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LM5170-Q1: How to get higher voltages without changing the topology of 2 switch solution

Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

Dear Experts,

I would like to develop a bidirectional dc-dc converter using 1 channel from the LM5170-Q1 IC.  I have noted these are limited to 60V on the LV side and 85V on the HV side. 

I have a bus rail of 48V on the LV battery side and about 200V on the HV side. 

What limits the IC to lower voltages? Is it possible to add additional components around the ic to allow high voltages? The power I need to transfer is 1.2kw.

Can use high side drivers to shift HO1 and LO1 and use an opamp to generate CSA 1and 2 signals? is there anything else I can do?

Many thanks in advance

Atish

  • Hi Atish,

    Do you not need isolation for this application?

    There are a number of external circuits that will allow for the HV rail to operate above 85V (note the absolute maximum voltage is 95V) .

    1. An external gate driver can be added drive the MOSFETs.
    -This will limit the gate drivers from seeing the high voltage.

    2. The VIN pin will need to be protected.
    - This can be done with a resistor divider, diode clamp or suppled by an external supply. Please note that this is going to effect the OVPA voltage setting

    Regarding the low voltage rail,

    1. What is the maximum voltage of the 48V battery?
    -If it is higher than the absolute maximum voltage rating of the CSAx and CSBx pins the LM5170 might not be the best solution due to the number of external components needed.

    Please let me know if you have any questions.

    Thanks,

    Garrett
  • Hi Garrett, Thank you for your valuable input.

    Isolation is not required as the battery terminals will not be accessible.

    To add a high side driver, It seems I will also need to generate a floating rail to power the high side driver IC. Is there another way? Can the LM5170 safely drive 2 gate drive transformers?

    What does CHB1 do (0.22uF)?

    How can I protect the Vin? Vin is used for sensing but its only 10R so a divider to GND would consume lots of power. Why is it so low? Also I cant generate this voltage as I need it to sense the HV for voltage control. Can I use a zener to simply loose the majority of volts?

    The battery voltage is expected to be 36-48V. The HV bus also varies 200-250V max. The highest voltage gain is 250/36 = 7 its high conversion ratio boost.

    It seems the best architecture is to use 2 of these buck boosts in series to make losses more optimized/ efficient. That's where the 2nd channel could come in or just use 2 independent LM5170. I might not need a separate voltage loop for the intermediate voltage?

    worst case

    36V -----> BOOST ---(126V)---> BOOST-----> 250V
                                   
     
     
    Best case

    48V------> BOOST----(126V)----->BOOST-------->200V

    This is a 2 or 4 switch solution using 1 or 2 inductors.  There is no other better solution I know of.

    Many thanks

    Atish


     

  • Hi Atish,

    The high side gate driver can just use a boot-strap capacitor. Most of the time these gate drivers come in a h-bridge configuration. High-side and low-side combination. A gate drive transformer can be used but the it is really just another circuit to debug if something goes wrong.

    CHB1 is the boot-strap capacitor. It provides energy to the high-side gate driver. The cap is charged through a diode when the low side switch is pulling the switch to ground.

    The VIN pin is used to monitor for OVPA and provides the voltage to supply the RAMP signals. The VIN pin can be supplied externally as well. The OVP circuit can be implemented externally. Changing the RAMP voltage will change the control loop slightly but I can help with explaining what it does. There really isn't much of a change.

    Since the LV (48V battery) is only going to go up to 48V then the internal current sense amplifer can be used no problem.

    Cascading is a possibility , however there will be more components. I think realistically to achieve 1.2kW a multi-phase design will be required.

    Thanks,

    Garrett
  • Hi Garrett,

    Have you got an example IC that could be used for the gate drivers?

    Why is Vin recommended to be only 10-20R? 

    Vin is also used by the COMP pin so if I supply the Vin externally, what voltage regulation mechanism can I use in boost mode? can you kindly explain the change in the loop?

    Im seems cascading will provide better architecture overall as I can use near 50% duty cycle and require lower voltage switch with better RDSon and gate charge.  interleaving will otherwise stress my poor synchronous high voltage switch with high duty.. 

    With cascading, how to tackle the current sense? use an opamp?

    Is the max freq limit of the PWM 100k or 500K?

    Best regard

    Atish

  • Hi Atish,

    First, please refer to www.ti.com/.../ucc27712 for high voltage driver.

    However, I would not recommend the LM5170 for such application for, because we recommend the RAMP pk to be 5V when VIN=48V. For 200V HV, this would bring RAMP to 25V, well exceeding the COMP pin rating.

    To answer your other questions,: Also, to avoid complexity with external current sense, we'd better use the internal current sense, therefore cascade approach is not recommended.

    The max freq is 500K, which is what we characterized. It doesn't mean it cannot work beyond.

    Thanks,
    Youhao