Step 10 - Additional Parameters - Power integrations LinkSwitch-4 LNK4*15D Series Application Note

Application Note
Step 10 – Additional Parameters
Bias Capacitor - CVCC
Bias Capacitor
CBIAS
DELTAV_BIAS
Figure 16. Bias Capacitor Selection.
The bias capacitor serves three purposes:
1. Energy storage during the start-up procedure. The bias capacitor
powers the controller until the bias winding generates enough
voltage, which is limited by the voltage on the output capacitor,
to power the controller.
2. Acts as an energy reserve between switching cycles to power the
controller, particularly at no-load where the switching frequency
is low.
3. Forms part of the timing mechanism for the switching frequency
oscillator at no-load.
For easy starting into large output capacitance the bias capacitor
value can be made large. However, it must be a ceramic type
capacitor, an electrolytic capacitor will leak and possibly prevent
start-up especially as it ages. Also the bias capacitor must not be so
large that the ripple voltage on the VCC pin will be less than 50 mV at
the zero load switching frequency. If the ripple is less than 50 mV at
zero load, the controller will cease switching and power cycle. It
must detect the step in VCC when the transformer discharges to
move to the next state in the switching cycle when at zero load.
If the design has a zero load to partial or full load transient require-
ment, as found for USB charger designs, where the output voltage
must not fall below a given limit, the bias capacitor value should not
exceed 2 mF. Larger values of bias capacitor result in a very shallow
discharge curve across the capacitor. A voltage shifted version of this
curve is used to intersect with the internal switching frequency
oscillator capacitor charge voltage to start the next switching cycle.
If the angle of intersection is too shallow, thermal noise will make the
trigger point less predictable and the zero load frequency will vary
erratically. The average frequency will be such as to keep the output
voltage under zero load to be within specification, but the resulting
minimum frequency will allow a transient load to pull the output
voltage below the minimum allowed, if the transient coincides with a
low frequency cycle.
The bias capacitor ripple should not be allowed to exceed 1.6 V. If
V falls by more than 1.6 V in a switching cycle, an extra minimum
CC
primary current pulse is issued. This will recharge the bias capacitor
Zero-load Collector peak current
VCS_MIN
RCS2
Figure 17. Zero-Load Peak Collector Current Selection.
Dummy load and no load
R_PRELOAD
P_PRELOAD
FSW_NOLOAD
Figure 18. Dummy Load Resistor Selection.
14
Rev. B 10/17
5.38 uF Bias capacitor is greater than 2uF! Transient response could be unpredictable.
For improved and repeatable transient response keep capacitor value < 2 uF
Voltage ripple on VCC capacitor at zero-load (should be between 0.05 V and
100 mV
1.6 V
to prevent V falling inadvertently to the sleep level, which would
CC
cause a power reset cycle. If this happens too often, the output
voltage will rise and may exceed the specification.
The spreadsheet calculates the bias capacitor size to give 100 mV of
ripple [E139]. Alternative values can be entered into [B138] and the
new ripple level (Delta_Vbias) will be given in [E139].
Zero-Load Collector Peak Current
VCS_MIN is the minimum peak voltage that the controller can set
across the R resistor. This sets the minimum primary current and
CS
hence the minimum energy per switching cycle. On LNK4xx3x,
LNK4x14D and LNK4x15D devices there are four discrete levels that
can be set, 58 mV, 73 mV. 94 mV and 127 mV. These are set by the
resistor R which can be 100 W, 270 W, 470 W or 1 kW respectively.
CS2
No other values can be used, there are no intermediate values of
VCS_MIN. LNK40x2S devices only have one level for VCS_MIN, 88 mV.
VCS_MIN is used to control the zero load behavior of the circuit,
particularly the zero load switching frequency. The higher the VCS_
MIN level, the lower the zero load switching frequency and the lower
the zero load power. However, the lower the zero load switching
frequency, the larger the output capacitors must be if there is a zero
load transient requirement. Larger output capacitors require a larger
bias capacitor which cannot be greater than 2 mF in this situation.
A zero load switching frequency of between 1 kHz and 2 kHz should
be the target, calculated in [E154].Use the drop down selection table
in [B145] to select the required VCS_MIN. The required value of R
is given in [E136]. The higher the frequency, the easier it is to
start-up and meet zero load transient requirements, but zero load
power will be greater.
Dummy Load Resistor R
The value of R_PRELOAD (R
switching frequency [E154] by entering a value in [B152]. Aim to
have the dummy load power dissipation [E153] no lower than the bias
winding power at no-load [E108]. This aids consistent zero load
behavior across production.
Drives peak current at zero load. Affects zero load frequency and consumption,
73
73 mV
and 0-100% load step
270 Ohm Resistance for setting VCS_MIN
Pre load resistor (1%). Affects FSW_NOLOAD, zero load consumption, and
5620 Ohm
0-100% load step dip
4.4 mW Preload resistor power consumption at no load
Estimated switching frequency at no load. Adjust with VCS_MIN and
1116 Hz
R_PRELOAD
AN-69
OUT
) can be used to trim the zero load
OUT
www.power.com
CS2