“MOSFETs are widely used in switching power supplies due to their low on-resistance and fast switching speed. The driver of the MOSFET often selects the appropriate circuit according to the parameters of the power supply IC and the MOSFET. Let’s discuss the driving circuit of MOSFET for switching power supply together.
MOSFETs are widely used in switching power supplies due to their low on-resistance and fast switching speed. The driver of the MOSFET often selects the appropriate circuit according to the parameters of the power supply IC and the MOSFET. Let’s discuss the driving circuit of MOSFET for switching power supply together.
When using MOSFETs to design switching power supplies, most people will consider the on-resistance, maximum voltage, and maximum current of the MOSFET. But in many cases, only these factors are considered. Such a circuit may work normally, but it is not a good design solution. More detailed, MOSFET should also consider its own parasitic parameters. For a certain MOSFET, its driving circuit, the peak current output by the driving pin, the rising rate, etc., will affect the switching performance of the MOSFET.
After the power supply IC and the MOS tube are selected, it is particularly important to select a suitable driving circuit to connect the power supply IC and the MOS tube.
A good MOSFET driver circuit has the following requirements:
(1) At the moment when the switch is turned on, the drive circuit should be able to provide a sufficiently large charging current to rapidly increase the voltage between the gate and source of the MOSFET to the required value, so as to ensure that the switch can be turned on quickly and there is no high-frequency oscillation on the rising edge.
(2) The drive circuit can ensure that the voltage between the gate and the source of the MOSFET remains stable and reliably turned on during the switch-on period.
(3) The drive circuit at the moment of turn-off can provide a path with the lowest possible impedance for the rapid discharge of the capacitor voltage between the gate and the source of the MOSFET, so as to ensure that the switch tube can be turned off quickly.
(4) The structure of the driving circuit is simple and reliable, and the loss is small.
(5) Apply isolation according to the situation.
The following introduces several MOSFET drive circuits commonly used in module power supplies.
1. The power IC directly drives the MOSFET
Figure 1 IC drives MOSFETs directly
Power IC direct drive is our most common driving method, and it is also the simplest driving method. When using this driving method, we should pay attention to several parameters and the influence of these parameters. First, check the power supply IC manual, its maximum driving peak current, because different chips have different driving capabilities in many cases. Second, understand the parasitic capacitance of the MOSFET, such as the values of C1 and C2 in Figure 1. If the values of C1 and C2 are relatively large, the energy required for the conduction of the MOS tube is relatively large. If the power supply IC does not have a relatively large driving peak current, the conduction speed of the tube is relatively slow. If the driving ability is insufficient, high-frequency oscillation may occur on the rising edge. Even if the Rg in Figure 1 is reduced, the problem cannot be solved! Factors such as IC driving ability, MOS parasitic capacitance, and MOS switching speed all affect the resistance of the driving resistor. choice, so Rg cannot decrease infinitely.
2. When the power IC driving capability is insufficient
If the parasitic capacitance of the MOS transistor is relatively large, and the driving ability inside the power IC is insufficient, the driving ability needs to be enhanced on the driving circuit. The totem pole circuit is often used to increase the driving ability of the power IC. The circuit is shown in the dotted box in Figure 2.
Figure 2 Totem pole drive MOS
The function of this driving circuit is to improve the current supply capability and quickly complete the charging process of the gate input capacitor charge. This topology increases the turn-on time, but reduces the turn-off time. The switch can be turned on quickly and avoid high-frequency oscillations on the rising edge.
3. The drive circuit accelerates the turn-off time of the MOS tube
Figure 3 Accelerated MOS turn-off
The drive circuit at the moment of turn-off can provide a path with the lowest possible impedance for the rapid discharge of the capacitor voltage between the gate and the source of the MOSFET, so as to ensure that the switch tube can be turned off quickly. In order to quickly discharge the capacitor voltage between the gate and the source, a resistor and a diode are often connected in parallel with the driving resistor, as shown in Figure 3, where D1 is commonly used as a fast recovery diode. This reduces the off-time while reducing losses during turn-off. Rg2 is to prevent the current from being too large when it is turned off and burn the power IC.
Figure 4 Improved accelerated MOS turn-off
The totem pole circuit introduced in the second point also has the effect of speeding up the turn-off. When the driving capability of the power supply IC is sufficient, the improvement of the circuit in Figure 2 can speed up the turn-off time of the MOS tube, and the circuit shown in Figure 4 is obtained. It is more common to use a triode to discharge the capacitor voltage between the gate and the source. If the emitter of Q1 has no resistance, when the PNP transistor is turned on, the capacitance between the gate and the source is short-circuited, so as to discharge the charge in the shortest time and minimize the cross loss when it is turned off. Compared with the topology in Figure 3, there is another advantage that the current does not pass through the power IC when the charge on the capacitor between the gate and source is discharged, which improves reliability.
4. The drive circuit accelerates the turn-off time of the MOS tube
Figure 5 Isolated driver
In order to meet the drive of the high-end MOS tube as shown in Figure 5, transformer drive is often used, and sometimes transformer drive is also used to meet safety isolation. The purpose of R1 is to suppress the parasitic inductance on the PCB board and C1 to form LC oscillation, and the purpose of C1 is to separate the DC, pass the AC, and also prevent the core from saturating.
In addition to the above drive circuits, there are many other forms of drive circuits. There is no one driving circuit that is the best for various driving circuits, and only the most suitable driver can be selected in combination with the specific application.