AN-7512| Application Note

Parallel Operation Of Insulated Gate Transistors
Application Note September 1993 AN-7512

/Title AN75 2) Subect Paralel peraion Of n ulated ate ran istors) Autho () Keyords Interil orpoation, emionuctor, vaanche nergy ated, witch ng ower uplie , ower witch ng

FORWARD VOLTAGE DROP (V)

In the November issue of Powertechnics, the general considerations of paralleling semiconductor switches were presented. Some of the important factors include the characteristics of different types of load reactances and the action of the switching device during its turn-on delay, rise time and turn-off delay times. Different types of switching devices must be handled differently when operated in parallel. Power bipolar transistors, SCRs, MOSFETs and IGTs all have different characteristics which must be taken into consideration. The IGT transistor combines the high input impedance, voltage controlled turn on/turn off capabilities of power MOSFETs and the low on-state conduction losses of bipolar transistors. Like MOSFETs, the output characteristics of IGTs are generated by plotting collector-emitter current, collector-emitter voltage and gate voltage. Unlike the MOSFET, there is an offset voltage generated by the collectoremitter junction of the npn transistor. However, once this offset is overcome, the effe
ctive on-resistance in the saturation region is much lower for the IGT than for the MOSFET. A steady state equivalent circuit is shown in Figure 1. Total device current equals MOSFET current (IMOS) plus bipolar current (IBIT) and since the MOSFET current is the base current of the pnp, these current components are related by the gain of the pnp.
C IMOS RMOD + VDS + VBE ITOT + VCE

We also know from measurements, the MOSFET's temperature coefficient in the epi-resistance is positive. We know further that as the device temperature increases, the bipolar transistor's gain increases, the VBE drop decreases, which both tend to reduce on-voltage drop. On the other hand, the MOSFET and epi-resistance voltage drop will increase with temperature, tending to increase on-voltage voltage. These effects cancel and the net result is that the IGT exhibits much less variation of on-voltage voltage with temperature than either bipolars or MOSFETs. The temperature coefficient goes from a bipolar like negative (at low currents) to zero (at rated current) and to a MOSFET-like positive coefficient as current density increases (Figure 2).
VCE(SAT) vs IC AND TJ FOR 10A DEVICE 5.0 CONDITIONS: GATE BIAS: 14V CURRENT LEVELS: PULSED, 300us 4.0 IC = 15A 3.0 ICH = 10A IC = 8A 2.0 IC = 6A IC = 4A IC = 2A 1.0 25 50 75 100 125 150 AMBIENT TEMPERATURE (oC) IC = 20A

IBJT

ITOT = IMOS + IBJT IBJT = BJTIMOS VBE + IMOSRMOD = VDS

FIGURE 2. VCE vs TA OF IGT, AT DIFFERENT COLLECTOR CURRENT

Turn-On Switching Performance Like the MOSFET, the IGT gate presents a capacitive load to the drive circuit. The IGT capacitive elements and their typical variation with voltage is analogous to the MOSFET, hence the IGT turn-on interval can be divided into three distinct regions (refer to Figure 3). In region I, the input capacitance is charged until the gate voltage reaches the value needed to initiate collector current conduction. In region II, turn-on is essentially completed as the collector voltage falls rapidly to the 10% level. The effective capacitance increases dramatically in this region due to the Miller effect. In region III, the collector voltage slowly settles to its saturation level. At the start of Section III, the effective input capacitance remains high because as the collector voltage is driven below the gate voltage, the polarity of the collector gate voltage reverses and CGC increases dramatically. When the collector reaches the saturation voltage level, the gate rises to the gate-emitte
r supply level (typically 15 volts).

FIGURE 1. N-CHANNEL IGT TRANSISTOR STEADY STATE EQUIVALENT CIRCUIT

To understand the unusual behavior of its temperature coefficient, negative at low current, almost zero at normal current, and positive at high current, we analyzed the IGT by treating the two branch currents comprising the conduction path as two separated devices. The IGT's on-state voltage drop is composed of the MOSFET voltage drop plus the bipolar VBE drop apparently parallel by a pnp-transistor. Note that the only part of the bipolar in parallel to the MOSFET and modulation resistance is the base-collector junction, but the baseemitter junction is common to both branches.

2002 Fairchild Semiconductor Corporation

Application Note 7512 Rev. A1