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This post asked a similar question to mine, but the problems that I have noticed are distinctly different.

I have a PCB that uses a buck converter to supply 12V from a 24V source, and I am boosting the 12V to 36V. The boost converter works well on its own from a power supply's 12V (150mA), but only outputs 12V (4.32mA) when supplied with the output of the buck converter.

Why might the boost converter be working incorrectly when supplied from the buck converter?

The buck converter should be able to supply up to 3A, and when a 68 Ohm resistor is connected 170mA are seen through the resistor.

When a 1 Ohm resistor is put between the converters, the characteristics worsen - more current drawn, output voltage of boost converter reduced.

I'm sure that the 24V source would have been sufficient to boost to 36V but no appropriate parts were found for this function.

Are there any other investigative steps I should make to ascertain the source of the issue?

The buck converter is connected as seen in the following schematic, where Vin is 24V.

Buck Converter External Circuitry

The boost converter is connected using Cin=220uF, Cout=100uF, Ctune=10nF, Rtune=30.1kOhm, Rtrim=6.98kOhm

Boost Converter External Circuitry

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  • \$\begingroup\$ Probably inrush current into the booster is tripping the buck module OCP. \$\endgroup\$ – Andy aka Oct 28 '20 at 14:52
  • \$\begingroup\$ @Andyaka shouldn't the relatively large capacitor on the 12V line buffer this somewhat? \$\endgroup\$ – kanoo Oct 28 '20 at 14:56
  • \$\begingroup\$ No not really. As soon as the cap starts heavily discharging the buck tries to supply current and cannot provide enough etc. Cin is ten times C29 <-- hint. And Cout being 100 uF is like 360 uF referred to the 12 volt line so, in effect, you are trying to instantly charge 580 uF. \$\endgroup\$ – Andy aka Oct 28 '20 at 14:58
  • \$\begingroup\$ @Andyaka a Scope Plus 440 (20MSa/s) measures a peak current of 0.147A. I'll continue to investigate this avenue though, thank you for suggesting a direction to look. \$\endgroup\$ – kanoo Oct 28 '20 at 16:10

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