An Intelligent IGBT Module for Single-Ended Induction Heating Applications: All-in-One Solution for Induction Cooking System

Wonjin Cho, Staff Application Engineer at Alpha and Omega Semiconductor, Inc.
Dino Ge, Marketing Manager at Alpha and Omega Semiconductor, Inc.
Bum-Seok Suh, Vice President at Alpha and Omega Semiconductor, Inc.

Induction heating (IH) application has developed with requirements of higher efficiency, compactness, cost effectiveness, and reliability. Even with simple structure, single-ended IH system operates in high power condition as a domestic application with various transient modes. Due to resonant operation, the IGBT, suffers ~1 kV of blocking voltage and ~40 A of peak conduction current. As a consequence, the system requires robust protections for certain abnormal conditions such as surge input and high temperature. An intelligent IGBT module, so called AlphaIPMTM-IPM6, shown in Figure 1, provides multiple protective functions to meet market requirement and boost system reliability. The footprint of the package is same as that of TO-247 standard package, which is popularly used in quasi-resonant IH applications.

Figure 1: AlphaIPMTM-IPM6, Intelligent IGBT for single-ended induction heating applications.

Conventional Protective Functions in IPM6

IPM6 provides general protective functions which are used in motion control IPMs such as over-voltage lock-out (OVLO), under-voltage lock-out (UVLO), temperature monitoring and over-temperature (OT) protection. When the supply of the control IC exceeds trigger level, 22 V, the OVLO function blocks the input PWM signal so that the output signal stays in low state until OVLO is reset at 21 V. The UVLO function disables to output at 11 V and reset at 12 V. These OVLO and UVLO are designed for conventional auxiliary voltage range of 15 to 18V. OT protection is activated at the temperature of 120°C triggering the fault-out (FO) signal and is deactivated when the temperature decreases to 85°C releasing FO signal.

Temperature value of the control IC is represented as   voltage value at the VOT pin of the IPM6 as shown in Figure 2. The monitored voltage has the range between 0 to 5 V for the temperature during the normal operational, which is compatible to the sensing input range of the conventional 5 V micro controller. OT protection level is 130°C with reset level of 100°C. With OT protection, fault out signal is activated to indicate abnormal working condition.

Figure 2: Temperature monitoring characteristic

Soft-Start Function

Practical single-ended IH systems start their operation by load detection control. Single or multiple pulses turn on the IGBT at the start-up, and corresponding current or voltage responses are fed back to micro controller. During the load detection, single-ended IH inverter inevitably suffers high inrush current because of empty resonant capacitor. Once the resonance capacitor is charged, single-ended IH inverter operates in zero-voltage switching (ZVS) turn-on. Thus the inrush current at the first turn-on has the most severe level in overall operation. IPM6 has built-in soft start logic, which is implemented by independent gate driver to limit inrush current. Soft-start function mitigates the peak of inrush current by ~70% as shown in Figure 3, and is disabled until the system restarts from halted operation.

Figure 3: Soft-start function

Abnormal Turn-On Disable (ATOD) Function

During the normal operation of single-ended IH system, the IGBT voltage reaches ~1 kV during off-state. If the IGBT is unexpectedly turned on due to noise signal, it suffers severe hard switching which can be problem in efficiency and reliability. ATOD function maintains the minimum turn-off duration for 10 μs by blocking input signal from the turn-off edge so that designated turn-off period is guaranteed. The ATOD function is implemented by internal timing logic of the built-in control IC.

Hard Turn-On Disable (HTOD) Function

HTOD function is designed to prohibit excessively hard turn-on switching of the IGBT in abnormal system conditions like ac supply swell and mal-functional control cases. The HTOD function is implemented by the fast comparator to block any input signals when the sensed voltage of the IGBT is greater than ~450 V. Desired protective level of the IGBT can be adjusted by the conversion ratio of external sensing resistors.

Maximum Duty Cycle Disable (MDCD) Function

The peak conduction current level is proportional to turn-on duration of the IGBT in single-ended IH system. Because the conduction current is converted to the resonant voltage, the longer turn-on duration more increases IGBT voltage during the resonance. With the conventional switching frequency range of 20 to 30 kHz of single-ended IH system, the designated turn-on duration range is 17 to 25 μs at the half duty cycle. IPM6 limit maximum turn-on duration at 37 μs to prevent excessively long turn-on and corresponding high voltage at the IGBT.

Over-Voltage Clamping Protection (OVCP) function

If an excessive abnormal voltages such as lightening surge or voltage swell are applied to ac input line of IH system, IGBT voltage can increased by several hundred volts even though surge absorbing circuit suppresses those excessive inputs. IGBT voltage rating is chosen to have blocking margin for such abnormal cases but the increased voltage can even exceed the dynamic breakdown voltage of the IGBT. The purpose OVCP function is to consume the resonant current that is converted to the resonant voltage by partially turning on the IGBT. Partially turned-on IGBT does not experience severe short circuit current because current channel between IGBT collector and emitter is limited by the gate voltage of ~8 V. Once the resonant current is consumed by IGBT, the resonant capacitor is not charged even discharged, so that IGBT voltage does not increase over the destructive level. The clamping level is determined by hysteresis band, defined by set and reset level, which can be controlled by the external sensing resistance ratio. Once IGBT voltage is settled in the reset level, OVCP function finishes its operation. In this case, the remaining current can charge the resonant capacitor again increasing the IGBT voltage, and then OVCP function is triggered again. The OVCP function can repeatedly work until IGBT voltage is permanently settled down as shown in Figure 4. The peak voltage of the IGBT is limited by ~1.4 kV which is under the dynamic breakdown voltage of the IGBT.

Figure 4: OVCP function

In summary, the new intelligent IGBT module, AlphaIPMTM-IPM, is designed for single-ended induction heating applications up to 2.3 kW. Besides conventional protective functions, designated functions for IH application surely give benefits in system reliability and make a completely new outlook for system designers enabling a far more optimum performance of the resonant converter and cost effectiveness as well.

More information: Alpha and Omega Semiconductor    Source: Bodo's Power Systems, August 2017