How to calculate the maximum switching frequency of MOS tube - Kia MOS tube

MOS How to calculate the switching frequency of the tube
MOS When the tube is on and off , It must not be done in an instant .MOS tube The voltage at both ends has a falling process , The current flowing through has a rising process , In the meantime ,MOS The loss of the tube is the product of voltage and current , It's called switching loss . Usually, the switching loss is much greater than the conduction loss , And the faster the switching frequency , The greater the loss .

With IRF840 The calculation of the parameters of , Assume gate voltage 10V, Then the capacitance is 63nC/10V=6.3nF. And 10 The time constant of kiloohm discharge resistance is 63us. however , Not after a time constant MOS The tube turns off , Instead, the gate voltage drops to Vg(th) following MOS The tube will turn off . This time with MOS The pipe model is related to , It is related to the voltage reached by gate charging ( The actual gate capacitance is not a linear capacitance ), An inaccurate estimate , The discharge time of gate capacitor can be estimated as 63us Of 2 times , namely 0.12ms.MOS The charging resistance of the gate electrode is small ( The first post is 3 Thousand euro ), The estimated charging time is 0.06ms. So the total charge and discharge time is 0.18ms. The MOS The maximum switching frequency of the tube in this circuit is 5.5kHz.

Two 、MOS Tube switch circuit
The following is a typical N Channel enhanced MOS Schematic diagram of tube switch circuit ：

D1 effect ：

The freewheeling diode

R1 effect ：

1、 Current limiting resistor , Reduce the instantaneous current value ：MOS The tube belongs to a voltage controlled device , Parasitic capacitance exists between two pins （Cgs、Cgd、Cds）:

The specification will generally indicate Ciss、Coss、Crss：

Ciss = Cgs + Cgd

Coss = Cds + Cgd

Crss = Cgd

Pictured Ciss=587pF, hypothesis VGs=24V,dt=Tr（ Rise time ）=20ns, be MOS The instantaneous current when the tube is switched on and off I = Ciss * dVgs / dt = 0.7A

When a resistor is connected in series at the grid （ A few Ω~ over a thousand Ω） when , Will be with Ciss formation RC Charge discharge circuit , So as to reduce the instantaneous current value

2、 Adjust the MOS The on-off speed of the tube , Conducive to control EMI： meanwhile , add R1 after ,MOS Tube on-off switching time will slow down , Conducive to control EMI; But if the series resistance is too large , This will cause the gate to reach the on voltage for a longer time , in other words MOS The tube has been semi conducting for too long , here MOS The internal resistance of the pipe is large ,Rds->Rdson It takes a long time ,Rds Will consume a lot of power , May lead to MOS The tube was damaged by heat

3、 Suppress grid oscillation ：

MOS After the tube is connected to the circuit , Lead in parasitic inductance , Will form with parasitic capacitance LC Oscillating circuit , For the square wave switching waveform signal, it contains many frequency components ：

Then it is possible to form a series resonant circuit when a resonant frequency is the same or similar , Connecting a resistor in series will reduce the power of the oscillation circuit Q value , So that the oscillation can be quickly attenuated

R2 effect ：

1、G Pole to ground resistance （ commonly 5KΩ~ Dozens of KΩ）, Drop down to MOS The tube provides a fixed offset , Avoid when IC When the drive port is in the high resistance state G Extremely disturbed signals make MOS The tube was accidentally turned on

2、 Discharge resistance , Discharge... Through this resistance G-S A small amount of static electricity between （G-S There is a great resistance between , A small amount of static electricity can pass through G-S The equivalent capacitance between produces a high voltage , At this time due to RGS It's big , Induced charge is difficult to release , So that the high pressure will MOS The thin insulating layer of the tube breaks down , damage MOS tube ） To protect MOS tube , Without this resistor ,MOS The tube is vulnerable to external interference, accidental conduction and burning , In addition to MOS When the tube is continuously turned on and off, the parasitic capacitor is properly discharged to protect MOS tube

KIA semiconductor MOS Tube has a great core competitiveness , It is the best choice for switching power supply manufacturers .KIA semiconductor MOS Tube manufacturers mainly research and develop 、 production 、 business ： Field effect tube （MOS tube ）、COOL MOS（ Superjunction FET ）、 Three terminal regulator 、 Fast recovery diode ; Widely used in inverter 、 Lithium battery protection board 、 Electric vehicle controller 、HID Automative lighting 、LED The lamp 、 No brush motor 、 Miner power supply 、 Industrial power supply 、 Adapter 、3D Printer and other fields ; Can apply for samples and quotations and have technical support , If there is any problem, technicians can help solve the problem ！ If you need or want to know, you can deduct ：2880195519