Gate Driver Optocouplers in Induction Cooker (4) - Finish

Summary
In this article, the half-bridge series resonant and quasi resonant induction cooker topologies along with three gate driver methods were discussed. In order to reduce the design size and audible switching noise while improving power efficiency, these resonant converters are chosen.

The discrete transistor gate driver circuit is cost effective but increases design complexity while providing no safety isolation. The gate drive transformer consumes board space due its size and requires additional work and cost to achieve higher switching duty cycle above 50%. Finally, gate drive optocoupler integrated ICs saves board space through high level feature integration while providing high voltage safe isolation and noise immunity all in one package.

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Gate Driver Optocouplers in Induction Cooker (3)

Gate Driver Circuits for IGBT Power Switches
Three types driver circuits, the discrete transistor circuit (Figure 5), gate driver optocoupler (Figure 6) and gate driver transformer (Figure 7) can be used to drive the power switches in induction cooker application. There are several issues associated with high-frequency gate drivers; the parasitic inductances, power dissipation in the gate-drive circuit and the losses in the power switching devices in the gate driver.

Typically, the switching frequency of an induction cooker is between 25kHz to 40kHz. In order to rapidly charge turn on and off the power switch, the gate current inductance loop between the driver and power switch should be as low as possible. Hence it is advisable to design the layout of the circuit to reduce the parasitic inductances. Since the driver rapidly charge and discharge the gate capacitor of the IGBT, a higher peak gate current may be needed for proper operation. Due to this, the power dissipation within the gate drive circuit is important to manage the increase switching speed. The higher peak current is also desirable to increase the charging and discharging during turn on and off as it will help reduce the switching losses of the IGBT.
The discrete gate drivers are constructed using the bipolar transistors. NPN and PNP emitter followers can achieve reasonable drive capability. However, using several discrete components to build the driver and other functions or protection operation like Under Voltage Lockout (UVLO) is not as space efficient as using integrated ICs. Moreover discrete transistor drivers do not provide sufficient safety isolation or noise immunity.
Two types of isolation method are discussed in this article; pulse transformer and gate driver optocoupler. The pulse transformer is a traditional and simple solution which suffers from saturation limitation for a given transformer size that can reduce efficiency. Normally, a transformer can only transmit AC information and have a limited duty cycle of up to 50% due to the transformer volt-second relationship. Additional capacitor and zener diode on the secondary size can be added to allow a higher duty cycle. However, this increase the design board size and parasitic inductances which in turn increases power losses in the driver circuit.
Gate driver optocoupler ic is an integration of LED for safety isolation, transistors to provide drive current and protection functions like UVLO or Desaturation Detector. Gate driver ICs are easy to design and will save PCB board space in the application. Due to the integrated design, the drive circuitry can be located very close to the power switch which not only saves PCB space but also improves the overall noise immunity of the system. However, like any integrated ICs, power dissipation is main concern observed by designers.
For the single switch resonant converter, designer has the option of the discrete gate driver topology, gate transformer or gate driver optocoupler. As discussed in the previous section, the quasi-converter resonant voltage can be higher compared to the DC link voltage and this voltage stresses the power semiconductor switch. In most commercial low cost single switch induction cooker design, the discrete gate driver circuit is used as there is no upper power switch and both controller and the power semiconductor are able to share the same power ground. However, in cases where safety isolation and reduction of driver losses becomes an issue, the gate drive optocoupler or transformer are excellent alternatives.
For the half-bridge converter, a floating or high-side power switch needs to be driven. A high side discrete solution would increase the component count while not providing any isolation. As shown, the pulse transformer galvanic isolation solution increases in complexity for duty cycle switching above 50%. Also, the solution size is larger because of the additional discrete components on top of the transformer size. The gate driver optocoupler IC provides a good level of protection, isolation, and common-mode noise rejection. This resolves much of the problems that are associated with transformer driver or transistor discrete solution as mentioned earlier.

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Gate Driver Optocouplers in Induction Cooker (2)

By applying the transformer equivalent circuit, designers are able to map the load pot (secondary of transformer) to the primary side of circuit where the resonant inductor, Lr and capacitor Cr are located. From this, we can obtain the equivalent circuit for half-bridge and quasi resonant circuit, shown in Figure 3 and Figure 4. From these equivalent circuits, the operation of the induction cooker, the sizing of the resonant inductor, capacitor and control algorithm can be conceived.

In order to reduce component size, minimize switching losses and reduce audible noise during operation (above 20kHz resonant frequency), induction cooker circuit typically utilizes resonant or soft switching techniques. This circuitsoft switching technique can be subcategorized into two methods: Zero-voltage switching and Zero-current switching.
Zero-voltage switching occurs when the transistor turn-on at zero voltage. Zero-current switching refers to elimination of turn-off switching loss at zero current flow. The voltage or current administered to the switching circuit can be made zero by using the resonance created by an L-C resonant circuit. This topology is named a “resonant converter.” This allows the application to utilize resonant frequency and obtain the benefits mentioned compared to conventional hard switching techniques.
The advantages of half-bridge series resonant are stable switching, and lower cost due to streamlined design. The voltage within the circuit is limited to the level of the input voltage which reduces the voltage stress across IGBT power switch. This in-turn allows the designer to lower the cost by choosing a lower rating IGBT. The disadvantage is that the overall half-bridge control is more complicated, the size of heatsink and PCB area is bigger and insulated gate driver circuits, especially on the upper IGBT (S1 in Figure 1).
The advantage of quasi-resonant converter is that it needs only 1 IGBT power switch which reduces design size PCB and heat sink. The disadvantages are that the quasi resonant switching the high resonant voltage which can be higher than the DC input voltage stressed on to IGBT power switches. This requires a higher cost and blocking voltage power components.

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Gate Driver Optocouplers in Induction Cooker (1)

What is Induction Cooking?
We start by first comparing the difference between conventional gas cooking and induction cooking. In induction cooking methods, energy is transferred directly to the pot or pan while conventional cooking first generate a fire and heat energy which is then transferred to the cooking pot. Hence due to this two step energy transfer of conventional cooking, the efficiency of the induction cooking is much better.

Figure 1 & 2 shows two circuit topologies for induction cooker, the half-bridge series resonant converter Figure 1 and quasi-resonant converter Figure 2 [2]. In both topologies, there exist the resonant elements Lr and Cr. For circuit simplification, the load pot, R is assumed to be of pure resistive element. In both topologies, an ac input supply of 220V 50Hz is converted into an uncontrolled dc voltage by a full-bridge rectifier. This DC voltage is then converted into a high frequency AC voltage by the inverter IGBT (insulated gate bipolar transistors) switches, S1 and S2 in the case of the half-bridge circuit, which can be controlled using a micro-controller. Due to the high frequency switching AC, the element coil will then produce a high frequency electromagnetic field which will penetrate the ferrous material cooking pot. From Faraday’s Law and skin effect, this generates eddy current within the cooking pot which then generates heat to cook the food inside the pot.

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IGBT Gate Drivers in High-Frequency Induction Cookers (3)

Gate driver circuits for IGBT power switches
Three types of driver circuits, using discrete transistors (Fig. 5), gate driver optocouplers (Fig. 6) or gate driver transformers (Fig. 7) can be used to drive the power switches in the induction cooker. There are several issues associated with high-frequency gate drivers: parasitic inductances, power dissipation in the gate-drive circuit and the losses in the power switching devices in the gate driver, all of which are involved when selecting an appropriate driver circuit.

Typically, the switching frequency of an induction cooker is between 25 kHz and 40 kHz. In order to rapidly turn on and off the power switch, the gate current inductance loop between the driver and power switch should be as low as possible. Hence it is advisable to design the layout of the circuit to reduce the parasitic inductances. Since the driver rapidly charges and discharges the gate capacitor of the IGBT, a relatively high peak gate current may be needed for proper operation. A higher peak current is also desirable to increase the charging and discharging rates during turn-on and turn-off, to help reduce the switching losses of the IGBT. Due to this, managing the power dissipation within the gate drive circuit becomes increasingly important as the switching speeds are increased.
Discrete gate drivers are constructed using bipolar transistors, and NPN and PNP emitter followers can achieve reasonable drive capability. However, using several discrete components to build the driver, while simultaneously incorporating necessary operational and protective functions such as under voltage lockout (UVLO), is not as space efficient as using integrated circuits. Moreover most discrete transistor driver designs do not provide sufficient safety isolation or noise immunity.
Two methods of providing electrical isolation are pulse transformers and gate driver optocouplers. The pulse transformer is a traditional and simple solution, which, however, suffers from the potential for core saturation in a reasonably-sized transformer, resulting in reduced efficiency. A pulse transformer can only transmit AC signals, and most designs have a limited duty cycle ranging up to 50 percent due to the transformer volt-second relationship.
An additional capacitor and zener diode on the transformer secondary can be added to permit a higher duty cycle. However, this increases the circuit board size and parasitic inductances, which, in turn, increases power losses in the driver circuit.
The gate driver optocoupler IC integrates an LED light source and optical receiver for safety isolation, with transistors to provide sufficient drive current, and protection functions such as UVLO or desaturation detection.
Gate driver ICs are easy to design with, and will save PCB board space. Due to the integrated design, the drive circuitry can be located very close to the power switch, which not only saves PCB space but also improves the overall noise immunity of the system. However, as with any ICs, power dissipation is a major concern.
For the single-switch resonant converter, the designer has the option of the discrete gate driver, gate transformer or gate driver optocoupler topologies. As discussed previously, the quasiconverter resonant voltage can be higher than the DC link voltage and this voltage stresses the power semiconductor switch. In most commercial low cost single switch induction cooker designs, the discrete gate driver circuit is used as there is no upper power switch, and both the controller and power semiconductor are able to share the same power ground. However, in cases where safety isolation and reduction of driver losses becomes an issue, the gate drive optocoupler or transformer are excellent alternatives.
For the half-bridge converter, a floating or high-side power switch needs to be driven. A high-side discrete solution would increase the component count, and not provide any isolation. As shown, the pulse transformer galvanic isolation solution becomes increasingly complicated for duty cycle switching above 50 percent. Also, the solution size is larger because of the additional discrete components on top of the transformer size. The gate driver optocoupler IC provides a good level of protection, isolation, and common-mode noise rejection. This resolves many of the problems that are associated with transformer or discrete transistor drivers.

Summary
In this article, the halfbridge series resonant and quasi resonant induction cooker topologies along with three gate driver methods were discussed. In order to reduce the design size and audible switching noise while improving power efficiency, these resonant converters are chosen. The discrete transistor gate driver circuit is cost effective but increases design complexity while providing no safety isolation. The required size of the gate drive transformer consumes board space, and requires additional work, cost and board space to achieve switching duty cycles above 50 percent. Finally, the use of gate drive optocoupler ICs saves board space through high level feature integration while providing high voltage safety isolation and noise immunity all in one package.

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IGBT Gate Drivers in High-Frequency Induction Cookers (2)

How does an induction cooker work?
Figures 1 and 2 show two circuit topologies for induction cookers: the half-bridge series resonant converter, Fig. 1, and the quasi-resonant converter, Fig. 2. In both topologies, there exist the resonant elements Lr and Cr. For circuit simplification, the load pot, R, is assumed to be a purely resistive element.

In both topologies, an AC input supply of 220V 50 Hz is converted into an unregulated DC voltage by a full-bridge rectifier. This DC voltage is then converted into a high frequency AC voltage by the inverter IGBT (insulated gate bipolar transistor) switches—S1 and S2 in the case
of the half-bridge circuit—which can be controlled using a microcontroller. Due to the high frequency switching AC, the element coil will produce a high frequency electromagnetic field which will penetrate the ferrous material of the cooking pot. From Faraday’s Law and skin effect, this generates eddy current within the cooking pot which then generates heat to cook the food inside the pot.
By applying the transformer equivalent circuit, designers are able to map the load pot (secondary of transformer) to the primary side of the circuit where the resonant inductor, Lr, and capacitor, Cr, are located. From this, we can obtain the equivalent circuit for the half-bridge and quasi resonant circuits, shown in Figs. 3 and 4. From these equivalent circuits, the operation of the induction cooker, and the values of the resonant inductor, capacitor and control algorithm
can be derived.
In order to reduce component size, minimize switching losses and reduce audible noise during operation, induction cooker circuits typically utilize resonant or soft switching techniques. Soft switching can be subcategorized into two methods: zero-voltage switching and zero-current switching. Zero-voltage switching occurs when the transistor turns-on at zero voltage. Zero-current switching refers to the elimination of turn-off switching loss at zero current flow. The voltage or current provided to the switching circuit can be made zero by using the resonance created by an L-C circuit. This topology is named a “resonant converter.”
The advantages of a half-bridge series resonant circuit are stable switching and lower cost due to simplified design. The voltage within the circuit is limited to the level of the input voltage, which reduces the voltage stress across IGBT power switch. This, in turn, allows the designer to lower the cost by choosing an IGBT with a lower voltage rating. The disadvantage of this approach is that the control of the half-bridge circuit is relatively complicated and the required size of the heatsink and PCB area is greater, because of the high side gate driver circuit required for the upper IGBT, S1 in Fig. 1)
The advantage of a quasi-resonant converter is that it needs only one IGBT power switch, which reduces the size of the PCB and heat sink. The disadvantages are that the quasi-resonant switching develops a resonant voltage which can be higher than the DC input voltage, increasing stresses on the IGBT power switches. This requires highercost components with higher blocking
voltage capabilities.

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