A brief and important analysis about the low side and high side FET when used as a switch.This is used in many applications including electronic ballasts. This analysis to be included in the R&D labs libraries.Also usable to reduce the radiations and go for EMI certification.
Can I add that designers do not have an easy way to time the gate drive in a reliable, low-cost circuit. No one device or circuit has appeared that fixes this problem at the right price and reliability point. I have seen many circuits loaded with hand-tweaked RC networks.
If you read carefully you can spot that this article is talking about a buck converter, but using terms like hi-side and lo-side which are normally associated with H-bridges. In H-bridges, timing is a symmetric problem and turning on the lo-side is equally critical.
The synchronous FET pair in this buck converter can also easily have such a symmetric mode, depending on the design parameters.
For synchronous buck topology I have used of the shelf PWM chips, which usually have built in optimization of ON/OFF timing between the hi-side and lo-side switches...not much to do with manual tuning.
There is no single schematic here to show, it depends on what you are driving--the point here is to show you what you need to be aware of when doing a circuit. It's not a "cookbook" situation, unfortunately.
The timing of the upper switch turn-on is helped by a few factors. For example, as we can see in Robert's figure 1C, the gate voltage is at a low level anyhow if the upper switch turns on too early. This limits the possible amount of shoot-through. More importantly, both Spice and O-scope plots have showed me that the parasitic source inductance plays a critical role in lowering shoot-through. High di/dt across Ls in the lower switch "pushes" the gate voltage higher even though the G-S voltage is falling, and is often mistaken for a Miller plateau. The upward rise in V-gate in figure 1A is this very effect. Remember: Vgate is not the same as Vgate-source.
D. Hambley, SENTEK Engineering
Some Drivers Do have Programmable Dead time to prevent shoot-through. How ever this limits the usable duty cycle range.
Robert has considered 12:1 buck reg, which is one of the difficult examples. This timing problem becomes more important when we need to achive duty cycles down to 1%
Why is the full linked list of all tips in the series a PDF file??? I can't even use that on some of my mobile devices, but I can read the articles.
The absurdity of a PDF file is clear when you see that for the next tip you have to change it, and I have to re-download it (or if I get busy and miss a few, I come back want to catch up, I need to download a PDF)
Why isn't that just a link to a home page for the Power Tips Series on eetimes.com ? On that home page is the hyperlinks to other articles in the series.
sorry, in my annoyance at the PDF, I forgot to mention that the Series is a favorite of mine, absolutely a wealth of quality information that is hard or impossible to find or know without the experience you clearly have. I am continually learning from you, thanks, but just hate the PDF :)