How to configure the driving resistance in parallel with the MOS tube and how to determine the turn-on voltage?

kal9623287

How to configure the driving resistance in parallel with the MOS tube and how to determine the turn-on voltage? [Copy link]

 

Connecting MOS tubes in parallel can increase the current capacity. What characteristics of MOS tubes should be paid attention to when connecting MOS tubes in parallel, such as the turn-on and turn-off delay time and the turn-on voltage?

Is the following connection correct?

maychang

In fact, the signal obtained by each MOS tube is the voltage divided by two resistors on the drive signal, and the amplitude is only half of the drive signal. Considering the input capacitance of the MOS tube, the opening and closing of the MOS tube will become slower.

maychang

In practical applications, R5R6R8 are often removed (open circuit), and the value of R4R7R9 is much smaller than 1 kilo-ohm, usually tens of ohms, and should not exceed 100 ohms.

chunyang

The principle of MOS tube parallel connection is also parameter consistency. The current MOS tube process can ensure that the parameter dispersion is not too large, so tubes of the same model and manufacturer can be directly connected in parallel in most cases. If you are worried about parameter dispersion and are afraid that it will affect reliability, pay attention to appropriately enlarging the parameter margin.

For the original poster's diagram, you can simply connect them in parallel and then use a low-value resistor to drive them uniformly. After the tubes are connected in parallel, the gate capacitance will double, resulting in a longer charging time. At the same time, considering the discreteness of the turn-on threshold, a low-value resistor is required.

kal9623287

maychang posted on 2022-4-25 16:49 In practical applications, R5R6R8 is often removed (open circuit), and the value of R4R7R9 is much smaller than 1 kilo ohm, usually tens of ohms are used, and should not exceed 100 ohms.

Thank you teacher for your answer

If the MOS tubes are not connected in parallel properly, they will heat up and burn out. The local heating will be more severe. How can this situation be avoided?

Can this be avoided by choosing a MOS tube with a relatively small Rds?

kal9623287

chunyang posted on 2022-4-25 22:40 The principle of MOS tube parallel connection is also the consistency of parameters. The current MOS tube process can ensure that the parameter discreteness is not too large, so tubes of the same model and the same manufacturer...

Thank you teacher for your answer

When MOS tubes are connected in parallel, local heating will be more severe. How can this situation be avoided?

Can this be avoided by choosing a MOS tube with a relatively small Rds?

Some people suggest strictly matching the Rds of the devices. How to match them? The Rds of common MOS tubes also needs to be measured before soldering them on the board?

chunyang

kal9623287 Published on 2022-4-26 09:29 Thank you for the teacher's answer. When MOS tubes are connected in parallel, local heating will be more severe. How can we avoid this situation? Can we avoid it by choosing MOS tubes with relatively small Rds?

What do you mean by "local heating"? If it is part of the parallel tube, it means that the on-resistance dispersion is too large. At this time, it is necessary to examine whether the heating situation will endanger the tube. If the temperature is too high, you need to increase the number of parallel connections or replace the tube with a larger specification. At the same time, pay attention to the PCB design or the installation of the heat sink (determined by how your MOS tube is installed) and try to balance the temperature. Of course, if the parameter dispersion is reduced through screening, it will also be improved.

In addition, pay attention to sufficient drive and well-controlled gate resistor value, which can reduce the impact of the discreteness of the turn-on/turn-off threshold voltage on the transient process of the MOS tube switch, as mentioned in the previous post.

chunyang

If you want to test and screen MOS tubes, you need to design a tool. The circuit on the tool is the simplest switch circuit. You can use a power resistor as a load to measure the current or load voltage for screening. The accuracy and stability of the instrument must be sufficient. The larger the current, the higher the resolution, but the heat dissipation of the tube on the tool must be considered.

kal9623287

chunyang posted on 2022-4-26 13:01 If you want to test and screen MOS tubes, you need to design a tooling. The circuit on the tooling is the simplest switching circuit. You can use a power resistor as a load to test...

Thanks for answering

Follow this idea to install the whole tooling, and ask again if you have any questions

se7ens

R5R6R8 is recommended to be retained to speed up the shutdown process and reduce shutdown loss

Of course, if you speed up the turn-on process, the resistance of R4R7R9 can be reduced.

If the drive level meets the requirements, the drive speed can be appropriately increased to further reduce switching losses.

kal9623287

se7ens published on 2022-4-27 09:47 It is recommended to keep R5R6R8 to speed up the shutdown process and reduce the shutdown loss. Of course, if the turn-on process is accelerated, the resistance value of R4R7R9 can be reduced. If the driver...

To what extent can the resistance of R4R7R9 be reduced, such as resistance value, and what formula is used to consider this resistance value?

PowerAnts

The host needs to provide working parameters, such as voltage, current, frequency, pulse width or duty cycle, drive level, load properties

PowerAnts

se7ens published on 2022-4-27 09:47 It is recommended to keep R5R6R8 to speed up the shutdown process and reduce the shutdown loss. Of course, if the turn-on process is accelerated, the resistance value of R4R7R9 can be reduced. If the driver...

This is the correct way to improve the problem. It is much better than the bad idea of sifting through tools here and there.

chunyang

When the driving resistor has a low resistance, the pull-down resistor is completely unnecessary, because the previous driver IO can provide enough source current, and it can also provide enough sink current, which is much more effective than the pull-down resistor. Don't just look at the details, think more about the overall picture.

The tooling problem is to deal with screening. Whether screening is necessary is a question that should be considered. The semantics of the previous post has clearly indicated that it is not recommended. How to screen is another question. If screening is necessary, then tooling is obviously the correct way to deal with it. Suggestions are just suggestions. Suggestions and decision-making are two different things. Don't interfere or make irresponsible remarks.

hjl2832

maychang posted on 2022-4-25 16:49 In practical applications, R5R6R8 is often removed (open circuit), and the value of R4R7R9 is much smaller than 1 kilo ohm, usually tens of ohms are used, and should not exceed 100 ohms.

It is best to keep the pull-down resistor. In my experience, in many cases, having a pull-down resistor can prevent the instantaneous voltage output by the driver chip at the moment of power-on (such as the voltage generated by the capacitor-coupled surge current) from causing the MOSFET to turn on instantly and cause problems. This is especially true in high-power applications. The large current shock caused by this instantaneous conduction may cause damage to the equipment.

lkh747566933

In the parallel state, it is better to use the same gate drive signal to drive the parallel MOS at the same time.

niuyite

Read the component parameter information carefully and design the circuit according to the optimal configuration of the information!