HEATING DEVICE

Disclosed is a heating device having a heating resistor assembly with a positive terminal, a negative terminal, and heating resistors connected in series between the positive terminal and the negative terminal, and a transistor switch connected in series with the heating resistors. A first half of the heating resistors connects the positive terminal to the transistor switch and a second half of the heating resistors connects the transistor switch to the negative terminal.

RELATED APPLICATIONS

This application claims priority to EP 23 172 379.2, filed May 9, 2023, the entire disclosure of which is hereby incorporated herein by reference.

BACKGROUND AND SUMMARY

This disclosure relates to a heating device, especially for an automobile, of the type generally known from, e.g., U.S. Publication No. 2022/0082297 A1.

Heating devices, like flow heaters, comprising a heating resistor assembly, e.g., a metallic heating plate with a resistive track, are used in automobiles for heating various fluids, e.g., air, water or aqueous solutions. As the on-board supply voltage of automobiles has been increased to several hundred volts, it is becoming more difficult to meet stringent requirements for electromagnetic compatibility, especially if the heating device is operated with a pulse width modulated voltage.

This disclosure shows how electromagnetic compatibility of a heating device for high voltage applications can be improved at low cost.

A heating device according to this disclosure comprises a heating resistor assembly which comprises a positive terminal, a negative terminal, and heating resistors connected in series between the positive terminal and the negative terminal.

According to this disclosure, electromagnetic emissions are minimized in such a way that a first half of the heating resistors connects a positive terminal of the heating resistor assembly to a transistor switch and a second half of the heating resistors connects the transistor switch to a negative terminal of the heating resistor assembly. The heating load is split into a first half and a second half arranged on a high side and a low side of the transistor switch, respectively. The transistor switch is arranged between the first half and the second half of the heating resistors. Parasitic capacitances of the heating load are thereby separated into parasitic capacitances of the first and the second half of the heating resistors and are thereby balanced.

When the transistor switch is open, parasitic capacitances between the first half of the heating load and the parasitic capacitances of the second half of the heating load are of opposite polarity so they are charged in opposite direction. When the transistor switch closes, the parasitic capacitances are discharged between them. The system is thereby balanced and currents charging and discharging the parasitic capacitances of each half of the heating load cancel each other to a large extent, reducing the common mode current generated by the system, especially if both halves of the heating load are electrically symmetrical. Radiated and conducted emissions are thereby greatly reduced and electromagnetic compatibility improved, which is especially important for heating devices in automobiles.

In a refinement of this disclosure, the heating resistor assembly is a heating plate comprising a metal sheet, on which the heating resistors are arranged as resistive tracks, and an insulation layer insulating the resistive tracks from the metal sheet. A first half of the resistive tracks then connects the positive terminal to the transistor switch and a second half of the resistive tracks connects the transistor switch to the negative terminal. Parasitic capacitances of the heating load are thereby separated into parasitic capacitances between the first half of the resistive tracks and the metal plate and parasitic capacitances between the second half of the tracks and the metal plate.

When the transistor switch is open, the parasitic capacitances between the first half of the tracks and the metal plate and the parasitic capacitances between the second half of the tracks and the metal plate are of opposite polarity so they are charged in opposite direction. When the transistor switch closes, the parasitic capacitances are discharged between them. The system is thereby balanced and the currents both charging and discharging of the parasitic capacitances of each half of the resistive tracks cancel each other to a large extent, reducing the common mode current generated by the system, especially if both halves of the resistive tracks are electrically symmetrical. Radiated and conducted emissions are thereby greatly reduced and electromagnetic compatibility improved. In a refinement of this disclosure, both halves of the resistive tracks load are physically or geometrically symmetrical. Thereby electrical symmetry can be achieved more easily to a larger extent.

As described above, the heating resistor assembly may be a heating plate with resistive tracks. In another embodiment of this disclosure, the heating resistor assembly may comprise PTC resistor plates or blocks, e.g., in heating rods of an air heater.

DESCRIPTION

The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of this disclosure.

The heating device shown inFIGS.1and2is a flow heater for an automobile. The flow heater has a housing1with an inlet2and an outlet3as well as electrical connectors4,5and a heating resistor assembly provided as a heating plate6. A flow channel for liquid to be heated extends in the housing1from the inlet2to the outlet3.

The flow channel runs along a heating plate6. In order to improve heat dissipation to the liquid in the flow channel, the metal plate6may carry a corrugated sheet7, which protrudes into the flow channel. On its dry side the heating plate6shown inFIG.4comprises heating resistors10provided in the as resistive tracks. In operation, heating current flows through these resistive tracks10of the heating plate6. In a dry compartment of the housing1is control circuitry8for controlling heating power by pulse with modulation of heating current.

The heating resistors10of the heating plate6may be electrically connected to the control circuitry by means of press-fit contacts that are pressed into openings of a circuit board and soldered to the heating plate6.

FIG.3shows schematically a circuit diagram of the flow heater that is configured to be operated at high voltage, e.g., voltage in the rage of 100 V to 1 kV. The control circuitry comprises a driver20that operates a transistor switch21for pulse width modulation. Switch21is a high side switch. While it is closed, a duty cycle of pulse width modulation is applied and heating current flows through a heating resistor provided as a resistive track of the heating plate6. As can be seen inFIG.3, the high side switch21splits the resistive tracks and thereby the heating resistors10into a first half10aand a second half10b. The switch21is arranged between the first half10aand the second half10bof the heating resistors. When switch21is open, the first half10aof the heating resistors10is on positive potential and the second half of the heating resistors10is on negative potential. Both halves10a,10bform a capacitance with a metallic substrate11of the resistive tracks, e.g., a plate made of steel or aluminum. The metallic substrate11may be grounded by an electrical connection to mass.

Parasitic capacitances of the first half10aof the resistive tracks and parasitic capacitances of the second half10bof the resistive tracks are therefore charged with opposite polarity. Therefore, any radiated and conducted emissions caused by these parasitic capacitances are canceled to a large extent, especially if the heating plate6is designed symmetrically on both side of the transistor switch21.

FIG.4shows an illustrative embodiment of the heating plate6. The heating plate6comprises a metal sheet11, e.g., a steel or aluminum sheet, as a substrate and heating resistors10provided as resistive tracks that are electrically insulated from the metal sheet11by means of an insulating layer12arranged between the metal sheet11and the resistive tracks. The resistive tracks might be printed onto the insulating layer12or deposited on it by other means, e.g., physical or chemical vapor deposition.

The heating plate6has a positive terminal13for connection to positive potential and a negative terminal14for connection to negative potential of a high voltage source. The positive terminal13and the negative terminal14are provided as solder pads at the ends of the resistive tracks such that a connection to circuit board8may be made by soldering a connector pin to the terminals13,14, e.g., as a surface mounted device.

The transistor switch21shown inFIG.3is electrically connected to solder pads15,16, e.g., by means of connector pins soldered to solder pads15,16and may be arranged on printed circuit board8. As can be seen inFIG.4, a first half10aof the resistive tracks runs from the positive terminal13to solder pad15and thereby electrically connects the positive terminal13to transistor switch21ofFIG.3. A second half10bof the resistive tracks runs from solder pad16to the negative terminal14and thereby electrically connects the transistor switch21to the negative terminal14.

The heating plate6may have a symmetrical design as shown inFIG.4to have electrical symmetry. Then the parasitic capacitances of the first half10aand the second half10bof the resistive tracks are balanced to the largest extent possible and emissions can thus be minimized.

In addition to the transistor switch21for pulse width modulation, the control circuitry may comprise a safety switch22, e.g., a low side switch, that is operated by a driver23, e.g., if overheating is detected.