Why does a MOSFET heat up with slow rise/fall times at gate?

In summary: This makes it possible to become more efficient at higher frequencies.In summary, the conversation discusses the use of a pulse width modulator composed of a triangular wave generator and a comparator to output a pulse width modulated signal with a set duty cycle. The slow rise and fall times of the output waveform can cause power losses in the MOSFET used to drive a DC motor, leading to the use of a BJT inverter to improve the rise and fall times. This is because when the FET is turning on and off, there can be a large product of voltage and current, resulting in heating of the FET. The conversation also mentions the different types of MOSFETs and their architecture, as well as the intrinsic diode built
  • #1
saad87
85
0
I have a pulse width modulator that's composed of a triangular wave generator and a comparator. This outputs a pulse width modulated signal, whose duty cycle I set by changing the comparator's V- voltage.

I have measured the rise and fall times of my output waveform and they are 27uS (for both). I have been told by someone that such a slow waveform will cause power losses in the MOSFET I am using to drive a simple, small, DC Motor. I was told that this because with with a slow rise/fall time, the current and voltage waveforms overlap by a small bit and since P=VI, this causes power loss.

Is this true? I'm skeptical because I don't understand why current and voltage are out of phase in the first - is this because the motor is an inductive load?
 
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  • #2
With a slow rise and fall, there is an increased time during which the MOSFET is carrying some current AND has some voltage drop across it. Result. It gets hot.
Bob S
 
  • #3
Thank you!

Does this phenomena occur in BJTs too? If so, why are inverter BJTs circuits used to improve the rise/fall times of the waveform?

In my case, I was recommended that I should insert a BJT inverter between the output of the comparator and the input of the MOSFET. There is a resistor connected between the collector and power supply. This resistor ends up making a RC circuit with the input capacitance of the MOSFET and due to a time constant of 1uS, the rise/fall time is much faster.

Does this make sense?

Also, do slow rise/fall times affect the switch speed of a transistor?
 
  • #4
saad87 said:
Thank you!

Does this phenomena occur in BJTs too? If so, why are inverter BJTs circuits used to improve the rise/fall times of the waveform?

In my case, I was recommended that I should insert a BJT inverter between the output of the comparator and the input of the MOSFET. There is a resistor connected between the collector and power supply. This resistor ends up making a RC circuit with the input capacitance of the MOSFET and due to a time constant of 1uS, the rise/fall time is much faster.

Does this make sense?

Also, do slow rise/fall times affect the switch speed of a transistor?

Exactly, an inverter circuit would be able to source/sink more current than the comperator, allowing the mosfet to switch on and off more quickly, meaning that it will be in the linear range for a shorter amount of time, thus reducing switching losses.
 
  • #5
Is this true? I'm skeptical because I don't understand why current and voltage are out of phase in the first - is this because the motor is an inductive load?

In the following circuit,

FET driver.PNG


Can you see that if the FET is ON, there will be very little voltage across it, so this multiplied by the current results in very little power.
If the FET is OFF, there will be maximum voltage across the FET but no current, so the power will be zero or very low.

It is only when the FET is turning ON and turning OFF that there can be a large product of voltage and current. If this takes a long time, it can result in heating of the FET.
 
  • #6
OSFETs come in four different types. They may be enhancement or depletion mode, and they may be n-channel or p-channel. We are only interested in n-channel enhancement mode MOSFETs, and these will be the only ones talked about from now on. There are also logic-level MOSFETs and normal MOSFETs. We can use either type.

The source terminal is normally the negative one, and the drain is the positive one (the names refer to the source and drain of electrons). The diagram above shows a diode connected across the MOSFET. This diode is called the "intrinsic diode", because it is built into the silicon structure of the MOSFET. It is a consequence of the way power MOSFETs are created in the layers of silicon, and can be very useful. In most MOSFET architectures, it is rated at the same current as the MOSFET itself.
 
  • #7
Eric McClean said:
OSFETs come in four different ...

Please don't blindly copy and paste stuff from other sites. If you have a link that you think might provide good information, just post it.

http://robots.freehostia.com/SpeedControl/MosfetBody.html

Also, do slow rise/fall times affect the switch speed of a transistor?

Absolutely. A faster transient time allows the resulting waveform to maintain its shape longer as frequency increases until it turns into mush.
 

Related to Why does a MOSFET heat up with slow rise/fall times at gate?

1. Why does a MOSFET heat up with slow rise/fall times at gate?

The MOSFET heats up with slow rise/fall times at the gate because of the increased switching losses. When the rise/fall times are slow, the MOSFET spends more time in the linear region, causing it to dissipate more power and heat up.

2. What are switching losses in a MOSFET?

Switching losses in a MOSFET refers to the energy dissipated during the switching process of the device. This includes both the conduction losses and the losses due to charging and discharging of the gate capacitance.

3. How do fast rise/fall times reduce MOSFET heating?

Fast rise/fall times reduce MOSFET heating by reducing the amount of time the device spends in the linear region. This means that the switching losses are reduced, resulting in less heat being dissipated.

4. What can cause slow rise/fall times in a MOSFET?

There are several factors that can cause slow rise/fall times in a MOSFET, such as a high gate resistance, a large gate capacitance, or a long gate drive circuit. External factors, such as a high load capacitance or inductance, can also contribute to slow rise/fall times.

5. How can I reduce the rise/fall times in a MOSFET?

To reduce the rise/fall times in a MOSFET, you can use a gate driver with a low output impedance, which can provide a higher current to charge and discharge the gate capacitance quickly. You can also reduce the gate resistance, minimize stray inductances, and optimize the layout of the circuit to improve the switching performance.

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