VNHD7008AY, brushed motor at 24V, 3A: burnt despite protection

[GDald.1]

Hi !

I'm using the VNHD7008AY, 24V powered, to drive a brushed DC motor, 3A. It works fine, but sometimes the VNHD7008AY burns. I think that the chip has a self protection, but it seams that it doesn't work or I'm doing something wrong. The problem happen using a 24V 3A power supply. Why the chip burn at a so low current ?

I follow the application note, the main difference are the mosfet, I'm using three PSMN021-100YL instead of the STL120N4F6AG and STL76DN4LF7AG. 

[GDald.1]

[Peter BENSCH]

What do your schematics look like?

Do you turn it backwards when it burns out?

Regards
/Peter

[GDald.1]

What do you mean: "What do your schematics look like?"

When it burns, we stop the motor, doing a brake using the PWM.

The sequence is this:

1. Motor running using a PWM at 70%.

2. Brake the motor, also using a variable PWM

3. All off about 200ms

It doesn't burn every time.

[Peter BENSCH]

I wanted to see your schematics, but I wrote my post almost at the same time as yours. Unfortunately, your image gives you knots in the lens of your eyes, because the stark yellow and the lines make it very difficult to recognise the actual schematics.

I suspect you have the same reason as in the thread where you first added it: as soon as the motor is braked, the energy of the rotor mass has to go somewhere. And if it is not temporarily stored somewhere or correctly converted into heat, voltage peaks occur that send the driver to the eternal hunting grounds. That was the reason for wanting to see your schematics, so that you could perhaps see what happens during braking.

[GDald.1]

Thanks for your quickly reply. 

Attached a better quality schematic.

I mount a SMAJ33CA Transil, in order to absorb the overvoltage. Do you think that it's too small ?

The brake isn't so huge, we try several time it doesn't burns.

It burns when the motor is stopped when is doing a strong mechanical traction and we drive it in order to brake it. 

[Peter BENSCH]

The SMBAJ33CA has a breakdown voltage VBR of typ. 38.6V -- at 1mA.

The generated voltage of the regenerated energy depends very much on the rotating mass and the braking acceleration. The faster/stronger you brake, the higher this voltage will be. Let's assume that the motor has a reverse current of 20A when it is switched off, then (for a conservatively assumed RD of 0.884ohms of the 8/20µs pulse) this results in a voltage difference Vdiff = 0.884ohms * 20A = 17.68V, which is added to the breakdown voltage. At 20A there is therefore a voltage of approx. 38.6V+17.68V = 56.28V at the SMAJ. Even at ‘only’ 10A, this would still be 47.44V. And for a 10/1000µs pulse with 1.7ohms, the voltage difference roughly doubles.

Now we look at the data sheet of the VNHD7008AY and see that the absolute max rating of VCC = 38V (table 4).
Can VNHD7008AY survive your scenario, even at VBR at 1mA?
Please note that exceeding the absolute max ratings will (not can) lead to destruction.

The problem is simply that a TVS has a non-negligible curve steepness that is relatively low for an application like this and the current does not go to infinity from VBR. You would therefore have to find a TVS whose voltage remains below the absolute max rating of VCC = 38V at the (break) current to be assumed and which, on the other hand, does not already conduct too strongly at normal operating voltage and thus produces power losses.

Otherwise I would like to refer you to the other thread mentioned above where the same problem was discussed.

Hope that helps?

Regards
/Peter