Parts, datasheets, and Background:
|Gate Driver IC on breakout board for testing (quite small)|
|From the data sheet - typical circuit configuration|
|Relevant values from data sheet and some picked for my design.|
|Relevant values from data sheet of MOSFET|
|Relevant values from data sheet of 1N4148 diode|
|PWM waveform characteristics decided by me|
The pwm carrier frequency was something that I was originally confused about. I was thinking that this frequency described by 1/tcyc was going to change based off of how fast the motor was rotating at any given time. However this is held constant no matter what RPM the motor is running at. It is the duty cycle around the carrier frequency that changes the speed of the motor. In some of my reading on the internet commercial (Electronic Speed Controllers) ESC's range in carrier pwm frequencies in the range of 8kHz to 25kHz. One of the most important considerations in selecting a carrier frequency is one that is well above the rate of the actual application switching. This is to avoid harmonic interference issues. In the case of the motor that I selected, the electrical frequency at full RPM is 1200 Hz. This can be seen below. So basically since I dont know how adjusting the carrier frequency will effect the performance of my motor driver, right now I will design for a high end frequency of 30kHz to give me a little head room. This gives a period of tcyc at about 33 microseconds.
|Electrical frequency of the motor at full RPM is 1200 Hz|
The following equations were aquired from a document that I found useful by Silicon Labs. This paper does a great job of explaining the need, functionality, and design behind a bootstrapped n-channel gate driver. I would recommend reading this document for learning more than my own blog post.
|Equations from the Silicon Labs document|
These equations are useful for getting your design in the right ball park. They are not necessarily meant to be met EXACTLY! A lot will determine what values of components you ultimately put into the design, like what you have available, and what is realistic. For example as the document lays out, at high switching frequencies the value of Rboot goes close to 0 and can be neglected. You might also not have the exact value of capacitance for Cboot as capacitors come in very specific values.
|Results from above equations, and the values that I selected for my design.|
As you can see the values that I decided on using were determined by what I had available and made sense. Once I selected my capacitor, I redid the calculation for the resistor and came out closer to a value that I had available. Another thing to look out for is to make sure the diode you chose can handle the IRBAV as well as the bootstrap resistor being able to handle the power dissipation. This should be no issue in most cases. My spreadsheet with all of the parameters and equations in it is located here.
For the resistor values that I had talked about earlier, R1 through R4, I went of standards and rules of thumb that I read online. Basically R1 and R3 are anywhere in the range from 1 to 100 ohms. While R2 and R4 are typically 10,000 times larger. This will ensure a good voltage divider that while the gate driver is sourcing current to the gate capacitor there isnt much of a loss in current. However there will still be a path to ground for the gate capacitor current when the gate driver disconnects itself from the circuit. In my case I selected the values of resistance to be 47 Ohms for R1 and R3 and 470k Ohms for R2 and R4.
My next post will be me putting together the circuit on a perf board / proto board to get ready to finally get a motor spinning.