How to calculate the switching frequency when MOS transistor is working — SUNMATE

How to calculate the switching frequency when MOS transistor is working

MOS transistors must not be completed in an instant when conducting and cutting off. The voltage at both ends of the MOS transistor undergoes a decreasing process, while the current flowing through it undergoes an increasing process. During this period, the loss of the MOS transistor is the product of voltage and current, known as switch loss. Usually, the switching loss is much greater than the conduction loss, and the faster the switching frequency, the greater the loss.

Based on the parameters of IRF840, assuming a gate voltage of 10V, the capacitance is 63nC/10V=6.3nF. The time constant with a discharge resistor of 10 kiloohms is 63us. However, it is not after a time constant that the MOS transistor turns off, but rather when the gate voltage drops below Vg (th) that the MOS transistor turns off. This period is related to the MOS transistor model and the voltage reached by the gate charging (the actual gate capacitor is not a linear capacitor). It is not accurate to estimate the discharge time of the gate capacitor as twice 63us, which is 0.12ms. The charging resistance of the MOS transistor gate is relatively small (3 kiloohms in the first image), and the estimated charging time is 0.06ms. So the total charging and discharging time is 0.18ms. The maximum switching frequency of the MOS transistor in this circuit is 5.5kHz.

MOS transistor switch circuit

The following is a typical N-channel enhanced MOS transistor switch circuit schematic diagram:

D1 function:

Freewheeling diode

R1 function:

1. Current limiting resistor to reduce instantaneous current value: MOS transistor is a voltage controlled device, and there are parasitic capacitors (Cgs, Cgd, Cds) between two pins. Generally, Ciss, Coss, and Crss are marked in the specification book:

Ciss=Cgs+Cgd

Coss=Cd s+Cgd

Crss=Cgd

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

When a resistor (several Ω to thousands of Ω) is connected in series at the gate, an RC charging and discharging circuit is formed with Ciss, thereby reducing the instantaneous current value

2. Adjusting the on/off speed of MOS tubes is beneficial for controlling EMI. At the same time, adding R1 will slow down the on/off switching time of MOS tubes, which is beneficial for controlling EMI; However, if the resistance in series is too large, it will lead to a longer time for the gate to reach the conduction voltage, which means that the MOS transistor is in a semi conduction state for too long. At this time, the internal resistance of the MOS transistor is relatively high, and the time for Rds ->Rdson is relatively long. Rds will consume a large amount of power, which may cause the MOS transistor to be damaged due to heating.

3. Suppressing gate oscillation: After the MOS transistor is connected to the circuit, the parasitic inductance of the lead is introduced, which forms an LC oscillation circuit with the parasitic capacitance. For square wave switching waveform signals, there are many frequency components:

So it is possible to form a series resonant circuit when a certain resonant frequency is the same or similar, and connecting a resistor in series will reduce the Q value of the oscillation circuit, thereby causing the oscillation to quickly decay.

R2 function:

1. The G pole to ground resistance (usually 5K Ω to tens of K Ω) provides a fixed bias for the MOS transistor through pull-down, to avoid the G pole being interfered with by signals when the IC driver port is in a high resistance state, causing the MOS transistor to unexpectedly conduct.

2. Discharge resistor, which releases a small amount of static electricity between G-S through this resistor (the resistance between G-S is very large, and 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, it is difficult to release the induced charge, resulting in high voltage breakdown of the thin insulation layer of the MOS tube and damage to the MOS tube) to protect the MOS tube. Without this resistor, the MOS tube is prone to external interference and accidental conduction and burning, In addition, when the MOS transistor is continuously turned on and off during operation, appropriate discharge of parasitic capacitors is carried out to protect the MOS transistor.