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Overcurrent Protection

The overcurrent protection scheme is usually necessary to built a part of the function in power integrated circuits. The protection scheme needs to distinguish different types of fault conditions, e.g. moderate over-load or sever short-circuit, and to react accordingly based on the device SOA limit. At the same time, the protection circuit should be concise and suitable for integration. The behaviors of the IGBT device under various fault conditions were studied in the literature and useful protection schemes were proposed (Biswas et al., 1991; Chokhawala et al., 1995; Valentine, 1995). The protection in practice should cover a wide range of overcurrent condition. Under moderate overload, the device current is higher than its continuous current rating (/ccr) but remains below the maximum pulse current level (/mpc). The failure mechanism for IGBT device under the moderate overload is thermal runaway. Under the short-circuit condition, the current is much larger and usually exceeds the Impc rating. Under such a high current, the IGBT device needs to be turned off immediately or less than a few microseconds to avoid fatal destruction.

Usually, a time-delay control is implemented for overcurrent protection to avoid unnecessary shutdown caused by transient current surges. In practice, it ranges from a few microseconds for the catastrophic fault to a few milliseconds for the moderate overload condition. The variation of delay time is a nonlinear function of load current as shown in Fig. 5.27. The delay-time limit is derived from the device SOA. In general, it covers four major zones, namely the thermal limitation, the maximum rating on pulsed current, the short-circuit withstanding capability, and the maximum current level limited by the transistor gain (/max). At room temperature, the boundary on thermal limitation at certain current level is determined by the time taken to reach 450 K junction temperature under single-pulse and normal-gate voltage condition. Depends on the gate voltage, the maximum short-circuit current level varies (Shen, 1996).

A resistor-capacitor first-order circuit can be used and to be charged by the anode voltage to serve as a simple timer for turn-off delay time. The voltage across the capacitor can be expressed as The turn-off delay time under different overcurrent conditions

Fig. 5.27. The turn-off delay time under different overcurrent conditions.

where R and C are the resistance and capacitance and K is the conversion gain factor between the anode voltage and anode current, i.e. К1д = Уд. Under overload condition, the IGBT device works in its saturation region, e.g. at high voltage and high current. The value K can approximately be assumed as a constant. If a voltage Vp is set as the reference voltage for the shutdown threshold, that is, when Vc is equal or higher than Vp, then the IGBT gate drive will be removed. The delay time can be found as

The curve of time delay using simple R-C timer is shown in Fig. 5.27. It can be seen that, although the delay time is controlled within the SOA transient limit, it has a far distance away from the actual thermal limitation boundary. This is because the time constant needs to be kept small to meet the critical limit on short-circuit withstanding limit. In this case, the allowable overload capability of the device is not fully utilized. One way to make the full use of the SOA transient limit is to insert a zener diode in parallel with the resistor to provide a voltage-dependent two-stage time constant. This is shown by (Z//R2) and C1 in the circuit of Fig. 5.28. When the anode voltage, which is approximately proportional to the anode current, goes up and the voltage across R2 goes above the zener diode breakdown voltage, VZ, the zener diode breaks down and the capacitor C1 will be charged up at a much smaller time constant. This is because the zener diode

Overcurrent protection circuit

Fig. 5.28. Overcurrent protection circuit.

has a smaller dynamic resistance after breakdown. The delay time can now be expressed as:

The curve with zener diode added is also shown in Fig. 5.27 which has a closer match to the profile of SOA transient boundary.

For new generation IGBTs, they have a higher I a /Vg gain for high conductivity and low anode voltage drop. But, they also have a lower short-circuit withstanding time (Otsuki et al., 1993; Seki et al., 1994; Iwamuro et al., 1995). The short-circuit withstanding time is typically below 5 ц-s. Although the delay time can be made very short by reducing the value of capacitance C1 to a minimum, in practice, it may suffer from parasitics and noise interference. Therefore, it would not be a reliable configuration for the R-C network to provide a delay time in the sub-microsecond zone. Also, the delay time should be relatively longer than the switching transition time to ensure its normal operation. One solution to prolong the allowable time before turn-off is to lower the gate voltage as the short-circuit withstanding capability is also related to the gate voltage.

The protection circuit in Fig. 5.28 has incorporate the feature to lower the gate voltage when a direct short-circuit occurs. When this happens, the voltage across the IGBT goes nearly the same level as the supply voltage. The diode

D2 provides the isolation and keeps voltage VAi to a constant level which is related to the magnitude of gate drive voltage. Resistors R4 and R5 divide the voltage and turn the MOSFET M2 on. When M2 is turned on, it pulls Rgi to ground and lowers the voltage to the IGBT gate contact. A lower gate bias will then prolong the short-circuit withstanding capability. At the same time, capacitor Ci keeps being charged up via zener diode and R2. When the capacitor voltage reaches the threshold value Vp, MOSFET Mi turns on and the IGBT gate contact is grounded. The device is now turned off.

The operation can be quantitatively analyzed. Under the normal operating condition, the anode voltage is low and diode D2 is at its on-state. The voltage

Va1 is

where VD2 is the voltage across diode D2, VD4 is the voltage across diode D4, Vth,Mi is the threshold voltage to turn on MOSFET Mi. When overcurrent occurs, Va will increase and this brings VAi to be higher as well. As a result, capacitor Cl is charged to a higher voltage. The time constant is

When Vci reaches the threshold voltage V^mi, Ml turns on to pull down the voltage to the gate contact. The IGBT is then turned off.

A zener diode is used to distinguish between the moderate overload and the catastrophic conditions. When VA rises above Vz + Vth,Mi, the device will be regarded as in the short-circuit operation, and the turn-off delay time must be shortened within its withstanding capability. The time constant is now

which is much lower than ri. As mentioned, M2, R4, R5, and Rgl are added to provide the current-limiting feature by reducing the voltage to the gate contact to prolong the short-circuit withstanding time. When the short-circuit occurs, diode D2 is turned off, and the voltage to the gate contact of MOSFET M2 is

where Vth,M2 is the threshold voltage of MOSFET M2. As the IGBT gate voltage is reduced, the fault current can be limited to a lower value. The response time for the current-limiting effect to appear is determined by the time constant of

where Cg,M2 is the gate input capacitance of MOSFET M2. This time constant is normally very short and below 1 ц-s. While the short-circuit withstanding capability is extended, the time delay circuit (with zener diode breakdown) functions concurrently to turn off the gate voltage to IGBT in the similar manner as described earlier, but with a more comfortable delay-time period. The turn-off delay-time is described as shown in Eq. (5.24). Figure 5.29 shows the measured circuit performance to protect an IGBT rated around 15 A. Based on the SOA limit at gate voltage of 15 V, the time constant of x is set to be 7ms and T2 is set to be 16 ц-s. The triangular data point is measured at a reduced gate voltage when the protection circuit lowers it from 15 V to 10 V to prolong the short-circuit withstanding capability.

The protection circuit can be fabricated together with the IGBT device to form the smart-power integration on the same silicon die, e.g. Fig. 5.30 showing the integration of key components and Fig. 5.31 showing part of the silicon die. The chip was fabricated with a total of eight masks on a single-side n- (5 x 1014 cm-3)(100) epi-wafer with zener diode as an external component. Resistors are not shown and they can be formed by polysilicon deposition. Short- circuit faults have been applied to verify the effectiveness of the proposed protection scheme. Two types of short-circuit faults, namely the “hard switch fault” and the “fault under load”, can occur during the operation of power electronic circuits. The “hard switch fault” condition is described as that the short-circuit fault exists before the IGBT is turned on, that is, the IGBT will be turned on

Measured turn-off delay time of the protection circuit of Fig. 5.25

Fig. 5.29. Measured turn-off delay time of the protection circuit of Fig. 5.25.

Integration of key components for the overcurrent protection circuit on bulk substrate

Fig. 5.30. Integration of key components for the overcurrent protection circuit on bulk substrate.

Micrography of key components of the overcurrent protection integrated circuit

Fig. 5.31. Micrography of key components of the overcurrent protection integrated circuit.

under short-circuit condition. The “fault under load” condition is described as that the short-circuit fault occurs during the normal IGBT conduction state.

Figure 5.32 shows the measured waveforms of anode voltage and current of hard switched fault (at 800 A/cm2) at supply voltage of 105 V and gate voltage of 10 V. The device recovers from the fault by shutting down the gate drive after about 10 ц-s. Another short-circuit fault under load (at 1060 A/cm2), as shown in Fig. 5.33, is applied to the device, and similar recovery is made after about 5 ^s.

Measured hard-switch fault (at 105 V supply voltage, 800 A/cm anode current density); time scale

Fig. 5.32. Measured hard-switch fault (at 105 V supply voltage, 800 A/cm2 anode current density); time scale: 2 ^s/div; anode voltage scale: 25 V/div; anode current scale: 2 A/div.

Measured fault under load (at 105 V supply voltage, 1068 A/cm anode current density); time scale

Fig. 5.33. Measured fault under load (at 105 V supply voltage, 1068 A/cm2 anode current density); time scale: 1 ^s/div; anode voltage scale 35 V/div; anode current scale: 2 A/div.

 
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