Film heater open circuit detection and power limiting method

Abstract

The application discloses a kind of membrane heater open circuit detection and power limiting method, it is related to membrane heater technical field, comprising the following steps: step 1. construct heating film grouping resistance value model;Step 2. real-time acquisition and resistance calculation;Step 3. open circuit state determination;Step 4. power dynamic adjustment and equal division control.In the present application, only by a whole current sensor can determine whether there is heating film open circuit and the number of open circuit, and the output power is limited by detection result, ensure that even in the case of failure, the output of each IGBT is still normal.Heating film resistance is relatively stable, with certain TCR effect, that is, the higher the temperature, the resistance will be larger.Current and voltage value can be used to calculate the equivalent resistance of the current heater, and the heater is determined whether a certain heating film is open circuit, and protection and control are carried out accordingly.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of film heater, in particular to a film heater open circuit detection and power limiting method. BACKGROUND

[0002] The film heater is used for heating cooling liquid and providing a heat source for a whole vehicle. When the required power of the heater is large, the resistance of the heating film is too small, which can cause large output current. The high current needs to be matched with IGBTs with higher current resistance. In order to limit the output current and adapt to IGBTs with lower current resistance, the heating film can be divided into several groups. Each group is independently controlled by corresponding phase shift control, the bus current can be reduced, and the IGBTs are prevented from being damaged by the large current.

[0003] Under the normal working output condition, the current of each heating film of the heater does not exceed the current resistance of the IGBT. However, when the open circuit fault of the heating film cannot be opened, the required power is provided by the remaining normal heating film. This can cause the current passing through the normal IGBT to increase, the temperature of the heating film to rise, and the risk of further damaging the heater to exist. Therefore, the state of the fault path needs to be identified. When the open circuit fault of the heating film in a certain path is detected, the requested power is reduced, the current output in the normal path is prevented from exceeding the IGBT limit, and the normal path is prevented from being affected by the fault.

[0004] In the prior art, in order to meet the heating power and select IGBTs with small current resistance, the heating film of the heater is generally divided into multiple groups. When a fault exists in an individual heating group, the heater needs to be able to detect and protect itself. The detection method can determine whether the group is working normally by detecting the current of each heating group. However, when the number of heating groups is large, setting a current sampling path for each group can increase the product cost and space requirement. SUMMARY

[0005] The application is applied to the film heater on the automobile. The film heater is used for heating cooling liquid and providing a heat source for a whole vehicle system. The application can determine whether the heating film is overheated or has an open circuit by calculating the resistance of the current heating film. If the open circuit fault exists, the output power is limited to prevent the product from being further damaged and improve the product reliability.

[0006] To achieve the above object, the application provides a technical scheme: a film heater open circuit detection and power limiting method, comprising the following steps:

[0007] Step 1. Constructing a heating film grouping resistance model:

[0008] Based on the TCR characteristics of the heating film and the grouping parallel structure, the resistance range threshold under different open circuit states is preset, including:

[0009] Normal state, all heating film work:

[0010] The lower limit of the resistance range Rnormal-min=R0 / N;

[0011] The upper limit of the resistance range Rnormal-max=R0 / N x (1+αx((TMAX-25) / 10 6 ))

[0012] The lower limit of the resistance range Rk-open-min=R0 / (N-k) when the kth group is open;

[0013] The upper limit Rk-open-max=R0 / (N-k) x (1+αx((TMAX-25) / 10 6 ));

[0014] Wherein, R0 is the cold resistance of a single group of heating film, α is the TCR value (ppm / ℃), Tmax is the safety temperature limit, and N is the total number of groups of heating film;

[0015] Step 2. Real-time acquisition and resistance calculation

[0016] The bus current I is acquired by a single current sensor, the input voltage U and the actual duty cycle D of IGBT are acquired synchronously, and the current overall resistance is calculated according to the formula:

[0017] Rcurrent=UD / I;

[0018] Wherein, the acquisition method of input voltage U and IGBT actual duty cycle D is as follows, first, the power output is controlled by the film heater:

[0019] (1) The heating receives the opening request and power value request W transmitted by communication;

[0020] (2) The heater can acquire the current voltage value U in real time through the sensor, in order to output the power request value W, the target current value I required to be output can be obtained by I=W / U;

[0021] (3) The software controls the IGBT duty cycle D to adjust the output current of the heater, when the output current is less than I, the duty cycle D is increased, when the total output current is greater than I, the duty cycle D is reduced, until the output current reaches I, the value of D is kept unchanged;

[0022] (4) As described above, the value of D is controlled by software to adjust the output, so as to acquire in real time through software.

[0023] Step 3. Open circuit state determination

[0024] Compare Rcurrent with the preset threshold value of step 1:

[0025] If Rcurrent∈(Rnormal-min, Rnormal-max), determine no open circuit;

[0026] If Rcurrent∈(Rk-open-min, Rk-open-max), determine k open circuits;

[0027] If Rcurrent> 50Ω or I = 0, determine all open circuits;

[0028] Step 4. Dynamic power adjustment and equalization control

[0029] If k open circuits are determined, adjust the output power to (N−k) / N of the original target power;

[0030] The adjusted power is evenly distributed by the (N−k) groups of normal working heating films, ensuring constant output power of each group and avoiding overcurrent.

[0031] In one embodiment, in step 1 of the embodiment: N = 3, R0 = 33.36Ω, α = 1500ppm / ℃, Tmax = 300℃;

[0032] Normal state resistance range: 11.12Ω to 15.707Ω;

[0033] One-way open circuit resistance range: 16.68Ω to 23.5605Ω;

[0034] Two-way open circuit resistance range: 33.36Ω to 47.121Ω.

[0035] Preferably, the power output in step 4 uses phase-shift control:

[0036] Control the phase difference of each group of IGBT output to 360° / N, so that the current waveform is staggered to reduce superposition and ensure equal distribution of power among groups.

[0037] Preferably, in step 1, precision optimization is also included: read the measured R0 and α values recorded in the label of each batch of heating films, update the resistance range threshold and store it to the control chip.

[0038] Preferably, the open circuit state determination logic in step 3 includes:

[0039] Step 3-1: Obtain current resistance

[0040] Read the overall resistance Rcurrent calculated in real time in step 2;

[0041] Step 3-2: Sequential range comparison

[0042] Match the resistance range in the following priority order:

[0043] (1) If Rcurrent∈(Rnormal-min, Rnormal-max), it is determined that there is no open circuit, and the process jumps to step 4 to perform full power output;

[0044] (2) If Rcurrent∈(R1-open-min, R1-open-max), it is determined that there is one open circuit, and the process jumps to step 4 to perform power reduction to (N−1) / N;

[0045] (3) If Rcurrent∈(R2-open-min, R2-open-max), it is determined that there are two open circuits, and the process jumps to step 4 to perform power reduction to (N−2) / N;

[0046] (4) If Rcurrent>R all-open or I=0, it is determined that there are all open circuits, and the process jumps to step 4 to perform zero power output.

[0047] Step 3-3: Fault-tolerant processing

[0048] If Rcurrent does not belong to any of the above ranges, the open circuit state determination result of the last period is maintained.

[0049] Preferably, it also includes a film heater open circuit detection and power limiting system, which has:

[0050] Grouped heating film modules, multiple groups in parallel, each group having consistent cold resistance;

[0051] A single current sensor is arranged in the bus loop to collect the overall current;

[0052] An IGBT driving module drives each group of heating films in a phase-shifted control manner;

[0053] A processing unit is configured to perform:

[0054] Real-time calculation of overall resistance Rcurrent=(U×D) / I;

[0055] Determination of the number of open circuits based on a preset resistance range;

[0056] Dynamic adjustment of output power and equal division according to the normal number of groups.

[0057] Preferably, in the film heater open circuit detection and power limiting system, the IGBT driving module includes multiple independent IGBT circuits, and the control signals of each IGBT have a phase difference of 120°, and the current overlap is reduced through phase-shifted control.

[0058] Preferably, the film heater open circuit detection and power limiting system further includes a resistance calibration module connected to the processing unit, which is used for:

[0059] Reading the measured cold resistance R0 and TCR value α stored in each batch of heating film label;

[0060] Updating the resistance range threshold and storing it to the processing unit to eliminate batch errors.

[0061] Compared with the prior art, the beneficial effects of the present application are:

[0062] 1. In the present application, only one overall current sensor is needed to determine whether there is an open circuit of the heating film and the number of open circuits, and the output power is limited by the detection result to ensure that the output of each IGBT is normal even in the presence of a fault. The resistance of the heating film is relatively stable and has a certain TCR effect, that is, the higher the temperature, the larger the resistance. The current and voltage values can be used to calculate the equivalent resistance of the current heater, and the heater can be determined whether a certain heating film is open, and protection and control are performed accordingly. This scheme can detect open circuits without increasing the cost and space requirements of the device, improving the reliability of the product.

[0063] 2. In the present application, the heating film of the heater is generally divided into multiple groups, so as to meet the heating power and select IGBTs with smaller current resistance. When an individual heating group has a fault, the heater needs to detect it and perform corresponding protection. The detection method can determine whether the group is working normally by detecting the current of each heating group, but when the number of heating groups is large, setting a current sampling for each group will increase the product cost and space requirement. In order to solve this problem without increasing the hardware cost, the present application realizes this open circuit detection method, which can detect the number of heating film groups with open circuit faults in the heater under the condition of multiple heating films and only one sampling, and limit the output power according to the detection result to ensure that the output current and power of each heating film group are consistent with those under normal working conditions. The algorithm realizes that even if part of the heating groups of the heater has an open circuit fault, the heater can still provide heat and ensure that the heating condition of the undamaged heating group remains the same as the original state, avoiding the situation that the product is further damaged while still providing heating function for the system. BRIEF DESCRIPTION OF DRAWINGS

[0064] Figure 1 The flowchart of the open circuit detection and power limiting method of the film heater of the present application;

[0065] Figure 2 The specific flowchart of step 3 in the open circuit detection and power limiting method of the film heater of the present application;

[0066] Figure 3 The output state diagram of the IGBT when controlled by PWM duty cycle in the open circuit detection and power limiting method of the film heater of the present application;

[0067] Figure 4 Another output state diagram when IGBT is controlled by PWM duty ratio in the open circuit detection and power limiting method of the film heater of the application. DETAILED DESCRIPTION

[0068] The technical solutions in the embodiments of the application will be apparently and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all the other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the application.

[0069] Refer to the drawings in the description Figure 1 In the embodiment, the heater is divided into three groups of heating films, the heating films are produced by the same batch process, and the resistances are basically consistent. The cold-state resistance of a single group of heating films is 33.36Ω at 25℃, and the TCR is 1500ppm / ℃. The three groups of identical films are connected in parallel, so that the overall resistance of the heater after parallel connection is 33.36 / 3=11.12Ω at 25℃. According to the safety limit value of 300℃, the overall resistance of the heating film at 300℃ is:

[0070] 11.12×(1+1500×(300-25) / 1000000)=15.707Ω

[0071] It is set that 300℃ is the protection value, and the output is turned off to protect the heater when the overall resistance of the heating film is greater than 15.707. That is, when the overall resistance of the heater is in the range of (11.12Ω-15.707Ω), the heating film works at a temperature of 25℃-300℃, which is the normal working temperature and resistance range of the heating film.

[0072] When one group of heating films is open, the overall resistance of the heating film will change.

[0073] When one group of heating films is open, the remaining two groups are connected in parallel, so that the overall resistance of the heater after parallel connection is 33.36 / 2=16.68Ω at 25℃. According to the safety limit value of 300℃, the overall resistance of the heating film at 300℃ is:

[0074] 16.68×(1+1500×(300-25) / 1000000)=23.5605Ω

[0075] When one group of heating films is open, the overall resistance of the heater is in the range of (16.68Ω-23.5605Ω), and the heating film works at a temperature of 25℃-300℃, which is the working temperature and resistance range of the heater when one group of heating films is open.

[0076] When two groups of heating films are open, one way works, and the overall resistance of the parallel heater at 25°C is 33.36Ω. According to the safety limit of 300°C, the overall resistance of the heating film at 300°C is:

[0077] 33.36*(1+1500*(300-25) / 1000000)=47.121Ω

[0078] When two groups of heating films are open, the overall resistance of the heater is in the range of (33.36Ω-47.121Ω) when the heating film works at a temperature of 25°C-300°C, which is the working temperature and resistance range of the heater when two groups of heating films are open.

[0079] When three groups of heating films are all open, the heater loses the heating function, and the system collects a current of 0. Thus, it can be determined that all three groups of the heater are open.

[0080] The resistance range calculated in the above manner can obtain the current open circuit state of the heater system only by one-way current sampling. The IGBT is controlled by the PWM duty cycle, and the output phase difference of the three IGBTs is 120°, which can ensure that the current of the three IGBTs is as small as possible. The output state is shown in the description. Figure 3 、 Figure 4 .

[0081] The resistance of the three groups of heating films is consistent, the heat dissipation environment is consistent, the output phase difference is 120°, which can ensure that the current and power output of each way are averaged to ensure that the temperature and resistance of each way are consistent.

[0082] During operation, the high-voltage input voltage and bus current are collected in real time. When the target output power is obtained, the duty cycle of the IGBT is controlled and adjusted to output the target value. In this process, the current resistance of the heater is calculated in real time:

[0083] Current resistance=input voltage*IGBT duty cycle / bus current

[0084] The calculated resistance is compared with the overall resistance of the heating film described above, and the open circuit state of the current heater can be obtained.

[0085] Referring to the description Figure 2 ,

[0086] When 11.12Ω≤current resistance≤15.707Ω, the heater is normal and the power is normally output.

[0087] When 16.68Ω≤current resistance≤23.5605Ω, one group of heating films of the heater is open, and the output power is reduced to 2 / 3 of the original output power.

[0088] When 33.36Ω≤current resistance≤47.121Ω, there are two groups of open-circuit heating films in the heater, and the output power is reduced to 1 / 3 of the original output power;

[0089] When the bus current is continuously 0 or the current resistance>50Ω, then three groups of heating films have open-circuit faults, and the output power is 0.

[0090] If the calculated resistance is not in the above range, it is determined that the product state is maintained as the previous state.

[0091] The three groups of heating films have the same state and work output at the same time, and the output power is evenly divided, that is, when there is power output, each group of heating films outputs 1 / 3 of the total output, when an open-circuit fault is detected, the output power is multiplied by the corresponding coefficient, and then evenly divided by the normal group number. Under this logic, when there is an open-circuit fault, it will not cause the output power of the unopened heating film to increase, but ensure that its power output remains unchanged, which will not increase the working temperature and current of the normal heating group, so that it is not affected by other roads, and the working state remains unchanged.

[0092] If you want to further improve the accuracy and accuracy of the resistance range, you can measure the resistance of each batch of heating films and calculate the reference resistance and TCR ppm value after writing it into the chip for storage. This can eliminate the influence of errors. Currently, the label information on each heating film has its reference resistance and TCR ppm value information.

[0093] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements to part of the technical features, and any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A film heater open circuit detection and power limiting method, characterized in that, Comprising the following steps: Step 1. Constructing the heating film grouping resistance value model: Based on the TCR characteristics of the heating film and the grouping parallel structure, preset the resistance value range threshold under different open circuit conditions, including: Normal state, all heating films work: The lower limit of the resistance value range Rnormal-min=R0 / N; Rnormal-max = R0 / N x (1 + a x ((T MAX - 25) / 10 6 )) The kth group is open: the lower limit of the resistance value range Rk-open-min=R0 / (N-k); Upper limit Rk-open-max = R0 / (N-k) x (1 + a x ((T MAX -25) / 10 6 )) ; Wherein, R0 is the cold resistance value of a single group of heating films, unit Ω; Alpha is the TCR value, unit ppm / ℃; Tmax is the safety temperature limit, unit ℃; N is the total number of heating film groups, unit group; Step 2. Real-time acquisition and resistance calculation Through a single current sensor to collect bus current I, synchronously acquire input voltage U and IGBT actual duty cycle D, calculate the current overall resistance value according to the formula: Rcurrent=UD / I; Step 3. Open circuit state determination Compare Rcurrent with the preset threshold in step 1: If Rcurrent∈(Rnormal-min,Rnormal-max), it is determined that there is no open circuit; If Rcurrent∈(Rk-open-min,Rk-open-max), it is determined that the kth group is open; If Rcurrent>50Ω or I=0, it is determined that all are open; Step 4. Power dynamic adjustment and equal distribution control If it is determined that the kth group is open, adjust the output power to (N-k) / N of the original target power; The adjusted power is evenly distributed by the (N-k) groups of heating films working normally, ensuring that the output power of each group is constant and avoiding overcurrent.

2. The film heater open circuit detection and power limiting method according to claim 1, characterized in that: In step 1: N=3, R0=33.36Ω, alpha=1500 ppm / ℃, Tmax=300℃; The resistance value range in the normal state is 11.12Ω to 15.707Ω; The resistance value range of one-way open circuit is 16.68Ω to 23.5605Ω; The resistance value range of two-way open circuit is 33.36Ω to 47.121Ω.

3. The film heater open circuit detection and power limiting method according to claim 1, characterized in that: The power output in step 4 uses phase shift control: Control the phase difference of each group of IGBT to 360° / N, so that the current waveform is staggered to reduce superposition and ensure equal distribution of power in each group.

4. The film heater open circuit detection and power limiting method according to claim 1, characterized in that: In step 1, it also includes precision optimization: read the measured R0 and alpha values recorded in the label of each batch of heating films, update the resistance value range threshold and store it to the control chip.

5. The film heater open circuit detection and power limiting method according to claim 1, characterized in that: The determination logic of open circuit state in step 3 includes: Step 3-1: Get the current resistance value Read the overall resistance value Rcurrent calculated in real time in step 2; Step 3-2: Sequential range comparison Match the resistance value range in the following priority order: (1) If Rcurrent∈(Rnormal-min, Rnormal-max), it is determined that there is no open circuit, and the process jumps to step 4 to perform full power output; (2) If Rcurrent∈(R1-open-min, R1-open-max), it is determined that there is one open circuit, and the process jumps to step 4 to perform power reduction to (N−1) / N; (3) If Rcurrent∈(R2-open-min, R2-open-max), it is determined that there are two open circuits, and the process jumps to step 4 to perform power reduction to (N−2) / N; (4) If Rcurrent>R all-open or I=0, it is determined that there are all open circuits, and the process jumps to step 4 to perform zero power output; Step 3-3: Fault-tolerant processing; If Rcurrent does not belong to any of the above ranges, the open circuit state determination result of the last period is maintained.

6. The open circuit detection and power limiting method of a membrane heater according to any one of claims 1-5, further comprising an open circuit detection and power limiting system of a membrane heater, the system having: grouped heating membrane modules, multiple groups in parallel, each group having consistent cold resistance; a single current sensor arranged in a bus loop to collect overall current; an IGBT driving module to drive each group of heating membranes in a phase-shifted control manner; a processing unit configured to perform: real-time calculation of overall resistance Rcurrent= (U x D) / I; determination of the number of open circuits based on preset resistance value ranges; dynamic adjustment of output power and equal division according to the normal number of groups.

7. The open circuit detection and power limiting method of a membrane heater according to claim 6, wherein in the open circuit detection and power limiting system of the membrane heater, the IGBT driving module includes multiple independent IGBT circuits, and the control signals of each IGBT have a phase difference of 120°, thereby reducing current overlap through phase-shifted control.

8. The open circuit detection and power limiting method of a membrane heater according to claim 6, wherein the open circuit detection and power limiting system of the membrane heater further includes a resistance calibration module connected to the processing unit, configured to: read the measured cold resistance R0 and TCR value α stored in the label of each batch of heating membranes; update the resistance value range threshold and store it in the processing unit to eliminate batch errors. ​ ​ ​

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