USB port breaking problem

Dear all,
first of all I hope to have made the question in the correct section.
I have a problem whit a device that I’ve built: it contains a LPC4300 micrcontroller and its supplied by a 24Vdc.
I’ve notice a strange problem when I connect it to a USB port. The device is use as a Slave.
First I have to say that this device has no usb protection (my fault) and all the usb pins of the micro are directly connected to the usb connector.
The problem is the following: when I connect the device to my desktop PC it works correctly. If I turn off the device and I connect it to my Laptop the device also work correctly.
But if I connect it first to my PC desktop, then I disconnect the USB cable and I connect the device to the laptop, the USB port results broken (I measure a short circuit between the VBUS and GND line).
This happens just when I don’t turn off the device.

Which could be the cause of this problem?
The laptop supply should be isolated (I measure more that 1MOhm between the Earth and the Output pin).


Best Regards

Hi,

Just an uneducated guess: Maybe there's a voltage spike/surge and the lack of TVS diode array or similar blows something. I'm probably wrong, but nevertheless...

This is about ethernet but may apply for USB input protection:

**broken link removed**

Hi,

Was reading about bi-directional switches and came across this, may be of interest/relevant, maybe not.

Reverse Current Protection in Load Switches: View attachment slva730.pdf

Page 2: "Reverse current is when there is a higher voltage at the output of a system than at the input, causing
current to flow backwards through the system. There are two common sources of reverse voltage: (1)
when power is disconnected from a system and (2) when the body diode of a FET becomes forwardbiased.
One source is when system input power is disconnected, a higher voltage can be left at the
output, which temporarily produces reverse current.
(...)
Reverse current can damage internal circuitry and power supplies. Depending on the path from output to
input, reverse current spikes can also damage cables and connectors."