Method for reducing magnetic field emission of a heated seat

Abstract

A system and method for controlling a power MOSFET to limit electromagnetic interference from a load is provided. The system and method includes a pulse width modulation (PWM) control voltage to operate the power MOSFET in accordance with an ohmic region (linear mode) of the power MOSFET. By operating the power MOSFET in the ohmic region of the power MOSFET, electromagnetic fields generated by the load are reduced without the need for a dedicated DC / DC converter that would otherwise increase the cost, size, and weight of the power electronics.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of U.S. Provisional Application 62 / 882,001, filed August 2, 2019, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] This invention relates to the operation of power MOSFETs in switching applications to reduce emissions and interference from electromagnetic fields, optionally for use in temperature-controlled vehicle seats. The invention can also reduce inrush current or startup current in capacitive and inductive loads, thereby providing cost savings for switching operations. Background Technology

[0004] Electric seat heaters for motor vehicles include a heating pad for converting electrical energy into heat. The heating pad includes a resistance heating element positioned near the trim cover, typically in a spiral pattern. Modern heating pads are characterized by a short preheating phase and uniform heat distribution to provide comfort in cold weather conditions and during long drives.

[0005] Heating pads are typically controlled by power MOSFETs. The power MOSFET provides pulse-width modulated drain-source current (Id). DS This heating pad is used to regulate the heat output of the seat cushion. However, when the heating pad is powered on, electromagnetic emissions or interference can be undesirably high, sometimes exceeding the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines for time-varying electromagnetic fields. Because the resistance of the heating pad changes with temperature, electromagnetic emissions can be particularly problematic at lower temperatures when the resistance of the heating pad decreases.

[0006] Therefore, there remains a need for a system and method for regulating the power directed to a heating pad or other resistive load in order to minimize the electromagnetic fields generated, particularly during the switching on and off of the resistive load. Summary of the Invention

[0007] A system and method are provided for controlling a power MOSFET to limit electromagnetic emissions and interference from a load. The system and method include pulse-width modulation (PWM) control voltage to operate the power MOSFET according to its ohmic region (linear mode). By operating the power MOSFET within its ohmic region, electromagnetic fields generated by the load are reduced without requiring a dedicated DC / DC converter, which would otherwise increase the cost, size, and weight of the electronic control module.

[0008] In one embodiment, a control circuit includes a power MOSFET and a resistive load, such as a heating pad. The power MOSFET includes a PWM gate voltage (Vm) connected to control the current flowing through it. GS The gate terminal of the power MOSFET is connected in series with the heating pad. The PWM gate voltage (V) is iteratively selected. GS The frequency and duty cycle of the power MOSFET are adjusted so that it operates only within its ohmic region, which can be measured, for example, optionally by the rate of change of current (di / dt) through a shunt resistor. Furthermore, the invention can control the rate of change of current (di / dt) and the rate of change of voltage (dv / dt) of any load, including resistive, capacitive, and inductive loads.

[0009] This method is uniquely adapted to dynamically determine the ohmic operating range of a power MOSFET for a given resistive load without prior knowledge of that resistive load. For example, a general-purpose power module can provide electromagnetic interference mitigation for a wide range of heating pads by internally calibrating its power MOSFET control voltage without requiring specific knowledge of the heating pad. An additional embodiment involves winding a heater coil in a first direction and a second opposite direction such that the magnetic field is canceled out along the length of the heater coil.

[0010] These and other features and advantages of the invention will become apparent from the following description of the invention when viewed in conjunction with the accompanying drawings and claims. Attached Figure Description

[0011] Figure 1 It is an operating region diagram that depicts the output characteristics of a power MOSFET, including the ohmic (linear) region and the saturation region.

[0012] Figure 2 This includes frequencies used to mitigate electromagnetic interference from resistive loads. f ) and duty cycle ( D Circuit diagram of a power MOSFET controlled by a controller.

[0013] Figure 3 The diagram illustrates the frequency of a power MOSFET used to mitigate electromagnetic interference from resistive loads. f ) and duty cycle ( D ) control flowchart.

[0014] Figure 4 The diagram illustrates the gate voltage (V). GS )frequency( f ) and duty cycle ( D ) as heating pad current (I DS The curve of the function ).

[0015] Figure 5 The diagram illustrates the drain-source voltage (V). DS ) and drain-source current (I DS A graph of a function of time. Detailed Implementation

[0016] I. Ohmic operation of power MOSFETs

[0017] In this embodiment, a power MOSFET is used as a switch to control the power flow to an electrical load, and particularly to a resistive heating element in an electric seat heater. As is known in the art, a MOSFET is a three-terminal device in which the gate voltage controls the current flow between the source and drain. The system and method of the present invention limit electromagnetic emissions and interference from the electric seat heater by operating the power MOSFET in its ohmic region. Before discussing the system and method in more detail, the general operation of the power MOSFET in its ohmic region will now be described.

[0018] refer to Figure 1 The output characteristics of an exemplary power MOSFET are shown, where the drain-source current (Id) is plotted on the y-axis. DS ), and the drain-source voltage (V) is plotted on the x-axis. DS In the Ohmic region (left), the drain-source current (I) DS ) and drain-source voltage (V DS The relationship between the drain and source currents follows Ohm's law because the drain-source current (Id) is... DS ) and drain-source voltage (V DS The ohmic region of this power MOSFET is linearly proportional to the ohmic region. Therefore, the ohmic region of this power MOSFET is often referred to as "linear mode".

[0019] In the saturation region (right side), the drain-source current (Id) DS Almost independent of the drain-source voltage (V) DS ), and instead heavily relies on gate voltage (V GS In other words, the load current (I) DS First, in the ohmic region, as the load path voltage (V) increases... DS As the load path voltage (V) increases, and then, in the saturation region, as the load path voltage (V) increases, the load path voltage (V) increases. DS The increase is only slight or disproportionate. For higher control voltages (V), GSThe saturation point at which it begins is relatively high, for example, 100A for a 2.9V control voltage and 800A for a 4V control voltage. To reduce electromagnetic interference (EMI) in the load, it is desirable for the MOSFET to operate only in its linear mode (ohmic region) or at the transition between the ohmic and saturation regions. As explained in Section II below, this is achieved by controlling the gate voltage (V... GS This is achieved through duty cycle and frequency. For example, the gate voltage (V) GS A pulse width modulation (PWM) signal allows the frequency and duration of individual control pulses, as well as the time interval between two consecutive pulses, to be varied.

[0020] II. Systems and methods for mitigating EMI

[0021] Now for reference Figure 2 The diagram illustrates a control circuit for mitigating EMI by operating the power MOSFET essentially within the ohmic region of the power MOSFET. The control circuit includes a resistive load 10, and more particularly a resistive heating element, connected in series between a 16V power supply 12 and the drain terminal of the power MOSFET 14. The MOSFET source terminal is connected for measuring the drain-source current (Id). DS The shunt resistor 16, the drain-source current (I) DS The current also flows through the resistive load 10. An additional shunt resistor 18 is shown connected in series between the power supply 12 and the resistive load 10 to measure the power output of the power supply 12.

[0022] For example Figure 2 As shown, the gate voltage (V) of power MOSFET 14 GS The first and second switching elements Q1 and Q2 are controlled by actuation and are depicted as BJT transistors, but may include other switching elements in other embodiments. Specifically, the first and second switching elements Q1 and Q2 cause the power MOSFET 14 to receive a gate voltage (V0) with a desired duty cycle and desired frequency. GS When switching element Q2 is closed, DC power supply 20 is selectively coupled to the gate terminal of MOSFET 14, which in turn closes in response to the actuation of switching element Q1. Switching element Q1 is voltage-controlled based on the output of power supply 22, which may optionally be microcontroller-controlled. Because power MOSFET 14 includes a short switching time through its ohmic region, in linear mode, the gate voltage (V0) remains constant regardless of the load. GS Millisecond duty cycle control is possible.

[0023] Combination Figure 3Functional block diagram to illustrate Figure 2 The operation of the control circuit. As indicated above, the gate voltage (V GS Duty cycle ( D —It is the ratio of the MOSFET's on-time to its switching time —as the gate voltage frequency ( f The method includes utilizing pulse width modulation (PWM) to change the gate voltage (V). GS The power MOSFET 14 is activated at a baseline frequency (e.g., 100 kHz) and a duty cycle (e.g., 0.02). At step 42, the method includes measuring the current (IC) in the load path. DS In this embodiment, the measurement is performed by measuring the voltage across the shunt resistor 16, which, as noted above, is connected in series with the resistive load 10. At determination step 44, the measured current (I...) is... DS ) and current limit value (I DS-Limit ) for comparison. If the measured current (I) DS Not less than the current limit value (I) DS-Limit The method includes maintaining the pulse-width modulated gate voltage (V) at step 46. GS ) frequency ( f ) and duty cycle ( D If the measured current (I) DS ) less than the current limit (I) DS-Limit The method then includes iteratively increasing the frequency ( f Decrease (taper) and iteratively increase the duty cycle ( D ) Increase (increment). For example, the frequency can be decreased by 2kHz at step 48. f The kHz frequency was reduced to 98 kHz, and the load current (I) was measured at step 50. DS If the current limit value (I) is satisfied at step 52. DS-Limit The method includes returning to the immediately preceding frequency at step 54. If the current limit value (I) is not met at step 52... DS-Limit If the duty cycle is increased at step 56, optionally by an increment of 0.02 up to a duty cycle of 0.04. D ). Measure the load current (I) again at step 58. DS ), and at step 60, the load current (I) DS ) and current limit value (I) DS-Limit Compare with the current limit value (I). If the current limit value is not met... DS-Limit Then, at step 48, the frequency is iteratively increased again ( f ) and duty cycle ( D), and measure the load current (I) at step 50. DS However, if the load current (I) DS ) equal to or greater than the current limit value (I DS-Limit If the method includes returning to the immediately preceding duty cycle at step 62, then the method includes... In other embodiments, the gate voltage is a digital output provided by a controller (e.g., a proportional-integral-derivative controller).

[0024] To further illustrate the frequency and duty cycle control of the power MOSFET Figure 4 The gate voltage (V) is plotted in the figure. GS ) frequency ( f ) and duty cycle ( D ) as heating pad current (I DS A function of ). For example Figure 4 As shown, the pulse width modulated gate voltage (V GS The frequency (solid line) is included, which decreases from 100kHz to approximately 30kHz as the corresponding increase in the heating pad current increases. Interleaved with this decrease in frequency is the pulse width modulation gate voltage (V). GS The duty cycle (dashed line) is as follows: as the corresponding increase in the heating pad current, this duty cycle increases from zero to approximately one. The linear operating mode of the power MOSFET in... Figure 4 The value is shown as being between 4A and 6.5A. In this example, the current limit (I) DS-Limit The current is selected to be approximately 6A, thereby ensuring that the power MOSFET operates in its ohmic region, which has a gate voltage frequency between 30kHz and 50kHz and a duty cycle (pulse width %) between 0.3 and 0.6. Figure 5 Drawing in Figure 2 The drain-source voltage (V) of the circuit DS ) and drain-source current (I DS The drain-source current (Id) is a function of time. In this simulation, the drain-source current (Id) is... DS (As the gate voltage duty cycle increases from zero) it gradually increases from zero to a maximum value of 8A, while the drain-source voltage (V) DS It decreases within the same time interval.

[0025] Laboratory tests revealed the ability of the system and method of this invention to reduce electromagnetic interference. Specifically, electromagnetic emissions were evaluated during the switching on and off of the 7A heating pad. During the on-time period, only 77% of the ICNIRP criterion for time-varying electromagnetic fields was detected. During the off-time period, only 65% ​​of the ICNIRP criterion for time-varying electromagnetic fields was detected. The system and method were also found to reduce the starting current of the heating pad at lower temperatures, resulting in greater flexibility for the heating pad.

[0026] In the aforementioned operating mode, when the load current (I) DS Gradually increase to the current limit value (I) DS-Limit When the load current change rate (di / dt) is reached, the controller monitors the load current change rate (di / dt). The load current change rate (di / dt) is maintained below a predetermined threshold stored in computer-readable memory, and by extending the load path voltage (V... DS The rate of change of the load current (di / dt) is also kept below a predetermined threshold. As an alternative operating mode, the rate of change of the load current (di / dt) is monitored via shunt resistor 16. At the gate voltage (V... GS As the current change rate (di / dt) increases, i.e., as di / dt falls below the threshold, the controller (not shown) interprets this situation as a transition point between linear and saturation modes. In this case, the controller varies the PWM gate voltage (V... GS The system iteratively identifies the range of PWM control voltages for a given load, within which the rate of change of current (di / dt) remains positive (above this threshold) throughout the entire activation of the power MOSFET. In various embodiments, the gate voltage is between 0 and 2.9V, the duty cycle is between 0.2 and 0.8, and the frequency is between 20kHz and 80kHz. For example, as... Figure 4 As shown, when the MOSFET is operating only in linear mode, a frequency between approximately 40 kHz and 60 kHz delivers up to 7 A of current to the heating pad.

[0027] As noted above, by operating the power MOSFET in its linear mode, a significantly lower electromagnetic field was found in the series-connected heating pads. Because this method operates iteratively, it is uniquely adapted to dynamically determine the linear mode of the MOSFET for a given resistive load without prior knowledge of the load itself. As a result, the invention can be implemented in conjunction with a wide variety of heating pads. For example, a general-purpose power module can provide EMI mitigation to a broad range of heating pads by internally calibrating its power MOSFET during normal operation.

[0028] As an alternative solution, the resistive element can extend in a spiral pattern in a first direction and in the same spiral pattern in a second direction opposite to the first direction, such that the magnetic field generated by the resistive element is self-cancelling. In other words, the resistive element can be double-backed within the heating pad, such that at any given point along the spiral winding, the two segments of the resistive element sit side by side. Therefore, the magnetic field generated by the DC current in the first direction is canceled out by the magnetic field generated by the DC current in the second opposite direction. In this respect, EMI generated by the heating pad is reduced at the load, rather than through the operation of the power MOSFET in linear mode. In yet another embodiment, the heating pad manufactured according to this configuration can be used with a power MOSFET in linear mode to further reduce EMI from the heating pad.

[0029] The above description is of the present embodiments of the present invention. Various changes and modifications can be made without departing from the spirit and broader aspects of the invention. This disclosure is presented for illustrative purposes and should not be construed as an exhaustive description of all embodiments of the invention or as limiting the scope of the claims to the specific elements illustrated or described in connection with these embodiments. Any reference to an element in the singular (e.g., the use of the articles “a,” “an,” “the,” or “described”) should not be construed as limiting that element to the singular.

Claims

1. A method for pulse width modulation (PWM) control of a power MOSFET having a load path, the method comprising: (a) Provide an electrical load connected in series with the load path of the power MOSFET; (b) The power MOSFET is activated using a PWM gate voltage having a frequency and a duty cycle; (c) Measure the current in the load path of the power MOSFET during activation; (d) Compare the measured current in the load path of the power MOSFET with the current limit value; (e) Iteratively decrease the frequency of the PWM gate voltage and iteratively increase the duty cycle of the PWM gate voltage until it is determined that the measured current in the load path exceeds the current limit value, and measure the current in the load path after each PWM gate voltage modification; as well as (f) If the measured current in the load path is not less than the current limit, then resume the frequency or duty cycle immediately preceding the PWM gate voltage.

2. The method according to claim 1, wherein the electrical load is a resistive element of the electric seat heater.

3. The method of claim 1, wherein the electrical load is coupled to the drain or source terminal of the power MOSFET.

4. The method of claim 1, further comprising a capacitor coupled in parallel between the gate terminal and the source terminal of the power MOSFET.

5. The method of claim 1, wherein varying the frequency and duty cycle of the PWM gate voltage comprises: Alternately decrease the frequency of the PWM gate voltage and increase the duty cycle of the PWM gate voltage.

6. The method of claim 1, wherein steps (b) to (f) are performed by a proportional-integral-derivative controller or by referencing a lookup table stored in memory.

7. A system for pulse width modulation (PWM) control of a power MOSFET, the system comprising: A power MOSFET that has a load path; An electrical load, which is connected in series with the load path of the power MOSFET; A current sensor adapted to measure the current value in the load path of the power MOSFET; A voltage source adapted to provide a variable PWM gate voltage to the power MOSFET, wherein the voltage source is operable to iteratively decrease the frequency of the PWM gate voltage and iteratively increase the duty cycle of the PWM gate voltage until a measured current in the load path is determined to exceed a current limit based on the output of the current sensor.

8. The system of claim 7, wherein the electrical load is a resistive element of the electric seat heater.

9. The system of claim 7, wherein the voltage source comprises at least one microcontroller-controlled switching element.

10. The system of claim 7, wherein the voltage source comprises a proportional-integral-derivative controller.

11. The system of claim 7, further comprising a capacitor coupled in parallel between the gate terminal and the source terminal of the power MOSFET.

12. The system of claim 7, wherein the frequency of the PWM gate voltage is between 20 kHz and 80 kHz, and wherein the duty cycle of the PWM gate voltage is between 0.2 and 0.

8.

13. The system of claim 7, wherein the electrical load is coupled to the drain or source terminal of the power MOSFET.

14. The system of claim 7, wherein the electrical load comprises a capacitive element or an inductive element.

Images