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PMP8740: TIDA-00779 and PMP8740

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Replies: 33

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Part Number: PMP8740

Hi,

Can We use TIDA-00779 (3.5-kW PFC board) to power the DC-DC converter board of PMP8740?

The switching frequency used in TIDA-00779 is 40kHz whereas the DC-DC converter part of PMP8740 uses 100kHz.  Will it cause issues?

Awaiting response...

Regards,

Anbarasan R

  • Hi Anbarasan,

    There is no problem using the TIDA-00779 to supply the DC/DC converter of the PMP8740. Please consider that I supplied this DC/DC section with 400V and not 390V, so I suggest to increase the output voltage of TIDA-00779 accordingly.

    Regarding the switching frequency, 40KHz and 100KHz are far away to create a beat frequency in the audible range, so also from this point of view there is no issue.

    Best regards,

    Roberto

  • In reply to Roberto S.:

    Hi Roberto,

     

    Thanks a lot for your input.

    I have few more queries listed below:

    • PMP8790 is designed for battery charging applications. However, we want to use it to power a motor load via INVERTER board. The load on the motor ramps to its maximum power in 2s. Should we change the compensation loop. 
    • In UCC288950, 600W design document -The controller goes into DCM mode at 15% load. Valley switching  below 50% load. My understating is the controller will realize the DCM mode by the voltage (Vrcs) set using the resistive divider (Which in turn depends upon the load). After which it goes into DCM mode from CCM mode and it will operate in valley switching to maximize efficiency.  But the % used for DCM and valley switching are different. Please comment on this.
    • We also aim for a current sharing between PMP8790 and LM5170EVM board. The LM5170 supports the PMP8790 during peak loads. Our plan is to use the PMP8790 as a voltage source and LM5170EVM board as a current source.  Will it work? Please suggest.

    Regards,

    Anbarasan R

  • In reply to a anbarasan:

    Hi Anbarasan,

    You are referring to the PMP8790 here, but this reference design is an Active Clamp Forward converter:

    Is it a typo and you are still referring to PMP8740? If so, there is no issue to use the PMP8740 as a normal power supply, because it´s suitable for both tasks, charging batteries and supplying loads. In fact, even though the voltage loop bandwidth is comprised in the range 100 Hz .... 500 Hz, there is no problem supplying a motor that ramps up in 2 seconds.

    The PMP8740 is designed to work in parallel with other modules, or other power stages, so I think it can work also together with the LM5170 EVM.

    Now the UCC28950 EVM:

    The "valley mode" for this topology is actually the boundary between full ZVS and starting the hard-switching region. This boundary is dependent on the leakage inductance of the transformer+ the shim inductor. The energy stored in these two magnetic components are driving up and down the switch nodes of the main FETs. When this energy is not neough to drive them down to zero or to Vin, then the converter works at the boubdary, or in "valley mode".

    But still, the converter works in CCM, therefore the output inductor current never reaches zero. According to the voltage on Vrcs, you can decide where the sync-FETs should be kept off; typically this is done just above the level that the converter enters DCM. If you force this level to zero, the converter never switches the sync-FET off (unless during soft start or protection mode), and it works in CCM mode all the time.

    Best regards,

    Roberto

  • In reply to Roberto S.:

    Hi Roberto,

    Sorry for the Typo error. 

    For the 1st query on using PMP8740 for a motor load: Thanks for the input and it will really help us.

    Regarding current sharing: Let me first summarize our goal:

    • We are looking to develop a 5kW PSU unit.
    • Out of the 5kW-3.5 kW is from PMP8740 design and 1.5 kW from LM5170EVM
    • For the 3.5 kW we planned to use 2 units of PMP8740 board in parallel. Similarly, for 1.5kW-2 units of 750W LM5170EVM board in parallel.
    • Will sharing work for this scenario i.e  70-30 ratio.
    • We have procured 2 units of LM5170EVM board and we tried paralleling both. Its working fine.
    • We are also in  process of fabricating the PMP8740 board. Once it its done we will work on sharing (70-30) between PMP8740 and LM5170EVM. Will update you, once it is done. In case if we face problems, we may need your inputs.

    Valley switching: Let  me understand it fully, and will get back back to you. Please try to share any relevant literature's on Valley switching.

    Regards,

    Anbarasan R

  • In reply to a anbarasan:

    Hi Anbarasan,

    OK, I understood the architecture that you are developing.

    Regarding the explanation about zero voltage switching, like the phase shift full bridge here, please download the follwing application report, which explains in detail this topology.

    Best regards,

    Roberto

  • In reply to Roberto S.:

    Hi Roberto,

    Thanks for the document. Will  revert back

    Regards,

    Anbarasan

  • In reply to Roberto S.:

    Hi Roberto,

    Hope things are fine... We have assembled the Buck part (DC-DC) of PMP8740 and started testing.

    We made the following the following to test the DC-DC part:

    1) Shorted PWM_Vref P2.4  and PWM_Iref P2.3 both to ground

    2) The delay time setting resistors (R9, R10, R11) are kept same as in the schematic of PMP8740.

    3) Fly back transformer turns ratio is 20:3. We designed it for a output voltage of 48(Nominal). Hence, we changed R19 from 2.37k to 1.5k.

    4) We also changed R34 to 20ohms, R7 to 10k and R69 to 200k.

    5) We added a 100K in parallel to R29, and increased the Vin higher than Vin(nominal)/2 (around 200V).  The converter entered regulation (we observed the waveform across primary side of T4-width of the pulse reduced).  The output voltage was around 32V. We did not apply any load, but the primary side MOSFETs were warmer-without fan and with fan it was fine.

    6)Then, we removed the 100K in parallel to R29 and increased the Vin. Around 305V, the converter entered regulation. The output voltage was around 50V. The load was not connected. The issue is, we are observing a low hissing noise when the converter enters regulation. Is it normal? Do we need to change (increase) the delay time?  Also, Light warm is observed in the primary side MOSFETS -even with fan. Please suggest.

    7) How should we tune tabset? Please help on the procedure.

    8) We used only the rectifier section of PFC board until now. Is it fine or should use the PFC part also for supplying Vin?

    9) Should we load and observe?

    Regards,

    Anbarasan R

  • In reply to a anbarasan:

    Hi Anbarasan,

    OK, I agree on all points.

    Regarding the noise when the converter enters regulation, is that due to burst mode? If so, it can depend on the transformer, maybe not glued.

    If it is not glued, nor varnished, you can check if the noise comes from it, by pressing (please use gloves) the core; the noise should stop.

    There is no need to use the PFC stage on, if the voltage on the DC/DC stage is enough for regulation. The difference between PFC on and off, is that when it's off there is a lot of 100 Hz ripple on 400V output, and also the diodes of the PFC will be more stressed because of the high 100 Hz peak current. But this happens of course at medium to high load current.

    Regarding the MOSFET temperatures, it's pretty normal that when ZVS is lost, there is some loss due to the switching (charging and discharging of Coss), but this should not be too high (you can calculate how much loss you have, but you should take into account the primary capacitance of your transformer).

    The Tab set should be fine tuned by watching the voltage on the switch node with a scope; when one MOSFET turns off, at a certain point of load current, you reach valley switching (here the energy stored in the shim inductor is exactly equal to the energy needed to charge/discharge the total switching capacitance (2 x Coss + transformer cap). Then adjust Tab delay so that the next MOSFET is turned on at 1/4 of the resonance, or at the valley.

    Best regards,

    Roberto

  • In reply to Roberto S.:

    Hi,

    Today, we loaded the converter up to 4 A. The output voltage was regulated at 50V and the noise also stopped.

    Please help on where exactly to measure , for varying Tab.

    We measured at two points indicated in the Figure attached (vgs and vds). Is it correct?  Should we use two isolation probes for measurement? Do help us out.

    Please try to reply ASAP.

     Regards,

    Anbarasan R

  • In reply to a anbarasan:

    Hi Anbarasan,

    That is correct. Please try to fine tune the turn on instant (Vgs) just after the voltage on Vds has reached the valley.

    Regards,

    Roberto