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Converter failure in short circuit, LLC Resonant Converter @ UCC25600

Anurag Sharma
Prodigy 10 points
Other Parts Discussed in Thread: UCC25600
We are working on 100W SMPS for LED Load based on LLC Resonant Converter topology and using UCC 25600D for LLC section and UCC 28810D for Boost section. We are facing problem in short circuit condition. Below mentioned are our design values referring to sluc 146,
VDC range=390-430 V
Fsw_nom= 130KHz
Fsw range= 85-350KHz
Plimit= 120W
Pout= 100W
Eff= 0.96
Vo= 24V
Ln= 5 & Qe=0.5
 
N=10.12
Lr(Leakage) = 100uH(used)
Cr= 9.4nF(used)
 
OC Pin parameters:
Rs= R51(in schematic) = 820Kohm
Cs= C45= 132pf (220 in parallel with 330pf)
Rp= R22= 12Kohm
Cp= C25= 10nF
 
For Vin=225, Iin=0.524 and Pin= 115.4 output voltage is 24.7V @103.3W.
Problems at Short circuit condition:

1.       Driver is consuming power during short circuit, i.e 28.2W. The driver doesn’t go in total shutdown mode.

2.       Voltage at OC pin is not above 2 V to start the latch off condition.

3.       Main LLC transformer is burning after 15 minutes of Short circuit test.

4.       Switching is observed in LLC Section.

 

Please help us to resolve this issue.  Let me know if you require any more information regarding this.

  • 0
    TI__Mastermind 25570 points
    Hello Anurag - I'm not sure if this is the same issue that I replied to a few days ago so I have to be a little careful here. I'm not sure if you saw the earlier post - Anyhow - here is a (very slightly) edited reply.

    Can you send me the full schematic - one that shows the LLC resonant tank components and the secondary rectification - are you using diodes or SRs and if SRs what SR controller. As a first step, if you are using SRs then I would disable them to see if they are part of the problem.

    I can only guess at this stage but there are a couple of things that could be going wrong here and which are worth checking.

    As the LLC load increases the frequency decreases, if the frequency decreases below the peak gain point then the control law reverses and the system will run down to minimum frequency and will stay there. In this condition the system loses ZVS, the dv/dt rates increase and the reverse currents in the MOSFETs significantly increase device dissipation - leading to destruction. You should monitor the switching frequency as the s/c is applied and see whether the control law reversal is happening.

    Is the gate drive transformer rated for operation at whatever frequency the system gets to in this overload condition - if the transformer saturates then both MOSFETs will turn off and the energy in the LLC resonant tank could cause a significant VDS overvoltage.

    The gate driver transformer sees a significant common mode voltage between the primary and the high side secondary. If the transformer has a large input to output capacitance then this can drive large currents into the controller output causing upset - this is more likely to happen if/when the LLC stage loses ZVS when the dv/dt rates increase significantly. On the other hand, if the transformer is built for low CM capacitance, then it is likely to have a large amount of leakage inductance. This leakage inductance will cause spiking in the voltage seen a the MOSFET gates - this can be large enough to cause failure. I note the lack of a primary/secondary shield in the schematic of the gate drive transformer so I suspect it is a low CM Capacitance design and I see no components which might limit the voltage spiking at the MOSFET gates.

    Self driven SRs can 'oscillate' as the PSU turns off - this can feed power from the output back to the input - potentially damaging the MOSFETS.

    How quickly after the s/c is applied does the failure happen?

    I need to see
    The full schematic
    The design drawings for the gate drive transformer - including leakage inductance and pri/sec capacitance specifications
    Circuit waveforms - gate drive waveforms - high side AND low side - under normal operation and when the short is applied.
    MOSFET waveforms, (at the MOSFET switched node)

    Is the resonant inductor saturating ? - this would increase the currents in the resonant tank.
    and, as noted by Peter Meaney (seperately)

    Can the customer send us the waveforms of the CS pin and the UCC25600 gate drive during the short circuit? Once the voltage on the CS pin exceeds 2V the UCC25600 will stop the gate drive until the voltage drops below 0.6V.

    Can you also tell us the reference design number the design is based on as well?

    If you measure the current on the primary then during s/c the secondary currents may not get high enough so that the primary currents aren't high enough to trip the fault condition.

    Some further comments.
    For Vin=225, Iin=0.524 and Pin= 115.4 output voltage is 24.7V @103.3W.
    Problems at Short circuit condition:

    1. Driver is consuming power during short circuit, i.e 28.2W. The driver doesn’t go in total shutdown mode. - This sounds improbably high - anyhow, driver power is probably a symptom rather than a cause.

    2. Voltage at OC pin is not above 2 V to start the latch off condition. - This is the central reason for lack of shutdown.

    3. Main LLC transformer is burning after 15 minutes of Short circuit test. -

    4. Switching is observed in LLC Section. - at what frequency, and does the system recover from a short term s/c, ie if you s/s the output for 5 seconds will it then recover back into a nominal load (100%, 50% etc). If it does not, then the system may be operating below peak gain frequency and suffered control law reversal. I would probably check this first.

    Regards
    Colin