RDK-239 Power Integrations, Inc., RDK-239 Datasheet - Page 17

• Manufacturer Part Number: RDK-239
• Description: Specifications: Family: Eval Boards - DC/DC & AC/DC (Off-Line) SMPS ; Series: HiperLCS™ ; Main Purpose: DC/DC, Step Down ; Outputs and Type: 1, Isolated ; Power - Output: 150W ; Voltage - Output: 24V ; Current - Output: 6.25A ; Voltage - Input:
• Manufacturer: Power Integrations, Inc.

The recommended series resistor value of 220 W and the bypass
capacitor form a low-pass filter, and its time constant must not
cause significant attenuation of the current sense signal at the
nominal operating frequency. The effect of the attenuation is
greatest for the first pulse in the start-up current waveform, and
can also affect proper shutdown during short-circuit testing,
which typically trips the 7-cycle current limit. Place a close-
coupled probe across the IS pin bypass capacitor and compare
the waveform to the primary current.
Burst Mode Operation and Tuning
Burst mode will produce a typical waveform such as in Figure 24.
During the burst pulse train, the switching frequency rises from
f
Figure 23. Capacitive Divider Current Sense Circuit.
Figure 24. Typical Waveform of Burst Mode. 24 V / 150 W HiperLCS Design at
www.powerint.com
START
GROUND
400
300
200
100
HB Pin
IS Pin
S Pin
Pin
0
to f
0
STOP
Zero Load. The Initial Spike (circled) Size is Dependent on Post-Filter
Electrolytic Capacitor ESR.
.
1 nF
C7
220 Ω
R12
47 pF
1 kV
C12
Time (ms)
24 Ω
R11
50
Burst Repetition Rate
Transformer
6.8 nF
LLC
1 kV
C11
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100
24.1
24.0
23.9
If the initial output ripple spike at the beginning of the burst
pulse train is ignored, the output ripple somewhat resembles a
sawtooth. See the output ripple waveform in Figure 24. When
the HiperLCS is switching, the output rises. When it stops
switching, the output falls. The top of the sawtooth is where the
burst pulse train ends, because the feedback loop has commanded
a frequency = f
burst pulse train begins, because the feedback loop has
commanded a frequency = f
control resembles a hysteretic controller, where the top and
bottom of the sawtooth are fixed by the feedback loop gain.
The downward slope of the sawtooth is merely the output
capacitors discharging into the load, with dv/dt:
Where I = load current. C is the total output capacitance.
The upward slope of the sawtooth is dependent on the difference
between the current delivered by the powertrain, and the current
drawn by the load. For a given design, the upward slope
increases with input voltage.
The burst repetition rate (frequency) then increases with load.
When the load reaches a point where the powertrain can
regulate at a frequency <f
load current decreases (from heavy load), frequency increases,
and when it reaches f
In a typical design, f
higher than the nominal switching frequency. Figure 18 shows
the relationship between f
ratio of f
some cases the designer may choose to change dead-time
slightly in order to change f
only enter burst mode at zero load and an input voltage above
nominal.
Figure 25. Zoom in of First Few Switching Cycles of Burst Pulse Train of Figure 24.
400
300
200
100
0
0
STOP
The First 2 Cycles Show That the High-Side Driver has not Turned
on yet. The Switching Frequency of the First Few Cycles is f
335 kHz in This Case. The Ringing on the Output is from the
Output Filter.
to f
STOP
START
. The bottom of the sawtooth is where the
START
vs. Burst Threshold setting number). In
STOP
must be chosen to be at least 20-40%
, bursting will commence.
STOP
START
I
START
=
Time ( s)
START
, the bursting will stop. When the
C
and dead-time, and Table 5 the
and f
25
. As such, the burst mode
#
LCS700-708
dv
dt
STOP
. Some designs may
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START
50
,
17
24.1
24.0
23.9