7 Ways of Isolated Drive to Avoid IGBT Adverse Reactions

IGBT technology is mostly used in high-power power supply applications. To be applied in these applications, it is necessary to have the ability to avoid undervoltage, Miller effect, overload and other undesirable reactions in the first place. Therefore, the design of isolated drive IGBT power devices requires corresponding design skills.

This article will introduce eight techniques for the design of IGBT power devices for the premise of avoiding adverse reactions.

How to Avoid the Miller Effect

One of the problems faced when operating an IGBT is the Miller effect of parasitic capacitance. This effect is evident in 0 to 15V type gate drivers (single supply drivers). The coupling between the gate collector-electrode lies during IGBT turn-off, and high dV/dt transients can induce parasitic IGBT channel pass (gate collector voltage spikes), which is potentially dangerous.

When the upper half-bridge IGBT turns on operation, the dVCE/dt voltage change occurs across the lower half-bridge IGBT. current will flow through Miller's parasitic capacitor, the gate resistor and the inner gate drive resistor. This will reverse to the gate resistor voltage generation. If this voltage exceeds the IGBT gate threshold voltage, a parasitic IGBT channel pass may result.

There are two conventional solutions. The first is to add a capacitor between the gate and emitter. The second solution is to use a negative gate drive. The first solution results in a loss of efficiency. The additional cost of the second solution is the negative supply voltage.

The solution is by shortening the gate-emitter path by using an additional transistor that lies between the gate-emitter. After reaching a certain threshold, the transistor will short out the gate-emitter region. This technique is known as active Miller clamping.

What are the fail-safe functions? Are they all integrated in the isolated driver?

All 3 failsafe functions are integrated into Avago's highly integrated gate driver ACPL-33xJ - UVLO (to avoid turning on the IGBT when the VCC2 level is not sufficient), DESAT (to protect the IGBT from overcurrent or short circuit), and Miller Clamp (to prevent false triggering of the IGBT due to parasitic Miller capacitance).

For 30~75A, 1200V IGBTs operating at 600V DC bus, can the ACPL-33x and ACPL-H342, five gate drive optocouplers with miller clamp protection, achieve high reliability driving with only a single power supply, and is it more reliable or less reliable than the traditional positive and negative power supply?

Avago ACPL-332J, ACPL-333J and ACPL-H342 gate drive optocouplers can output 2.5A. These products are suitable for driving 1200V, 100A type IGBTs.

1、When negative power supply is used, there is no need to use Miller clamps, but extra cost is needed for negative power supply.

2. If only a single power supply is available, then the designer can use the internal built-in active Miller clamps.

Both solutions are equally reliable. The Miller clamping pin needs to be connected to VEE when not in use.

How can undervoltage and undersaturation be better avoided?

AVAGO gate-drive optocouplers come with an undervoltage lockout (UVLO) protection function. When the IGBT fails, the voltage supplied by the gate drive optocoupler may fall below the threshold value. With this blocking protection function it is ensured that the IGBT continues to be in a low resistance state.

Smart gate drive optocouplers, HCPL-316J and ACPL-33xJ, come with a DESAT detection function. When the voltage on the DESAT pin exceeds the internal reference voltage of about 7V while the IGBT is still running, after about 5μs, the Fault pin changes to a logic low state to notify the MCU/DSP.

At the same time, that 1X small grain transistor will turn on, discharging the gate level of the IGBT through the RG resistor. Since this transistor is about 50 times smaller than the actual shutdown transistor, the IGBT gate voltage will be gradually discharged leading to the so-called soft shutdown.

Operating ambient temperature range of the optocoupler

The operating ambient temperature range can reach -40°C to 105°C. This is sufficient for industrial applications. If customers require higher operating temperatures, R2 Coupler optocouplers can operate at extended temperatures up to 125°C.

Optocoupler insulation withstand voltage

Gate drive optocouplers are available in different packages. Each package has its own characteristics - such as different creepage distances and clearances - to suit different applications. The different creepage distances and clearances correspond to different operating insulation voltages, Viorm. maximum Viorm ranges from 566V to 2262V.

Optocoupler gate driver maximum output current

Depending on the device type selected, Avago's optocoupler gate drivers can reach maximum output currents of 0.4A, 0.6A, 1.0A, 1.5A, 2.5A, 3.0A, 4.0A, and 5.0A.

This article introduces how to avoid short circuit, Miller effect and other IGBT bad reaction situation in circuit design through 7 aspects of isolated drive IGBT power devices, this article has more content, for the beginning readers may have to read repeatedly to understand the knowledge, you may wish to collect this article, convenient to read at any time.