TPS43060: Over heating

Hi Hugh,

Thank you for posting.  We will take a look into this in the next few days, and get back to you.

At the mean time, would you please show us some key waveforms like LDRV, SW, and VOUT?

Thanks,

Youhao

Hi Youhao,

Thanks for your reply.

Below are some of the waveforms. The input voltage is about 25V and the output about 35V. The output current is about 200mA. The IC is running at about about 60C - 70C (rather hot). A good chip runs at about 35C.

Regards,

Hugh

Hi Hugh,

Thanks for the info. The waveforms look normal.  You circuit seems running in deep DCM.  You may consider to increase your inductance to move the operation into CCM for higher efficiency.  

I reviewed your layout and it does not look bad, although you may add more vias for the input capacitor ground polygon to lower the ground impedance between the input and output. 

When you say "A good chip runs at about 35C", do you mean this IC overheating issue just happens to a few parts?  How many have you built and how many has such overheating issue?  

Thanks,

Youhao

Hello Youhao,

We have about 15 prototype PCBs, each with 2 TPS43060 ICs to provide 2 35V 4A supplies. Out of the 15 boards about 8 have at least 1 faulty IC. The boards seem to be fine until the supplies were loaded to about 4A with an air temp of about 40C and the IC case temperatures reached about 80C - 85C.

I found on 2 of the ICs the voltage across the 10U 16V cap had reduced from about 7.5V to only about 4V and, the voltage drop across the 2R2 ohm resistor was about 110mV, so it looks like the VCC supply was in current limit. I found on these ICs the internal resistance between SW and BOOT had reduced to about 100 ohms (normally >100k). On other ICs the voltage across the 2R2 resistor was about 25mV and the VCC supply was between 6.5V and 7.5V and appeared normal, but they ran hot (typically about 70C with an air temp of about 20C) with minimal load, where they used to run at about 30C.

I tested 1 PCB at 20C and could load the power supply to 4A with no damage, with the IC case temperature reaching a maximum of about 65C.  When I placed the PCB in our environmental chamber at 45C the IC case temperature reached about 83C - 86C. The two 35V supplies still functioned correctly, but when I cooled the PCB back down to about 20C one IC now runs at 51C, and the other at about 71C with no load, where as before they both ran at about 30C. The ICs didn't seem to have reached the thermal shutdown threshold of 165C as they kept running.

So it seems the ICs are being damage somehow when the IC case temperature exceeds 65C.

Regards,

Hugh 

We measured the voltage between SW and BOOT (e.g. across the 100N cap) and it was sometimes exceeding the maximum voltage of 8V (up to about 15V in some cases).  I've add 2 4R7 resistors (Rcharge and Rldrv shown below from the IC data sheet) in case this was casing the damage to the ICs. The improved waveform is shown below.

Since we last used this IC on another product about a year ago  (with no noticeable problems)  we changed the inductor from a Coilcraft XAL7070-562ME (5.6uH 13A) to a XAL1010-562ME (5.6uH 23A) to reduce heat from the coil but this may have increase ringing etc as its resistance was about half (7mOhm vs 14mOhm, I will try the other coil shortly to see what difference it makes.

We are also using the BSZ100N06LS3G FET for both FETs. Workbench shows it just for the high side FET. I'm not sure if this FET is considered to be a 'low Qg, low-voltage threshold device. It didn't appear to be that different to the other FETs listed. 

When minimal current is drawn from the supply (<50mA) the IC's temp is about 25C (room temp is about 20C) so hardly gets warm. But when the load on the 35V supply is increased to about 300mA the temperature of the IC increases to about 45C to 50C.  It doesn't get a lot hotter when the current reaches abot 4A. As its just the controller IC I have no idea why increasing the load current should cause the IC to run any hotter (its not being heated by the FETs or coil to any degree). I assume the heat is due to the LDO regulator which would be dropping the voltage from 25V (VIN) to 7.5V (VCC) so if wouldn't take a lot of current to get hot. It looks like I could run VIN of a separate supply (e.g. 12V) which may reduce the heat.

Hi Hugh,

Thank you for the info.  Note that the datasheet Abs Max ratings are Absolute Max, and they should not be exceeded.

The SW ringing is too high when the low side FET is on, which can cause the boot cap to be overcharged.   Adding a series resistor to the boot diode as you already shown should help. If you can add a zener diode across the Boot and SW pin can further protect the IC pins. But the key is to reduce the SW ringing.  Can you slow down your gate by adding a gate resistor to the low side FET and see any improvement? 

Thanks,

Youhao