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LM5022 project in WEBENCH

Aleksander Vershinin
Prodigy 140 points
Other Parts Discussed in Thread: LM5022, TPS40210, LM3481

Hi.

I try to design not isolated boost converter in WEBENCH. Vin= 24V DC, Vout= 110V DC, Iout= 25 mA. Customer prefers to use LM5022.

WEBENCH cannot find suitable FET for these design parameters. Design with other IC, for instance with TPS40210, finds solution with FET FDB28N30 with Id up to 28 A. Parameters of LM5022 and TPS40210 are very similar. So why does project with LM5022 fail?

I suppose

1. There is a mistake in WEBENCH.

2. LM5022 is not suitable for driving this FET.

There are very good TI reference designs for example PMP7129 with LM3481. Can I use it for startup for design with LM5022?

Could you give an advice whether FET STD1NK60 is good for this application?

Thank You! 
  • 0
    TI__Expert 6365 points

    Hi Aleksander,

    We will take a look at the LM5022 as to why WEBENCH was not able to create the design.

    However, just looking at the datasheets, the TPS40210 has 10x of the drive current capability vs. the LM5022. And the osc. min. frequency is lower (35KHz vs 200KHz ) .  So thermally, a chosen FET that operates at a lower frequency will be much cooler.  Thermal is a critical parameter whether WEBENCH creates a design or not using the actual selected components.

  • 0 in reply to Howard Chen-WEBENCH Design Center
    Prodigy 140 points

    Hi Chen,

    Thank yor for answer.

    1. Could you give an advice for driver current for LM5022 and TPS40210. Looking in to datasheets I try to compare drive currents. Please see below.

    The table for LM5022 contains Vsathi and Vsatlo at the test conditions 50mA and 100mA respectively. It is not maximum drive current. Suggesting datasheets peak current is 1A for LM5022. Please see below.

    Driver current is 400mA for TPS40210. But it is not noted that is current average, RMS or peak. Is TPS40210 capable to drive peak current up to 10A? Could you give more accurate definition of currents for both chips?

    2) WEBENCH successfully performs simulation with TPS40210 at oscillation frequency up to 500 kHz. Temperature of FET is 78C, power dissipation of the FET is 0.5W, total efficiency = 64%. So probably the reason LM5022 is not valid for this project is that it has low FET driver capability. Unfortunately, it is not clear from datasheet and WEBENCH message.

    Kind regards, Alexander.

  • 0 in reply to Aleksander Vershinin
    TI__Expert 6365 points

    Hi Alexander,

    The main concern with all controllers is the start up where the current is limited by the LDO (See captured section on Startup).   Yes, the driver is capable of supplying 1A by using an external supply feeding the VCC pin.  BTW, LM5022 VSSOP package won't be able to dissipate the power of the LDO supplying 1A. It will fail for thermal. 

    Sorry I cannot help with the LM5022 in WEBENCH.  Sounds like the TPS40210 is a good fit, why not just use it.?

  • 0 in reply to Howard Chen-WEBENCH Design Center
    Prodigy 140 points

    Hi Howard.

    Yes, LDO power dissipation is essential factor for operating of the IC. BTW, power dissipation calculates in accordance with equations 54 and 55 and is much less than LDO voltage drop with 1A current. Driver has peak value up to 1A and middle approximately 13mA in the datasheet's example. So, power dissipation is a small part of (Vin-Vcc)*1A, isn't it?

    Kind regards,

    Alexander.

  • 0 in reply to Aleksander Vershinin
    TI__Expert 6365 points
    Hi Alexander,

    Yes, that is one aspect of the check WEBENCH won't allow for the design to be created.