Electronic Design

Linear-RMS Phase Control Improves Thyristor-Based Thermostat

Precision temperature control circuits for small thermal loads like oscillator crystals and voltage references tend to be fairly easy to design. This is because simple, well-behaved, linear-output drivers running from regulated dc supplies are often used for straightforward heater control. While handy for low-wattage heaters, linear drive is an unappealing solution for satisfying the demands of bigger loads and their bigger heaters. Those thirsty power consumers call for more efficient switch-mode power-handling circuits.

This pursuit of efficiency frequently leads to the use of power-control circuits based on thyristors with 60-Hz phase-angle triggering. Cheap and robust, thyristor phase-angle circuits can easily drive multikilowatt heaters while achieving greater than 90% efficiency. Unfortunately, a serious obstacle hinders the use of these devices in precision control applications.

The problem can be seen in this typical thyristor phase-control response plot (Fig. 1, curve A). Here, 80% of the thyristor rms output range (0.1 to 0.9 of full scale) is spanned by only 55% of the control-input range (0.12 to 0.67). This severe nonlinearity complicates the simultaneous achievement of adequate system feedback gain and non-oscillatory stability over the full range of heater control inputs.

Figure 2's thermostat dodges this bullet by incorporating a control-voltage-to-trigger-angle converter circuit (A4, D1, C3, Q5, and R2). The circuit achieves an essentially linear relationship between the control voltage and the rms heater drive (Fig. 1, curve B). On each positive half-cycle of the silicon controlled rectifier (SCR) anode voltage, VAC, timing capacitor C3 begins charging through D1 and R2. A4 compares the accumulating voltage (VC3) to the heater control voltage on C2. A trigger pulse is issued to the SCR gate when VC3 = VC2. This, in turn, generates the VC2/SCR phase-conversion function and also resets C3 via Q5.

It's the sigmoidal (rather than triangular) shape of the VC3 waveform that produces the excellent linearity of the VC2/rms-heater voltage conversion. This wave shape results in an inverse-sigmoidal relationship between VC2 and the SCR trigger timing. Consequently, the sigmoidal functionality of firing-angle-to-thyristor/heater current is accurately compensated and linearized. Although as illustrated the converter implements a 0° to 180° half-wave ac-control function, modification for full-wave 360° operation would be straightforward.

Figure 2's circuit was de-signed to serve an Air Curtain Incubator application that satisfies a requirement for accurate thermostasis of biological samples and culture dishes when transferred from a cabinet incubator to a microscope viewing stage. This application requires the generation of a flow of temperature-controlled air. In this case, the airflow is conveniently and cheaply produced by an ordinary, unmodified household hair dryer.

The airstream temperature is sensed using Q3's VBE temperature coefficient (−2 mV/°C). Q3's VBE (VT) is compared to the 0.43- to 0.65-V setpoint voltage (VS). After being integrated, the VS − VT error signal is applied to A4's voltage-to-trigger-angle converter circuit.

Due to the finite velocity of the heat-transporting airstream, there's a thermal time lag between heater H and sensor Q3. This interval complicates servo stability issues and requires the use of a robust, integrating, convergence-by-bisection feedback control algorithm (see "Take Back Half: A Novel Integrating Temperature-Control Algorithm," Electronic Design, Dec. 4, 2000, p. 132).

Take Back Half (TBH) damps oscillations and stabilizes the servo loop by revising the estimate of the optimum heater input at each setpoint (VT = VS) crossing. Comparator A2 detects these setpoint crossings by going high when VT is less than VS and low when VT is greater than VS. The positive feedback network around A2 keeps these logic transitions snappy.

Meanwhile, the role of TBH variables HO and H are served by the sample-and-hold capacitor C1 and integrator cap C2, respectively. Further details of the workings of the analog TBH controller circuitry can be found in, "Circuit Enables Precision Control In Radiant Heating Systems," Electronic Design, Jan. 8, 2001, p. 131.

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