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Isolated Power Supply with Vout = 0-10V, sourcing and sinking up to 1A

Niels Koll
Expert 1950 points
Other Parts Discussed in Thread: TPS40055

Hello,

a customer asks for an isolated power supply with

  • Vin = 12V
  • Vout = 0V to 10V.

It shall be able to source up to 1A. In addition, if a higher voltage is connected to the power supply output, it shall ba able to sink up to 1A.

Does anybody have an idea which parts from TI could be used for this application?

Regards, Niels

  • 0
    Prodigy 5 points
    Hello Neils,
    Thank you for posting.

    How high above 12V could the applied Vout voltage be?
    Also how much energy, capacitance, will this applied voltage have, that the output converter will have to sink?
    Is there a slew rate of the applied Vout or will it be more of a cable connection?

    At the moment I am thinking:
    1. DC-DC isolated to 12V
    2. secondary DCDC with synchronous rectification that can source sink.

    Need a few more details to fine tune device selections.
  • 0 in reply to Ed Walker
    Expert 1950 points

    Hello Ed,

    you can assume that the applied Vout voltage will always stay below 12V.

    The voltage applied to vout will be connected via cables from an external component.
    I have no further information about the maximum energy, capacitance or slew rate of the voltage that can be applied to Vout.

    Regards, Niels

  • 0 in reply to Niels Koll
    Prodigy 5 points
    Hi Niels,
    I would first start with the isolated converter.
    Here is a link for a design I made in Webench.
    webench.ti.com/.../SDP.cgi

    12Vin, 12Vout @ 1Amp, isolated.

    Then use a synchronous buck from the 12Vout for stepdown to 10V and lower.

    Consider TPS40055 for the sync buck controller.
    It will both source and sink output current.
    It is a controller that needs external fets, but it will work to sink an applied Vout to the regulation set point.

    Getting a linear ramp down to 0V will be difficult.
    All IC's have a reference voltage Vref, usually around 0.7V, that you can regulate Vout down to that point.
    But getting Vout below Vref is a difficult task.

    So, Isolated converter to 12Vout, source/sink synchronous buck for the controlled output.
    This would be the concept, many ways to do it.
  • 0 in reply to Ed Walker
    Expert 1950 points

    Hi Ed,

    thank you for the Webench design.

    The subsequent buck controller with external FETs is a good idea. It helps to dissipate the heat during current sinking.

    Regarding the 0V output voltage:
    What about setting the output voltage with a DAC? When I connect a DAC (via a series resistor) to the switcher's FB pin, I can change Vout by changing the DAC's output voltage.
    If V(DAC) is less than the switchers Vref, I will increase Vout. And in case V(DAC) is gigher than the switchers Vref, it sould result in a decreased Vout.
    Sould'nt it be possible this way to force the switcher to output voltages below Vref?

    Regards, Niels

  • 0 in reply to Niels Koll
    Prodigy 5 points

    Good Day Niels,

    With Vfb below Vref a switcher will revert to full duty cycle.
    PWM is a very simple operation.
    See this app note ; DN-62 Switching Power Supply Topology Voltage Mode vs. Current Mode

    There are ways to offset Vref to produce a voltage lower than Vref.
    It requires some calculations and support circuitry as shown in this app note: Regulating V OUT Below 1.2 V Using an External Reference
    However it will not extend it to zero Vout.

    Here is another useful app note ; Understanding output voltage limitations of DC/DC buck converters
    Consider minimum duty cycle achievable in a PWM converter. It will also limit the minimum Vout.

    On the Webench design, the output current should be increased above 1Amp due to the secondary DCDC efficiency not being 100% and to account for startup current surges.
    To be safe I would try setting it at 1.5A, or whatever seems to work best.

    You might consider adding a linear element after the DCDC.
    I do not have any examples, but a linear regulator with connection to a slightly negative rail could provide a slight adjustment at this low end and allow Vout down to zero.
    But where to get this negative offset? It starts getting complicated.