UCC21220: Output protection

Hi John,

thank you very much for the prompt reply.

I'm working on a circuit that should behave as pass transistor and current limiter at pretty high voltage ( about 150 V dc ).

In normal operation we expect about 200 mA on our circuit and we want to limit / turn off the switch if current rise over let's say 500 mA.

We put togherter a couple of power mosfet connected back to back and we  think to drive them with UCC21220 because we use it in another application and it has two galvanicaly isolated outputs.

Unfortunatelly the way we think to limit current could put a temporary ( 1 -2 us ) low impedance between gate drive outputs in case of fault.

Of course low impednace is not a short but I think that without protection the results will be the same.

Thank in advance

Stefano

Hi Stefano,

Can you share the schematic of the current limit circuit and how it implements low impedance between gate drive outputs and VSS?

We can brainstorm how to manage this circuit to prevent damage to the UCC21220.

Thanks,

John

Of course,

is fairly simple idea; basically we plan to use a standard mosfet current limiter circuit but instead modulate the mosfet we turn on an optocoupler that disable the gate drive.

Here a schematic were E2 could be UCC21220. as you can see if the drop across R1 goes over the Q1 Vbe a current start to flow inside the optocoupler and we can get a signal that switch off the gate drive in less than a us but the current is limited only by the gate resistor. If we want high commutation speed I think i'll we reach value uncompatible with the UCC21220 output.

What do you think ? Is there a most suitable solution to get an high voltage bidirectional switch with us switching time and current limit ?

Thank in advance

Stefano

Hi Stefano,

Thanks for sharing your schematic. I assume you mean E1 could be UCC21220, instead of E2, please correct me if I’m wrong. Is the 1 Ohm the final value? What is the desired current limit fault level? Could you share a bit more of your schematic?

Does the fault signal/current limit need to latch on? Whenever E1 pulls low, it will also stop the flow of current through the opto-coupler, turning the Fault signal low.

What is the value of Vin and Vout? Do you need basic isolation, or just functional isolation?

You might also be interested in our single channel isolated drivers with protections, like the ISO5451. This driver integrates a DESAT protection circuit with integrated fault monitoring output, which could replace the optocouplers and BJTs in your circuit. The DESAT threshold circuit can be level shifted using Zener diodes, and additional resistance in series with the FET drain-source path like R1 and R2.

Thanks and best regards,

John