TOPTriac: A Temperature and Overload Protected Triac


Nick Ham, Matjaz Rozman, Ed Huang, and Stephen Wood at WeEn Semiconductors

For the past several decades, Triacs have been used as power switches. The emergence of smart appliances and greater emphasis on safety has suggested the need for Triacs with self-protection features and smart monitoring capabilities. TOPTriac meets this need.

Triacs are the most widely used solid-state AC power switches for controlling AC mains loads of 50/60Hz or even 400Hz, home appliances, commercial, and industrial equipment. They are designed to withstand high blocking voltage while conducting very low leakage current or latch into the on-state to conduct load current in response to a low-level current pulse on the gate.

As with all power semiconductors, Triacs are available at many different current levels from <1A upwards. At higher currents, a heatsink will be required to remove the heat. Due to the fixed voltage drop inherent with their technology, Triacs will typically dissipate around 1 Watt per Ampere of load current.

If the Triac should become too hot for any reason (Tj > Tj(max)), it may lose control, i.e. be unable to commutate (turn itself off at the end of a half-cycle) and continue to conduct even without trigger commands applied to the gate. This may lead to thermal runaway and destructive failure of the Triac and possibly the load. To avoid such danger, the application must be over-designed to allow for all possible fault or overload conditions. Over-design strategies may include:

  • A larger Triac than necessary to allow for fault conditions;
  • A larger heatsink than necessary to allow for fault conditions;
  • A larger enclosure than necessary to allow for fault condition cooling;
  • Additional thermal protection;
  • Additional overcurrent protection;
  • Artificial limitation of duty cycle on intermittent loads.

A Temperature and Overload-Protected Triac (TOPTriac) does not require any such over-design because it protects itself against overheating, turning itself off well below the temperature at which it would lose control. This allows the application to be designed exclusively for normal operation only, in the safe knowledge that TOPTriac will take care of its own thermal overload conditions.

In addition to over-temperature protection with indefinite latch-off until reset, TOPTriac can communicate its status back to the system microcontroller via gate feedback. The system can report when it is conducting or blocking normally or whether it is in an over-temperature trip state, with or without gate trigger commands applied. Its capability to communicate its status to the microcontroller gives the designer a unique opportunity to implement a smart overload monitoring function, with manual or automatic reset applied according to the needs of the application.

The over-temperature protection and smart monitoring features of TOPTriac improve the overall safety of the end-product, remove the need to over-design for fault conditions, remove the need for additional protection components and reduce the chance of field failures.

 

Triac and TOPTriac symbols

Figure 1. Triac and TOPTriac symbols.

 

How it Works

TOPTriac is based on WeEn’s planar-passivated three-quadrant Hi-Com Triac technology, it is a solid-state AC power switch with semiconductor-based on-chip thermal protection. The protection disables the Triac element, preventing conduction before excessive die temperature causes loss of control and damage to the device or the circuit. Protection is activated at a junction temperature between 125°C and 150°C.

As a two-quadrant device, TOPTriac is triggered exclusively by negative gate current.

TOPTriac is triggered like a standard Triac by applying current to its gate. Trigger current can be DC (for continuous conduction) or pulsed (for any phase angle). Pulsed triggering requires an additional low-level continuous DC bleed current to be applied to the gate. Bleed current is in the range 0.5mA – 2mA.

When TOPTriac goes into the over-temperature protection condition, “tripped”, it will remain latched-off indefinitely even after cooling, thanks to the continuous DC gate trigger current or bleed current.

Reset is achieved by removing and re-applying the DC gate drive. Normal conduction will be restored, provided by then Tj is below the trip temperature.

 

Table 1: A Comparison of the Three-quadrant Hi-com Triac and TOPTriac

Three-quadrant Hi-com Triac and TOPTriac compared

 

Application Circuits

TOPTriac triggering and gate monitoring circuits can be as simple or as complicated as the application requires. A simple discrete circuit (open loop, no monitoring or feedback, manual reset) can be applied to low-cost designs and simple systems, while for higher-end applications with microcontrollers, full status monitoring and any combination of manual or automatic reset become possible.

 

Discrete Phase Control with Manual Reset

TOPTriac is a two-quadrant device with IGT of 5mA min to 35mA max. Negative gate current pulses of at least 35mA amplitude and adjustable phase angle are applied for variable power control. A bleed current of ~0.8mA is applied via R7 to ensure continued latch-off after an over-temperature trip. Reset is achieved by opening the switch momentarily in series with the gate.

 

Discrete phase control circuit

Figure 2. Discrete phase control circuit.

 

Table 2. Bill of Materials for a Discrete Phase Control Circuit

Bill of materials for discrete phase control circuit

 

Microcontroller Controlled

Combining TOPTriac with a microcontroller allows full functionality. Triggering can be continuous DC or pulsed with bleed current (minimum pulse duration 10µs). Gate voltage feedback indicates TOPTriac’s status:

  • Presence of mains frequency square wave means load current is flowing;
  • Low-level continuous DC offset and no AC signal means over-temperature tripped.

Gate feedback also allows detection of load current zero-crossing because the gate voltage AC signal reflects load current. This is critical for Triacs, which commutate at current (not voltage) zero-crossing, so pulse triggering needs to be synchronized to current zero-crossing for continuous conduction.

When the TOPTriac has gone into an over-temperature trip condition, the microcontroller will spontaneously detect its status and automatically send an alert signal after a programmed time delay. For instance, it could be hazardous to allow cycling through an over-temperature trip (e.g. it may be unsafe to have an unexpected start-up of the machine). In such a scenario, the system can be programmed to send an early warning signal that the user could self-evaluate and intervene in the process allowing the circuit to be manually reset.

 

DC and pulse trigger using 3.3V microcontroller

Figure 3. DC and pulse trigger using a 3.3V microcontroller.

 

TOPTriac Vs. Standard Triac

The performance of TOPTriac was evaluated and compared with a standard Triac during this analysis. The test board used was designed to function well for both TOPTriac and standard Triacs. WeEn’s standard Triac, BTA312B-600CT was compared with WeEn’s TOPTriac TOP12-800CO. Devices were controlled by a microcontroller and triggered at the phase angle of 900 while a 1200W heating element was used as the load. Functionality was evaluated under general environment working conditions and an extreme temperature environment which exceeded Tjmax.

In figure 4a, both the waveforms of the gate triggering pulses and the load current of the, standard Triac during general environment conditions are presented. The Triac expectedly opens at each half cycle, when the gate pulse appears on its gate. However, the waveforms of the TOPTriac presented in figure 4b, were no different from figure 4a. Both devices functioned as specified in the datasheet.

 

TOPTriac versus a standard Triac performance during common working condition

Figure 4. TOPTriac and a standard Triac performance during common working condition. Left: BTA312B-600CT during normal operating conditions and at 900 triggering angle. Right: TOP12-800CO during normal operating conditions and at 900 triggering angle.     

 

The major differences between the standard Triac and the TOPTriac can be observed when the ambient temperature exceeds Tjmax. As can be seen from figure 5a, a standard Triac continues to conduct for full half cycle without the gate signal being applied to the gate instead of the intended phase-controlled conduction. In this condition, the power supplied to the load cannot be controlled and higher power is dissipated in the load and the Triac. This leads to the Triac having a higher risk of overheating and device failure. By contrast, as figure 5b indicates, when subjected to the same over-temperature conditions a TOPTriac self-protects and switches off as it reaches its temperature trip limit. Not only so, after the over-temperature protection has been triggered, a TOPTriac does not conduct again regardless of the gate pulses being applied to the gate.

 

 TOPTriac and a standard Triac performance during over-temperature condition.

Figure 5. TOPTriac and a standard Triac performance during over-temperature condition. Left: BTA312B-600CT during over-temperature condition and at 900 triggering angle. Right: TOP12-800CO during over-temperature condition and at 900 triggering angle.

 

Conclusion

In this paper, TOPTriac and a standard Triac have been compared up to and beyond normal operating temperature conditions. TOPTriac operates as a standard Triac with the additional unique capability to protect itself against overheating and overcurrent scenarios and so prevent loss of control of power supplied to the load. In this way TOPTriac provides an enhanced safety and are a reliable option for designers of appliance applications, greatly reducing total ownership by eliminating the need for over-designs and the risk of field failures.

 

More information: WeEn Semiconductors    Source: Bodo's Power Systems, June 2018

 

  More News Like This?

Sign up to receive our weekly email newsletter and never miss an update!