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LM74502H: Use in high current 24 Volt application

Dan Schrader
Prodigy 10 points
Part Number: LM74502H
Other Parts Discussed in Thread: TLV1821, , LM74502

Tool/software:

    I have prototyped a circuit using two LM74502H parts each with two Infineon IPF009N04NF2SATMA1 MOSFETs connected source to source as in the data sheet figure 10-1. There is also a TI TLV1821 comparator fed from the two power sources via a small diode OR gate that compares the primary voltage source against a reference voltage and is intended to switch the load from the primary source to the secondary source upon the primary dropping out of a predefined voltage range and then restoring it again if the voltage again comes back into range. The other LM74502H circuit for the secondary power source has no such comparator circuit. The two sources must remain isolated from one another and cannot ever be connected together. 

    My problem is that on the initial application of power to this board, if the secondary source is not already connected and supplying power to the board, the primary side cannot get into conduction mode which causes the primary voltage supply to go into limiting. This seems to be caused by the huge amount of Qg in the IPF009N04NF2SATMA1 parts which is around 210 - 315 nC per part! There are 10Ω gate resistors on each part. Even with the enhanced gate drive of the "H" parts, I believe there is simply not enough power available to turn the MOSFETs on quickly and allow the power supply to operate even into a 1 Amp load. The evaluation design I created was targeted to operate with load currents up to 125 Amps at 24VDC, and I deliberately over specified the MOSFET parts in order to help them survive the testing cycles during this evaluation process. And the 0.9mΩ RDS(ON) is ideal as there is minimal room in the physical device for heat sinking and the PCB is able to dissipate the small amount of heat these MOSFETs produce at full power. I am exploring alternative MOSFETs that have less power handling capacity, and therefore, less total Qg but I am wondering just how much is "too much" for the LM74502H's gate drive circuit. Can anyone help me with a potential maximum total gate capacitance for the LM74502?

    Is there another way to boost the amount of gate drive available during start up, like bootstrapping? Any ideas?

    I am a contract engineer, so I cannot post the schematic publicly due to my NDA agreement. I can do so via PM if needed. Please let me know if you need further data and thank you for the assistance!

  • 0
    TI__Mastermind 33165 points

    Hi Dan,

    LM74502H should be able to drive the used FETs. Unfortunately drive strength of the controller cannot be increased.

    By "not getting into conduction mode" do you mean that the FETs are violating SOA because of high load during startup?

    Are you facing any FET damages?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 10 points

    Hi Shiven,

        Thanks for the reply! The data I am seeing suggests that the MOSFETs are not getting driven out of their linear regions quickly enough to allow the connected power supplies to ramp up fully. If the connected electronic load is configured for a zero power (no load) load, the circuit powers up consistently. If the electronic load is configured to, for example, a 1 Amp load, then the primary (the side with the voltage divider/comparator circuit driving the ICs' OV pins) side of the circuit will not allow the power supply to start up and it ends up in current limiting mode, rapidly cycling on and off. 

        There have been zero MOSFET failures during all of the testing. I can make the circuit work in several ways, one is to first connect the secondary input to a source of 24VDC power, which being the non-voltage monitored side of the circuit, works perfectly fine. Then, when I connect the primary side of the circuit to +24 Volts, it will power up correctly. This behavior works up until a load of ≈8 Amps is applied at the output where the same issue as above reappears. Also, I can manually hold the primary side's OV pin high while applying the power and the supply will come up to full voltage (as the IC is being held OFF) and then release the holding voltage from the OV pin allowing the IC to turn on and the circuit will then power the load correctly. This also fails at ≈>8 Amps of loading or so. Finally, when the electronic load is set below ≈8 Amps, the circuit (once it is forced to work correctly using one of these two methods) will successfully allow either power supply to be removed and restored without dropping the power being supplied to the load as expected. If the load is set above ≈8 Amps of current and the primary supply is removed and restored, the output drops to zero and the power supply goes into current limiting again. I have used five different bench top supplies from 3 Amps to 100 Amps, both linear and switching for testing with the same results. 

        I am estimating the pair of MOSFET's Qg ​≈315nC × 2 = 630nC. Including the Miller plateau, this means the time to fully charge the gates of the two MOSFETs is ≈70μs to 57us. with the 9mA - 11mA available gate drive from the IC. With the drain of the power input MOSFET being somewhere between 12 and 20 volts at the startup (and being toggled on/off by the power supply limiting circuit), I believe this is causing the MOSFETs to become stuck in the Miller plateau region and this is why we cannot start up properly. In theory, a more powerful gate drive should turn the MOSFET on so quickly that it comes out of its linear region and the power supply does not have time to detect it as a "short". None of this however, explains why the secondary side of the circuit seems to work fine. 

        Please let me know if you need further information. I can PM the schematics to you if needed.

    Best wishes,

    Dan

  • 0 in reply to Dan Schrader
    TI__Mastermind 33165 points

    Hi Dan,

    Please PM me the schematics and startup waveforms. 

    Please probe VIN,VOUT,GATE,VCAP

    Regards,

    Shiven Dhir