[MUSIC PLAYING] Greetings. I'm Dave Priscak, your host for the Power Tips video series. I'm back with Robert Taylor, power expert, to talk about the layout for the power stage of a boost converter. Thank you, very much. Welcome.
Thanks, Dave. So a boost converter, generally what we're doing is we're taking a smaller input voltage and boosting that to a higher output voltage level. And the way we're going to do that is with this simplified circuit that I have here. So again, in a typical DC to DC converter, we have three main elements. So we have our inductor, we have a switching FET, and then we have a rectifying diode that we're going to use here. OK?
So first, let's take a look at what happens to the currents during the PWM on and a PWM off state. So we're going to draw the PWM current in blue for the on state. So what's going to happen is I'm going to turn this switching MOSFET on, and I'm going to begin to build up current in this inductor. So current is going to flow from the input, through the inductor, to ground.
And the rectifier is holding that voltage back.
The rectifier is off, at this point, because the current is flowing this way.
Then, during the PWM off period, I've built up the energy in this inductor. And the current still needs to keep flowing in this direction, so what's going to happen is this voltage is going to rise up until the current can flow through this rectifier to the output voltage. OK, that's during the PWM off time.
Now then, when we get to this high frequency current that we're trying to mitigate in the loop, that's going to be this current that's right here. So when I turn on this MOSFET, there's a brief time in which I need to recover the energy from this switching dial. So we have high frequency current that flows like that. And so that high frequency loop is the one we're going to try to minimize in the layout.
So let's take a look at the actual layout of these components that we have here. So I've taken these components and I've placed them as I would in a real power supply layout and connected them with planes, vias, and other traces to see how everything goes together.
So during the PWM in on time, we're going to use this blue color to represent that. So if we look here, the current is flowing from the input, through the inductor, through the FET to ground. So the current's going to flow like this, through the inductor, through the FET, towards ground. OK? Just like that.
So also, because the current in the inductor cannot change instantaneously, when I turn this FET off, it's going to want to keep flowing in this direction. So the current's going to flow like this, directly to the output in the off state. So what that leaves us with is this period here, once I transition from the off period to turning this that back on, current's going to flow like this, with very high frequency. And so this is the loop that we're trying to minimize.
And so you can see, we basically tied these grounds directly together of the output cap-- so these grounds right here-- to the ground of the MOSFET over here. And we've done that with a number of vias, and by placing the components close together.
So again, giving them tight minimizes the parasitics of both capacitance, as well some of the inductance of the traces as well.
Absolutely Yeah. And in the boost converter, the output current is not continuous. It's pulsed current. And that's very important to keep these caps really close and tightly coupled with the ground of this MOSFET.
And I also know that we have some the reference designs on TI Designs that replace this rectifier with another switch, a two-switch boost. Does all of this still come into play with this?
That's correct. So as long as we're operating in continuous conduction mode where the current is always positive in the inductor, then these same currents will apply with a synchronous boost as well.
Thank you very much, Robert. This has been very informative about the tricks on making a better layout for higher performance boost converters. For more information, please go to ti.com, Power Tips Video Series. Thank you, very much.
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