Tpwm = 800 ns
324 ns
30 31 32 33 34
10 ns steps
37.5%
38.8%
40.0%
41.3%
42.5%
Demanded
duty (40.5%)
EPWM1A
0
79
Enhanced PWM (ePWM) Module
15.2.2.10.5.1 Edge Positioning
In a typical power control loop (switch modes, digital motor control (DMC), uninterruptible power supply
(UPS)), a digital controller (PID, 2pole/2zero, lag/lead, etc.) issues a duty command, usually expressed in
a per unit or percentage terms.
In the following example, assume that for a particular operating point, the demanded duty cycle is 0.405 or
40.5% on-time and the required converter PWM frequency is 1.25 MHz. In conventional PWM generation
with a system clock of 100 MHz, the duty cycle choices are in the vicinity of 40.5%. In
, a
compare value of 32 counts (duty = 40%) is the closest to 40.5% that you can attain. This is equivalent to
an edge position of 320 ns instead of the desired 324 ns. This data is shown in
.
By utilizing the MEP, you can achieve an edge position much closer to the desired point of 324 ns.
shows that in addition to the CMPA value, 22 steps of the MEP (CMPAHR register) will
position the edge at 323.96 ns, resulting in almost zero error. In this example, it is assumed that the MEP
has a step resolution of 180 ns.
Figure 15-51. Required PWM Waveform for a Requested Duty = 40.5%
Table 15-42. CMPA vs Duty (left), and [CMPA:CMPAHR] vs Duty (right)
CMPA
DUTY
High Time
CMPA
CMPAHR
Duty
High Time
(count)
(1) (2) (3)
(%)
(ns)
(count)
(count)
(%)
(ns)
28
35.0
280
32
18
40.405
323.24
29
36.3
290
32
19
40.428
323.42
30
37.5
300
32
20
40.450
323.60
31
38.8
310
32
21
40.473
323.78
32
40.0
320
32
22
40.495
323.96
33
41.3
330
32
23
40.518
324.14
34
42.5
340
32
24
40.540
324.32
32
25
40.563
324.50
Required
32
26
40.585
324.68
32.40
40.5
324
32
27
40.608
324.86
(1)
System clock, SYSCLKOUT and TBCLK = 100 MHz, 10 ns
(2)
For a PWM Period register value of 80 counts, PWM Period = 80 × 10 ns = 800 ns, PWM frequency = 1/800 ns = 1.25 MHz
(3)
Assumed MEP step size for the above example = 180 ps
1554
Pulse-Width Modulation Subsystem (PWMSS)
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated