Power Devices and Drivers

Three options:

  • Level-shifting for lower power/voltages
  • Optocoupler isolation for medium power/voltages (preferred)
  • Fibre optic isolation for high power/voltages

Either use:

  • Isolated power supply, optocoupler isolation, non-isolated gate driver
  • Isolated power supply, isolated gate driver.

Ideally our circuit will be inputting: ground, 5V, control; outputting: gate, source/emitter (depending on MOSFET/IGBT).

MOSFET is generally lower currents/voltages, and IGBTs are higher currents/voltages. CoolMOS is the only MOSFET range I can find that is good for 600V+. MOSFETs can switch much faster than IGBTs. IGBT have a constant voltage drop when on (like a diode), but MOSFETs behave more like a resistor (so voltage drop is proportional to current). IGBTs come in discretes and modules.

The driver needs to be able to deliver enough current to charge the gate capacitance quickly. Drive can turn on with positive voltage, but need to make a decision on whether to turn off with 0V or negative voltage.

Driver should be placed close to gate terminal. Use on/off resistors, and various diodes etc. Can power the gate driver with a bootstrap circuit or with an isolated power supply.

A device like the ACPL-302J does gate driving, UVLO, etc. plus has an integrated DC-DC controller so with some external components, the isolated power supply is provided too. This isolated supply is regulated, so the input can be unregulated. The ACNW3410 is a more basic version, without fault indication and without DC-DC controller, so a fully independent isolated power supply is needed.

A device like the ISO5451ACPL-344JT or 1ED020I12-B2 provides all the advanced gate driver features with fault reporting, however it doesn’t have an integrated DC-DC controller. The main benefit of this device over the Avago gate drivers is that it allows the gate to be driven with a negative voltage. The UCC21520 or 2ED020I12-FI is a similar version, but is a 2 channel gate driver meaning that it can drive the entire leg. The advantage of this is that it provides features like dead time. Note that the newer Infineon 2ED020I12FA is a dual channel driver but it doesn’t have dead time as the channels are completely independent. The UCC21520 doesn’t have other safety features like overcurrent (desat) detection. Some desirable features:

  • Dual channel with deadtime/interlock
  • Desaturation protection/detection (overcurrent), “soft” turn-off, fault feedback
  • UVLO with feedback (this refers to the driver chip, making sure it has sufficient supply voltage on input and output sides)
  • Active miller clamping
  • Short circuit clamping
  • Split outputs, for a separate source/sink control.

Because the DC bus experiences common mode voltage, both the high and low side drivers need to be isolated. For a full-bridge (single phase), the positive rail jumps between 0V and +VDC (say 400V), and the negative rail jumps between -VDC (say -400V) and 0V. For a three-phase two-level converter, the positive rail jumps between 0V, 1/3VDC, 2/3VDC and VDC (say 650V) and the negative rail jumps between 0V, -1/3VDC, -2/3VDC and -VDC (say -650V). The mid-point of the DC bus (if available) in the three-phase two-level converter jumps between -1/2VDC, -1/6VDC, 1/6VDC, 1/2VDC.

AV02-0258EN-AN_5324-ACPL-332J-09Jan20120 and ACPL337J000E contain excellent information on desat detection (blanking time, threshold voltage, reason for needing series resistor). http://www.evdl.org/docs/igbt_shortoff.pdf also contains excellent information.