Thermal flow sensor

1. A thermal flow sensor comprising:

a semiconductor substrate;

a cavity portion provided in the semiconductor substrate;

a dielectric film provided on the semiconductor substrate so as to cover the cavity portion;

a thin layer area formed as a result of the dielectric film covering the cavity portion;

a heating resistor provided in the thin layer area on the dielectric film;

a first temperature-sensitive resistor provided in the thin layer area on the dielectric film, a resistance value of the first temperature-sensitive resistor varying depending upon temperature;

a heating controller that controls temperature of the heating resistor on the basis of temperature of the first temperature-sensitive resistor;

a second temperature-sensitive resistor provided near the heating resistor, a resistance value of the second temperature-sensitive resistor varying depending upon temperature; and

a flow rate detector that detects a flow rate of a fluid on the basis of temperature of the second temperature-sensitive resistor,

wherein the heating controller controls the temperature of the heating resistor on the basis of a first reference temperature, which is a target temperature of the first temperature-sensitive resistor, and a second reference temperature, which is an escape temperature of the first temperature-sensitive resistor.

2. The thermal flow sensor according to claim 1 , wherein

the second reference temperature is set at a temperature higher than the first reference temperature, and

the heating controller comprises:

a first temperature controller that brings a temperature of the first temperature-sensitive resistor close to the first reference temperature; and

a second temperature controller that holds temperature of the first temperature-sensitive resistor at a temperature equal to or lower than the second reference temperature.

3. The thermal flow sensor according to claim 2 , wherein

the first temperature controller comprises a feedback controller that receives signals based on the temperature of the first temperature-sensitive resistor and outputs either power to be supplied to the heating resistor or a power indication value.

4. The thermal flow sensor according to claim 2 , wherein

the second temperature controller comprises a determining unit that receives signals based on the temperature of the first temperature-sensitive resistor and determines whether or not the temperature of the first temperature-sensitive resistor is equal to or greater than the second reference temperature.

5. The thermal flow sensor according to claim 2 , wherein

the second temperature controller comprises a limit unit that limits either power to be supplied to the heating resistor or a power indication value on the basis of a temperature difference between a temperature of the first temperature-sensitive resistor and the second reference temperature.

6. The thermal flow sensor according to claim 5 , wherein

the limit unit comprises a disconnecting unit that disconnects a transmission path for the power or the power indication value.

7. The thermal flow sensor according to claim 5 , wherein

the limit unit comprises a fixing unit that fixes the power or the power indication value to a predetermined value.

8. The thermal flow sensor according to claim 5 , wherein

the limit unit comprises load resistors electrically connectable to the heating resistor and having at least two resistance values and a connection unit that provides electrical connection to the heating resistor by selecting one of the resistance values of the load resistors.

9. The thermal flow sensor according to claim 3 , wherein

the second temperature controller comprises an accelerator that accelerates an input/output response of the feedback controller on the basis of a temperature difference between the temperature of the first temperature-sensitive resistor and the second reference temperature.

10. The thermal flow sensor according to claim 9 , wherein

the feedback controller comprises a digital converter that converts signals based on the temperature of the first temperature-sensitive resistor into digital signals and a digital signal processor that performs processing on the basis of a first clock signal;

the accelerator comprises a second clock generating unit that generates a second clock signal having a period different from a period of the first clock signal and a clock switch that switches between clock signals for operating the digital signal processor; and

the clock switch switches between operation clocks of the digital signal processor on the basis of a difference between the temperature of the first temperature-sensitive resistor and the second reference temperature.

11. The thermal flow sensor according to claim 9 , wherein

the feedback controller comprises a digital converter that converts signals based on the temperature of the first temperature-sensitive resistor into digital signals and a digital signal processor that performs processing on the basis of the digital signals, and

the accelerator comprises a gain changing unit that changes an input/output gain of the digital signal processor and that maximizes the input/output gain of the digital signal processor on the basis of a difference between the temperature of the first temperature-sensitive resistor and the second reference temperature.

12. The thermal flow sensor according to claim 9 , wherein

the feedback controller comprises a digital converter that converts signals based on temperature of the first temperature-sensitive resistor into digital signals, a digital signal processor that performs processing on the basis of the digital signals, and storage in which at least two different processing programs are stored;

the accelerator comprises a program selecting unit that selects a process of the digital signal processor; and

the program selecting unit changes a process of the digital signal processor on the basis of a difference between the temperature of the first temperature-sensitive resistor and the second reference temperature.