# Dead time calculation for Insulated Gate Bipolar Transistors (IGBT) – KBA237369

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Community Translation: 絶縁ゲート型バイポーラトランジスタ（IGBT）のデッドタイム計算 - KBA237369

In motor drive systems, the voltage source inverter is an important component. The basic building block of a voltage source inverter is a phase leg which has two switches in a series. The IGBT is used as these switches.

To ensure proper operation, avoid the leg shoot-through. Leg shoot-through generates undesired additional losses or potentially triggers a thermal runaway. Therefore, both IGBT devices and the inverter as a whole may fail. To prevent the generation of a leg shoot-through current, add an “interlock delay time” or the more popular “dead time” into the control scheme. With this additional time, turn off one of the two IGBTs, the other one will turn on after the dead time has expired. This prevents a bridge shoot-through caused by the asymmetrical turn on and off times of the IGBTs.

## Figure 1           Voltage inverter phase leg

The dead time should be selected to prevent the bridge shoot-through and must be as short as possible to ensure the correct operation of the voltage source inverter. Therefore, it is a challenge to calculate the correct dead time for a dedicated IGBT device and driver.

 Knowledge Base Article Please read the Important Notice and Warnings at the end of this document 002-37369 Rev. ** www.infineon.com page 1 2023-04-04

To calculate the control dead time, use the following equation:

𝒕𝒅𝒆𝒂𝒅 = [(𝒕𝒅_𝒐𝒇𝒇_𝒎𝒂𝒙 − 𝒕𝒅_𝒐𝒏_𝒎𝒊𝒏) + (𝒕𝒑𝒅𝒅_𝒎𝒂𝒙 − 𝒕𝒑𝒅𝒅_𝒎𝒊𝒏)] ∗ 𝟏. 𝟐

Where,

• 𝑡𝑑_𝑜𝑓𝑓_𝑚𝑎𝑥: Maximum turn off delay time
• 𝑡𝑑_𝑜𝑛_𝑚𝑖𝑛: Minimum turn on delay time
• 𝑡𝑝𝑑𝑑_𝑚𝑎𝑥: Maximum propagation delay of driver
• 𝑡𝑝𝑑𝑑_𝑚𝑖𝑛: Minimum propagation delay of driver
• 1.2: Safety margin value to be multiplied. In this equation,
• (𝑡𝑑_𝑜𝑓𝑓_𝑚𝑎𝑥 − 𝑡𝑑_𝑜𝑛_𝑚𝑖𝑛) is the difference between the maximum turn off delay time and the minimum turn on delay time. It describes the characteristics of the IGBT device controlled by its gate driver and gate resistor circuit. The fall and rise time is usually much shorter than the delay time and is not considered here.
• (𝑡𝑝𝑑𝑑_𝑚𝑎𝑥 − 𝑡𝑝𝑑𝑑_𝑚𝑖𝑛) is the propagation delay time difference (delay time mismatch) determined by the driver. This parameter is usually found in the driver datasheet of the driver manufacturer.

At times, dead time is calculated by simply multiplying the values from the typical datasheet with a safety factor obtained from field experience. This method works in a few cases but is generally not accurate enough. The measurements shown here intend to provide a more precise approach.

The IGBT datasheet lists the typical values for standardized operating conditions. A series of measurements are made to get the correct value for the delay time and then calculate the dead time to determine the maximum values for those conditions.

The following parameters have a significant effect on the delay time:

• Gate resistor
• Collector current
• Gate driver supply voltage

For more details on dead time for IGBTs, see the AN2007-04 application note.

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