As a hobbyist, I am working for a long time on some circuits which require a power MOSFET driven with PWM to control various loads.
I have tried low side switching with N channel MOSFETs and high side switching with P channel MOSFETs. I know about the threshold gate source voltages, about RDS ON values etc. Static switching (without PWM) is OK. But as soon as I start using PWM (from a microcontroller) problems begin to arise.
I exactly know about the parasitic capacitances of a MOSFET and that the best way for PWM switching is using dedicated gate drivers but I want to learn something and try the gate drive using discrete components.
I know that I must charge and discharge the famous gate capacitor as quickly as possible to be able to switch very fast and to achieve the shortest time period where the MOSFET is partially conducting (producing waste heat).
I decided to use a totem pole driver for the logic level MOSFET (IRLZ44). You can see the circuit in the first figure. The load is a simple low voltage light bulb. I don’t think it has a significant inductance and for this reason I didn’t use an inductor and flyback diode.
In the totem pole I used a pair of BD137 / BD 138. They have a continuous Ic of about 1.5A (pulsed 3.0 A) and hfe of about 160.
The totem pole has been driven by two consecutive 2N2222 transistors in common emitter configuration.
The circuit is being fed by an Arduino with a PWM frequency of 490 Hz.
The logic part of the circuit is supplied with 5V and the light bulb is powered by a separate 1S (and 2S) LiPo battery (4.2V – 8.4 V respectively).
With this configuration I have a very clean PWM signal at the output of the totem pole but almost a sawtooth waveform (with a short rise time and long falling fime) at the MOSFET output (measured at the terminals of the light bulb).
The waveform suggests that I can drive the gate to ON position without any problems but I cannot discharge the gate capacitor in a short time.
Now my questions:
1- Is a circuit modification (with a series inductor and a flywheel diode) like in the Figure 2 necessary even if the load is a simple light bulb?
2- Is the collector resistor of the second 2N2222 too high? Should I lower it to maybe 470 ohms so that the totem pole can produce a higher collector current to drive the MOSFET gate?
3- Even if I set a low duty cycle for the PWM signal, the bulb gets brighter and brighter until it burns out in about 30 – 40 seconds. I cannot find any explanation for this.
4- Are there any other factors I am overlooking or unaware of?
(Please bear in mind that I am a hobbyist and not a professional).
Thank you very much...
I have tried low side switching with N channel MOSFETs and high side switching with P channel MOSFETs. I know about the threshold gate source voltages, about RDS ON values etc. Static switching (without PWM) is OK. But as soon as I start using PWM (from a microcontroller) problems begin to arise.
I exactly know about the parasitic capacitances of a MOSFET and that the best way for PWM switching is using dedicated gate drivers but I want to learn something and try the gate drive using discrete components.
I know that I must charge and discharge the famous gate capacitor as quickly as possible to be able to switch very fast and to achieve the shortest time period where the MOSFET is partially conducting (producing waste heat).
I decided to use a totem pole driver for the logic level MOSFET (IRLZ44). You can see the circuit in the first figure. The load is a simple low voltage light bulb. I don’t think it has a significant inductance and for this reason I didn’t use an inductor and flyback diode.
In the totem pole I used a pair of BD137 / BD 138. They have a continuous Ic of about 1.5A (pulsed 3.0 A) and hfe of about 160.
The totem pole has been driven by two consecutive 2N2222 transistors in common emitter configuration.
The circuit is being fed by an Arduino with a PWM frequency of 490 Hz.
The logic part of the circuit is supplied with 5V and the light bulb is powered by a separate 1S (and 2S) LiPo battery (4.2V – 8.4 V respectively).
With this configuration I have a very clean PWM signal at the output of the totem pole but almost a sawtooth waveform (with a short rise time and long falling fime) at the MOSFET output (measured at the terminals of the light bulb).
The waveform suggests that I can drive the gate to ON position without any problems but I cannot discharge the gate capacitor in a short time.
Now my questions:
1- Is a circuit modification (with a series inductor and a flywheel diode) like in the Figure 2 necessary even if the load is a simple light bulb?
2- Is the collector resistor of the second 2N2222 too high? Should I lower it to maybe 470 ohms so that the totem pole can produce a higher collector current to drive the MOSFET gate?
3- Even if I set a low duty cycle for the PWM signal, the bulb gets brighter and brighter until it burns out in about 30 – 40 seconds. I cannot find any explanation for this.
4- Are there any other factors I am overlooking or unaware of?
(Please bear in mind that I am a hobbyist and not a professional).
Thank you very much...
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