1. Field of the Invention
The present invention relates to a self-oscillating full-bridge driver IC, and more particularly to a driver having a pre-charging circuit, which may include a bootstrap circuit for pre-charging a bootstrap capacitor before starting up or transferring signals from the oscillator.
2. Related Art
A self-oscillating full-bridge driver of background interest is the IR2153 integrated circuit. A functional block diagram of the IR2153 is shown in FIG. 1. The IR2153 has several features of particular interest:
Under voltage lockout (UVLO) is the name for a protection feature built into several control IC's manufactured by International Rectifier. This block, marked as UV Detect on FIG. 1, ensures that the gate drive outputs, HO and LO are both low should bias become too marginal for comfortable gate drive to the output transistors. The UVLO circuit also assures a repeatable start-up sequence and controls bias current needed for various elements of the IC. The UVLO circuit monitors the bias supply Vcc.
The UVLO circuit is further provided with hysteresis to improve noise immunity. Instead of switching on/off at one voltage level, two threshold levels are defined; one sensitive to a rising edge and the other to a falling edge. In product data sheets describing Vcc undervoltage lockout, these two thresholds are listed as VccUV+ and VccUV−. Hysteresis is a measure of the difference between these two thresholds and is separately listed on data sheets as VccUVH. VccUV+ is the threshold at which the chip is first enabled, when Vcc is rising. Once enabled, Vcc is allowed to drop slightly without adverse effects.
The IR2153 also has micropower start-up, which means that the bias current needed when the IC is in a quiescent state (i.e. not oscillating and Vcc<VccUV) is reduced to a very low level. Micropower start-up is beneficial because off-line applications usually have a single resistor from a high voltage bus for start-up bias to keep component cost and complexity low. In some cases this resistor provides bias for all modes of operation(both start-up and running, where current demand is highest). However, an auxiliary low voltage source is more common for this purpose.
“Bootstrapping” is beneficial because an auxiliary supply can be derived with much higher efficiency than from a simple dropping resistor connected to the line. In bootstrap start-up schemes, the micropower feature allows the pull-up resistor to be increased in value compared to schemes in which a single dropping resistor provides all the bias. This is because the resistor needs only to overcome the quiescent bias current requirements for the IC and raise Vcc to UV+. When the IC starts oscillating, the additional bias requirement is met by an auxiliary supply.
The term “bootstrap” is also used to describe the diode and Vbs capacitor arrangement used in the typical circuit configuration. This part of the circuit serves a different function than the resistor in a bootstrap “start up” scheme. Bootstrap diode/capacitor circuits are described in Design Tip DT-2, by Jonathan Adams, which is available at www.irf.com. incorporated by reference.
The IR2153 further incorporates a fast shutdown mode providing a protective action against fault conditions that would otherwise destroy output switches. Fusing is the simplest option but is often ineffective at preventing damage to semiconductors and is not self-resetting, unless comparatively more expensive devices are used. If the load is capacitively coupled or resonant in nature, for example as in most electronic ballast designs, ample protection may be afforded by merely halting the oscillator and preventing further switching cycles. However, adequate protection sometimes calls for both FET's to be rapidly turned off.
One way to implement shutdown is to pull Vcc below VccUV− so as to turn off both FETs. This requirement may to an extent conflict with the need to provide a healthy charge reservoir and solid decoupling. The shutdown network must be capable of rapidly pulling charge from the Vcc reservoir capacitor (and the decoupling capacitor, if present). Low holding current thyristors can be used to crowbar Vcc and latch the chip off until the supply is recycled. This approach is both simple and inexpensive but does not support automatic power-on reset.
The IR2153 offers a convenient shutdown solution by adding a second function to the Ct pin. A third functional threshold is added to Ct and set at Vcc/6. Below this threshold a fast shutdown mode is invoked and both output buffers are set low with minimal delay. This allows a simple open-collector NPN transistor or similar to be used as part of an inexpensive protection scheme.
IR2153 can be if desired be driven directly as a slave device, for example to complete a fill bridge. The Ct input can be directly fed from a square wave source, provided Vct high>⅔ Vcc and Vct low<⅓ Vcc. The Rt output of any self-oscillating control IC can generate a master clock to feed directly into the Ct input of another, effectively bypassing the internal oscillator of the slaved device. This can be a useful way to implement a full bridge using just one type of IC or hybrid.
IR2153 further has improved dead-time accuracy with zero average temperature coefficient. Dead time is the period during which both HO and LO outputs are intentionally low. This period is fixed inside the IC and serves several essential functions. The primary function of dead-time is to prevent cross conduction or shoot-through in half-bridge designs. If the load is resonant in nature, such as in electronic ballasts and resonant mode power supplies, dead-time also helps maintain zero voltage switching (ZVS). Sometimes called soft switching, this technique significantly reduces switching losses.
Cross-conduction will occur in half bridge circuits if both high and low side transistors are either fully or partially on at the same time. The resulting short across the supply exacerbates EMI, increases dissipation and may destroy power switches, the control IC, or both if extreme. MOSFET turn on/off times are often inequal and vary in production, so dead-time offers a guard band to account for these differences. To prevent cross conduction, or shoot-through, the IR2153 is available with various dead time options. Moreover, switching times can be easily modified using small signal diodes across series resistors in the gate drive loop if necessary.
When the load is inductive or resonant, circuit efficiency is generally highest when zero voltage switching (ZVS or soft switching) occurs. In ZVS, there is just enough time when both FETs are off (approximately equal to the dead-time) to allow load energy to swing the output to the opposite rail, where current will flow into the FET body drain diode. This desirable process is called self-commutation.
Soft switching (ZVS) is more efficient than hard switching. MOSFETSs become hot when the load is removed.
Self-commuation is almost completely lossless because output capacitance of the half bridge rings with the inductive component of the load when both half bridge transistors are off. When the next transistor turns on, the voltage across it is already at or close to zero, so its internal capacitance is already discharged. When its load is not resonant or inductive in nature, the output cannot self-commute to the opposite rail during off time, so the voltage across the next transistor to turn on is high. It must therefore discharge its own output capacitance in addition to providing load current. This explains why half bridge transistors often get hot when there is no load at all, or in ballasts when the lamp is removed and the oscillator is allowed to continue running.
If ZVS is required but the dead-time is too short, there is insufficient time for the half-bridge output voltage to completely self-commutate to the opposite rail. This leaves a fraction of the bus voltage remaining across the output capacitance of both transistors which the next transistor to switch on must discharge. This is partial hard switching (or partial soft switching). Conversely, if the dead-time is too long, the output voltage will self-commutate to the opposite rail, but a short time later, the load current will reverse or ring, producing a voltage transient. Again, the results is partial hard switching and less than optimal switching efficiency. For ZVS, optimal dead time occurs when there is just enough time for self commutation to occur. IR2153 is available with several dead-time options to help meet this requirement. If further adjustments are necessary, small signal diodes can be used in the gate drive loop to independently change charge/discharge timing.
IR2153 output buffers also have a reduced di/dt output stage. This means that the peak rate of change of current (rate of current rise and fall for turn-on and turn-off) has been deliberately reduced. In effect the IR2153 output buffers are turned on and off at a controlled rate.
Benefits of reduced di/dt output buffers include lower gate ringing (Vgs overshoot/undershoot) from a combination of di/dt and gate circuit parasitics. In some cases gate resistors can be eliminated altogether, however this depends on several factors such as layout and transistor type used. Rf interference may also be reduced.