ISO5852S-Q1: ISO5852S DESAT Protection variations

Gaurav,

Gaurav Sharma said:

1. Generally when someone design a gate driver using ISO5852 with high current buffer in the output then a RC circuit is connected in between Vee and OUTL pins. what is the reason of connecting this RC circuit?

The RC is used to tune and control the STO current when using booster.

Because the bottom PNP is current controller, it will gain the base current (the ISO5852S STO current) to the collector current by the PNPs BETA.

The gate of course is discharged by the IB+IC of the PNP.

Because of the PNP current gain, we might actually end up with a larger STO current than we want. Discharging too fast can lead to VCE/VGS overshoot as a result of stray inductance, which is why STO is useful in the first place.

That is where the RC circuit comes in (shown below). With this addition, the STO current source internal to the IC now pulls current from both the CSTO cap thru RSTO, as well as thru the normal gate drive pulldown path (thru PNP), effectively reducing the current.

At the Point of STO, CSTO should be FULLY charged to VCC2. The RCHG / RG(OFF) ratio roughly controls how much current comes from each.

CSTO cap store a charge of CSTO*VCC2. This way, we also have a control of the time, and can size up the CSTO cap to store more charge and thus affect STO over the full duration.

Recommend to choose approximate values and confirm via simulation since the behavior of gate cap is not perfect like an ideal cap, for one.

Gaurav Sharma said:

2. Sometime the desat protection of the gate driver like ISO5852S is connected to the device using an Opamp instead of connecting directly with the collector or drain of the device. Can you please provide me the advantages of doing so?

I have not seen OPAMP used before in conjuction with the onboard DESAT detection. If it was used, i doubt it would be used for signal conditioning of the Collector/Drain, since (1) this is HV node (2) not sure what benefit it has. It would have to be isolated, AND have its own supply on the HV isolated side and its own isolated reference in order to be connected to the collector/drain.

I could however see them using an opamp to amplify something the voltage across say a shunt resistor and in that way, provide an "Overcurrent" detection rather than DESAT by directly measuring drain/collector current. In some instances, such as for SiC, true OC detection might be preferred over DESAT which is tuned for IGBTs.

Other parts like UCC217xx family have OC detection built in as well as DESAT variants, but using OPAMP with ISO5852S, which only have DESAT detection option, could be a way of doing Overcurrent detection, or something else.

But OPAMP probably is not used in conjuction with a DESAT detection setup.

The main way of changing DESAT threshold is by changing the numbers of series resistors. More series resistors, = Desat threshold is reduced by their VFs.

You can also add series zener to increase it by the zener voltage.

If this answers all your questions please let me know by pressing the green button.

Best

Dimitri