For an application I am building I need a custom DC/DC convertor that can convert a voltage ranging from 60V-600V to a contant voltage of around 5V.

Using the TI Webench with the paramaters Vin_min = 55V, Vin_max = 650V, V_out = 5V and Iout = 0.5A gave me the following schematic: TI Webench DC/DC schematic

I tried to buy as many of the components that were provided by the TI Webench, and where I wasn't able to find the exact suggested component I bought an equivalent one. The design also required me to wind my own transformer (T1).

After building the PCB and testing it I've gotten mixed results which do not satisfy my need. I've already tried to understand the schematic, went through all the datasheets and asked around with my colleagues but no one seemed to be able to explain to me how this schematic works.

The output I get is a sawtooth with Vpeak of ~4.8V and Vmean of ~3V, which I think means the switcher turns off too fast since the output doesn't reach the desired 5V. The UCC2813-0D does behave like it should we think, since it's Vcc is around 7.5V (turn-on threshold is 7.2V) and we can see switching happening on the output. However, the switching frequency is around 66kHz instead of the 102kHz we expect (can be calculated with F = 1.5/(Rt*Ct)). I will add some oscilloscope screenshots later.

After some testing and measuring we noticed that when the CS pin (current sense comparator used for overcurrent protection) on the UCC2813-0D went above ~1.35V, the IC seemed to shut down. This is reflected in the electrical characteristics, which state that overcurrent threshold on CS has a minimum of 1.32V. We tried replacing the Rsns (current sense resistor) with a lower resistance value, but still reach the over current threshold and get the same behavior on the output.

I was hoping someone on this forum could explain to me how this schematic works, and maybe point me to a possible solution as to why this design doesn't work for me. Pointing me in another direction that could also satisfy my design requirements is also greatly appreciated (req: Vin_min = 55V, Vin_max = 650V, Vout = 5V, Iout ~= 50mA).

Thanks in advance!

Note: in my design I connected both grounds with each other, since I didn't need and isolated DC/DC. I don't know if this matters for the functionality of the schematic but I think it is worth mentioning anyway.

  • \$\begingroup\$ The values of Rfbt and Rfbb imply to me that the output should be 10 volts (assuming that VR is a programmable reference like the TL431). Does it work stably at any range of input voltage? \$\endgroup\$ – Andy aka Jul 4 '18 at 9:33

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