Part # AN1263 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: USING THE INTERNAL BOOTSTRAP CHARGE CAPABILITY OF THE L6384, 85 AND 86 IN DRIVING A SIX TRANSISTOR INVERTER BRIDGE


Part Details:

AN1263 APPLICATION NOTE USING THE INTERNAL BOOTSTRAP CHARGE CAPABILITYOF THE L6384, 85, AND 86 IN DRIVING A SIX TRANSISTOR INVERTER BRIDGE by: Dennis Nolan Review of Traditional Bootstrap Circuit: Figure 1.0 depicts a typical six transistor, three phase DC-to-AC inverter bridge which uses the BOOTSTRAPmethod to power the floating gate drivers for the three upper power transistors. This bridge may employ eitherMOSFETs or IGBTs so in this text I will use the generic term, transistor, which applies to both. Please note thatthe gate drivers for the three lower transistors are powered directly by the VCC power supply which is referredto signal common, which is the same circuit node as the negative side of the main DC supply bus and the source(or emitter) terminals of the lower transistors. Since the gate must be driven high with respect to the source(emitter) of the transistor, the gate drivers for the upper transistors and their respective power supplies must bereferenced to the nodes labeled Va, Vb, and Vc, respectively and float up and down with these motor terminalvoltages. Consider the gate driver for transistor Qah. The power supply for this driver is the stored charge in capacitorC1. When the control for this bridge is first powered-up, but before any transistors have been gated, gating sig-nals are applied to all three of the lower transistors for a time interval of about one or two milliseconds. Duringthis "pre-charge" period, the negative terminals of C1, C2, and C3 are connected via a low impedance path tocircuit common, which will cause current to flow through diodes D1, D2, and D3 until the bootstrap capacitorshave been charged up to approximately the voltage level of Vcc. Whenever an upper transistor is turned on, itssource (emitter) terminal will be pulled up nearly to the level of the plus side of the DC bus (which is normallyhigher than Vcc) and the positive terminal of the bootstrap capacitor will "fly up" to the level of the DC bus plusVcc (the initial voltage it was charged up to). In this instance the bootstrap capacitor can be thought of like a dry cell battery that is free to float up and downwith the motor terminal and continue to power the gate driver. Unfortunately, unlike a battery which can contin-ually supply output current and maintain its output voltage, a capacitor will discharge, with its voltage decreasingin direct proportion to the time integral of the current it supplies. The discharging of the bootstrap capacitors isnot normally a problem, however, because the current drawn by the gate driver is very small and the normaloperation of the inverter bridge offers periodic opportunities to recharge the cap. If the inverter bridge is being used to power either an induction motor or a sinusoidally driven brushless DC mo-tor (also called permanent magnet synchronous), then the three individual phases ( Qah & Qal, Qbh & Qbl, andQch & Qcl ) are each operated simultaneously in a complementary mode so as to produce a close approxima-tion of a three phase set of sinusoidal phase voltages, displaced from each other by 120 electrical degrees.Consider phase A. Transistor Qah and Qal are alternately gated, one gating signal being essentially the logicinverse of the other except for a short "dead time" of one or two microseconds at the transition from upper on tolower on or vice versa when both are left off. During the latter or "off" part of each pulse width modulation cycle,Qal is gated, which pulls Va to ground and provides a low impedance DC path to recharge the bootstrap capac-itor. May 2000 1/6 AN1263 APPLICATION NOTE Another gating scheme commonly used with this bridge is the so-called six step commutation method. Six stepcommutation is used primarily with brushless DC motors which employ three Hall effect Sensors to provide rotorposition information which is used to control commutation. With six step commutation, no more than two of thesix transistors are active at any one time. These transistor pairs consist of one upper and one lower transistorand are referred to as phases AB, AC, BC, BA, CA, and CB. The first letter indicates which one of the uppertransistors is on while the second indicates the lower. Note that with six step commutation one of the three mo-tor wires is always de-energized, with no current flowing in it. Pulse width modulation is most often accom-plished by leaving the upper transistor on during the entire sixty degree interval for the given phase andmodulating the lower one, or leaving the lower one on and modulating the upper, or sometimes modulating bothon and off together. Consider phase AB, modulating the lower transistor. During the ON part of the PWM cycle, Va is pulled up tothe DC bus, Vb is pulled down to ground, C2 is being charged and C1 has lost charge equal to the required gatecharge of Qah and continues to be discharged by the quiescent current requirements of the Qah gate driver pluswhatever leakage currents flow in the transistor gate circuit and on the circuit board itself. During the off part ofthe PWM cycle, Va is still pulled up to the DC bus and Vb is also pulled up to the DC bus since the motor currentthat was flowing down through Qbl is now "freewheeling" up through the diode associated with Qbh. None ofthe bootstrap capacitors charge during this interval. If the motor should get "stuck" on this phase, either becausethe motor is stalled by an excessive load or it is being used in a positioning type application and being com-manded to hold position (perhaps against a gravity load as would be common in a robotics type application),then the charge on C1 will eventually be depleted to the point where operation of the bridge cannot continue.Many integrated gate drivers monitor the bootstrap capacitor voltage and will shutdown when it falls below apreset threshold. We should note here that if the motor does move on to the next phase ( which would be ACin normal ABC rotation sequencing), C3 will get charged but poor C1 will still get none. The next phase, which is BC, will be of no help to our starving capacitor either, but if we can keep the motormoving into the next phase, BA, then C1 will finally get its much needed refresh charge. Now let us consider phase AB, modulating the upper transistor. The on part of the PWM cycle is, of course, thesame situation as that described previously for bottom modulation. During the off part of the PWM cycle, Vb isstill pulled down to ground and Va is also pulled down to ground since the motor current that was flowing downthrough Qah is now "freewheeling" up through the diode associated with Qal. Both bootstrap capacitors C1 andC2 will charge during this interval. As long as the off time of the PWM is adequately long, and we should notneed much time since the impedance in the charging circuit is low, this situation can be sustained indefinitelyand we can loiter on any given phase for as long as we wish. It is for this reason that modulation of the uppertransistor has been generally preferred when the bootstrap method is used. Bootstrap Circuit Using the STMicroelectronics L6384, 5, and 6: Now let us consider specifically usage of the STMicroelectronics L6384, L6385, and L6386 integrated gate driv-ers. These chips contain one gate driver which is referred to ground and one floating gate driver. Three of thesechips would be used to operate a six transistor inverter bridge. An examination of the block diagram given inthe data sheet for one of these parts will show that a major advance has been made in the design of these partscompared to previous floating driver designs since the external high voltage, fast recovery, diode is no longerrequired for charging the bootstrap capacitor. There are some caveats which must be observed when usingthese parts compared to the traditional approach using an external diode. These differences stem from the factthat, although the integrated charging circuit is referred to as a "bootstrap diode" and is drawn as such in theblock diagram, this circuit is actually realized by a diode in series with a MOSFET transistor switch which is gat-ed synchronously with the lower gate driver. This diode/MOSFET combination can be accurately modeled by 2/6 AN1263 APPLICATION NOTE one of the two circuits presented in figure 3. Circuit one, which consists of an ideal (no forward drop) diode inseries with a 0.7VDC independent voltage source and a 125 ohm resistor, is valid whenever the lower driver inon. Circuit two, the same circuit except that the voltage source is now 3.2VDC, is valid whenever the lower driv-er is off. Figure 2 shows the new circuit with the diodes replaced by a diode/MOSFET series circuit. For the most part,the synchronously gated MOSFET operates very similarly to a diode in this circuit, but there can arise somesubtle differences which must be considered. In the case of sinusoidal commutation, where the upper and lower transistors of each phase are alternately gat-ed in a nearly complimentary (except for dead time) manner, the bootstrap caps get depleted during the on partof the PWM period for each independent phase, but then get recharged during the off part, when the lower tran-sistor is on (providing a charging path to ground) and the charging MOSFET is on (providing a charging pathfrom Vcc). Some attention should be paid to the sizing of the bootstrap capacitor so that the time constantformed between the cap and the 125 ohms Rdson of the charging mosfet is not too large compared to the PWMoff time. This will not normally present a problem, however, since the charge accumulated during the off part ofthe PWM cycle will probably be more than the charge lost during the on part of the cycle. Now consider six step, phase AB again. Whether upper or lower modulation is employed, the main concernhere is that while we are on phase AB, for


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