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Posted on 01 July 2019

600V Trench IGBTs

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Optimized for 20 kHz Operation

The new generation of 600 Volt IGBTs is targeted for DC-to-AC inverter applications. These new devices use Depletion Stop Trench technology to reduce both conduction and switching losses in high frequency switching applications such as UPS and solar inverters enabling higher efficiency power conversion. These devices have been optimized for switching at 20 kHz with low short circuit requirements.

By Carl Blake and Wibawa Chou, International Rectifier

 

Compared to previous generation IGBTs operated at 20 kHz, trench IGBTs have lower conduction and lower switching losses at the same operating conditions. These devices are also rated for 175°C maximum junction temperature to increase design margin further. Positive Vce(on) temperature coefficient allows easier paralleling of the devices for higher inverter power applications. In addition, 100 percent of the parts are clamped inductive tested (ILM) at 4x the rated current to guarantee robustness.

Trench IGBT

One of the major improvements in IGBT designs is the introduction of trench gate IGBT. In a trench IGBT, the gate is trenched down into the p+ body region. This results in reduction of saturation voltage, Vce(on), when the IGBT is conducting. Another major improvement in trench IGBT is the deploying of a depletion stop layer which allows thinner IGBT with reduced tail current at turn off.

Figure 1 shows parametric comparisons between planar punch through (planar) IGBT and trench depletion stop (trench) IGBT of the same dimensions. This table shows important parameters of the IGBTs in 20 kHz application. Trench IGBT parameters are superior to those of planar IGBTs. Comparing parameter differences on Figure 1, trench IGBT’s advantages can be summarized as follows: lower conduction voltage, Vce(on), lower switching losses and lower steady state thermal resistance (Rth) compared to planar IGBTs.

Comparisons of planar punch through with trench depletion stop IGBTs

Power Dissipation at 20 kHz

Power dissipation difference at 20 kHz is analyzed in Figure 2. 20 kHz is typically used for DC-AC inverter in applications such as solar or UPS inverters. Calculation shows that there will be up to 30 percent reduction in power dissipation at 20 kHz. This reduction means end-products can be designed smaller or more output power can be obtained from existing power board assembly.

Trench IGBT offers 30 percent power dissipation reduction compared to planar IGBT in a 20 kHz DC to AC inverter application

Short Circuit Requirements

In a typical DC-AC inverter, output inductors and capacitors filter out 20 kHz switching harmonics. This results in an output voltage that is very close to a true sine wave. When the output of the inverter is shorted, the output inductors will limit the rate of change (di/dt) of the current into the IGBT. Current will ramp linearly and the protection circuit should detect an over-current condition which terminates signals to the gates of the IGBTs. This condition is different from the one in a motor drive application where a short circuit usually means direct short of IGBT terminals without inductor to limit di/dt of the current. The waveform differences are shown in Figure 3. The first oscilloscope waveform shows current ramps linearly when an output inductor is present on the output of the inverter. The controller sets over current limit at 12 Amperes and stops the gate signal to the IGBT when this limit is exceeded. When current ramps linearly, the IGBT is not saturated and voltage drop, Vce(on), across it is small (1.7V). Therefore, the instantaneous power dissipation is also relatively small at 20.4 Watts (12A x 1.7V).

Output inductor limits the rate of change of current into the IGBT when the output is shorted.

Assuming that the controller takes 500 nano seconds to react to this over-current condition, the increase in the junction temperature of the IGBT is calculated to be 0.204°C above the steady state junction temperature. This is assuming an instantaneous thermal resistance of 0.01°C/W and following the formula: dT = Zth x Pdissipation, where dT is the increase in junction temperature, Zth is the instantaneous thermal resistance and Pdissipation is power dissipation on the IGBT die. This increase in the junction temperature is insignificant in most applications and the IGBT should be able to survive this condition without any problem.

On the second waveform, there is no output inductor that limits di/dt of the current. IGBT reaches its current saturation level almost at the instant the short circuit occurs. The entire bus voltage is dropped across the IGBT and power dissipation is very large. In this example, the current saturates at 33 Amperes and the entire bus voltage of 400V is dropped across the IGBT. The instantaneous power dissipation is approximately 13.2 kilowatts (33A x 400V). Using the previous formula, the increase in junction temperature will be 132°C above the steady state junction temperature. This increase is very significant such that the gate signal to the IGBT must be extinguished immediately to prevent catastrophic device failure.

600V trench IGBTs for 20 kHz inverter application offer a trade-off between short circuit time and Vce(on). Typically, the higher the short circuit time, the higher Vce(on) will be. However, since output inductors are always present on the application, current ramps linearly when the output is shorted. Therefore, short circuit time handling capability of the IGBTs does not have to be as long as that for motor drive applications. These trench IGBTs have 5 usec short circuit time at room temperature and 3 usec at 150°C. This specification is sufficient for modern protection circuit and controllers to detect abnormality on the inverter and to terminate gate signals to the IGBTs to prevent catastrophic failure. Increasing short circuit time is not necessary as it will increase the Vce(on) significantly which reduces efficiency of the inverter.

Paralleling and 175°C Operations

Since 600V trench IGBTs are based on depletion stop thin wafer design, the temperature coefficient of the Vce(on) is positive, similar to that of a power MOSFET. This means increasing junction temperature will increase Vce(on) of the IGBT. This has an advantage when multiple IGBTs are put in parallel to increase power handling capability of the inverter. With positive temperature coefficient, when one device takes more current than the others, it will heat up more and its Vce(on) will increase. When Vce(on) increases, current through the IGBT will automatically reduce and current balance is automatically restored. On punch through planar IGBTs without thin wafer design, the temperature coefficient is negative. Increasing temperature of the IGBT will result in reduction in Vce(on). It is, therefore, more difficult to achieve current balance in parallel operation since the device that takes more current will have its Vce(on) reduced and continues taking more and more current leading to device destruction.

Another improvement made on the 600V trench IGBT is its capability to operate up to 175°C junction temperature. This improvement allows design engineers to have more design margin for reliability. It also means that the device can provide more power density out of the same package. This allows design engineers to provide more output current capability out of the same power board assembly. On planar IGBTs, the maximum junction temperature is limited to 150°C.

4x Clamped Inductive Load

On an inverter with inductor at the output, the IGBT will see high current and high voltage simultaneously at the instant the device switches off. This is due to the transition time of the IGBT from conducting to blocking (usually specified as tdoff and tf on the datasheet) and parasitic inductances of the circuit. These inductances are shown in Figure 4 for clarity.

Output inductor and parasitic inductors on a typical half bridge inverter

During transition time, current on the output inductor can not change instantaneously. This means that current will continue flowing on the co-pack diode of the complementary IGBT. However, it takes time for the co-pack diode to conduct, during which time, the voltage across the IGBT will spike above the bus voltage. This high voltage spike will coincide with the high current to be turned off causing a lot of power dissipation across the IGBT. In order to guarantee robustness, 600V trench IGBTs are designed to be able to turn off four times its rated current. This is guaranteed by 100 percent testing each device in production for clamped inductive load test. Typical waveform across the IGBT at turn off transition is shown in Figure 5.

Typical voltage and current across the IGBT at turn off transition

Conclusions

600V trench IGBTs offer several benefits compared to planar punch through IGBTs. These advantages are increased power density, increased reliability and ease of paralleling. This generation of IGBTs is targeted for DC-AC inverter application at 20 kHz with low short circuit rating requirements.

 

 

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