ST AN2002 APPLICATION NOTE

AN2002

APPLICATION NOTE

Using the Demoboard for the TD350 Advanced IGBT Driver

1 Introduction

TD350 is an advanced IGBT/MOSFET driver with integrated control and protection functions. Principles of operation and application examples for the TD350 are described extensively in application note AN1944. The present application note concerns the characteristics and use of the demoboard available for testing the TD350.

Figure 1: TD350 pinout

IN 1

VREF 2

FAULT 3

NC 4

COFF 5

NC 6

LVOFF 7

code date

TD350I

	14	DESAT
	13	VH
	12	OUT	Bevelled edge
	11	OUTL	indicates pin 1
			location
	10	VL
	9	CLAMP
	8	GND

2 Printed circuit board presentation

The demoboard is a 26x35mm double-sided PCB with the TD350 and its peripheral components mounted in a typical configuration (pictured in Figure 2). To aid in the understanding and easy modification of the board, the PCB top-side and bottom-side layers, as well as the silk-screen previews are shown in Figure 3.

The demoboard electrical schematic is presented on Figure 4, and the associated bill of material can be found on Table 1 on page 5.

Let’s study step by step the demoboard original configuration, and the way it can be modified to be adapted to the application.

July 2004

Revision A

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AN2002

Using the Demoboard for the TD350 Advanced IGBT Driver

Figure 2: Demoboard layout

Figure 3: Demoboard PCB: Silk-screen, top-layer and bottom-layer previews

2.1 Input stage

The TD350 can be driven by either a pulse transformer or an optocoupler. The demoboard is set up to use an optocoupler. The TD350 IN pin is clamped to 5V by a zener diode D2 and its bleeding resistor R11=10K (see demoboard schematic on Figure 4). To interface the demoboard with the system, the IN1 pin of the input connector can simply be connected to a low-side optocoupler, as show on Figure 5. An optional filtering capacitor can be added (for instance 47pF) in the event of a highly noisy environment, although the TD350 already includes a filtering on input signals and rejects signals smaller than 100ns (tonmin specification).

The demoboard can be modified to be driven by a pulse transformer by implementing the optional components R2, R3, R4 and C8. The principle of operation with a pulse transformer is explained in AN1944.

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Using the Demoboard for the TD350 Advanced IGBT Driver

AN2002

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).

Figure 4: Demoboard PCB: Electrical Schematic

				VH
	R2*			R11			R1
				D2		C1
	C8*	R3*
In2	1			TD350			VH
	VH	C2
VH	2	R4*	1	IN	DESAT 14
Gnd 3			2	VREF	VH	13	C6
		R5				C3		Desat
							1
In1	4		3	FAULT	OUTH	12
		VREF				R6	2	Out
Fault	5		4	NC	OUTL	11
		R8				R7	3	Gnd
Gnd 6			5	COFF	VL	10
	VH					C5	4	Clamp
VL 7		C4	6	NC	CLAMP		C7
						9	R9
	VL	R10
Lvoff	8		7	LVOFF	GND	8	VL
		D1
		(*): optional parts not mounted on demoboard original configuration

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 turnoff, 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

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