DRV8235: Excessive heat generation and chip damage.

SREEHARI P

Part Number: DRV8235

Tool/software:

Hi TI,
I am trying to control a linear actuator of 24V and 55watt.

First i tried to control a 6v gear motor and 24v gear motor Its working fine.(Soft start / soft stop working properly) and it draws current: 6V gear motor = 50 mA and 24V gear motor = 200mA.

When comes to 24 V Linear actuator which draws 1.2 A, hitting some problems.

Please note the following configurations i done.

1. Enabled the soft start and soft stop.
2. ALL_DRIVER_FET_IN_HIGH_Z initially.
3. Enabled OVP, STALL, TSD
4. I2C BRIDGE CONTROL MODE.
5. PH_EN_MODE
6. STALL REPORT ON NFAULT PIN.
7.CURRENT REGULATION AT ALL TIME.
8. LATCHED DISABLE WITH INDICATION
9. VREF fixed internally at 3V
10. TINRUSH LSB = 0x26
    TINRUSH_MSB = 0x02
11. WSET_VSET = 0x8F
12. MOTOR_SOFT_START_STOP_ENABLED
13. CURRENT_REGULATION_SCHEME = MOTOR_VOLTAGE

circuit
----------------------

1. 220uF electrolytic bulk capacitor and 0.1 uF ceramic capacitor in parallel betweeen VM and ground.
2. RIPROPI = 820 ohm to limit current at 2.6A.
3. The linear actuator have limit switches at both ends, which will disconnect the connection to motor.

Working sequence
----------------------
while(1)
{
1. Set Motor Forward
   1. Configure: SET_MOSFET_IN_HIGH_Z,
   2. EN_IN1 = 1, PH_IN2 = 1, enable MOSFET driver output
   3. Delay: 5 seconds

2. Stop Motor
    1. Configure: SET_MOSFET_IN_HIGH_Z,
    2. EN_IN1 = 0
    3. Delay: 5 seconds

3. Set Motor Backward
    1. Configure: SET_MOSFET_IN_HIGH_Z
    2. EN_IN1 = 1, PH_IN2 = 0, enable MOSFET driver output
    3. Delay: 5 seconds

4. Stop Motor
    1. Configure: SET_MOSFET_IN_HIGH_Z,
    2. EN_IN1 = 0
    3. Delay: 5 seconds
}

NB: If motor(actuator) exceeds the limit, suddenly the limit switch will open.
Please find the attached circuit diagram of DRV8235+Linear actuator

Issues
----------------------
1. Excess heat at soft-start/break time.
2. When the actuator hitting the limit switch, the VM and Vref are getting short to ground internally.But its working well in between the min max position of actuator. Even though its getting internally short when we reset the chip, its working until once again hitting the limit switch. But after some time the VM is permanently short to the ground.

Queries
----------------------
1.Is there a power limitation for the chip? The datasheet specifies it can operate under 38V at 3.7A, which equals 140.6 watts. Is this an absolute rating, or is there a maximum wattage limit?
2. We are currently switching the chip to High-Z mode before changing the direction, does it cause any issues or does it necessary?
_CIRCUIT_DIAGRAM.drawio.pdf

8 months ago

0 Murugavel4637 8 months ago

TI__Mastermind 38945 points

Hello Sreehari,

SREEHARI P said:
The datasheet specifies it can operate under 38V at 3.7A

Note 3.7 A is peak current only which can be supported for very short durations. Continuous current is specified at 2 A RMS which is also subject to PCBA, ambient temperature Ta.

SREEHARI P said:
1.Is there a power limitation for the chip? The datasheet specifies it can operate under 38V at 3.7A, which equals 140.6 watts. Is this an absolute rating, or is there a maximum wattage limit?

Please refer to the thermal information section of the specifications in the datasheet. These values are for the JEDEC standard test board which is explained in the "Semiconductor and IC Package Thermal Metrics" application report, http://www.ti.com/lit/SPRA953.

You can use the RΘJA to estimate the IC junction temperature. The PCB you shared the image of may have even higher thermal resistance. For example with 1.2 A the power dissipated due to FET conduction losses would be I²Rdson = 1.2 x 1.2 x 0.72 = 1.04 W, 0.72 Ω is based on MAX Rdson at 25 ºC HS + LS from the datasheet. So for 1.04 W the junction temp increase would be RΘJA x power = 48.2 x 1.04 = 50.13 ºC, the actual temp would be Ta + 50.13, @ Ta = 25 ºC estimated junction temp would be 75.13 ºC which would be hot but withing the thermal shutdown limit for the IC. However at higher currents and/or higher Ta the temperature increase would be high enough to cause thermal shutdown. Also note this calculation only accounted for conduction losses. With PWM you need to account for switching losses which could be a significant portion.

SREEHARI P said:
2. We are currently switching the chip to High-Z mode before changing the direction, does it cause any issues or does it necessary?

So when you switch the driver to Hi-Z how much time do you wait for the motor to spin down and dissipate the back EMF. This may take 10s or 100s of ms depending on your system load. If you reverse instantly after Hi-Z there would likely be stored mechanical energy due to inertia that could over stress the driver potentially damaging the device.

Regards, Murugavel

0 Yves Gohy 8 months ago

Prodigy 60 points

Hi Sreehari,

If you have placed your component on the demonstration board as in the photo, there is no ground plane on the other side to make a heatsink.

I made the same mistake. ;)

Best,

Yves.

0 Murugavel4637 8 months ago in reply to Yves Gohy

TI__Mastermind 38945 points

No ground plane on the bottom to sink the heat would be inefficient for thermals. Thanks for the pointer.

Regards, Murugavel

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DRV8235: Excessive heat generation and chip damage.