SM72445: SM72445 - Deadtime issue

Hello Sang,

Please share your schematic.

Did you measure both gate signals with reference to GND or the gate signal on the top transistor with reference to the switch node? You need to measure the gate signal of the top transistors with reference to the switch node (you can as well measure with reference to GND and as well measure the switch node with reference to GND and then use the scope to calculate the voltage on the gate with reference to the switch node).

Hi Brigitte,

Thank you very much for your reply;

Here is the schematic around the gate driver and MOSFETs:

The LIA and HIA are form the pins 26 & 27 of SM72445.

For measures: C5 & C7 are measured with reference to GND because they are the output signal of TI controller.

C1 and C2 are measured with the differential probe: LS with reference to GND, HS with reference to the middle point.

Our issue here is the dead-time of the output signals of the TI controller? Some time there is no dead-time at all, sometimes, there is the dead-time but not good enough (only on the fall time of C5, no on the rise time of C5). We changed to 110kHz, 135kHz, 215Khz, the problem is always the same.

Best regards,

Sang.

Hello Sang,

Thank you for the information. Please do me the favor and exchange the probes for the output signals of the TI controller to check if the dead time changes or not.

Hi Brigitte,

Thank you for your reply.

I will change the probes to verify the problem might come from the measurements.

However, we see directly the thermal problems on the MOSFETs through the thermal camera because of the dead-time of SM72445. MOSFETs become too hot without dead-time.

In fact, when we used a function generator with the right dead-time on the PWM, there is no thermal issue.

Best regards,

Sang.

Hello Sang,

UCC27211 is typically adding 2 ns deadtime,

Please check as well if there is a change when you remove the gate resistors for a test from the MOSFETs. MOSFETs are getting hot, when there is no deadtime, but they are getting hot as well if the turn-on is too slow.

In addition it might be worth a test adding a Schottky diode in parallel to D-S of the low-side MOSFET.

Are both MOSFETs getting hot or is one getting hotter than the other?

Hi Sang,

Brigitte is out of office.  Glad that you have resolved the issues with external fix.  Have you replaced the SM72445 IC with a fresh one?  The reason for this question is, the controller should have adequate dead time.  There are other customers using the IC, but this is the first time we have heard about the problem of no dead time. 

Anyway, we will continue to investigate this, but you can save your effort with your current solution if you wish.

Best Regards,

Youhao