Designers spend much time combating thermal effects in circuitry. The close relationship between temperature and electronic devices is the source of more design headaches than any other consideration.

　　In fact， instead of eliminating or compensating for thermal parasitics in circuits， it is possible to utilize them. In particular， applying thermal techniques to measurement and control circuits allows novel solutions to diffi cult problems. The most obvious example is temperature control. Familiarity with thermal considerations in temperature control loops permits less obvious， but very useful， thermallybased circuits to be built.

　　Temperature Controller Figure 1 shows a precision temperature controller for a small components oven. When power is applied， the thermistor， a negative TC device， is at a high value. A1 saturates positive. This forces the LT?3525A switching regulator’s output low， biasing Q1. As the heater warms， the thermistor ’s value decreases. When its inputs fi nally balance， A1 comes out of saturation and the LT3525A pulse width modulates the heater via Q1， completing a feedback path. A1 provides gain and the LT3523A furnishes high effi ciency. The 2kHz pulse width modulated heater power is much faster than the thermal loop’s response and the oven sees an even， continuous heat fl ow.

　　The key to high performance control is matching the gain bandwidth of A1 to the thermal feedback path. Theoretically， it is a simple matter to do this using conventional servo-feedback techniques. Practically， the long time constants and uncertain delays inherent in thermal systems present a challenge. The unfortunate relationship between servo systems and oscillators is very apparent in thermal control systems.