AN303 STMICROELECTRONICS | Alldatasheet

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Technical content

1 Application examples

The importance of the latching current is highlighted by the following application examples.

1.1 Example 1: low power lamp control

In the application circuit shown in Figure 2 a TRIAC is used to control a 10 W signaling light. Figure 2. Control of a low-power lamp continuous gate current should be applied or a longer gate current pulse should be applied. the Z0103 IL max is 15 mA in QII). a. The minimum current that keeps a TRIAC conducti ng is called the hypostatic or holding current IH.

1.2 Example 2: inductive load control

When a TRIAC controls an inductive load (L), the rise of the load current IT is slowed down. where VA is the mains voltage when the gate signal is applied. In Figure 3 the impact of the gate pulse width on the TRIAC conduction is shown.

  • In continuous lines: A short gate signal (T1) is applied. The TRIAC doesn’t remain in the on state because the load current IT doesn’t reach the TRIAC latching current level before the gate current removal.
  • In dotted lines: A longer gate signal (T2) is applied. In this case, the TRIAC turns on and remains in the on state. The TRIAC turns off when the load current reaches zero after the gate current removal.

Figure 3. Control of an alternating current (AC) motor TRIAC latching current. The control mode shown in Figure 3 is a square pulse. some TRIAC control circuits specially designed to work with inductive loads.

1.3 Example 3: varying power load control

welding (see Figure 4), the controlled power can be subjected to considerable variations. triggering issue could occur. transformer, which heats its coils and can cause transformer failure. Figure 4. Arc welding control

2 Latching current – the details

made according to temperature variations.

2.1 Effect of RC snubber circuit at turn on

Figure 5. RC snubber circuit. Figure 6. Snubber capacitor discharge at TRIAC turn on

the resistor (R) must be higher than a minimum value (typically 47 ohms for most TRIACs).

2.2 Latching current measurement

set to a higher value than the gate current specified in the datasheet (IG = 1.2 x IGT). measured with a 1 kΩ resistor connected between gate and cathode. Figure 7. Circuit for the measurement of the latching current (I L)

  • The pulse width of the gate current must be at least equal to 1 ms.
  • The applied gate current (IG) must by higher than the specified triggering gate current (IGT) of the device measured. An IG / IGT ratio higher than or equal to 1.2 is recommended. There are four different latching current levels; corresponding to the four triggering quadrants (refer to Figure 8). These quadrants are defined according to VT polarity (VT is positive if A2 is set to a higher bias voltage than A1) and gate polarity (IG is positive if it is sourced to the gate, so it circulates from G to A1):
  • Quadrant I (QI): VT > 0 and IG > 0
  • Quadrant II (QII): VT > 0 and IG < 0
  • Quadrant III (QIII): VT < 0 and IG < 0
  • Quadrant IV (QIV): VT < 0 and IG > 0 IL in QIV is not specified for devices only controlled in the three first quadrants, as SnubberlessTM, logic level and high temperature TRIACs. TM: Snubberless is a trademark of STMicroelectronics IG IT G A V = ±12V V = ±12V C R2 R1 R1 ≥ 33 Ω

Figure 8. TRIAC triggering quadrants

2.3 Variation of the latching current

2.3.1 Typical variation of I L with device sensitivity and quadrant

on the triggering quadrant as shown in Table 1. then IL (QI) ≈ 3 mA, IL (QII) ≈ 6 mA and IL (QIII) ≈ 3.5 mA.

2.3.2 Typical variation between I L and IH

than the IH value. The IL /I H ratio is related to the TRIACs current rating, as shown Table 2. Table 1. Approximate ratio between I L and IGT for sensitive and standard TRIACs Sensitive TRIAC 12 A rms (TW type) 3 6 3.5 N.A. Table 2. Approximate ratio between I L and IH according to devices current rating

  1. First quadrant in the case of TRIACs

2.3.3 Typical variation of I L with junction temperature

parameters vary with the junction temperature as shown in Figure 9. Figure 9. Relative variation of I L with the junction temperature Tj

2.3.4 Influence of the extern al gate-cathode resistor

latching current as shown in Figure 10. Figure 10. Variation of I

Figure 10. Thus, in certain applications, the designer may want to use a high-impedance control circuit. The drawback will then be that the SCR sustainable dV/dt will be lower. control circuit to trigger the device.

2.3.5 Typical variation of the latching current IL with the control signal

The latching current IL varies with the amplitude and the pulse width tp of the gate current IG. lead to the IL amplitude increase (refer to Figure 11). Figure 11. Variation of I L with the width tp and the amplitude IG of the gate pulse

  • Increase if a negative gate current is applied at the end of the pulse, as shown in Figure 12.
  • Decrease if the decreasing slope of the gate current is low (compared to dashed line), as shown in Figure 13. For a decreasing slope of the gate current lower than 0.5 A/µs, the IL value is typically closed to the IH value. IL (mA) 100 0 10 20 30 40 50 IG = 10 IGT IG = 5 IGT IG = 2 IGT tp [pulse width] t IG IGT Tj = 25 °C tp (µs) TRIAC BTB16-600B

3 Conclusion

The SCR or TRIAC choice does not depend only on the voltage, the rated current and the sensitivity. Other parameters must be taken into account to ensure appropriate operation. Among them, the latching current, IL, plays an important role in many circuits. The value of this parameter varies with

  • gate current pulse (amplitude, shape and width)
  • temperature
  • control circuit (in the case of sensitive SCRs)
  • direction of current flow TRIAC and SCR applications, involving highly inductive loads or loads with considerable power variations, are the main applications for which the latching current must be given particular consideration. Taking into account these aspects, the designer can obtain satisfactory operation of the circuit in industrial applications.

4 Revision history

Table 3. Document revision history 15-May-2004 2 Style sheet update. No content change.