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in my design i need to place a buck converter 10-30V input with a fixed 12V output. my concern is that when the battery volatge level is low (for instance 10v) the device will stop bucking. so i want to make my buck converter to bypass the input voltage when it is lower than the required output(which in this case 12). any ideas on such designs? or which components to use?

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  • \$\begingroup\$ Is your shift key broken? Look for P channel buck converters by bypass functionality. \$\endgroup\$
    – winny
    Apr 3 '19 at 14:34
  • \$\begingroup\$ When the input voltage reduces, the PWM of the buck converter increases. Some IC's allow for clipping at 100% PWM: the mosfet in series with the input source will be closed contineously. \$\endgroup\$
    – Huisman
    Apr 3 '19 at 14:35
  • \$\begingroup\$ my concern is that when the battery volatge level is low (for instance 10v) the device will stop bucking. You mean "stop switching". I think it depends on the design of the buck converter. It could be designed such that the converter will simply "do its best" to make the required output voltage. When the input voltage is too low it might simply use a 100% duty cycle (stop switching) which isn't bad, that's like the pass switch you're looking for. You could just grab a Buck converter module (like LM2596) and measure what it does. \$\endgroup\$ Apr 3 '19 at 14:35
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    \$\begingroup\$ When the battery is low, I think you do want to disable the converter in order to safe the battery? \$\endgroup\$
    – Huisman
    Apr 3 '19 at 14:38
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    \$\begingroup\$ Why not use a buck-boost instead? Buck is made for step-down. If your output voltage is between input range, use buck-boost. Buck makes no sense. \$\endgroup\$ Apr 3 '19 at 14:39
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This is why the SEPIC converter was invented.

The single-ended primary-inductor converter (SEPIC) is a type of DC/DC converter that allows the electrical potential (voltage) at its output to be greater than, less than, or equal to that at its input.

SEPIC on wikipedia

TPS40210 from TI is a SEPIC converter that'll meet your needs.

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