DNP and  49.9K as DELAB and DELCD resistor, and i observed some problem at high Load. Then i changed DELAB and DELCD resistor to 82K. After that i could able to Load my converter fully.

    Then only i observed the MOSFET heating problem below 20% of Load. Now i have only two options

    1. Start with a 49.9K as DELAB and DELCD again then my dead time will decrease when CS voltage increases in this configuration

    2. Connect ADEL Pin with a resistor divider to CS Pin(Mount R8 In PMP8740) My Dead time will increase when CS voltage increases in this configuration

    Both are working opposite way(One decrease the dead time and one increase the dead time) . So which option from above you recommend for me?

     Second point is Regarding the peak current mode there are only two options right?

    2.1. Either add a capacitor series to shim inductor then connect RSUM resistor(R20 in PMP8740) to VREF to operate in Voltage mode control

    2.2. Or dont add any series capacitor and connect RSUM resistor(R20 in PMP8740) to GROUND to operate in Peak current mode control

    Adding Series capacitor and connecting RSUM resistor(R20 in PMP8740) to GROUND is not recommended 

    Correct me if i am wrong

    Regards

    Aneesh

  • In reply to Aneesh TS:

    Hi Aneesh,

    What kind of light-load problems you experienced when you used 49.9K on DELAB and DELCD resistors? Were spikes in the current? This might be coming from lost ZVS and hard switching. Anyway this is normal, and is reduced by using the latest CFD or CFD2 technology from FETs. You can check out if the converter works in zero voltage switching or not, by watching at the same time the VDS and the VGS of each MOSFET: if it is driven when the VDS is zero...it´s ZVS :-).

    So, I suggest to work with lower delay times, and solve the issues with noise (by filtering, for example).

    Now "Connect ADEL Pin with a resistor divider to CS Pin(Mount R8 In PMP8740" means that the higher the load current, the lower the delay times, because the higher the primary current, the faster the switch nodes are swinging from ground to Vin, and the lower is the delay needed at the MOSFET.

    In addition: your points 2.1 and 2.2 are correct. For sure working in peak current mode by setting the controller with Rsum resistor, and adding the series capacitor to shim indutcor is not recommended.

    Regards,

    Roberto

  • In reply to Roberto S.:

    Hi Roberto,

    Thank you for the feedback

    1. I will work with small dead time with 49.9K delay resistors

    2. Will remove the Capacitor series to shim inductor and will work in Peak current mode

    Here resolved my issue....

    Will come back to you if any further assistance is required

    Thank you so much

    Regards

    Aneesh