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TPS40200EVM form 48VDC input.

Knighty
Intellectual 380 points

Other Parts Discussed in Thread: TPS40200EVM-002, TPS40200

Hi All,

We would like to get 3.3V/2.5A from 48V input. can any one suggest how to get this done base on TPS40200EVM?

NT

  • 0
    TI__Guru 56985 points

    Knightly,

     

    Step 1:  Start with TPS40200EVM-002

    Step 2: Check all components expected to VIN for >48V rating

    Input Capacitors - C1 rated for 63V, C3, C4 & C8 rated for 50V

    C3, C4 & C8 50V rating might be a little close.  Might consider 100V rated ceramic capacitors

    MOSFET: Q1 rated for -60V

    Diode: D1 rated for 60V

    D1 Average Forward Current Rating = 3A, should handle 2.5A @ 93% duty cycle.

    Check Inductor Peak to Peak Ripple Current: (48V - 3.3V ) x 3.3V / 48V x 1/200kHz x 1/33uH = 0.465A (18.6%)  - Should work, could go to a 22uH inductor if you wanted.

    Since the Output voltage and current is the same, output capacitance should be fine.

    If Inductor and output capacitor doesn't change, compensation loop should be ok since TPS40200 uses voltage feed-forward. (Without voltage feed forward, you would need to reduce the error amplifier gain by 6dB to compensate for the increased PWM modulation gain.

    Check Minimum Pulse Width: 3.3 / 48 * 1/200kHz =  343ns. - Minimum On-time (max) @ 30V = 200ns, so that should be OK.

     

    So, it looks like the TPS40200EVM-002 design with some minor changes to the input capacitors should work to test a 48V to 3.3V application.

  • 0 in reply to Peter James Miller
    Intellectual 380 points

    Quote,

    Thank for your comment, we are considering another design, 48V input  down to 5V/2.5A output, it seem minor chage forTPS40200EVM-002 too as this EVM can change to 5V output but we concern about compensation, any major concern about compensation if we still use original circuit?

    NT

  • 0 in reply to Knighty
    TI__Guru 56985 points

    If you change the inductor or the output capacitors, you should always revisit the compensation.

    If you don't change the inductor or output capacitors, the compensation will generally be pretty close.

     

    Check the Inductor ripple current shift with a simple Volt-Second check.

    48V to 3.3V applies 48V-3.3V * 3.3V/48V = 3.073V-s (normallized to 1Hz)

    48V to 5V applies 48V - 5V * 5V/48V = 4.479

    Ripple current will INCREASE by 46% - Unless the original inductor was undersized, that's a pretty big increase.  Since we calculed abotu 20% ripple current on the EVM at 48V to 3.3V, it'll be about 30% at 5V output and should be ok.

     

    Next check the voltage rating of the ouptut capacitors.  6.3V ceramic capacitors have a pretty heavy DC bias derating.  At 3.3V they typically have 70-80% of their "rated" capacitance (capacitance measured at 0Vdc with a 0.1V AC ripple, generally at 100Hz - 1kHz.  With a 5V input, these capacitors would be 50-60% of their rated capacitance.  That alone should be cause to check the loop compensation.  You can use the TPS40k Loop Stability Excel tool provided in the TPS40200 product folder under "Tools and Software" - Model the ceramic ouptut capacitors at 50% of their rated value and make sure the loop is still stable.

    Check MOSFET Power Dissipation

    At 48V to 5V, you're running close to the same duty cycle as the original 24V to 3.3V, but with 2x the input voltage you'll have 2x the switching losses.  Run a quick check on the MOSFET losses to make sure you're not over-stressing the MOSFET. 

    If you are over-stressing the MOSFET, you'll need to reduce total losses.  Since conduction loses are about the same (same current and similar duty cycle) but switching losses have double (due to 2x the input voltage) most likely reducing over-all losses can be achieved by selecting a similar MOSFET with slightly higher Rdson, and thus lower gate charge.

  • 0
    Intellectual 855 points

    What do you mean "if the inductor and output capacitor doesnt change? What if the output capactor is changed to a different manufacturer but the value remains the same? This new capacitor would have a different ESR. Does the compensation loop need to be changed? 

    "If Inductor and output capacitor doesn't change, compensation loop should be ok since TPS40200 uses voltage feed-forward. (Without voltage feed forward, you would need to reduce the error amplifier gain by 6dB to compensate for the increased PWM modulation gain."