How to calculate the switching frequency of MOS tube

When the MOS tube is turned on and off, it must not be completed in an instant. The voltage across the MOS tube has a falling process, and the current flowing through it has a rising process. During this period, the loss of the MOS tube is the product of the voltage and the current, which is called switching loss. Usually switching losses are much larger than conduction losses, and the faster the switching frequency, the greater the losses.

Calculated with the parameters of IRF840, assuming that the gate voltage is 10V, then the capacitance is 63nC/10V=6.3nF. With a 10kΩ discharge resistor the time constant is 63us. However, instead of turning off the MOS tube after a time constant, the MOS tube will be turned off when the gate voltage drops below Vg(th). This period of time is related to the model of the MOS tube and to the voltage reached by the gate charge (in fact, the gate capacitor is not a linear capacitor). For an inaccurate estimate, the discharge time of the gate capacitor can be estimated to be twice as long as 63us, that is, 0.12ms . The gate charging resistance of the MOS tube is small (3 kiloohms in the first picture), and the estimated charging time is 0.06ms. Then the total charging and discharging time is 0.18ms. The maximum switching frequency of the MOS tube in this circuit is 5.5kHz.

MOS tube switch circuit

The following is a schematic diagram of a typical N-channel enhancement mode MOS switch circuit:

D1 role:

Freewheeling diode

R1 role:

1. Current limiting resistor to reduce the instantaneous current value: MOS tube is a voltage-controlled device, and there is parasitic capacitance (Cgs, Cgd, Cds) between the two pins: Ciss, Ciss, and Cds are generally marked in the specification.

Coss, Crss:

Ciss = Cgs + Cgd

Coss = Cds + Cgd

Crss = Cgd

As shown in Ciss=587pF, assuming VGs=24V, dt=Tr (rise time)=20ns, then the instantaneous current of the MOS tube when switching is I = Ciss * dVgs / dt = 0.7A

When a resistor (several Ω ~ thousands of Ω) is connected in series with the gate, an RC charge and discharge loop will be formed with Ciss, thereby reducing the instantaneous current value

2. Adjust the on-off speed of the MOS tube, which is beneficial to control EMI: at the same time, after adding R1, the on-off switching time of the MOS tube will be slower, which is conducive to controlling EMI; but if the series resistance is too large, it will cause the gate The time to reach the on-voltage becomes longer, that is to say, the time of the MOS tube in the semi-conductive state is too long. At this time, the internal resistance of the MOS tube is relatively large, and the time between Rds->Rdson is relatively long, and Rds will consume a lot of power. As a result, the MOS tube is damaged due to heat.

3. Suppress gate oscillation: After the MOS tube is connected to the circuit, the parasitic inductance of the lead is introduced, which will form an LC oscillation circuit with the parasitic capacitance. For the switching waveform signal of the square wave, it contains many frequencies

Element:

Then it is possible to form a series resonant circuit when a certain resonant frequency is the same or similar. After connecting a resistor in series, the Q value of the oscillation circuit will be reduced, so that the oscillation will decay rapidly.

R2 role:

1. The resistance of the G pole to ground (generally 5KΩ~tens of KΩ) provides a fixed bias for the MOS tube by pulling down, so as to avoid the G pole being disturbed by the interference signal when the IC driver port is in a high resistance state, causing the MOS tube to be accidentally turned on .

2. The discharge resistor, through which a small amount of static electricity between G-S is discharged (the resistance between G-S is very large, a small amount of static electricity can generate a high voltage through the equivalent capacitance between G-S, at this time Due to the large RGS, the induced charge is difficult to release, so that the high voltage will break down the thin insulating layer of the MOS tube and damage the MOS tube) to protect the MOS tube. Without this resistance, the MOS tube is vulnerable to external interference. In addition, when the MOS tube is continuously turned on and off, the parasitic capacitance is properly discharged to protect the MOS tube.