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LM5045 full bridge switch protection

azadeh vkl
Prodigy 170 points
Other Parts Discussed in Thread: LM5045


There are two issues that makes me to ask for help:
First, In schematics of evaluation board and application notes related to LM5045, I couldn't find any type of snubber circuit on switches to reduce spikes.
does it mean that there is no needs of snubber for these circuits? If not required, how to reduce spikes on transformer windings?

The second issue is Powermos protection. In application notes Mosfets are connected directly to the driver pins from LM5045.
I did the same, but switches burned easily. Is there any need for mosfet protection?
I tried to add protection by placing 10ohm resistor in series with gate of each one.
Also since the absolute rating of LO1/LO2 , HO1/HO2 to HS1/HS2 are near vdd, and LM5045 would be damaged by spikes
I placed zener diode(12v) between gate and source of each mosfet which helped to elliminate this kind of LM5045 damage.
but the waveform of primary side winding of transformer altered and become asymmetric.

I would be appreciate any helps about snubber and protection.

the schematic is attached if it helps.

  • 0
    TI__Mastermind 34465 points
    The voltage on the centre point of the full bridge can never exceed VIN or else the body diode of the top MOSFET will conduct and the voltage can never dip below 0.7V or the body diode of the bottom MOSFET will conduct.
    So a snubber will really have no effect. However it is still important to design the transformer with low leakage since the energy stored in a leakage inductance does nothing for circuit performance and represents energy loss.

    Your MOSFETS may be burning up because of two issues:
    (1) You are using slow diodes (UF4007) on level-shifting boost power rail. You need to use ultra fast diodes like the DFLS1100 here. The voltage generated from BST1 and HS1 and between BST2 and HS2 need to be about equal to VCC .
    This would be about 9.5V in your application (since there is no auxiliary winding on the transformer)
    (2) You are using a 10uF on the level-shifting BST1 and BST2. This seems excessive. Usually 0.1uF or 1uF is enough here.

    I think the drive voltage on the MOSFETS is not high enough and they may be not fully on.
    There should not be any spikes on the gate drive signals. The components you added should not be required but I believe they will not do anything bad to the performance.
    Regards,
    John Griffin