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Posted on 18 February 2019

Driver Parameters and Switching Properties of IGBTs and MOSFETs

 

 

 

 

 

 

 

The important features of driven power MOSFET or IGBT are dependent on VGG+, VGG-, and RG ratings. The table below lists many of these features and provides an initial overview of how they relate to VGG+, VGG-, and RG. The symbols in the table are to be interpreted as follows: < ↔ increases; > ↔ decreases; - ↔ remains unaffected.

rating characteristic

Forward characteristics (RDS(on) , VCEsat )

The dependencies of the power MOSFET and IGBT forward characteristics on the driver parameters can be derived from their output characteristics. In Figure 1 this is explained by way of an example each for SEMITRANS MOSFET and IGBT modules using the data given in the datasheets.

Forward characteristics versus control voltage V GG+

Figure 1. Forward characteristics versus control voltage VGG+ a) Power MOSFET module b) IGBT module

Switching times and energy dissipation (ton , toff , Eon , Eoff )

Control voltages and gate resistances affect the various components of turn-on time ton = td(on) + tr , turn-off time toff = td(off) + tf and tail time tt of the IGBT.

Due to the gate capacitance amounting to absolute ratings of VGG+ and VGG- before switching, the recharge time between switching will decrease (turn-on delay time td(on), turn-off delay time td(off)) in proportion to the decreasing gate series resistance.

On the other hand, switching times tr and tf, and consequently, a large part of the switching losses Eon and Eoff are greatly affected by the switching control voltages VGG+ or VGG- and the gate resistor RG .

IGBT datasheets include diagrams showing the dependencies of switching times and energy dissipation on RG, in most cases given for rated current values and on condition of hard switching under ohmic-inductive load (Figure 2).

IGBT switching times (a) and switching losses (b) versus gate series resistance R G

Figure 2. IGBT switching times (a) and switching losses (b) versus gate series resistance RG; here at Tj = 125°C, VCE = 600 V, IC = 75 A, VGE = ± 15 V and on condition of hard switching under ohmic-inductive load

Dynamic turn-off behaviour of the free-wheeling diode (reverse recovery) and turn-on peak current of the transistor

The IGBT turn-on losses indicated in Figure 2b already includes the influence of the turn-off behavior of the integrated free-wheeling diode on turn-on peak current and turn-on losses.

Recovered charge Q rr

Figure 3. Recovered charge Qrr (a) and peak reverse recovery current I RRM (b) of the free-wheeling diodes in an IGBT module versus commutation speed -diF /dt of the diode current

The drain or collector current (iD , iC) rise time tr will decrease as the gate current rises (higher VGG+ or smaller RG). This in turn will increase the current commutation speed -diF/dt in the free-wheeling diode which the recovered charge Qrr and peak reverse recovery current IRRM depend on.

These dependencies are depicted in the datasheets for the fast free-wheeling diodes used in IGBT modules (Figure 3 and Figure 4).

An increase in Qrr and IRRM will cause higher turn-off losses in the internal free-wheeling diode.

Since a higher -diF /dt results in a higher Qrr and IRRM and, since the load current is increased by IRRM within the collector or drain current, the turn-on peak current and turn-on switching losses of the transistor will increase in line with its turn-on speed (Figure 2).

Free-wheeling diode turn-off losses E offD versus R G of the transistor during turn-on

Figure 4. Free-wheeling diode turn-off losses EoffD versus RG of the transistor during turn-on

Turn-off peak voltage

If VGG- increases or RG decreases, the turn-off gate current of the transistor being turned on will rise. As shown in Figure 2a, the drain or collector current fall time tf decreases, i.e. -diD/dt or -diC/dt increases. The voltage Δu = -Lσ·di/dt induced during di/dt over the parasitic commutation circuit inductance Lσ increases in proportion to di/dt.

 

For more information, please read:

Driver Circuits

Gate Current and Gate Voltage Characteristics for Drivers

Overload and Short Circuit Behavior of IGBTs and MOSFETs

Dynamic Properties of MOSFETs

 

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