Driving Technology of Power MOSFET in High Power Switching Power Supply

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Driving Technology of Power MOSFET in High Power Switching Power Supply [Copy link]

Power MOSFET has the advantages of low on-resistance and large load current, so it is very suitable for use as a rectifier component of a switch-mode power supply (SMPS). However, there are some precautions when selecting MOSFET.
　　Unlike bipolar transistors, power MOSFET has a relatively large gate capacitance. The capacitor must be charged before it is turned on. When the capacitor voltage exceeds the threshold voltage (VGS-TH), the MOSFET starts to turn on. Therefore, the load capacity of the gate driver must be large enough to ensure that the equivalent gate capacitance (CEI) is charged within the time required by the system.
　　When calculating the gate drive current, the most common mistake is to confuse the input capacitance (CISS) of the MOSFET with CEI, so the following formula is used to calculate the peak gate current.
　　I = C(dv/dt)
　　In fact, the value of CEI is much higher than CISS, and it must be calculated based on the gate charge (QG) indicator provided by the MOSFET manufacturer.
　　QG is part of the gate capacitance of the MOSFET and is calculated as follows:
　　QG = QGS + QGD + QOD
　　Where:
　　QG--Total gate charge
　　QGS--Gate-source charge
　　QGD--Gate-drain charge (Miller)
　　QOD--Overcharge after Miller capacitor is fully charged
　　A typical MOSFET curve is shown in Figure 1 and is provided by many MOSFET manufacturers. It can be seen that in order to ensure that the MOSFET is turned on, the VGS used to charge CGS must be higher than the rated value, and CGS must also be higher than VTH. The gate charge divided by VGS equals CEI, and the gate charge divided by the on-time equals the required drive current (to turn on within the specified time).
　　It can be expressed as follows:
　　QG = (CEI)(VGS)
　　IG = QG/tON
　　Where:
　　● QG Total gate charge, defined as above.
　　● CEI Equivalent gate capacitance
　　● VGS Gate-source voltage
　　● IG Gate drive current required to turn on the MOSFET within the specified time

Figure 1

　　In the past, the driver was directly integrated into the SMPS controller, which was very practical for some products. However, the application range was limited because the output peak current of such a driver was generally less than 1A. In addition, the heat generated by the driver would cause the voltage reference to drift.
　　As the market demand for "intelligent" power devices became stronger, more functional SMPS controllers were developed. These new controllers all use fine CMOS technology, the supply voltage is less than 12V, and the integrated MOSFET driver can also be used as a level converter to convert TTL level to MOSFET drive level. Taking the TC4427A as an example, the input voltage range (VIL = 0.8V, VIH = 2.4V) and output voltage range (equal to the maximum supply voltage, up to 18V) of the device meet the requirements of end-to-end (rail-to-rail) output.
　　Anti-latch ability is a very important indicator because MOSFETs are generally connected to inductive circuits, which will generate relatively strong reverse surge currents. The output of the TC4427 MOSFET driver can withstand up to 0.5A of reverse current without damage, and the performance is not affected at all.
Another issue that needs attention is the ability to withstand instantaneous short-circuit current, especially for high-frequency SMPS. The generation of instantaneous short-circuit current is usually due to the fact that the rise or fall process of the driving level pulse is too long, or the transmission delay is too large. At this time, the MOSFETs on the high-voltage side and the low-voltage side are in a state of simultaneous conduction for a very short time, forming a short circuit between the power supply and the ground. The instantaneous short-circuit current will significantly reduce the efficiency of the power supply. Using a dedicated MOSFET driver can improve this problem in two ways:
　　1. The rise time and fall time of the MOSFET gate drive level must be equal and as short as possible. When the TC4427 driver is equipped with a 1000pF load, the pulse rise time tR and fall time tF are about 25ns. These two indicators of some other drivers with larger output peak currents can be even shorter.
　　

Figure 2

　　
　　2. The propagation delay of the drive pulse must be very short (matching the switching frequency) to ensure that the MOSFETs on the high-voltage side and the low-voltage side have equal turn-on delay and cut-off delay. For example, the propagation delay of the pulse rising edge and falling edge of the TC4427A driver is less than 2ns (as shown in Figure 2). These two indicators will vary with voltage and temperature. Microchip's products have already taken the lead in this indicator (comparable products have this indicator at least 4 times larger, and the driver integrated in the SMPS controller has this indicator is even worse).

　　These issues (which are directly related to cost and reliability) have been relatively well addressed in stand-alone, dedicated drivers, but not in integrated devices or traditional discrete device circuits.

　　Typical Applications
　　Portable Computer Power Supply
　　Figure 3 shows a circuit for a high-efficiency synchronous boost converter with an input voltage range of 5V to 30V. It can be connected to an AC/DC rectifier (14V/30V) or powered by a battery (7.2V to 10.8V).

Figure 3

　
　　The TC1411N in Figure 3 is a low-voltage side driver. The output peak current of TC1411N is 1A. Since it uses a +5V power supply, the cut-off delay caused by gate overcharge can be reduced. TC4431 is a high-voltage side driver with an output peak current of up to 1.5A. The MOSFET driven by these two devices can withstand a drain current of 10A for 30ns.
　
　　Desktop computer power supply
　　
Figure 4 is a power supply circuit for a desktop computer, in which the synchronous buck converter is generally used for the power supply of the CPU, and its output current is generally not less than 6A. This circuit can provide an adjustable voltage, which is not possible with the current common discrete device power supply.
　　The circuit in Figure 4 is simpler than that in Figure 3. TC4428A is used as a driver for the high-voltage side and the low-voltage side here, and shares the power supply VDD; in order to reduce costs, N-channel MOSFET is used in the circuit. The output capability of TC4428A is strong, and it can drive the MOSFET to withstand a drain current of 10A for 25ns.
　　

Figure 4

　　Power MOSFET has become the best choice for switch components in SMPS controllers due to its outstanding advantages of low on-resistance and large load current. The emergence of dedicated MOSFET drivers has brought opportunities for optimizing SMPS controllers. Drivers integrated with SMPS controllers are only suitable for products with simple circuits and small output currents; and drive circuits built with discrete active or passive devices can neither meet the requirements for high performance nor obtain the cost advantages of dedicated monolithic driver devices. The pulse rise delay, fall delay and propagation delay of dedicated drivers are very short, and the circuit types are also very complete, which can meet the design needs of various products.

mutoudonggua

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破茧佼龙

Very good information

gongquan

XIEXIE

zq276

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chenliangyan

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selinna2011

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