What is a TG-FS-IGBT? In this breakdown of a new IGBT from STMicroelectronics, we'll briefly define this predecessor to the typical IGBT and discuss available app notes to check it out.

A trench-gate field-stop insulated gate bipolar transistor (IGBT) is a device that might be used in such applications as motor controllers, welding machines, induction heating, and power inverters. In very broad terms, MOSFETs are better for low-voltage, low-current switching at high frequencies and IGBTs are better for high-voltage, high-current switching at lower frequencies.

In this article, we'll take a look at the STGP8M120DF3 trench-gate field-stop IGBT from STMicroelectronics.

 

The STGP8M120DF3's TO-220 package. Image from the datasheet.

 

What’s Inside the Package?

A field-stop IGBT is similar to a non-punch-through (NPT) IGBT with the exception that a field-stop IGBT has an additional n layer near the collector (shown below as a yellow horizontal layer). This n layer causes the electric field in the nearby n- drift layer to fall off abruptly when it reaches the p+ collector. This leads to a thinner device with lower internal resistance.

The trench gate is an actual trench etched vertically in the silicon wafer that contains the gate electrode (shown below in gray). This causes the electric field characteristics inside the n- drift layer to become primarily vertical—lowering the saturation voltage without significantly affecting the breakdown voltage of the region.

 

Cross section of trench-gate field-stop IGBT from page 19 of app note AN4544

 

Considerations

The current that can flow through the STGP8M120DF3 device begins to drop significantly at frequencies above 1 kHz, as shown in the graphic below.

 

This plot is a modified version of Figure 7 from this datasheet.

 

This low switching frequency is characteristic of IGBT-type devices.

 

Switching waveform, from page 10 of the datasheet

 

On-Time

The turn-on delay time is the time interval that begins when the gate-emitter voltage reaches 10% of its final value during a low-to-high transition and ends when the collector current reaches 10% of its final value. This happens quickly—in about 20 nanoseconds. An additional 8.4 ns is required for the current rise time, the time interval measured from when the collector current begins at 10% and reaches 90% of its final value.

 

Off-Time

When the gate voltage is removed, the current does not stop immediately. The turn-off time is defined as the interval that begins when the gate-emitter potential reaches 90% of the initial value (during a high-to-low transition) and ends when the collector current reaches 90% of its initial value. For this device, that time is 126 ns (see page 4 of the datasheet).

Additionally, the current fall time is defined as the interval that begins when the collector current is at 90% of its initial value and ends when the collector current is at 10% of its initial value. That requires an additional 136 ns.

As you may have noticed, the device turns on much more quickly than it turns off.

 

Thermal Characteristics

These devices can survive temperatures from -55 °C to 175 °C, but high temperatures lead to significant performance degradation. As the operating temperature increases past 25 °C, the current capacity decreases. The ST application note AN544, page 15, indicates that every reduction in operating junction temperature of 10 °C doubles the device’s lifetime.

 

The power that the device can safely dissipate vs. case temperature, from Figure 1 of the datasheet

 

Using These Devices in Circuits

ST knows that electrical engineers have grown bored with H-bridge motor controllers. So for their AN4929 application note, they detail how to use these devices to create a welding machine. And not just any welder—a welder built around a 3-phase full-bridge DC/DC power converter.

 

DC/DC converter topology from AN4929

 

Their lab-built welder appears to use four of these IGBTs and can generate currents between 100 A and 200 A, more than enough current to weld a coworker’s desk-drawers shut.

 

Information about EMI

For those engineers that haven’t grown tired of creating H-bridge motor controllers—application note AN4694 provides EMC design guides for motor-control applications. Here you will find PCB design and layout guidelines, as well as a variety of case studies and theory to help you minimize the impact of EMI on your final circuit.

 

Figure 22 from page 30 of AN4694

 


 

Trench-gate field-stop insulated gate bipolar transistors (TG-FS-IGBTs) are a thinner, faster version of their predecessor, the IGBT. Soon, they will likely completely replace the older-style non-punch-through IGBTs. ST has provided several resources to help you to understand how to use them in your next switching design. You can check out AN4694 (EMI design guide), AN4544 (IGBT datasheet tutorial), and Infineon’s Trenchstop-IGBT app note to learn more.

 

Are you designing a circuit with these next-generation IGBTs? Let us know in the forums, or in the comment section below.

 

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