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WEBENCH® Tools/LM3150: Power Supply Design fails in the real world

Geoffrey Engel
Prodigy 50 points
Part Number: LM3150

Tool/software: WEBENCH® Design Tools

Hi,

I am a highly experienced electronics engineer (>20 years) and I have been frustrated by my inability to get a simple LM3150 circuit (from Webench) to work.   Can someone please help ?

I used Webench to design a power supply using the LM3150, where Vin=35 to 37VDC (nominal 36VDC).   The output voltage needs to vary between 2.0 Volts and 28.0 Volts, with an output current of up to 8.0 Amps.

The recommended circuit from Webench for 28.0 VDC output @ 8.0 amps is:

The only changes from this circuit that I have implemented on a PCB is that Rfb2 is a 100K trimpot, and Rfb1 is a fixed 2K2 resistor.

Under "No Load" conditions, the circuit on my PCB can successfully generate output voltages from 2.0 to 28.0 volts.   Great.

However, with a 3.6R 300W load (very large resistor) attached to the output of this circuit, the output voltage can only achieve from 2.0 to 9.8 volts before the circuit starts making a "squeeling" noises and the output voltage folds back.

The output current (for 9.8 volts output) is only 2.7 Amps - which is nowhere near the desired/intended rating of 8.0 amps.

I have re-examined the LM3150 datasheet many, many times, and re-examined the components on (now) 2 different builds of the PCB.   Both PCBs are producing the same result, ie. inability to produce high current at high voltage.

Can anyone help me ?

Thanks.

  • 0
    TI__Mastermind 22301 points
    Hi Geoff,

    Were you able to get full load output at 2V?

    The LM3150 is a constant on-time converter, and this means that the frequency depends on VOUT. By having adjustable VOUT in this converter you will have variable frequency as well as variable inductor ripple current.

    Your RON is 2.1MΩ. Using on-time equation 1 in the datasheet and combining that with ripple current equation 9, inductor ripple current range from 9A(2V) to 2A(28V).

    What is most likely happening is that as VOUT increases through a resistive load, average current increases faster than your ripple current amplitude decreases, and the inductor peak current is hitting current limit or worse, causing saturation.

    First I would confirm that the same issue appears at similar load current for lower VIN. If that's true the solution would be to increase inductance(~2x-3x) to reduce inductor current ripple, or to decrease RON resistor to decrease on-time, which will also decrease inductor ripple current.

    I hope that helped,
    -Orlando
  • 0 in reply to Orlando Murray
    Prodigy 50 points

    Hi Orlando.

    Thank you for your quick response.

    The load that this system needs to drive is highly resistive in nature.   ie. effectively a large 3R5 resistor.   As a result the output current required at low voltages is only low, and output current required at up to 28 Volts is up to 8.0 amps.

    The testing performed on the assembled LM3150 circuit showed that at 2.0 volts, the output current is less than 1 amp - and the output voltage is quite stable.    And at 9.3 volts, the output current is around 2.6 Amps - and the output voltage is quite stable.   However, above output voltage of 9.3 volts - or above output current of around 2.6 amps - the system fails (making squeeling noises - which I assume is the switching frequency becoming unstable).

    Please note that the inductor that we are using is https://www.digikey.com/products/en?keywords=PQ2614BHA-220K-ND  which has a max current of 30A and a saturation of 8.6A - which the circuit is not getting anywhere near.

    I decided to test your theory at Vout = 2.0 volts & Iout = 8.0 amps.   This will require a resistive load of 0.25 ohm (>16W).    

    I adjusted the output voltage (trimmer resistor) to be 2.081 volts under an open-circuit load.    I then placed a 0.26 ohm (40W) load resistor across the output of the circuit.   The output voltage dropped to 0.559 volts and the output current was 2.22 Amps.    Any attempt to increase the output voltage (above causes squeeling sounds and the output voltage drops back).  There is no permanent damage to the circuit as this process is quite repeatable - providing I don't try to push the output voltage higher.

    I decreased the value of RON resistor from 2M to 1M5 to see what happens, and upon adjusting the output voltage slightly higher, the LM3150 stopped working.  Dead.

    It looks like the Webench recommended circuit for LM3150 @ 30V output & 8A will never work.   I will now look for an alternative to the LM3150.

    Regards,

    Geoff