Below are several important criteria that should be considered when selecting IGBT and MOSFET modules.
1.) Operating voltage
Since most power modules are used in DC voltage links which are AC-voltage supplied via single-phase or three-phase rectifier bridges, the blocking voltages of IGBT and MOSFET modules are adjusted to common line voltage levels for general-purpose use (600 V, 1200 V, 1700 V). For this reason, a rough selection is first made in the DC link from line voltage (control angle 0° for controlled rectifiers) VN or no-load direct voltage VCC (VDD) as given in the following Table:
Afterwards, it is necessary to check whether, under maximum voltage stress, the blocking voltage is not exceeded. The maximum voltage VCEmax,T or VDSmax,T present at the module terminals must be limited, for example, to VCEmax,T≤VCES−LCE×0.8ICmax÷tf (ICmax). For the DC link voltage, including every possible stationary or transient overvoltage, the following must be true, for example: VCCmax,T≤VCES−Lσ×0.8ICmax÷tf (ICmax). For IGBT modules, the voltage applied to the chips can be approximately verified by performing measurements between the terminals Cx and Ex. The measurement-based dimensioning requirements for any possible operating condition have become even stricter as a result of the introduction of the IGBT4. This is owing to the fact that the different interaction between application conditions and IGBT properties is far more complex today than for older IGBT generations.
As an additional method of damping overvoltages – especially for DC link voltages > 700 V (for 1200 V-IGBT) and collector currents of some 100 A – we recommend placing suitable foil capacitors near the DC terminals of the module as snubbers (+DC: collector TOP IGBT, -DC: emitter BOT IGBT).
Coordination of insulation
Coordination of insulation brings the voltage stress requirements for electrical insulation in line with the necessary withstand capability. Such coordination values, which have been obtained from past experience, have been laid down in standards and must be observed for equipment design.
For power electronics components, such as power modules, functional insulation should be used between the module terminals. Basic insulation should be applied between the module base plate (earthed in the device through the heatsink) and the module terminals and reinforced or double insulation between the module terminals and its insulated internal sensors (e.g. for current, voltage, temperature), whose outputs may be connected to the voltage potential of information electronics by the user without any additional measures being taken.
The user must already be aware of the electrical and environmental conditions to be expected when choosing a power module, i.e. at the beginning of the device design stage, since these conditions strongly influence insulation coordination. For this reason, the following requirements must be analyzed in addition to selecting the proper voltage class of IGBT or MOSFET in line with the highest peak voltage that will be encountered:
- Mains overvoltage category in accordance with EN60664
- Maximum altitude of application
- Earthing of the supply network
- Maximum conductor-to-conductor voltage or highest DC supply voltage
- Maximum DC-link voltage
- Insulation reqirements for sensors and potential isolationg spots for isolation from control circuits.
- Maximum control voltage
2.) On-state current
As a rule, the output current of a power electronics circuit that can be gained in field applications is limited by the entire balance of power losses (forward, reverse, and switching losses) of the transistors and freewheeling diodes in the power modules and the possible heat dissipation from the chips through the module and the cooling system to the cooling medium:
There is no stationary or dynamic operating condition (with the exception of short-circuit turn-off which may only be repeated to a limited extent) where the maximum rated chip temperature of IGBTs, diodes, or MOSFETs may be exceeded. The temperature gradients that occur due to load and temperature changes must not result in wear-induced module destruction before the end of the expected module lifetime.
Further limits are the switching capacity of transistors in operation and in the event of overloads up to the maximum current being turned off, i.e. within the limits of the rated transistor operating areas, the necessary selectivity of active and passive overcurrent protection measures, and the switching overvoltages that depend on the current being turned off.
3.) Stress conditions of freewheeling diodes in rectifier and inverter mode
In order to be able to feed energy back to the grid, drive converters are often rated for 4-quadrant operation, which means they consist of 2 topologically identical converters at the line side (LSC: Line Side Converter) and at the machine side (MSC: Machine Side Converter). Depending on the direction of current flow (rectifier or inverter mode), the freewheeling diodes of the two converters are under different stresses in regards to resultant power dissipation for the same power transmitted.
In inverter mode, the average energy flow is directed from the DC link to the AC side, i.e. the AC side supplies a consumer, e.g. a three-phase motor or a power system. On the other hand, the average energy flow in rectifier mode is directed from the AC side to the DC link. In this case, the converter works as a pulse rectifier connected to an AC mains or generator. Although the power performance in both cases is the same, the power semiconductors are subject to different power losses essentially due to the opposite phase shift between the voltage and current fundamental frequency on the AC side that occurs in rectifier or inverter mode.
Most available IGBT and MOSFET modules with integrated freewheeling diodes are dimensioned for use in inverters in reference to the power losses that can be dissipated at rated current (e.g. cos φ = 0.6…1). Due to their reduced on-state and total losses, diodes have been designed for a far lower dissipation of power losses than for IGBT (ratio IGBT : Diode ≈2…3:1). When dimensioning a converter for use as a pulse rectifier, the diode load must be taken into particular consideration.
4.) Switching frequency
Since switching losses increase in proportion to the frequency, they limit the switching frequency, although this can still be increased by oversizing the power module. Other limitations may be set by the transistor turn-on and turn-off delay times td(on), td(off), the reverse recovery times of the freewheeling diodes, the driver control power which increases proportionate to the frequency, as well as by the minimum turn-on, turn-off or dead times necessary for driver, interlocking, measuring, protection and monitoring functions. If switching losses are to be shifted to passive networks (snubbers), or overvoltages are to be limited by snubbers, the recharge time of such networks required after low-loss switching has to be considered as deadtime.
Switching times of MOSFET and IGBT power modules are within the range of some ns to some 100 ns. While the switching times of MOSFETs and older IGBTs can be influenced within relatively wide limits by control parameters, many new Trench IGBTs provide this option for turn-on to a limited extent only and for turn-off barely at all, unless drastic increases in switching losses – owing to very high gate resistances – are accepted.
Today, the following guideline values for switching frequencies in standard modules apply:
|For hard switching||MOSFETT modules||low voltage||up to 250 kHz|
|high voltage||up to 100 kHz|
|IGBT modules||600 V||up to 30 kHz|
|1200 V||up to 20 kHz|
|1700 V||up to 10 kHz|
|3300 V||up to 3 kHz|
|For soft switching||MOSFETT modules||low voltage||up to 500 kHz|
|high voltage||up to 250 kHz|
|IGBT modules||up to 150 kHz|
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