ST AN1944 APPLICATION NOTE

AN1944

Application note

Developing IGBT applications using an

TD350 advanced IGBT driver

Introduction

The TD350 is an advanced Insulated Gate Bipolar Transistor (IGBT) driver with integrated control and protection functions. The TD350 is especially adapted for driving 1200V IGBTs with current ratings from 15 to 75A in Ecopak-like modules.

Main features are:

●Minimum1.2A sink / 0.75A source peak output current over full temperature range (-20°C to 125°C)

●Desaturation protection with adjustable blanking time and fault status signal

●Active Miller clamp function to reduce the risk of induced turn-on in high dV/dt conditions without the need of negative gate drive in most cases

●Optional 2-step turn-off sequence to reduce over-voltage in case of over-current or short-circuit event to protect IGBT and avoid RBSOA problems

●Input stage compatible with both optocouplers and pulse transformers

Applications include a three-phase full-bridge inverter used for motor speed control and UPS systems.

TD350 in 1200V 3-phase inverter application

HV DC

TD350					TD350					TD350

Load

TD350					TD350					TD350

GND

IGBT modules

October 2006

Rev 4

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www.st.com

Contents	AN1944

Contents

1	TD350 application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 3
2	Input stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 4
3	Output stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
4	Active Miller clamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
5	2-Level turn-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
6	Desaturation protection feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
7	Application schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
8	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
9	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20

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AN1944	TD350 application example

1 TD350 application example

Figure 1 shows an example of a TD350 application where the device is supplied by a +16V/-10V isolated voltage source, but a single voltage source can also be used. A pulse transformer is used for input signal galvanic isolation. Gate resistors at OUTH and OUTL pins (here 47 Ohms) are to be chosen depending on the IGBT specifications and the manufacturer recommendations. Sink and source resistor values can be independently tuned to optimize the turn-on and turn-off behaviors and can help to solve EMI issues.

The pull-down resistor (10kOhms in this example) connected between gate and emitter of the external IGBT ensures that the external IGBT remains OFF during the TD350 power-up sequence.

As the driver may be used in a very noisy environment, care should be taken to decouple the supplies. The use of 100nF ceramic capacitors connected from VH to GND (and from VL to GND if applicable) is recommended. The capacitors should be located as close as possible to the TD350 and the ground loops should be reduced as much as possible.

Figure 1. TD350 application example showing all the features

10K
10K
	10nF	TD350				1kV
10K					1K	diode
VH	1	IN	DESAT 14		16V
	2	VREF	VH	13	100pF
	10K				100nF
	3	FAULT	OUTH	12	47R
VREF
	4	NC	OUTL	11	47R
	4,7K
					10K
	5	COFF	VL	10	100nF
VH	470pF				-10V
		NC	CLAMP	9
	6
10K
	7	LVOFF	GND	8
11V

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Input stage	AN1944

2 Input stage

The TD350 is compatible with both pulse transformers or optocouplers. The schematic diagram shown in Figure 2 can be considered as example of use with both solutions.

When using an optocoupler, the IN input must be limited to approximately 5V. The pull-up resistor to VH must be between 5kOhms and 20kOhms, depending on optocoupler characteristics. An optional filtering capacitor can be added 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).

When using a pulse transformer, a 2.5V reference point can be built from the 5V VREF pin with a resistor divider. The capacitor between the VREF pin and the resistor divider middlepoint provides decoupling of the 2.5V reference, and also ensures a high level on the IN input pin at power-up to start the TD350 in OFF state.

The waveform from the pulse transformer must comply with the tONMIN and VtON/VtOFF specifications. To turn ON the TD350 outputs, the input signal must be lower than 0.8V for at

least 220ns. Conversely, the input signal must be higher than 4.2V for at least 200ns to turn OFF TD350 outputs. A pulse width of about 500ns at these threshold levels is recommended. In all cases, the input signal at the IN pin must be between 0 and 5V.

Figure 2. Application schematic (pulse transformer: left / optocoupler: right)

		VH
	TD350	4K7		TD350
1	IN		1	IN
10K	VREF
2
10nF
10K		47pF	5,1V
10nF
10K

Figure 3. Typical input signal waveforms with pulse transformer (left) or optocoupler (right)

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AN1944	Output stage

3 Output stage

The output stage is able to sink/source about 2A/1.5A typical at 25°C with a voltage drop VOL/VOH of 5V (Figure 4). The minimum sink/source currents over the full temperature range (-20°C/+125°C) are 1.2A sink and 0.75A source. VOL and VOH voltage drops at 0.5A are guaranteed to 3V and 4V maximum respectively, over the temperature range (Figure 5). This current capability sets the limit of IGBT driving, and the IGBT gate resistor should not be lower than approximately 15Ω.

The TD350 uses separate sink and source outputs (OUTL/OUTH) for easy gate driving. Output current capability can be increased by using an external buffer with two low-cost bipolar transistors.

Figure 4. Typical output stage current capability at 25°C (VH = 16V, VL = -10V)

OUT source current versus voltage (turn-on)

	3
	2,5
	2
(A)	1,5
Iout
	1
	0,5
	0
	-10	-5	0	5	10	15	20
				Vout (V)

OUT sink current versus voltage (turn-off)

	3
	2,5
	2
(A)	1,5
Iout
	1
	0,5
	0
	-10	-5	0	5	10	15	20
				Vout (V)

Figure 5. Typical VOL and VOH voltage variation with temperature

	3.0									4.0
										3.0
(V)	2.0								(V)					Iosource=500mA
						Iosink=500mA
VOL-VL									VH-VOH	2.0
														Iosource=200mA
	1.0
						Iosink=200mA				1.0					Iosource=20mA
	0.0					Iosink=20mA
										0.0
	-50	-25	0	25	50	75	100	125		-50	-25	0	25	50	75	100	125
				Temp (°C)									Temp (°C)

During the power-on sequence, it is not guaranteed that the Goff signal, which controls the OUTL-MOS (see TD350 output stage schematic diagram in Figure 6), stays HIGH. In this case when TD350 goes out from UVLO condition, the OUTL-MOS is turned off and OUTL is in High-Impedance state until the first IN transition occurs. In these conditions some leakage effects might slowly charge the external IGBT gate-emitter capacitance.

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Output stage	AN1944

Thus, it is recommended the use of a pull-down resistor of 10 kOhm or less (R3 in Figure 6) connected between the gate and emitter of the external IGBT.

Figure 6. TD350 output stage schematic

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AN1944	Active Miller clamp

4 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 (see Figure 7). 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. The main benefit is that negative voltage can be avoided in many cases, allowing a bootstrap technique for the high side driver supply.

The waveform shown in Figure 8 proves how using the Active Miller clamp provides a consistent reduction of the voltage spike on IGBT gate.

Figure 7. Active Miller clamp: principle of operation

TD350
Miller current	Miller current
high dV/dt !	high dV/dt !
active clamp
voltage spike on IGBT gate !	reduced voltage spike

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