Thumbs & Switches
Imagine you are using a hammer to strike nails into boards, and while you are holding the nails you graze your thumb ever so slightly. You may feel it, but because it’s just a graze it doesn’t seem like that big of a deal. After 50 times of this happening, you may notice some throbbing. After 500 the continuous strikes in the same spot add up, and your thumb is quite sore. Now imagine the 500 times happening in half a second – a 1Hz speed. That is what is happening to your switch at a 1hz switching speed, except instead of a hammer making contact it instead a massive current surging through and instead of your thumb, it’s your electronic device taking the beating. 1Hz is also quite slow for many switches, so now multiply the frequency by a few tens, or even hundreds, of thousands. Ouch.
So, what exactly is happening when the ‘hammer strikes’, per say? To understand this, it is important to understand the configuration of the switches. For ease of explanation and understanding, I am going to use MOSFETs as the switch going forward but know that it could just as easily be BJTs, IGBTs, SCRs or GTOs. They do have differences, however for the high-level explanation those difference won’t matter. In addition to this, we should also assume that the switches are being operated in a manner that maximizes their fully on or fully off time in order avoid the linear range in which the most power is dissipated as heat. The final assumption is that we are using PWM to control the output voltage, so if you’re not familiar with PWM it may be beneficial to familiarize yourself with it, however it is not crucial in understanding the concept of shoot-though.
Continue reading the tutorial below:
Keeping your Switches Alive with Deadtime.docx (59.5 KB)