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Circuit 1 is powered by (SMPS no 1) = 5V to 8V 700mA isolated dc to dc step up SMPS

Circuit 2 is powered by (SMPS no 2) = 5V to 50V 200mA isolated dc to dc step up SMPS

application - both power supplies used for two separate circuits, but circuit's GNDs are connected together, (50 isolated-SMPS GND and 8V isolated-SMPS GND are connected)

can current from 50V SMPS draw into 8V SMPS ?

or

can current from 50V circuit draw into 8V circuit due to voltage difference?

I need to avoid current draw from one power supply to the other while gnds of each power supply is connected together. does isolated power supplies avoid current draw from one to other?

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If there is no leakage from the higher voltage to the lower voltage, then no current flows between them. Each PSU sees only its own return current on its ground terminal. But you need to make sure your design guarantees that.

This mixing of voltages with a common ground is actually quite common. For example, a controller powered by 5V, running a higher-voltage higher-power motor. The motor drive part has an isolated voltage translation designed to block any leakage.

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  • \$\begingroup\$ The motor drive part has an isolated voltage translation to block any leakage. is isolated voltage translation block a isolated power supply? \$\endgroup\$
    – kobi89
    Sep 9, 2020 at 4:24
  • \$\begingroup\$ how to make sure there is no leakage from the higher voltage to the lower? \$\endgroup\$
    – kobi89
    Sep 9, 2020 at 4:25
  • \$\begingroup\$ If your parts (especially IC's) share the two different V+, there may be a chance to have an issue. You didn't post enough schematic detail to know.... \$\endgroup\$
    – Kyle B
    Sep 9, 2020 at 4:47
  • \$\begingroup\$ The driver would need to do voltage translation, in a way that blocks current between the two supplies. A relay for example provides that kind of isolation. \$\endgroup\$
    – hacktastical
    Sep 9, 2020 at 4:50
  • \$\begingroup\$ @hacktastical lets assume worst case isolated 50V shorting to isolated 8V and 50V_gnd shorting to 8V_gnd, will the current flow from 50V to 8V? and burn up output capacitor(15V 10uf) of 8V iso power supply? \$\endgroup\$
    – kobi89
    Sep 9, 2020 at 4:57

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