For the bootstrap converter fb00t. St raPP edconverter), the input capacitor can play two major operational functions. First, the input capacitor acts as a power source during the soft-start process, providing not only a current source for the converter gate drive, but also all other circuitry connected to the integrated circuit (Ic) during the soft-start process. Second, the input capacitor is capable of filtering noise from the converter circuit or other circuitry. When the converter is running, the input capacitor of the control IC can provide a high current pulse to the gate of the field effect transistor fFET), avoiding large current transients causing large noise in the Ic input.
These two major functions require a single capacitor to have multiple characteristics at the same time, and these characteristics are often incompatible with each other.
The energy storage capacity and size of the capacitor must be large enough to provide sufficient power to the system during the soft start process. The type and size of the capacitors dictate that they often fail to provide the required low impedance characteristics at higher frequencies.
The high frequency gate drive current required to switch the FET requires the capacitor to handle high frequency current. Multilayer ceramic capacitors provide the required characteristics. Due to the high frequency electrical requirements of the system, the capacitor must be close to Ic and the electrical inductance between the two is as small as possible.
When determining the size specifications of large capacitors, it is also important to recognize that the IC needs its own internal power supply. Because once the IC reaches its "on" voltage, all of its internal circuitry is energized and begins to sink current. These circuits include an oscillator that provides timing for the switch, an error amplifier, and a comparator for control and reference. Prior to this, the components were not powered. Because of this, we usually recommend using two capacitors at the same time.
One of the two capacitors can be relatively small in size, with a low equivalent series resistance fESR) and equivalent series inductance (ESL), and can be placed directly on the IC's power supply pin (Vcc) and grounded. between. The capacitor is capable of handling high frequency currents associated with internal and external load switches, such as the gate drive of a power FET.
The location of another large capacitor should be close to Vcc and ground according to the actual situation. However, since the capacitor does not handle the above high frequency current components, its position is not critical. This capacitor provides most of the power required during startup.
Studying the startup procedure of the converter helps to identify some potential problems. First, we assume that only one small high frequency capacitor is needed. At this time, in the case of using only a small capacitor, the relevant results emphasize that we should use another large capacitor.
For the sake of analysis, my flf~ is driven by an FET that requires a gate drive charge of 60 nanoliters of fnanocoloumb and operates at a frequency of 100 kHz each time it is "on". That is to say, once the circuit is energized, the current of the gate drive itself is 6 mA. The relevant calculation formula is as follows:
60 纳库 & TImes; 100 kHz = 6mA
If the total soft-start time is assumed to be 10ms, the FET drive current plus Ic's 6 mA internal current (12mA total) can be used to determine the voltage drop. The minimum hysteresis voltage hysteresis of the UCC2817 hysteresis is 5.8V. Can a 0.1pF Vcc capacitor be sufficient to power Ic during a soft start? "If the minimum hysteresis voltage hysteresis is unknown, then the minimum hysteresis voltage hysteresis value should be determined from the undervoltage lockout (UVLO) data using the minimum open voltage and the highest turn-off voltage.
If the answer to this question is no, it means that the 0.1pF Vcc capacitor is not enough. With a soft-start time of 10ms and a current of 12mA, the 0.1pF capacitor voltage will change by 1,200V. Even if the capacitance is 10PF, the voltage change will be 12V. In this case, we will need a capacitance of 1OOpF, at which point the Vcc voltage drop is 1.2V. Now, we understand that Ic and FET require 1OOpF of capacitance to handle power consumption during startup.
However, after studying a high frequency switching cycle, how do you determine the voltage ripple on a 0.1u capacitor? Due to the low ESR and ESL that affect the high frequency characteristics of l00BF capacitors, we often overlook large capacitors. A ripple voltage will appear on the Vcc pin and is related to the ground pin. The ripple voltage generated by the FET conversion is derived from the following equation:
[(60 nal/0.1 gF) = 0.6 V].
Unless there is another high frequency capacitor nearby or a higher capacitance capacitor used as a high frequency filter, a negative peak of 0,6v will occur when the gate is charged. In this case, the lgF capacitor is a better choice.
It should be acknowledged that this seems to be an extreme situation, but for PFC controllers such as UCC28l7, this is a very realistic situation. Of course, this can cause charging time problems for the l00BF capacitor (not discussed here).
For other circuits, such as a circuit with a soft-start time of 500 BF, the soft-start time is much shorter and the internal power consumption is lower. In the case of UCC28C42, the minimum turn-on and maximum turn-off voltages are 13.5v and 10.0V, respectively, and the maximum IC current consumption is 3.0mA. Assuming that the total gate drive charge is only half of the above example, 30 nanoliters, and the input capacitor has a capacity of 0.1 gF, the voltage will drop by 30V. In this case, a 1.0 gF capacitor will produce a 3V drop. This seems to be acceptable, everything is fine, but we have to consider other issues.
At startup, the voltage on the 1.0 gF capacitor is 13.5V. Once the capacitor reaches this voltage, the capacitor on the reference voltage pin rises to 5V. If the total capacitance on the reference voltage pin is 0.1gF, then the charge transfer will cause the voltage to drop further by 0.5V. In this way, it seems that the device just has enough power for normal startup. If the tolerance of the 1.0gF capacitor is low, the device may not be able to start properly.
Designers sometimes use the reference voltage V to power other loads. For example, the UCC28C42's V... maintains its regulation to achieve a current of 20mA. If this load is loaded on the v... pin, then the input capacitor should be sized to handle the extra current during the soft-start process because the current will eventually sink from the v (Ic input supply pin) capacitor. In order to limit the input voltage drop to 2.6V and ensure safe start, we need a 4.7 v capacitor. However, if the capacitance deviation of the 4.7 F capacitor is 20%, then the device may sometimes be difficult to start normally because when the capacitance deviation is . At 20%, the total pressure drop may exceed 3.5V.
in conclusion:
When selecting an input capacitor for a power control IC, there are a few steps to take first. First, it is necessary to clarify how much power is required to raise the converter voltage to full voltage and to supply current from the bootstrap coil to the IC. Second, it is necessary to clarify what kind of transient fluctuation effect the load on the capacitor will produce.
In addition, the power must be estimated. The power energy mainly includes the total load of the capacitor from Ic (including V... and other Vcc line loads), the drive requirements, and the operating current of Ic. The charge required to raise the V capacitor to a certain voltage is also taken into account because the current is drawn from the Vcc capacitor.
Next, it is necessary to clarify the minimum turn-on and maximum turn-off voltage of Ic, which is known as the UVLO threshold or the minimum hysteresis voltage hysteresis.
Finally, it is necessary to clarify the maximum time required from the start of the startup to the completion of the startup. At the same time, remember to consider the deviation of the capacitance of the capacitor.
The above three steps will clarify what kind of large capacitors are needed at the minimum. When selecting a high-frequency capacitor, it is necessary to analyze not only the high-frequency current that should be supplied by the small capacitor on the Ic, but also the voltage fluctuation generated by the current pulse absorbed from the load or the FET gate.
Only by considering all the above factors can we design a safe and reliable Ic input capacitor to meet the requirements of the bootstrap power converter.
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