Please advise, how to calculate the switching frequency of MOS tube?

sky999

Please advise, how to calculate the switching frequency of MOS tube? [Copy link]

 

I would like to ask, how to calculate the switching frequency of MOS tube? For example, in the figure below, the optocoupler is 521 and the MOS tube is 3205. What is the highest switching frequency of this figure?

maychang

TLP521 is a relatively slow optocoupler:

According to the parameters in the picture above, at 25kHz, the output of the optocoupler will no longer have a flat top and will become a triangular wave.

According to the parameters in the OP's electrical schematic, the optocoupler operates more slowly.

sky999

maychang published on 2019-9-23 07:40 TLP521 is a relatively slow optocoupler: According to the parameters in the above picture, at 25kHz, the optocoupler output will no longer have a flat top and will become a triangular wave. According to...

Hello, I am not mainly asking about the switching speed of the optocoupler, but whether the switching speed of the MOS tube is related to the gate resistance. Assuming that the optocoupler outputs a switching speed of 20KHZ, according to this graph, what can the switching speed of the MOS tube be?

maychang

sky999 posted on 2019-9-23 20:21 Hello, I am not mainly asking about the switching speed of the optocoupler, but whether the switching speed of the MOS tube is related to the gate resistance. Assuming that the optocoupler...

"According to this diagram, what is the switching speed of the MOS tube?"

Quite low.

There is a capacitor between the gate and source of the MOS tube, and there is also a capacitor between the gate and drain. As can be seen in the figure, the source of the MOS tube is grounded, so the tube must be drain output. In this case, the Miller effect will occur, which will increase the capacitance seen from the gate to the MOS tube a lot (the power MOS tube manual has a parameter on how much charge is needed to charge the gate at one time). The capacitance seen from the gate (input capacitance plus Miller capacitance) is charged by ZR1 and discharged by MR1. Both resistors are quite large, resulting in very slow charging and discharging speeds, especially discharging.

maychang

sky999 posted on 2019-9-23 20:21 Hello, I am not mainly asking about the switching speed of the optocoupler, but whether the switching speed of the MOS tube is related to the gate resistance. Assuming that the optocoupler...

"According to this diagram, what is the switching speed of the MOS tube?"

It is estimated that the maximum working frequency is several kHz. This depends on the tube model.

maychang

sky999 posted on 2019-9-23 20:21 Hello, I am not mainly asking about the switching speed of the optocoupler, but whether the switching speed of the MOS tube is related to the gate resistance. Assuming that the optocoupler...

Usually, power MOS tubes operating at higher switching frequencies require dedicated driver circuit chips. Although the average gate current of the MOS tube (capacitor charging and discharging current) is only a few mA, the output current of the driver chip can often reach hundreds of mA, or even more than 1A.

sky999

maychang posted on 2019-9-23 20:43 "According to this diagram, what is the switching speed of the MOS tube?" Quite low. There is a capacitor between the gate and source of the MOS tube, and the gate...

Thank you very much for your detailed explanation. The tube is IRF3205. Which parameter is it that indicates how much charge is needed to charge the gate at one time? There are many time parameters in the manual, such as conversion delay time, rise time, fall time, etc. Are these related?

maychang

sky999 posted on 2019-9-25 21:30 Thank you very much for your detailed explanation. The tube is IRF3205. What is the parameter for how much charge is needed for the gate to charge at one time? There are many...

"What is the parameter for how much charge is needed to charge the gate at one time?"

I haven't checked the information of IRF3205. The picture below is a screenshot of IRF840 parameters. Of course, the parameters of each model of power MOS are different, but they are not different by an order of magnitude.

maychang

sky999 posted on 2019-9-25 21:30 Thank you very much for your detailed explanation. The tube is IRF3205. What is the parameter for how much charge is needed for the gate to charge at one time? There are many...

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

maychang

sky999 posted on 2019-9-25 21:30 Thank you very much for your detailed explanation. The tube is IRF3205. What is the parameter for how much charge is needed for the gate to charge at one time? There are many...

In fact, when working at 5.5kHz, the drain current is already a sawtooth wave, that is, the drain current has no flat top part and no flat bottom part. At this time, it is difficult to say that it is working in a switching state. Usually, switching operation requires that the current rise time and fall time are less than 10% of the switching frequency, so that it can be called switching operation.

maychang

sky999 posted on 2019-9-25 21:30 Thank you very much for your detailed explanation. The tube is IRF3205. What is the parameter for how much charge is needed for the gate to charge at one time? There are many...

"There are many time parameters in the manual, such as conversion delay time, rise time, fall time, etc. Are these related?"

Of course it is related. But if you look closely at these time parameters (all in the figure on the 8th floor), they are only a dozen ns to tens of ns, which is at least two orders of magnitude smaller than your 120us, and can be completely ignored.

These rise times, fall times, etc., only need to be carefully considered when the switching operating frequency reaches hundreds of kHz.

sky999

maychang published on 2019-9-26 10:29 According to the parameters of IRF840, assuming the gate voltage is 10V, the capacitance is 63nC/10V=6.3nF. The time constant with the 10k ohm discharge resistor is 63us. ...

The 3205 is 146nC. Assuming the gate voltage is 20V, 146/20=7.3, the discharge resistor time constant is 73us. When the gate voltage drops below Vg(th), the MOS tube will be turned off. Does it mean that if the gate voltage is 2V at the lowest, the MOS tube will not be turned off until it reaches below 2V after 73us? But why is the charging time of the 3K resistor 0.06ms? How is this obtained?

sky999

maychang posted on 2019-9-26 10:32 In fact, when working at 5.5kHz, the drain current is already a sawtooth wave, that is, the drain current has no flat top part and no flat bottom part. At this time, it is difficult to say that it is turned on...

Switching requires that the current rise time and fall time be less than 10% of the switching frequency. Does the rise and fall time here refer to the input signal of the drain? Can it be understood that if the maximum operating speed of the tube is 10KHZ, then working within 9KHZ is considered switching?

sky999

maychang posted on 2019-9-26 10:36 "There are many time parameters in the manual, such as conversion delay time, rise time, fall time, etc. Are these related?" ...

I understand. By the way, the VGS turn-on voltage is generally 2V-4V, and it needs to be fully turned on to be above 10V, while the maximum VGS input voltage is generally ±20V. Why do some modules bought on Taobao use 24V as the control signal for VGS, but the tubes are fine? Moreover, my module was bought a few years ago, and it is still for sale now. The circuit still uses 24V as VGS, and there is no charging resistor on the gate, only a 10K discharge resistor. Why can it still be used?

sky999

Push it up!!!!