6. Temperature Detection Application of
This section shows an example about a temperature
detection circuit of a secondary rectifier diode in off-line
Figure 6-1 shows the reference of temperature
detection circuit with a NTC thermistor. The NTC
thermistor, coupled thermally with DS secondary
rectifier diode, is connected to the REF pin of the output
voltage detection circuit in the converter.
As shown in Figure 6-2, as the temperature rises, the
resistance of the NTC thermistor decreases.
When the temperature of DS rises due to such a cause
as overload state, the resistance of NTC thermistor
decreases, and the ratio of resistance voltage divider is
changed. When the voltage of RS shown in Figure 6-1
reaches the reference voltage of U1 shunt regulator, the
current flows to PC1 optocoupler, and the converter IC
in the primary limits the output power. Thus, the rise of
DS temperature can be limited.
Figure 6-1 Reference temperature detection circuit
with NTC thermistor
● Increasing the accuracy of temperature detection by
reducing the thermal resistance between DS and the
thermistor, it is necessary to attach the thermistor to
DS with high thermal conuctivity material between
In contrast with the temperature detection of
thermistor, the FMKS series can achieve high accuracy
of temperature detection by the following.
● The internal structure is formed a Schottky barrier
diode for temperature detection, SBD, and a fast
recovery diode, FRD, on the same die as shown in
Figure 6-3. Thus, the temperature is about the same
between SBD and FRD.
● The temperature detection uses the temperature
characteristics of the leakage current for SBD, which
increases as the temperature rises as shown in Figure
The temperature detection circuit with FMKS series
has the following advantages.
● Highly accurate and stable temperature detection of
● Real time temperature detection of FRD.
● Circuit component reduction such as thermistor, and
● Power supply downsizing.
(1) (2) (3)
Figure 6-3 Internal structure of FMKS series
Figure 6-2 Reference characteristics of NTC thermistor
The temperature detection circuit with thermistor has
the following issues.
● Since some attachment distance occurs between the
thermistor and DS, the accurate temperature of DS
cannot be detected.
● Thermistor cannot follow the rapid temperature
Figure 6-4 Reference temperature characteristics of
SBD leakage current
SANKEN ELECTRIC CO.,LTD.
© SANKEN ELECTRIC CO.,LTD. 2016