When power is applied, the thermistor, a negative tc device, is at a high value. A1 saturates positive. This forces the LT3525A switching regulator`s output low, biasing Ql. As the heater warms, the thermistor`s value decreases. When itsinputs finally balance, A1 comes out of saturation and the LT3525A pulsewidth modulates the heater via Q1, completing a feedback path.
A1 provides gain and the LT3525A is highly efficient. The 2-kHz, pulse-width modulated heater power is much faster than the thermal loop"s response, and the oven sees an even, continuous heat flow.