When a switch-mode power-supply controller, such as NCP1200, operates at a high ambient temperature, you should protect the entire power supply against lethal thermal runaway. The NCP1200 operates directly from the power mains without an auxiliary winding; therefore, the die in the IC dissipates power (Figure 1).
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| Figure 1. | A controller IC implements a low parts-count offline power supply. |
Unfortunately, the internal temperature-shutdown circuitry cannot perform its protection function because the die is not at ambient temperature but at a temperature that's higher than ambient by a few tens of degrees. To overcome this problem, you can implement a thermistor-based design, but this solution compromises the system's cost. Fortunately, you can use standard bipolar transistors to implement a low-cost thermal-shutdown circuit. Figure 2 shows how to build a classic thyristor circuit using two inexpensive bipolar transistors: a BC557B pnp and a BC547B npn. The idea is to use the negative-temperature coefficient of the silicon ~ –2 mV/°C) to fire the thyristor.
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| Figure 2. | Two bipolar transistors configure a thyristor-based temperature- shutdown circuit. |
In the inactive state, both the upper and the lower transistors in Figure 2 are in the off state because of the presence of the 10-kΩ resistors. The thyristor structure connects between the feedback pin, FB, and ground. One feature of the NCP1200 is to skip unwanted switching cycles when the power demand diminishes. The IC performs this function internally by constantly monitoring the FB pin. When the voltage on this pin falls below a certain level, the IC internally blanks the cycles, and the power transistor turns off. If the thyristor permanently pulls the FB pin to ground, the NCP1200 no longer delivers pulses. Once latched, the thyristor prevents any restart, until you disconnect the power supply from the power mains. The 316-kΩ resistor combines with the 10-kΩ resistor to form a voltage divider from the VCC rail. This rail, on average, varies from lot to lot from 10.3 to 10.6 V, for a total ΔV of 300 mV. This variation translates to less than 10 mV at the transistor's base. When the temperature rises, the BC547's turn-on VBE diminishes until it reaches the divider voltage on its base. (This voltage is approximately 320 mV, but you can alter it to accommodate other temperature levels.) At this point, the BC547 conducts current, and the BC557's base voltage starts toward ground. The BC557's collector current further biases the BC547's base, and the thyristor latches, thereby permanently stopping the NCP1200's pulses. Once you remove the supply from the power mains, the thyristor resets. The 0.1-µF capacitor prevents spurious noise from triggering the thyristor.
We conducted tests on the thyristor-based temperature-protection scheme using BC547B and BC557B transistors. The “B” extension is important because it corresponds to a narrow hFE range of 200 to 450. This design uses transistors in TO-92 packages, mounted close to each other. If only one transistor heats up, thermal results vary. Therefore, you should mount these two components close to each other on the pc-board-component side so that they will operate at approximately the same junction temperatures. From 20 bipolar-transistor combinations, you can obtain the results shown in Table 1. You can see that the latch-off threshold temperature varies by only approximately 5 °C for all combinations of transistors (Reference 1).
| Table 1. | Temperature shutdown versus VBE | |||||||||||||||||||||||||||||||||
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Reference
- Basso, Christophe. “Bipolars provide safe latch-off against opto failures.”
