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IXBD4411PI Datasheet(PDF) 8 Page - IXYS Corporation |
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IXBD4411PI Datasheet(HTML) 8 Page - IXYS Corporation |
8 / 11 page © 2004 IXYS All rights reserved IXBD4410 IXBD4411 terminals (C7, C11), and an output reservoir capacitor between V EE and GND (C10, C14). A 0.1 µF charge pump capacitor (C7, C11) is recom- mended. The voltage regulation method used in the IXBD4410/4411 allows a 1 to 2 V ripple frequency depends on the size of the V EE output reservoir capacitor (C10, C14) and the average load current. The minimum recommended output reservoir (C10, C14) is 4.7 µF tantalum, or 10 µF if aluminium electrolytic construction is chosen. Note that this reservoir capacitor is in addition to a good quality high frequency bypass capaci- tor (0.1 µF) that should be placed from VEE to GND (C9, C13). A small resistor in series with the charge pump capacitor, (R7, R8) reduces the peak charging currents of the charge pump. A value or 68 Ω or greater is recommended, as illustrated in the applications example in Fig. 6. Current Sense / Desaturation Detection Circuit All members of the ISOSMART™ driver family provide a very flexible overcurrent/short circuit protection capability that works with both standard three-terminal power transistors, and with 4- and 5-terminal current sensing power devices. Overcurrent detection is accomplished as illustrated in Fig. 7a (for a current mirror power device) and Fig. 7b (for a standard three terminal power transistor). Desaturation detection is accomplished with the same internal circuits by measuring the voltage across the power transistor in the on-state with an external resistor divider (Fig. 7c). The IM input trip point V TIM, typically 300 mV, is referenced to the Kelvin ground pin KG. Current Mirror MOSFET and IGBT allow good control of peak let-through currents and excellent short circuit protection when combined with the ISOSMART™ driver family of devices. The sense resistor is chosen to develop 300 mV at the desired peak transistor current, assuming a mirror ration of 1400:1, and a trip point of 30 A is desired: R s = 300 mV • 1400/30 A = 14 Ω (use 15 Ω CC). It is important to realize that C oss per Fig. 7: Alternative overcurrent protection circuits a b c Desaturation Detection with Standard MOSFET/IGBT With Standard MOSFET/IGBT With Current Mirror IM KG GND IM KG GND IM KG GND unit area of the mirror cells is much larger that C oss per unit area of the bulk of the chip due to periphery effects. This causes a large transient current pulse at the mirror output whenever the transistor switches (C • dv/dt currents), which can cause false overcurrent trigger. The RC filter indicated in Fig. 7a will eliminate this problem. Standard three-terminal MOSFET and IGBT devices (in discrete as well as modern industrial single transistor and phase-leg modules) can also be protected from short circuit with the ISOSMART™ driver family devices. In discrete device designs, where the source/emitter terminal is available, overcurrent protection with an external power resistor can be implemented. The resistor is placed in series with the device emitter, with the full device current flowing through it (Fig. 7b). The sense resistor is again selected to develop 300 mV at the desired peak transistor current, assuming a trip point of 30 A is desired: R s = 300 mV / 30 A = 10 mΩ (use 10 m Ω, noninductive current sense resistor). It is important to recognize that "noninductive" is a relative term, especially when applied to current sense resistor construction and characterization. There is always significant series inductance inserted with the sense resistor, and L • di/dt voltage transients can cause false overcurrent trigger. The RC filter indicated in Figure 7b will eliminate this problem. Choosing the RC pole at the current sense resistor RL zero should exactly compensate for series inductance. Because the exact value is not normally known (and can vary depending on PC layout and component lead dress) this is not normally a good idea. Usually, the RC time constant should be two to ten times longer than the suspected RL time constant. Desaturation detection as in Figure 7c is probably the most common method of short circuit protection in use today. While not strictly an "overcurrent" detector, if the power transistor gain, and consequently short circuit let- through current, is well controlled (as with modern MOSFET and IGBT) this methodology offers very effective protection. The IXBD4410/4411 half-bridge circuits in Fig. 6 uses desaturation detection. In Fig. 6, the voltage across the two power MOSFET devices (or IGBTs) are monitored by two sets of voltage- divider networks, R10 and R11 for the high-side gate driver, and R13 and R14 for the low-side gate driver. The dividers are set to trip the IM input comparators when either Power MOSFET device V DS exceeds a reasonable value, perhaps 50 V (usually a value of 10 % of the nominal DC bus voltage works well). R10 or R13 are chosen to tolerate the applied steady state DC bus voltage at an acceptable power dissipation. Dielectric withstand capability, power handling, temperature rise, and PC board creep and strike spacings, must all be carefully considered in the design of the voltage-divider networks. In the off-state, the voltage across the Power MOSFET device may go as high as the DC bus potential. To keep this normal condition from setting the internal fault flip-flop of the IXBD4410 or the IXBD4411, an internal CMOS switch is turned on and placed across lM and KG pins shorting them together. This effectively discharges C8 or C12 in Fig. 6 and maintains zero potential R s R s R s |
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