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Differences Between MOSFET IGBT IGBT combines advantages bipolar
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IGBT (Insulated Gate Bipolar Transistor)
Differences Between MOSFET IGBT
IGBT combines advantages bipolar field effect transistor. seen from structures shown below, only difference lies additional p-zone IGBT. presence this layer, holes injected into highly resistive n-layer carrier overflow created. This increase conductivity n-layer allows reduce on-state voltage IGBT.
Figure SIPMOS-Transistor (MOSFET)
Figure IGBT (Insulated Gate Bipolar Transistor) Comparison Output Characteristics
Figure Output Characteristics MOSFET (1000
Figure Output Characteristics IGBT (1000
RDSon MOSFET on-state mainly influenced doped center region, which essential voltage blocking capability. additional p-layer IGBT causes carrier overflow center region. spite threshold voltage, which created pn-junction collector side, 1000 V-IGBT "on-state resistance", which reduced factor compared MOSFET with similar blocking characterstics identical chip area. Equivalent Circuit IGBT
Figure Equivalent Circuit IGBT
equivalent circuit IGBT depicted quite accurately pnp-transistor, where base current controlled transistor. conductivity resistor base branch increased (modulated) when IGBT turned-on. This way, greater part load current flowing over base branch. These effects only show user turn-on delay time tail current turn-off. this reason, device simply considered transistor with corresponding capacities (see Figure
Today, different solutions known realizing IGBT that suitable existing applications: PT-structure NPT-structure, which been developed with Siemens.
Figure NPT-IGBT ("homogeneous structure")
Figure PT-IGBT ("epitaxial-structure")
(punch through) structure shows characteristic epitaxial layers with -doped region (buffer layer) -region p-doped substrate wafer. carrier life time minimized heavy metal diffusion highly energetic radiation. base material (non punch through) structure homogeneous -doped wafer. backside, specially formed p-layer created during wafer processing. necessary limit carrier life time. both cases typical IGBT cell structure formed front side.
Switching Behavior General
IGBTs mainly used switches, e.g. chopper frequency converter applications. these applications adaptation (freewheeling) diode essential, because after switching IGBT current driven load, which inductive most cases. attaching suitable diodes, this current flow enabled. When IGBT turned again, current flown diode (flooded charge carriers) first works like short. stored charge removed first diode block voltage. This appears surplus current additional load current which called reverse recovery current diode Irr. maximum occurs (di/dt when instantaneous voltages across IGBT diode equals supply voltage (Figure Switching-off IGBT results current change this makes overvoltage spike current change parasitic inductances according di/dt (Figure
Miller-effect nothing else than feedback collector-emitter voltage gate-collector capacitance gate. This means change same effect internal current source into bias circuit, where current given expression (VCE) dVCE/dt. Unfortunately constant, changes value with VCE. strongest change results small VCE. This explains that: During turning-on (starting with: high, zero negative) with constant gate charging current linear increase gate voltage results. With falling collector-emitter voltage gate bias current used changing charge (CGC dVCE/dt) gate voltage remains constant. Later, when collector-emitter voltage come down becomes larger much that also reduced slope still bias supplied gate current used Only when finally current needed charging becomes smaller than bias supplied current gate voltage rise again (Figure 10).
Figure Switch-on with Current Commutating from Freewheeling Circuit
turning-off: (starting with: low, positive greater than threshold voltage Vth) gate voltage first decreases nearly linearly constant gate discharge current). With still collector-emitter voltage with only moderate increase there strongest change (decrease) CGC. Decrease capacitance constant charge increases voltage. there bias source which drawing current gate, gate-emitter voltage remains constant. Subsequently increases most gate discharge current used dVCE/dt; gate voltage further remains constant. charge over process finally finished when roughly reaches operating voltage. further decrease gate voltage possible (Figure 11).
Figure Turn-off Inductive Load into Freewheeling Circuit Miller-effect gate current during turn-on turn-off first used changing charge CGC. This charging down gate slowed down. should mentioned that CGC-change VCE-change regulate itself that available gate current used more. This means that with larger gate series resistor events take longer time, i.e. turning-on turning-off last longer.
Turn-Off Behavior PT/NPT-IGBT
turn-off behavior both IGBT-types different with respect temperature dependence.
Figure Strong Increase Tail-Current With (left right
Figure Tail-Current Nearly Independent Temperature; Tail Starting Level Lower Fades Slower (left right
Influence Gate-Series-Resistor Switching Losses
Switching losses have their origin overlap current voltage waveforms during turn-on turn-off. They depend magnitude current voltage.
Turn-on speed that also turn-on losses influenced very easily gate series resistor. turn-off only current fall-time influenced gate resistor but, tail current.
Short-circuit Behavior IGBT
negative temperature coefficient short-circuit current causes negative thermal feedback device. This most important condition easy paralleling IGBTs. Short-circuit Type
This case short-circuit describes turn-on IGBT during existing short-circuit output circuit (see Figure 16).
Figure Schematic Short-circuit
Figure Output Current/Output Voltage During Short-circuit
short-circuit mode IGBT limits maximum collector current according output characteristics. high voltage while short-circuit current flows through IGBT device withstand extremely high power loss. this case IGBT turned between 5.10
Short-circuit type exists when short-circuit output circuit occurs during phase", IGBT (Figure 18). Limited inductivity current output circuit increases.
Figure Schematic Short-circuit Type
Figure Output Current/Output Voltage Shortcircuit Type
collector-emitter-voltage increases just when output current reaches level corresponding gate-emitter-voltage. increase output voltage leads strong decrease gate-collector-capacity. This causes internal current which charges gate-emitter-capacity (see: Miller-Effect, chapter 3.1). some cases gateemitter-voltage increases above allowed level According higher gate-emitter-voltage there dynamic short-circuit current peak. It's value higher than stationary short-circuit current depending actual gate-voltage. Similar situations appear also short-circuit type when there inductively limited slow increase current IGBT nearly turns short period time, which means that collector-emitter-voltage breaks down some
Clamping Short-circuit Type
Figure Bias Circuit With Actively Clamped Gate
Figure Current-/Voltage Response Without Active Clamping
described Miller-Effect increase collector-emitter-voltage causes current that elevates gate-voltage gate cannot discharged fast enough. This especially critical when IGBT controlled source with series gate resistor. Therefore very important gate clamping such short-circuit cases. Here active clamping (see Figure best method today fast diode used). Active clamping advantages compared Zener clamping between gate emitter, because clamped voltage independent from spread Z-diode voltage slope output characteristics. addition when fast diode used charge caused MillerEffect removed very fast. dynamic short-circuit current peak kept much lower compared bias supply without active clamping (Figure 21).
Safe Operating Area During Short-circuit (SCSOA)
normalized shortcut current 25°C
1200V-IGBT-2nd generation 12,00 10,00 8,00 Isc/Irated 6,00
0,00 1000 1200 1400
Figure Safe Operating Area Short-circuit Case level short-circuit current determined gate voltage IGBT. under normal on-state conditions gate-emitter-voltage causes increase (sat) higher forward loss. resulting short-circuit current lower than times nominal current. short-circuit safely turned-off between full breakdown voltage IGBT. Therefore protection circuitry kept relatively small. inductive voltage peak di/dt) caused set-up must kept mind.
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