Insulated-gate bipolar transistors (IGBTs) are frequently used in power electronics for high voltage and high current switching. These power transistors are voltage controlled and generate the majority of their losses during switching. Short switching times are preferable for minimizing switching losses.
Fast switching, on the other hand, hides the danger of high voltage transients, which could potentially affect or even damage the processor logic. As a result, gate drivers that supply the appropriate gate signals to the IGBTs perform the function of providing short-circuit protection and influencing the switching speed. Certain characteristics, however, are critical in the selection of the gate driver.
Current-Driving Capability
The transistor briefly enters a state in which both a high voltage and a high current are applied during the switching process. Ohm’s law predicts that this will result in certain losses based on the durations of these states (see Figure 1).
The goal is to keep these periods as short as possible. The transistor’s gate capacitance, which must be charged/discharged for switching, has a significant impact here. This process is sped up by higher transient currents.
Figure 1.Individual transistor loss components are represented in a simplified manner.
Drivers that can provide higher gate currents for a longer period of time reduce switching losses. Depending on the IGBT, this could result in switching times as low as a few ns.
Timing
The output rise time (tR), fall time (tF), and propagation delay are all important factors in minimizing switching times (tD). The propagation delay is defined as the time required for an input edge to reach the output and is affected by the IGBT driver output current and the output load.
The propagation delay is typically characterized by small differences between the rising and falling edges, resulting in a pulse width distortion (PWD):
PWD = |tDLH – tDHL|
Because drivers frequently have multiple output channels with different response times despite being driven by the same input, a small additional offset, known as the propagation delay skew (tSKEW), is produced.
Figure 2. Timing characteristics of a gate driver with multiple outputs.
Insulation Withstand Voltage
Insulation is required in power electronics for functional as well as safety reasons. Insulation is unavoidable in drive technology because gate drivers, for example, are used in the form of a half-bridge topology and thus come into contact with high bus voltages and currents.
The functional reason is that power stage actuation usually occurs from the low voltage circuit, and thus actuation of the half bridge’s high-side switch would be impossible due to the higher potential with a simultaneously open low-side switch.
Simultaneously, the insulation represents reliable isolation of the high voltage section from the control circuit in the event of a fault, allowing humans to make contact. Dielectric strengths of 5 kV (rms)/min or greater are typical for insulated gate drivers.
Immunity
Harsh industrial environments necessitate applications with the highest possible immunity or interference resistance to sources of interference.
RF noise, common-mode transients, and magnetic interference fields, for example, are problematic because they can couple into the gate driver and excite the power stage, causing it to switch at inconvenient times. The ability to reject common-mode transients between the input and the output of insulated gate drivers is defined as common-mode transient immunity (CMTI).
The addressed parameters are only a subset of the gate driver specifications and do not constitute an exhaustive list. Other deciding factors may include the operating voltage, supply voltage, temperature range, and optional integrated functions such as a Miller clamp and desaturation protection. As a result, depending on the application’s requirements, a wide range of different gate drivers can be chosen.
Igbt Overcurrent / Short Circuit Protection
The current is typically given a margin of more than 10% when designing an IGBT. When the power IGBT inverter is turned on, however, the load side fault causes overcurrent due to the short circuit of the component and load, and the load side has a particularly large inductive load. A high harmonic current will be produced when starting and stopping.
The inverter output current will rise sharply at this point, causing the IGBT operating current to rise sharply as well. The IGBT short circuit is classified into two types: through-current and type-I short circuit. The type-I short circuit occurs in the bridge arm of the converter. The type-II short circuit occurs when the converter’s short-circuit point is on the load side and the equivalent short-circuit impedance is high. In general, a type II short circuit is considered a severe overcurrent in an IGBT inverter.
If the relevant measures are not taken when a short circuit occurs, the IGBT will quickly enter desaturation, and the transient power consumption will exceed the limit and be damaged, because the IGBT can only sustain the overcurrent for a few microseconds.
As a result, when a short circuit occurs, the IGBT should be turned off as soon as possible, with a gentle turn-off speed, to ensure that the rate of current change is within a certain range, avoiding the voltage being cut too quickly, causing the voltage stress to exceed the limit and damaging the IGBT. The quick response measure is included in the active clamping scheme, allowing the IGBT driver to operate as quickly as possible.