Patent Application
20100222937
This invention relates generally to a heater control system, and more particularly to a heater control system utilizing power/current amplitude modulation.
Increasing demands for improved fuel economy and reduced emissions have lead to improvements and developments in hybrid vehicles, electric vehicles, and vehicles powered by fuel cells and diesel fuel. Such vehicles, however, characteristically generate little or no supplemental engine heat for cabin heating, windshield defrosting, and the like. As a result, supplemental heaters, typically positive temperature-coefficient (PTC) heaters, are employed to provide the requisite supplemental heat. The heating stages of PTC heaters (both the high voltage and low voltage stages) have traditionally been driven/controlled using voltage pulse-width-modulation (PWM) switching control. However, this technique results in incomplete overlapping of the individual heater circuits as the heater output is continuously varied. The result is the generation of significant ripple current on the supply line. This ripple current may be compensated for through the use of additional capacitance, and/or inductance, and/or higher frequency switching of the individual heating elements. Each of these solutions, however, is costly and complex, especially for heating systems having additional control parameters. Additionally, with such solutions, the usable control range of a PWM heater can be affected as system voltage varies.
It would therefore be desirable to provide an improved heater control system having enhanced power control as the system voltage varies. It is further desirable to provide an improved heater control system having reduced ripple current. It is still further desirable that the improved heater control system be simpler and less costly than known heater control systems.
A heater control system is provided that comprises a plurality of n banks of heating elements (H1, H2, H3, H4, . . . , Hn) each containing at least one heating element. Each one of a plurality of switches Q1, Q2, Q3, Q4, . . . , Qn is coupled to a respective one of the plurality of banks for selectively activating the plurality of banks of heating elements. Each one of a plurality of control inputs representing a desired heating level is coupled to a different one of the plurality of switches.
Further, there is provided a heater control system that comprises a plurality of n banks of heating elements (H1, H2, H3, H4, . . . , Hn) each bank comprising Xn heating elements. Each of a plurality of switches Q1, Q2, Q3, Q4, . . . , Qn is coupled to a different one of the plurality of n banks, and each of a plurality of driver circuits is coupled to one of the plurality of switches. A controller has an input for receiving a signal indicative of a desired heating level and has n outputs each one coupled to a respective one of the plurality of driver circuits. The controller is configured to generate a binary representation of the signal on the n outputs to selectively activate the plurality of n banks of heating elements
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and:
The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.
The following description refers to elements or features being “connected” or “coupled” together. As used herein, “connected” may refer to one element/feature being directly joined to (or directly communicating with) another element/feature, and not necessarily mechanically. Likewise, “coupled” may refer to one element/feature being directly or indirectly joined to (or directly or indirectly communicating with) another element/feature, and not necessarily mechanically. However, it should be understood that although two elements may be described below, in one embodiment, as being “connected,” in alternative embodiments similar elements may be “coupled,” and vice versa. Thus, although the schematic diagrams shown herein depict example arrangements of elements, additional intervening elements, devices, features, or components may be present in an actual embodiment. It should also be understood that
The inputs of switch driver circuits D1, D2, D3, D4, . . . , Dn are respectively coupled to outputs of B1, B2, B3, B4, . . . , Bn of controller 22 that, in turn, has an input coupled to a signal Tin that is indicative of a desired temperature or heating level. Tin may result from the manipulation of a control mechanism located within the passenger compartment of a vehicle that is necessary to achieve a desired temperature. Controller 22 converts Tin to the plurality of output signals (B1, B2, B3, B4, . . . , Bn) corresponding to a digital representation of Tin (with B1 corresponding to the least significant bit, B2 corresponding to the next least significant bit, and so on) so as to selectively turn power switches Q1, Q2, Q3, Q4, . . . , Qn ON via drivers D1, D2, D3, D4, . . . , Dn to achieve the desired heating level. For example, if the digital representation of Tin is 01010 . . . 0, only power switches Q2 and Q4 will be turned on resulting in current being drawn through the two heating elements in H2 and the eight heating elements in H4.
Stage 0/15 corresponds to minimum power; (i.e. B1=B2=B3=B4=0; thus, Q1=Q2=Q3=Q4=OFF). In this case, little or no current flows through any of the heating elements in banks H1, H2, H3, and H4 and therefore little or no heat is generated. Stage 15/15 corresponds to maximum power; (i.e. B1=B2=B3=B4=1; thus, Q1=Q2=Q3=Q4=ON). In this case, current flows through the single heating element in H1, the two heating elements in H2, the four heating elements in H3, and the eight heating elements in H4, thus generating maximum heat. It should thus be apparent that between minimum power stage 0/15 and maximum power stage 15/15, power may be incrementally increased or decreased as is shown by stages 2/15 through 14/15.
Certain strategies can be employed to minimize peak currents; for example, by providing a suitable time delay before switching up to the next power stage depending upon the relative size of the total resistance of the heating stage being turned ON. This permits in-rush current to subside prior to switching up to the next power stage. In addition, when maximum heating power is required, the process should begin with power stage 8/15, jump to power stage 12/15, then jump to power stage 14/15, and finally jump to power stage 15/15. This procedure minimizes peak currents and reaches maximum power as quickly as possible.
Peak currents can be further minimized by utilizing brake-before-make switching. For example, when switching from power stage 11/15 to power stage 12/15, Q1 and Q2 should be turned OFF before turning Q3 ON.
Thus, there has been described a simplified heater control system particularly suitable for controlling a PTC heater. The system employs power/current amplitude modulation through the use of discrete power switches that drive various banks of heating elements. The result is a control system characterized by lower cost, reduced electromagnetic compatibility, and improved control.
The above description is given by way of example only. Changes in form and details may be made by one skilled in the art without departing from the scope of the invention. For example, heating banks H1, . . . Hn have been illustrated as being configured in a binary configuration where each successive heating bank comprises two times the number in the previous heating bank. It should be appreciated that other configurations and/or groupings may be employed. For example, the number of heating elements in each grouping Hn may equal Xn-1 where X is a positive integer.
