a
2
a
1
a
3
a
4
fs < fr
fr
fs > fr
fs = fr_max
PWM
Mode
LLC Mode
Tr = 1/fr
Tr = 1/fr
SynFET
Primary
i
s
Automatic Mode Switching
48
SNIU028A – February 2016 – Revised April 2016
Copyright © 2016, Texas Instruments Incorporated
Digital Pulse Width Modulator (DPWM)
2.10 Automatic Mode Switching
Automatic Mode switching enables the DPWM module to switch between modes automatically, with no
firmware intervention. This is useful to increase efficiency and power range and also to achieve smooth
system performance (such as monotonic start-up) in certain topologies. An example of mode switching in
LLC topology is provided next.
2.10.1 Resonant LLC Example
In Resonant LLC topology, three modes are used. At the lowest power, a pulse width modulated mode
(Multi Mode) is used. As power increases and frequency decreases, Resonant mode is used. As the
frequency gets still lower, resonant mode is still used, however the Sync FET driver changes so that the
on-time is fixed and does not increase (SR Pulse Width is clamped). Here are the waveforms for the LLC:
Figure 2-11. Resonant LLC implementation in UCD3138 with Automatic Mode Switching
2.10.2 Mechanism for Automatic Mode Switching
Many of the configuration parameters for the DPWM, including the mode, are in DPWM Control Register
1. For automatic mode switching, some of these parameters are duplicated in the Auto Config Mid and
Auto Config High registers.
If automatic mode switching is enabled, the Filter Duty signal is used to select which of these three
registers is used. There are 4 registers which are used to select the points at which the mode switching
takes place. They are used as shown below: