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Protection of power devices (triacs) from a short circuit of heating devices

Serhii Bashkirov
Prodigy 30 points
Other Parts Discussed in Thread: INA302, INA303, TMCS1100

Hello.
I need to organize the protection of power devices (triacs) from a short circuit of heating devices (heaters). There are many heaters, 3,6,9 or 12. The power of each is 3-3.5 kW. I thought to put shunt resistors to ground, but with a resistance of 0.1 ohms, a power dissipation of 23 watts is obtained. It's too much, of course. If the shunt is set to 0.01 Ohm, then 2.3 W, it seems to be nothing, but the voltage from it will be only 0.15 volts of alternating voltage, which must also be turned into a constant one, and the rectifier diode will not help. He will eat all this low voltage, as you understand. Maybe you need to put an amplifier for this voltage? I’m already silent that now all the heating elements are turned on by a triangle (220/380 V), here the shunts will not help either. I can put heating elements on 230 volts with a neutral wire, as I wrote above, but this will increase the triac currents and the reliability of the heating elements. And putting 6, 9 or 12 current transformers will be quite expensive. Can you direct my thoughts in the right direction? Does TI have solutions?

  • 0
    TI__Mastermind 34220 points

    Hi,

    I think your idea of putting shunt resistors to ground and measure the voltage drop with a current sense amplifier might work. A bidirectional amplifier would be required, as illustrated in this picture:

    The INA302/INA303 family offers such bidirectional functionality, and comes with two independent comparators or a window comparator. The integration of amplifier and comparator makes it convenient in over current protection.  

    Another alternative is magnetic current sensors such as TMCS1100. These devices can be used on the high side or low side. The isolation property might come in handy if it is needed.

    Regards, Guang 

  • 0 in reply to Guang Zhou
    Prodigy 30 points

    Hello. Thank you very much for responding to my question. I got acquainted a little with the products that you suggested to me. INA302/INA303 operate at a maximum of 36 volts, and DC voltage. I have AC voltage 380V 50Hz. The second option should work for me. TMCS1100 current sensor 20 Amp. I have about 9-10 amps. So within acceptable limits. But I don't understand the following:
    The current sensor is connected in series with the load and measures this current, right? Let's say I set up the control circuit in such a way that when the current is exceeded, say at 12 Amperes and above, the load will be disconnected (the triac control will be turned off), right? But I have another task - I need protection against short circuits! When the heating element shorts out during an accident, the currents will be significantly higher! They will just burn this current sensor. The entire 380 V voltage will be applied to the resistance of the TMCS1100 sensor, i.e. 1.8 mOhm (380 / 1.8 * 10 * -3 \u003d 211111 A ideally).
    Will he have time to turn off the triac?

  • 0 in reply to Serhii Bashkirov
    TI__Mastermind 34220 points

    Hi,

    The challenge of short circuit protection would be present regardless which device you ultimately choose. Depending on the reaction time of the shut-down circuitry (reaction time of trac and TMCS1100, and loop delay which hopefully is negligible).

    The TMCS1100 has a step response time of a few uS, please refer to section 9.3.3.4 of the datasheet for a detailed description on transient response.

    Single higher-current events that are shorter duration can be tolerated by the TMCS1100. Section 8.3.3 discusses this scenario.

    Regards, Guang