jtw11:
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. Intelligent Power and Sensing Technologies | onsemi - 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;
- 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.
- 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.
- 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.
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).