Design Article
Getting the most out of power conversion stages
Shane Callanan, Excelsys
8/7/2012 10:40 AM EDT
In today's environment, we are continuously being challenged by our customers to make our designs smaller, more efficient and more reliable. In order to achieve this, we have to look at every last element of the design. One item where we concentrate on is the dead time control of switching MOSFETs. Excelsys engineers have gathered a significant amount of information on this as we continue to research new control technology.
Increasing efficiency by choosing the right algorithm
Deadtime can be defined as the period by which both the forward FET and freewheel FET are turned off during the switching sequence in a synchronous buck – see figure 1. The problem with this is that we get body diode conduction during this period, (when the forward FET switches 'off', and the freewheel should be turned 'on'). By reducing dead time, losses can be reduced thus increasing the overall converter efficiency.
If we look at a simplified synchronous buck converter in figure 1, you can see that there are three main sources for losses in the design.
. The switching FETs, due to conduction and switching losses
. The output inductor in terms of AC and DC losses
. The controller contribution due to driving losses
It becomes clear very quickly that in reducing the switching losses we can significantly improve the performance of our conversion stages. In order to gain a good understanding of what we are trying to achieve, let us first look at what we need to achieve in order to optimize the turn on and turn off of a FET. Figure 2 shows the gate-to-source and drain-to-source voltages of the FET as it is turned on. We see the non-optimal turn-off in figure 2(a). The Optimal timing occurs when VGSQR reaches the VTH of FET and VDSQR is approaching zero.
Figure 3 shows the gate-to-source and drain-to-source voltages of the N-type FET as it is turned off. We see the non-optimal turn-off in figure 3(a). The Optimal timing occurs when VGSQR reaches the VTH of FET as VDS starts to increase.
Maximum efficiency is achieved when the body diode conduction is reduced to a minimum. Various types of intelligent controllers can manage this operation. Two such methods are known as Predictive and Adaptive dead time control. In sections three and four, we discuss each of these approaches in more detail.
Increasing efficiency by choosing the right algorithm
Deadtime can be defined as the period by which both the forward FET and freewheel FET are turned off during the switching sequence in a synchronous buck – see figure 1. The problem with this is that we get body diode conduction during this period, (when the forward FET switches 'off', and the freewheel should be turned 'on'). By reducing dead time, losses can be reduced thus increasing the overall converter efficiency.
Figure 1: Synchronous buck topology
If we look at a simplified synchronous buck converter in figure 1, you can see that there are three main sources for losses in the design.
. The switching FETs, due to conduction and switching losses
. The output inductor in terms of AC and DC losses
. The controller contribution due to driving losses
It becomes clear very quickly that in reducing the switching losses we can significantly improve the performance of our conversion stages. In order to gain a good understanding of what we are trying to achieve, let us first look at what we need to achieve in order to optimize the turn on and turn off of a FET. Figure 2 shows the gate-to-source and drain-to-source voltages of the FET as it is turned on. We see the non-optimal turn-off in figure 2(a). The Optimal timing occurs when VGSQR reaches the VTH of FET and VDSQR is approaching zero.
Figure 2: Turn-on sequence of an N-type FET
Figure 3 shows the gate-to-source and drain-to-source voltages of the N-type FET as it is turned off. We see the non-optimal turn-off in figure 3(a). The Optimal timing occurs when VGSQR reaches the VTH of FET as VDS starts to increase.
Figure 3: Turn-off sequence of an N-type FET
Maximum efficiency is achieved when the body diode conduction is reduced to a minimum. Various types of intelligent controllers can manage this operation. Two such methods are known as Predictive and Adaptive dead time control. In sections three and four, we discuss each of these approaches in more detail.
Figure 4: Controller monitors Vgs and Vds of forward and flyback FETs
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shawnmk
8/10/2012 6:47 PM EDT
I was asked the question: what are some of the losses because of switching elements in a DC-DC converter? My interviewer helped me through to getting the answer and now, I see this article on the same topic. It is a good article. Thank you,
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