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LMG5200: Thermal capacity of junction / losses when blocking pulses

beat Ronner
Intellectual 405 points
Part Number: LMG5200

Hello TI
I'm using the LMG5200 FET phase leg for a 5A DC/DC converter for high precision current control of magnets in a particle accelerator. My loads are (due to their nature) inductive, ranging from 40 .. 250 mH
Normally, the current is ramped down by control before switching pulses to the FETs are blocked. However, if the control has a problem, switching pulses might be blocked at maximal current (in my case 5A).

The voltage drop over the diode (3rd quadrant) is ca. 3V @ 5A, down to a little bit more than 2V @ 0A, which fits with the datasheet.
This means that I have initially 15 W losses over a diode, dropping to 0W
- within ca. 6 ms for L_Load = 40 mH,
- within 40 ms for L_Load=250 mH.

At project start, the predicted L_Load were around 10 mH, so this was no issue. Recently, the predicted L were increased by more than a decade. So the 3rd quadrant losses in the diode when blocking pulses now becomes an issue.
The thermal information part of the datasheet of LMG5200 has only static (Rth) information, but no dynamic information such as Cth.

Since the chip is rated for 10A, other users might even have higher losses when blocking pulses (I estimate 30..35 W), just probably for smaller times. So probably you already have experience in this topic from other users.

Do you have experience on how long LMG5200 can handle such high losses?
Can you provide dynamic thermal information such as Cth, or Rth as function of the pulselenght or similar?

Best regards
Beat Ronner

  • 0
    TI__Genius 16390 points

    Hello Beat,

    Thanks for contacting us. Unfortunately, we don't have dynamic thermal information at this moment. The best way to figure it out is to try it out and see if device could survive in this situation. Also, monitoring the device temperature and make sure it's under 125 degree C would be helpful.

    What's the maximum time device is in 3rd quadrant you could anticipate?

    Regards,

  • 0 in reply to Yichi Zhang.
    Intellectual 405 points

    Hello Yichi

    As written in the post, the longest time with the current magnets is 40 ms, during which the current approx. linearly drops from 5A to 0.

    You suggest measuring the device temperature. How and where exactly do you propose to measure?

    Also if 40 ms is pretty long for the semiconductor, it's still way too fast for the temperature probes I have. They react within a few seconds, and after this time, the chip would already have cooled down again a lot. Also, I of course can't access junction for measurement, I can only measure on chip top side, so I'd have to estimate the delta from there to junction.

    Regards
    Beat

  • 0 in reply to beat Ronner
    TI__Genius 16390 points

    Hello Beat,

    First you will see if the FET is functioning properly in your case. If so, by monitoring the temperature, you can have a rough idea of the impact of the high power loss during the current magnet. 

    You can place the thermocouple on top of the device. Based on the thermal characterization parameter (1.8 degree C per watt) and the estimated power loss (use the worst case 5A), you can estimate the junction temperature.

    Please let me know if there are any additional questions!

    Regards,