I want to protect some microprocessor equipment ( 5.0v, max 700mA ) which is powered by usb cell phone type chargers from damage due to such things as polarity reversal, over-voltage, power surges etc. I don't want a voltage drop (such as from a series diode) and it should sustain, without damage, a prolonged fault condition.

Is it simply enough to have a polyfuse in series with power input and afterwards a unidirectional transient voltage supressor (with 5.0v standoff voltage) reversed across power input ?


1PMT5.0AT1G 5.0v unidirectional TVS diode

LITTELFUSE 60R050XPR Polyfuse 1A tripping current. http://uk.farnell.com/littelfuse/60r050xpr/resettable-fuse-60vdc-40a-radial/dp/2383313

Follow up question: Under what circumstances would a bi-directional TVS be used in this scenario ?

  • \$\begingroup\$ TVS can only sustain the harsh voltage for short time. Polyfuse takes considerable amount of time before jumping into action. How fast should the protection should act? \$\endgroup\$ – Umar Mar 1 '17 at 16:30
  • \$\begingroup\$ The combination of a polyfuse and a high power TVS can work. As Umar noted, the TVS must be capable of dissipating considerable power for a robust design. BUT, a TVS is not a precision voltage limiter. An incoming over-voltage may still expose VBUS to 7V or more before the polyfuse trips. If you need precision voltage protection, you may need a more precise voltage reference and a comparator based switch. \$\endgroup\$ – mkeith Mar 1 '17 at 16:38
  • \$\begingroup\$ USB ports are usually protected with Polyfuse at source and Schottky power diode for reverse voltage protection. Anything better requires an IC with dual MOSFETs, comparator and current sensing. \$\endgroup\$ – Sunnyskyguy EE75 Mar 1 '17 at 16:40
  • \$\begingroup\$ There are also USB protection IC's available that simplify the design. Personally, I have never seen a Schottky on a USB device. My experience is with handheld, battery powered devices. We used USB protection IC's. Nowadays a lot of LDO's and USB USB chargers and are only rated to 6 or 7 V absolute maximum, so it is basically impossible to protect them without some kind of active switching. I have not used reverse polarity protection on USB devices because the connector is polarized. \$\endgroup\$ – mkeith Mar 1 '17 at 16:47
  • \$\begingroup\$ Only for large OEM's maximintegrated.com/en/products/power/protection-control/… \$\endgroup\$ – Sunnyskyguy EE75 Mar 1 '17 at 16:47

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