• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 519 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3478: Total gate charge specifics and minimum Vin in SEPIC converter

Pietro Di Buono 88
Intellectual 360 points
Part Number: LM3478
Other Parts Discussed in Thread: LM5022

Hi!

I've designed a DC-DC Sepic Converter with 20-28 Vin and 24Vout 5A, switching frequency 500 kHz. In the prototipe developped, I've observed some troubles.

1. The choiced switching mosfet IRFR540ZPBF have very heat sinking problem over 3 A of output current
2. Also the LM3478 become hot.
3. SRF1260-6R8Y maybe it isn't the right choose.
4. When the Vin fall under 20 V, the high input current don't diminish, also if the load was disconnected.
5. When load increases suddenly, Vout goes down and it grows up too slowly.

So, I've some questions:

1. I can't find information in the datasheet, about the maximum total gate charge that LM3478 can drive. I need this parameters to choise the right mosfet.
2. Why the current doesn't diminish with disconnected load for low input voltage? How can I switch off the converter when the input voltage goes under a specific value?
3. How can I resolve the trouble 5?

Thanks for your help

  • 0
    TI__Guru* 78260 points
    The inductor for sure does not support your full load. Please change to a larger inductor able to handle the full load. Please follow the datasheet and also the SEPIC design app note (www.ti.com/.../snva168e.pdf) to select proper components.

    The gate drive peak current is 1A (see datasheet max rating). Refer to equation (10) of the SEPIC design app note for MOSFET selection.

    Trouble 4 is not very clear to us. What do you meant high input current. Can you be more specific? Do you have a scope picture to show this?

    Trouble 5 may be related to your loop speed, which may be able to improved by make a faster loop. However, you need to be more specific about the speed that you wish to achieve.
  • 0 in reply to Youhao Xi
    Intellectual 360 points

    Hi,
    thanks for the answers.

    About the inductor, do you think that WE 74485540101 or SRF1280A-8R2Y could be a right choice? From my calculations, for uncoupled inductance, the min value is 9.3 uH, ΔIL = 2.4 A and IL(peak) 7.5 A. If not, can you suggest me a suitable inductance?

    Equation 10 refers only to MOSFET power dissipation, so any choice of MOSFET is right if it can dissipate this power; but, I suppose that, increasing the Total Gate Gharge, at a specific switching frequency, it is required more power to drive the mosfet. LM3478 has to provide this power, that can't be limitless, and none information are in the datasheet or in the application note about this; unless you're telling me that, under any capacitive load and frequency conditions, LM3478 automatically limits the power supplied via the DR pin to avoid overheating.

    About trouble 4, I've further investigate it. The problem doesn't depend by the input voltage, but by the input current. Limiting the input current, under a specific value at a specific input voltage (for example 20 V, 2 A), the regulator fails to reach the proper output voltage, also without load, because, at the power on, the regulator drains all input current, the input voltage falls (at 4 V in the example) and the output voltage is instable (between 6 and 10 V). In this condition, the gate voltage is almost constant and equal to the input voltage, so all the input current flows in the inductor and in the mosfet.
    Could not this be a problem?

    I've solved trouble 5 adjusting compensation filter.

    Thanks for your help

  • 0 in reply to Pietro Di Buono 88
    Intellectual 360 points

    Hi!

    Can anyone give me support, please?

  • 0 in reply to Pietro Di Buono 88
    TI__Guru* 78260 points
    I have not been able to find the datasheet of 74485540101 from Wurth's website. If the Isat is greater than your worst case max peak current, it should be okay.

    Regarding MOSFET losses, you know the gate drive peak current, then you can calculate the MOSFET turn-on and turn-off interval during which the gate charge Qg is fully supplied or drained, then you can calculate the switching losses. You will find that above the certain level of Qg, the switching losses would become prohibitive, and this sets a constrain for your FET selection.

    About trouble 4, it seems your source has a low current limit. Do you have excessive output capacitors that take a lot of current at startup to charge up? The LM3478 has internal soft start. If this is a problem, you should consider our other devices able to program the soft start, like the LM5022.