When the high-power switching amplifier circuit based on IGBT operates, the power is large and the voltage is high. It is often impacted by capacitive or inductive loads, and is overloaded. The operating area of ​​the device exceeds the given safe working area. At the same time, due to the resistance of the IGBT Poor electrical stress impact capability can easily lead to device damage.

Therefore, it is very necessary to design the protection circuit of the IGBT power amplifier circuit to solve the potential fault factors such as bridge arm shoot-through, overcurrent, and overheating of the IGBT.

Different forms of circuit protection

In view of the three problems mentioned above, such as bridge arm shoot-through, overcurrent, and overheating, this paper proposes different forms of circuits to meet the protection requirements of IGBT devices.

Drive signal detection (to prevent bridge arm from passing through): In the IGBT-based power switch amplifier circuit, the drive signal must be a square wave signal with a dead zone. Once the signal “dead zone” disappears, the control sequence will be disordered, resulting in the same side bridge. The result is that the IGBT device is damaged, and the response time of the control “dead zone” must be microseconds; although discrete devices can be used to design the detection circuit, the debugging is more cumbersome; this paper chooses CPLD as the carrier of the logic circuit to play its role in The ease of use in logic circuit design, write the corresponding detection circuit code, program the CPLD, and obtain the core processing unit of the pulse width detection circuit of the driving signal. At the same time, design the corresponding external signal interface circuit, so as to realize the driving signal “Dead-band” detection of the pulse width;

Overcurrent protection: The overcurrent of the IGBT power switch circuit will cause the IGBT to exit the saturation region abnormally, thereby causing damage to the IGBT; high-speed devices such as Hall sensors, comparators, and drivers are used to design an “overcurrent” protection circuit to prevent overcurrent. The protection circuit can control the IGBT as the DC power switch, so that the DC power supply can accept the opening and closing control of the protection circuit, which is a feasible method.

Overheating protection: monitor the junction temperature of the IGBT, when the junction temperature exceeds the set value, output an alarm, cut off the input of the IGBT’s DC power supply and its driving signal, and design a suitable circuit, so as to achieve the protection of the IGBT-based power switch circuit Purpose.

Design of protection circuit

Overcurrent detection

The design schematic diagram of the overcurrent detection circuit is shown in Figure 1. In the main power supply circuit of the IGBT of the power amplifier circuit, a current transformer is installed in series, and the current transformer samples and outputs the DC current value passing through the IGBT in real time; the sampled value is sampled The resistor converts the current signal into a voltage signal and sends it to the input of the comparator. The threshold voltage of the comparator is adjusted and set by the potentiometer RP (in actual engineering, the set value can be obtained by pre-calculation or by experiment) ;In this way, the voltage on the sampling resistor and the threshold voltage are compared by the comparator; when the “overcurrent” of the DC power supply is detected, that is, the voltage of the overcurrent signal at both ends of the sampling resistor RM exceeds the preset threshold voltage upper limit, and the voltage comparison The device responds to the action and outputs a high level, this level signal triggers the conduction of the thyristor Q1, then the resistor R2 node in series with the cathode of the thyristor will output a level signal, this signal is in a continuous lock state, this power The flat signal (that is, the overcurrent output signal response action) is output to the signal driver board of the IGBT, and the driver board immediately blocks the output of the driving signal, so that the IGBT is disconnected, the DC power supply provided to the power amplifier circuit is cut off, and the IGBT device is protected.

 

 

Figure 1: Principle of overcurrent detection circuit

In addition, when the overcurrent protection circuit outputs the overcurrent signal, the LED turns on and emits light, indicating the detection of the overcurrent signal; to determine that the overcurrent state of the IGBT power amplifier circuit disappears, the switch S can be closed by external control, and the thyristor Reset, cancel the drive signal output lock level signal of the IGBT driver.

Drive signal detection

The two driving signals on the same bridge arm based on the IGBT power amplifier circuit must have a “dead zone” time, that is, they cannot be high at the same time, otherwise the bridge arm of the switching circuit will be connected and short-circuited, and the pulse of the driving signal will be too high. Wide and too narrow, as well as no pulse output, will seriously affect the stable operation of the power amplifier circuit.

When the power switch amplifier circuit uses a fixed drive signal, the pulse width detection method can be used, an appropriate frequency signal can be selected as the count time scale unit, the high level of the drive signal can be continuously counted, and the number of counted values ​​can be counted. Converting to time gives the pulse width value.

Circuit implementation: Set a preset count threshold value in the CPLD, that is, the maximum count value of the high level or low level of the drive signal; continue to count the high level or low level of the input drive signal, and the sample will be sampled. The obtained unit count value is compared with the preset threshold count value. If the count value is less than or greater than the preset value, the detection module outputs a blocking signal, cuts off the signal output of the drive signal output module, and completes the high-level pulse of the drive signal. Width detection; at the same time, the module converts the drive signal to inverse logic, and then detects the high-level pulse width formed after the conversion, so that the low-level pulse width detection of the drive signal is realized.

 

 

Figure 2: Pulse width detection logic diagram

The drive signal detection module includes three modules: counting, latching and control, as shown in Figure 2. Among them, the counting module counts the number of time-scale frequency signals at the high level of the driving signal, and outputs a latch clear signal, the latch module latches the data, and the control module determines the number of time-scale frequency signals within the high level of the driving signal. Whether the number exceeds the preset threshold.

Overheat detection

The power loss of IGBT increases with the increase of switching frequency. During high power operation, the power loss tends to increase rapidly and generate heat. Since the junction temperature of IGBT does not exceed 125°C, the power switching circuit based on IGBT cannot work at the upper limit of junction temperature for a long time. Otherwise, the IGBT will be overheated and damaged. Therefore, a temperature detection circuit is designed to detect the junction temperature of the IGBT to achieve the purpose of protection.

When designing an overheat protection circuit, first select an appropriate operating temperature value as the alarm and protection action threshold value of the external control system. The circuit design can be implemented in a mode similar to the overcurrent detection circuit, which is suitable for the comparison circuit; The mounted temperature measurement module is installed on the surface of the radiator of the IGBT module, and the temperature sensor monitors the operating temperature of the output IGBT.

In this way, when the temperature exceeds the preset alarm temperature critical point, the protection circuit can sample the temperature signal in time, and make the overheat protection circuit respond and output.

In addition, for the IGBT power amplifier circuit that uses water circulation for heat dissipation, set the cooling water flow monitoring to detect the cooling water flow in real time when the power amplifier unit is working, and monitor whether the flow exceeds the flow limit value required by the appropriate temperature value when the IGBT is working; when the cooling water flow rate When the flow rate is lower than the set lower limit, the overheat detection circuit outputs an overheat protection signal.

 

 

When the above two overheat detection circuits are actually running, the signal output by the overheat alarm trigger is logically associated with the drive signal of the IGBT drive board of the DC power switch, which blocks the drive signal output of the IGBT driver and cuts off the IGBT DC power supply of the power amplifier. supply power, thereby protecting the IGBT devices of the power switching circuit.

Simulation and experiment

For the driving signal pulse width detection circuit designed above, the function is verified by simulation. The simulation results are shown in Figure 3. When the driving signal (signal) pulse is too narrow, the detection module outputs a high level (signal_error) signal, which means that the driving When the high-level signal pulse width of the signal is less than the upper limit of the preset pulse width threshold, a high-level alarm is output.

Experimental verification

In order to further verify the effectiveness of the protection circuit designed in this paper, an actual test is carried out on the IGBT power amplifier. Figures 4 and 5 illustrate the experimental waveforms after the overcurrent protection is activated. The curves in the figures are the output voltage waveform and output current waveform of the power amplifier respectively.

Observing the results of the analysis curve, it is concluded that when the IGBT is running under different working parameters, once the overcurrent signal is detected by the protection circuit, the IGBT driver board blocks the output of the driving signal, the IGBT is turned off, and the voltage and current of the IGBT power amplifier circuit pass through. In a very short time to zero, the IGBT is protected.

Epilogue

This paper introduces three protection methods based on IGBT power amplifiers: an IGBT DC power supply overcurrent detection circuit, a CPLD-based drive signal pulse width detection logic circuit, and a circuit designed to detect the IGBT junction temperature; The function of the protection circuit is verified by example simulation and experimental examples.

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