Hello everyone, I am Li Gong, I hope you can support me. Today I would like to share with you an article about MOS tube drive circuit. (There are some of my own methods, and some suggestions from others.)
mos tubes are widely used in switching circuits because of the low internal resistance of and the slow development speed of . The mos tube often selects an appropriate drive circuit based on the parameters of the power IC and mos tube.
When using mos tubes to design switching power supply , most people will consider parameters such as the on-resistance, maximum voltage and maximum current of the mos tube. But that's it, we often only consider these factors. A circuit designed like this is far from a good circuit. We should take a closer look at its own parasitic parameters.
For a certain mos tube, its drive circuit, peak output drive current, rising rate, etc. will all affect the switching performance of the mos tube.
Parameters that need to be paid attention to when designing a driver circuit using mos tubes
1, Id (Package Limited) - the maximum theoretical drain current of the package
You must not exceed this value when designing the circuit, but it does not mean that you can actually Drive the load at this current. Before reaching this specification, the mos tube will always burn out due to heat.
2, Vgs - The maximum gate-source voltage
applied to the gate relative to the source cannot exceed this value. For example, the voltage value of the IRFS7530mos tube is plus or minus 20V. If we use a voltage of 12V, it is not close to this value.
3, Vdss – Another self-evident specification parameter, maximum drain-source voltage
The voltage difference from drain to source cannot exceed the parameter value marked on the datasheet.
4, Rds(on) – Maximum expected on-resistance from drain to source at a given gate voltage.
For example, the worst case Rds(on) of IRFS7530mos tube is 1m4.
The following figure shows common charts in the mos tube datasheet.
We can see that Rds(on) levels off, with its minimum value above ~8V. This is expected from a non-logic level MOSFET, and is acceptable when driven at 12V.
Rds(on) and Vgs
5, RthetaJA (junction to environment) at different temperatures - This is the thermal resistance from the chip junction to the outside of the package to the ambient air.
is specified using a specific amount of copper on the PCB.
6, Qg - The total charge required to inject into the gate to make the MOSFET fully "on", Gate Charge
This takes into account the gate-source charge, gate-drain charge, and any other internal parasitic effects. This is the simplest specification for calculating the maximum "theoretical" switching speed of a MOS tube.
Requirements for designing drive circuits using mos tubes
When selecting power ICs and mos tubes, it is particularly important to choose a suitable drive circuit to connect the IC to the mos tube.
A good MOSFET drive circuit has the following requirements:
1. When the switch tube is turned on, the drive circuit should be able to provide a large enough charging current , so that the voltage between the gate and source of the mos quickly rises to The required value ensures that not only the switch tube can be turned on quickly but also there is no high-frequency oscillation on the rising edge.
2. During the switch conduction period, the drive circuit must ensure that the voltage between the gate and source of the mos tube remains stable and reliable to turn on .
3. At the moment of turn-off, the drive circuit requires to provide a path with the lowest possible impedance to quickly discharge the capacitor voltage between the gate and source of the MOSFET to ensure that the switch tube can be turned off quickly.
4, the circuit structure should be simple, efficient and reliable .
5, correspondingly applies to electrically isolate .
What will happen if there is no mos tube gate resistor in the circuit?
Since the gate of the mos tube is essentially a capacitor , what happens at the moment when voltage is applied to the gate without a gate resistor? The circuit will treat the gate as a complete short (technically incorrect as the traces and wires have parasitic resistance and inductance, but close enough.)
This has some potential problems, If it can't supply that much current quickly, then This current surge can damage the driver circuit. Secondly, there is a danger of gate "ringing" due to parasitic inductance and gate capacitance. This kind of ringing can force the gate to oscillate between on/off states, or worse, overvoltage and completely destroy the mos tube .
A 100R resistor controls the charging rate of the gate. This limits the maximum amount of current
Another problem with not having a gate resistor is that the parasitic resistance becomes an important part of the gate calculation. When using the 100R gate resistor shown above, a few ohms of parasitic resistance will not meaningfully affect the calculations.
When switching high power mos tubes or any low speed circuit, the exact value of the gate resistor is usually not important. Typically use a 10R or 100R resistor and adjust it while testing if needed. As switching speeds approach several kilohertz, it becomes important to do some calculations on the maximum gate resistance to use.
logic level mos tube circuit basics
has a basic mos tube circuit of an N-channel mos tube that controls an LED.
In the following section, we mainly discuss N-channel MOS transistors. These are easier to drive and are the most common type for driving high power loads.
The picture above shows the simplest mos tube circuit. R1 is the gate resistor that limits the amount of current and prevents any ringing on the gate. R2 is a 10K pull-down resistor to ensure that the mos tube is always in a known state.
Q1 is a logic level MOS tube and is a suitable choice for controlling a 10 mA indicator LED. Since Q1 is a logic level MOS transistor, the CNTRL_MOSFET can come from the I/O lines of the standard microcontroller .
"ordinary" mos tube circuit
IRRFS753mos tube driving a 12A resistive load at 12V
driving "ordinary" mos tubes is slightly more complicated because they require at least 10V on the gate to fully conduct.
mos tube driver circuit
12A The load is switched by IRFS7530, which is a high power D2PAK mos tube . Since the rate of the PWM resistor is very low, a 100R resistor is an effective choice.
Using the integrated mos tube low-side driver to control the mos tube
drive power The simplest (and often the best) way to use the mos tube is to use a dedicated integrated driver.
These chips use internal logic level mos tube circuitry to get the logic level input from the microcontroller and turn on/off the mos tube .
The above picture shows a circuit based on NCP81074A, which is one of the commonly used drivers because it has a separate source and receive pins.
This allows fine-tuning of the switching speed of the on and off times if desired. Depending on the gate resistor, the chip is capable of delivering 10A to charge the gate extremely quickly, minimizing power loss.
An important point to note is the decoupling capacitor C1/C2. It is recommended to use a larger value capacitor than indicated in the datasheet. Since the driver needs to provide a large amount of current to turn on the mos tube , the larger the available capacitance, the better.
The trace length from pin U1.4/5 to R1 to Q2.1 should also be as short as possible to minimize parasitic inductance that can slow down switching speed.
An alternative to using NPN transistors to drive mos tubes from logic level signals
The only real reason not to use dedicated gate drivers on high power mos tubes is to keep costs as low as possible .
The picture above shows a cheaper way to drive a mos tube. This approach is only recommended when absolutely necessary though, as dedicated gate drivers are easier to implement and tend to have better performance.
The above are some methods and cases about mos drive circuit design. I hope it can be helpful to you.
If there is anything wrong or to add, please leave a message in the comment area. Please give me your advice.
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