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Topic: Gate drive requirements of IGBTs (Read 1 time) previous topic - next topic

jtw11

Hi all,

Quick question - that I feel I know the answer to already. Is it neccesary to drive the gate of an IGBT with a gate driver, given that there's a MOSFET before the BJT?

I'd imagine the answer is the same as for a MOSFET on its own "not essential, no - but for higher switching frequencies yes to reduce switching losses", correct?

Thanks in advance!

retrolefty

The root of the answer is in the datasheet for the specific IGBT device you are using. It will state it's input capacitance and also it's output current curve Vs input gate voltage at a given collector voltage. From that you can determine what kind of drive capacity you must supply. As most IGBT devices are used in very high voltage and/or current circuits the use of special gate drivers is commonly called for.

Lefty


MarkT

IGBTs are typically driven to +15V and -5V on the gate (since IGBTs are designed for higher voltages the
gate voltages are typically higher and the negative gate drive helps reduce the slow turn-off time.  If the
application isn't demanding you probably can just use 12V and a standard MOSFET driver...
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jtw11

#3
Jan 08, 2013, 02:18 pm Last Edit: Jan 08, 2013, 02:21 pm by jtw11 Reason: 1

IGBTs are typically driven to +15V and -5V on the gate (since IGBTs are designed for higher voltages the
gate voltages are typically higher and the negative gate drive helps reduce the slow turn-off time.  If the
application isn't demanding you probably can just use 12V and a standard MOSFET driver...


The device I'm using is a dedicated ignition IGBT - a Fairchild FGD3440G2. http://www.fairchildsemi.com/ds/FG/FGD3440G2.pdf - I chose this device as it has the lowest saturation voltage of all the easily available ignition IGBTs.

The voltage I'm switching is only small, ~13V - the inductive spike of course is significantly higher - which leads me to my first question;

1) There is no clamping voltage given, only clamping energy capability. How would one go about determining at what voltage this device clamps the inductive voltage spike?

2) Total gate charge seems to be somewhat higher than most MOSFETs I've worked with, at 24nC - however, the max gate emitter voltage is +- 10V, so I'll drive the gate using a MOSFET driver with 5V, given all the test conditions in the datasheet are quoted at 5V. Does this sound right?

3) All the examples, and test data use a series gate resistor - however, the symbol on page 1 shows a gate resistor, R1 - so why the external gate resistor too?

On the subject on gate resistors, with a 'normal' MOSFET on its own - should one use a gate resistor even when driving the gate with a gate driver?

MarkT



IGBTs are typically driven to +15V and -5V on the gate (since IGBTs are designed for higher voltages the
gate voltages are typically higher and the negative gate drive helps reduce the slow turn-off time.  If the
application isn't demanding you probably can just use 12V and a standard MOSFET driver...


The device I'm using is a dedicated ignition IGBT - a Fairchild FGD3440G2. http://www.fairchildsemi.com/ds/FG/FGD3440G2.pdf - I chose this device as it has the lowest saturation voltage of all the easily available ignition IGBTs.

The voltage I'm switching is only small, ~13V - the inductive spike of course is significantly higher - which leads me to my first question;

1) There is no clamping voltage given, only clamping energy capability. How would one go about determining at what voltage this device clamps the inductive voltage spike?

Well the collector-emitter breakdown voltage is 400V - should be a clue...  You probably don't want to clamp the spike anyway - for an ignition coil the spark plugs conduct before the primary voltage goes too high.
Quote


2) Total gate charge seems to be somewhat higher than most MOSFETs I've worked with, at 24nC - however, the max gate emitter voltage is +- 10V, so I'll drive the gate using a MOSFET driver with 5V, given all the test conditions in the datasheet are quoted at 5V. Does this sound right?

Yup, +/-5V is probably a reasonable range to restrict to (the device has internal zeners I note).  Driving with +5 and 0V will be OK, just
a bit slower turning off.  BTW a logic-level IGBT is not something I've seen before - will see how cheap these are.
Quote


3) All the examples, and test data use a series gate resistor - however, the symbol on page 1 shows a gate resistor, R1 - so why the external gate resistor too?

To limit rise/fall times and reduce gate currents - reducing RFI, to reduce dissipation in the gate and the gate driver.  Also can reducing
ringing on the gate circuit.
Quote


On the subject on gate resistors, with a 'normal' MOSFET on its own - should one use a gate resistor even when driving the gate with a gate driver?

Again depends on how fast you want to switch (faster reduces dissipation in the main circuit, but increases noise and RFI).
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