AN302 STMICROELECTRONICS | Alldatasheet

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Technical content

1 Application examples

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

1.1 Example 1: light dimmer

Figure 2. Dimmer with interference suppression filter (coil and capacitor) Figure 3. Current in the dimmer TRIAC In the dimmer circuit of Figure 2 the interference suppression filter can produce oscillations. known as the flicker effect.

1.2 Example 2: motor control (1)

Figure 4. Control of a small motor by TRIAC production, the manufacturer complains of low torque in the motor and blames the TRIAC. Figure 5. Voltage across the TRIAC and current for the motor control

1.3 Example 3: motor control (2)

This time, the designer selects a TRIAC with a lower maximum specified holding current, IH. The small high-impedance motor (Figure 4) seems to operate without problems. However, the motor is intended for mounting on out-door equipment. It is installed in summer and works well. But in winter, the fault described above occurs. What has happened? The designer studied the operation of his circuit at an ambient temperature of 25 °C. But the holding current varies inversely with the temperature. Thus, as the temperature decreases, the holding current increases and the phenomenon described in example 2 occurs. Again, it is essential to take into account the temperature effects on the device parameters for circuits which have to operate at extremes of temperature. It is not sufficient to use the values given for an ambient temperature of 25 °C.

2 Holding current - the details

used ensures an operating current sufficiently high. maximum value. Then, corrections must be made to compensate for temperature variations.

2.1 Measuring the holding current

  • The initial value of current IT must be more than five times the latching current IL before the test can begin.
  • If the holding current is measured by pulses (by an automatic tester, for example), the TRIAC should be conducting for at least 500 µs before starting the test.

Figure 6. Circuit for measurement of the holding current I

  • sensitive TRIAC: IGT (QI): 5 mA (type TW), 2 mA < IH < 8 mA (specified IH max: 10 mA)
  • standard TRIAC: IGT (QI): 50 mA (type B), 8 mA < IH < 40 mA (specified IH max: 50 mA) The minimum value of the IH parameter is not specified in the data sheets.

2.2 Variation of the holding current

2.2.1 Typical variation of I H with device sensitivity and direction of

The holding current, IH, is related to the gate firing current, IGT as shown in Ta bl e 1. BTA/BTB12-600C: if IGT (QI) = 10 mA then IH+ = 14 mA. rated current and the device technology.

2.2.2 Variation of I H with junction temperature

with the junction temperature as shown in Figure 7. Triac TO-220AB, type BTA/BTB12-600C: IH = 20 mA at Tj = 25 °C and 14 mA at 110 °C. Table 1. Ratio between I H+ and IGT (QI) for sensitive and standard TRIACs Table 2. Ratio between I H+ and IH- for sensitive and standard TRIACs

Figure 7. Relative variation of I H, with the junction temperature, Tj

2.2.3 Effect of reapplied voltage

have no effect on the value of its holding current, IH.

2.2.4 Influence of the exte rnal gate cathode resistor

Figure 8. Variation of I H, of a sensitive thyristor with RGK component, affects the holding current as shown in Figure 8. Figure 9. Thus, in certain applications, the designer may want to use a high-impedance is, RGK should be greater than 20 Ω. Note: The hypostatic current for sensitive thyristors is always specified for RGK = 1000ohms.

Figure 9. A Darlington TRIAC for high sensitivity with high holding current

2.3 Combining characteristics

3 Conclusions

and the sensitivity. Other parameters must be taken into account to ensure reliability.

  • dispersion of the characteristics at manufacture
  • temperature
  • control circuit (in the case of sensitive thyristors)
  • direction of current flow. Taking into account these elements, the designer can obtain satisfactory operation of his circuit in industrial real-life applications.

4 Revision history

Table 3. Document revision history 30-Mar-2004 2 Stylesheet update. No content change.