Limit inrush current of PTC heater

Bojan Jovanovic

Other Parts Discussed in Thread: LM317, TPS2595, TPS1653, TPS2640, LM5069

Hello, guys.

We need to drive the PTC Heater element that has a rated voltage of 12V and temperature of 70C as you can see below.

We are experiencing a high inrush current (above 4A) during the initial few seconds when we start the system (e.g. turn on the 12V power supply switch).

What would be the best way to protect the system from that inrush current?

We used LM317 configured as a current limiter in series with the PTC element with the idea to limit the current to 1.2A. The system seems to work well, the initial current is not above 1.2Amps but we have a side effect of heat dissipation on LM317 (temperature on LM317 can go up to 100C).

Is there any better way? Perhaps to use some load switch with adjusted rise-time or even buck-boost regulator (e.g. TPS552882) that will give us 12V output in a configuration where the current will be limited to ~1.2Amps.

I noticed that some of your PWM motor drivers (e.g. DRV2510) have optional second-order Butterworth low-pass filters with the purpose of limiting peak current (see image below). Would that help in our case?

Thanks in advance for your time and efforts. Looking forward to reading from you.

Sincerely,

Bojan.

over 3 years ago

0 Rakesh over 3 years ago

TI__Guru** 103876 points

Hi Bojan,

Thanks for reaching out!

Please look at TPS2595 and let us know. What is the effective impedance of PTC Heater element during startup.? How does the inrush current profile looks like if you power it from DC power supply.

Best Regards, Rakesh

0 Bojan Jovanovic over 3 years ago in reply to Rakesh

Prodigy 145 points

Hello, Rakesh.

Thanks for your reply. TPS2595 seems promising. Is there any similar chip that can withstand 24V input voltage? It might happen we decide to go with a 24V PTC heater and 24V DC power supply.

Concerning PTC impedance, if we use 12V PTC, its impedance is in the range from 2 to 5 Ohms. Consequently, inrush currents can go up to 6Amps.

I did not have a chance to record inrush's current profile so far.

Those eFuses you propose will not break the circuit in the case of overcurrent detection, they will just limit the current, right?

Regards,

Bojan.

0 Rakesh over 3 years ago in reply to Bojan Jovanovic

TI__Guru** 103876 points

Hi Bojan,

eFuses does current limiting during startup for overload condition and they do till they go into thermal shutdown.

Please refer below thread for details.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1053287/faq-tps2660-how-much-time-does-the-efuse-takes-to-thermal-shutdown-during-overcurrent-fault

For 24V operation, please refer TPS2640, TPS1653 devices.

Best Regards, Rakesh

0 Bojan Jovanovic over 3 years ago in reply to Rakesh

Prodigy 145 points

Hi, Rakesh.

When eFuse goes into thermal shutdown, it breaks the circuit (turns it off), until it cools down and starts limiting current again. I think this is not what we want.

Do you have any chip similar to LM317 that can accept 12/24V input and enable us to limit the current to 3A? LM317 can go up to 1.25A.

Regards,

Bojan.

0 Bojan Jovanovic over 3 years ago in reply to Bojan Jovanovic

Prodigy 145 points

What about LM350-N:

Will this type of circuitry protect us during a few seconds when we experience ~7A inrush current?

+1 Rakesh over 3 years ago in reply to Bojan Jovanovic

TI__Guru** 103876 points

The average power dissipation during 7A inrush current is 1/2*Vin*7A

@12V Vin, the power dissipation will be 42W. Handling 42W for couple of seconds is challenging with integrated solutions.

You would need to consider external FET based solutions like LM5069.

0 Bojan Jovanovic over 3 years ago in reply to Rakesh

Prodigy 145 points

Hello, Rakesh.

LM5069 seems indeed like a better solution.

However, even LM5069 will break the circuit sometime after an overcurrent event is detected.

We need a design that will limit the current but keep the system working.I don't ask this only because inrush current peak but also because of normal system run-time. We would like to try finding a sweet spot for the PTC heater and deliver less current than it is asking for in order to get lower power consumption that will be paid by increased PTC heating time.

Can we use some step-down buck converter whose output voltage and current limit we are gonna fine-tune with external resistors? For example, LM2673. We can adjust output voltage through FB pin and there is a possibility to adjust the current limit with an external resistor.

Would that approach withstand the overcurrent event and continue delivering limited current to the load even though the load is "asking" for more?

Thanks once again!

Regards,

Bojan.

0 Rakesh over 3 years ago in reply to Bojan Jovanovic

TI__Guru** 103876 points

Hi Bojan,

I would like to have a short call to understand more on this requirement. I have sent you a private message. Please reply.

Best Regards, Rakesh

0 Rakesh over 3 years ago in reply to Rakesh

TI__Guru** 103876 points

Hi Bojan,

As suggested, please explore on those options and come back if you have any follow-up questions. I am closing the thread for the time

Best Regards, Rakesh

0 Bojan Jovanovic over 3 years ago in reply to Rakesh

Prodigy 145 points

Thank you very much for all your support, Rakesh!

You've been of great help!

Cheers,

Bojan

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Limit inrush current of PTC heater