2.1 kW electromagnetic induction heating design based on IPD Protect

0 Preface

Due to the advantages of fast heating time, no open flame, high power, high electrical and thermal energy conversion efficiency and low system cost, induction heating has been widely used in the home appliance market on a large scale. However, manufacturers and end users still put forward more and more requirements for induction heating, such as reducing system size and weight, reducing system costs, and reducing failure rates and repair rates. Due to working in a fast heating and high-power environment, it causes a great stress impact on the induction heating power device IGBT, causing a considerable proportion of market failures. Manufacturers need to spend a lot of cost to do after-sales maintenance, and it also affects the brand reputation. In addition, fierce market competition has led to more stringent cost requirements. In order to reduce BOM costs, designers often ignore or even remove the protection function of IGBT under extreme conditions. At the same time, some alarm functions of the system can remind users to run under less continuous severe working conditions, and can also reduce the failure rate of the system.

Therefore, it is necessary to design a high-power, high-reliability, and highly integrated induction heating system. This article introduces a 2.1 kW induction heating evaluation board to meet the various needs of customers. At the same time, it comes with complete protection functions and optimized PCB design to provide designers with reference.

Figure 1 IPD Protect device picture and internal functional block diagram

1 Built-in protection function TRENCHSTOP™ IPD Protect device introduction

For induction heating market applications, Infineon has launched a novel F series TRENCHSTOP™ IPD (Integrated Power Device), also known as “IPD Protect”, TO-247-6 pin package IPD Protect device EWS20R5135IPB integrated 1350 V 20 A RCH5IGBT and independent driver with self-protection, integrated body diode can realize soft commutation^[1].

The device package TO247-6 achieves a flying distance of 3 mm between collector and emitter, and a creepage distance of 5.7 mm and 7.5 mm^[4]. VDET pin detection voltage VCEshi realizes over-voltage protection ^[3]; CS pin detects the current flowing through IPDProtect to achieve overcurrent protection: INN pin realizes over-temperature alarm and protection; VCC pin realizes undervoltage protection.

2 Induction heating system design

2.1 Introduction to the principle of induction heating

In August 1831, British physicist Faraday discovered the phenomenon of electromagnetic induction. The principle of electromagnetic induction heating is that the alternating current generated by the induction heating power supply generates an alternating magnetic field through an inductor (ie, a coil), and a magnetically conductive object is placed in it to cut the alternating current. The magnetic field lines are changed to produce alternating currents (or eddy currents) inside the object. The eddy currents cause the atoms inside the object to move randomly at high speed, and the atoms collide and rub against each other to generate heat energy, thereby heating the object. A heating method that converts electrical energy into magnetic energy and causes the heated steel body to induce magnetic energy to generate heat. This method fundamentally solves the problem of low efficiency of electric heating plates, electric heating coils and other resistive heating methods by heat conduction.

2.2 Overall introduction of the system

Induction heating applications generally use single-ended parallel-resonant-SEPR (singleended parallel-resonant-SEPR). This topology is relatively simple and can achieve efficient energy conversion while reducing EMI.However, this kind of electricity

The Circuit topology also has some shortcomings. For example, when working in resonance, the input voltage is amplified after resonance, and the voltage stress is applied to the IGBT. Uncontrollable high voltage can easily cause the overvoltage failure of the IGBT. At the same time, the current spike in resonance mode is also very easy to cause overcurrent damage to the IGBT.

The overall block diagram of the system is shown in Figure 2. After the 220 V AC voltage is filtered and rectified, it undergoes high-frequency resonance through resonant capacitors, large coils, IPD Protect, etc. The auxiliary power supply outputs 18 V to IPD Protect and 5 V to the microcontroller. The single-chip microcomputer performs closed-loop control by detecting the input voltage, resonance voltage and output voltage, as well as detecting the protection signal. Human-computer interaction interfaces such as buttons and LED Displays are used to control and monitor the running status of the system. The physical photo of the PCB board is shown in Figure 3.

3 System design and test results

3.1 Normal full-load working state

Set the system output full load power to 2.1 kW when running, the waveform of the test IPD Protect device is shown in Figure 4, V_CE Peak voltage 1 036 V, ICE peak current 50 A, I_NN The high and low levels are 2.5 V and 0 V respectively. It can be seen that the power device has been working in the soft switching state, I_NN The delay time from rising edge to VCE voltage rising is 1.8 μs.

3.2 Overvoltage protection circuit

As shown in Figure 5, the V of IPD Protect_CE The voltage is connected to V after passing through the voltage divider resistor_DET Pin, compared with the internally set overvoltage trigger voltage threshold, when it reaches V_DET+1 When the overvoltage protection function is triggered, V_CE Clamped to V_Clamp1, When it reaches VDET+2, the internal closed loop starts, V_E Constantly clamped at V_Clamp2, If after troubleshooting, V_CE Down to V_RST When, exit the overvoltage protection mode.

Where V_Clamp1 It is the IPD Protect V that the design expects_CE ClampVDET+1 is the internal overvoltage trigger threshold voltage. V_DET+2=4.36 V and V_RST-=1.37 Substituting in V, you can get V_Clamp2=673.1 V, V_RST=211.5 V

3.3 Overcurrent protection circuit

As shown in Figure 6, the current flowing through the emitter of the internal IGBT of the IPD Protect passes through the sampling resistor R8, divides the voltage through the resistor R27 and RCS, and connects to the CS pin of the IPD Protect, which is connected to the internal reference voltage V_CETH- Compare if it is greater than V_CETH-, The overcurrent protection function will be triggered.

The formula for calculating the overcurrent protection point of IPD Protect is shown below. After calculation, it can be obtained that the I of IPD Protect in the system_CE The overcurrent protection point is 68 A.

3.4 Over-temperature protection circuit

IPD Protect has built-in over-temperature protection function. When the internal junction temperature of the chip reaches the junction temperature alarm point TvjTW (typical value 75 ℃), I_NN The PWM voltage will be raised from 2.5 V to 4 V, so that the MCU can detect this raised alarm signal and can perform some derating processing. When the junction temperature of the chip continues to rise and reaches the junction temperature protection shutdown point TvjSD (typical value 150 ℃), the driver of IPD Protect will be pulled down and the system will shut down. When the temperature drops below 75 ℃, the system will automatically restart.

3.5 Input voltage drop test

When the input voltage changes suddenly, the resonant current suddenly increases, which may cause the power device to fail. The test condition is that when working with a full load of 2.1 kW, the input voltage drops from 312 V to 56 V for 200 μs. Check whether the system is abnormal. The test waveform is shown in Figure 7. When the input voltage suddenly rises, the bus voltage overshoot will cause the power device IPD Protect voltage VCE and current ICE to rise rapidly, which may cause the device to fail. Due to the well-integrated current limit function of IPD Protect, it can limit the rise of device voltage and protect IPD Protect from damage.

3.6 Moving the pot test during operation

When the system is running, sudden movement of the heated pot will cause a sudden change in the system load, which will impact the stability of the system and the stress of the device under the continuous resonant working state. In the actual test, the waveform is very stable and smooth, and there is no current and voltage stress problem of the IPD Protect device.

3.7 PCB layout and overall design suggestions

1) Place the peripheral bypass circuit close to the device

Since IPD Protect integrates many analog and power Circuits inside, the chip is very sensitive to the signals collected by the pins. In order to ensure that the function of the chip is not interfered, the peripheral circuits of the chip should be placed as close as possible to the chip.

For example, the bypass capacitor of the chip +18 V supply voltage needs to be close to the VCC pin, the filter capacitor of the VCE voltage sampling circuit needs to be close to the VDET pin, and the RC filter circuit of the ICE current sampling needs to be close to the CS pin.

2) The LC filter circuit and resonant circuit should be as short as possible. The single-ended parallel resonant circuit topology has always been working in resonant mode, so the power circuit must be as short as possible to avoid some parasitic parameters causing unstable operation.The inductors, IPD Protect, resonant capacitors and bus capacitors working in resonant mode should be placed as close as possible[2].

3) Safety distance

Because the system has high-voltage dangerous voltage signals, and there are some human-computer interaction single-chip digital circuits, the safety distance of the high-voltage circuit must be considered when laying out.

Figure 5 Overvoltage protection circuit and V_CEWave

4) Heat dissipation design

Although IPD Protect integrates over-temperature alarm and shutdown functions, due to the accuracy of the over-temperature protection point and the response time of the over-temperature protection, and the resonance state of the system will cause large junction temperature fluctuations during system operation, the system is still designed Good heat dissipation is required, such as forced heat dissipation using a fan with sufficient air volume, close contact between the IPD Protect device and the heat sink, etc.

Figure 6 Overcurrent protection circuit and I_CEWave

4 Conclusion

Aiming at some technical problems and customer pain points of induction heating products, this paper designs a novel 2.1 kW induction heating system, using Infineon’s highly integrated IPD Protect power device, which greatly improves the integration and reliability of the system. It also simplifies the difficulty of design, and the application in the small home appliance market will have great appeal and prospects.