Using the Demoboard for the TD350 Advanced IGBT Driver

AN2002

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2.2 Output stage

The TD350 uses separate sink and source outputs (OUTL/OUTH) for easy gate driving. The integrated

circuit output stage is able to sink/source about 2A/1.5A typical at 25°C, but the IGBT turn-on and turn-off

current can be limited independently by the gate resistors R6 and R7 included on the demoboard. The

original value for these resistors are R6=R7=47R.

Output current capability can be increased by using an external buffer with two low-cost bipolar

transistors. This configuration is described in Section 4 on page 7.

2.3 Active Miller Clamp

The TD350 offers an alternative solution to the problem of the Miller current in IGBT switching

applications. Instead of driving the IGBT gate to a negative voltage to increase the safety margin, the

TD350 uses a dedicated CLAMP pin to control the Miller current. When the IGBT is off, a low impedance

path is established between IGBT gate and emitter to carry the Miller current, and the voltage spike on

the IGBT gate is greatly reduced. The CLAMP switch is opened when the input is activated and is closed

when the actual gate voltage goes close to the ground level. In this way, the CLAMP function doesn’t

affect the turn-off characteristic, but only keeps the gate to the low level throughout the off time.

On the demoboard, the CLAMP pin is connected to the IGBT gate by strap R9. This strap can be

removed when the clamp feature is not needed, or when an external buffer is used (see reference

schematic on Figure 6).

2.4 2-level turn-off

In the event of a short-circuit or overcurrent in the load, a large voltage overshoot can occur across the

IGBT at turn-off and can exceed the IGBT breakdown voltage. By reducing the gate voltage before turn-

off, the IGBT current is limited and the potential overvoltage is reduced. This technique is called 2-level

turn-off. Both the level and duration of the intermediate off level are adjustable. Duration is set by the

external resistor R8 and capacitor C4 in conjunction with the integrated voltage reference for accurate

Figure 4: Demoboard PCB: Electrical Schematic

R3*

R4*

R2*

C2

VH

R5

VREF

VH

R7

R1

R8

C4

R10

D1

C1

R6

C3

C5

C7

C6

7

2

4

5

1

3

6

8

2

4

1

3

R9

Desat

Out

Gnd

Clamp

VH

VL

In1

Lvoff

VL

Gnd

Fault

Gnd

VH

In2

VL

C8*

VH

R11

D2

TD350

13

12

9

11

10

8

14

7

2

4

5

1

3

VH

OUTH

CLAMP

GND

VL

OUTL

FAULT

6

IN

VREF

NC

COFF

LVOFF

DESAT

NC

(*): optional parts not mounted on demoboard original configuration