Design guide IDP2303(A)
LLC design
Application Note
32
Revision 2.0
2017-05-03
With some interpolation, the frequency value at 0 V is calculated to be around 170 kHz.
4.3.1.3
𝑽
_𝑳𝑳𝑽𝑪𝑶
: 𝒇
_𝑳𝑳𝑽𝑪𝑶
(light load) :
𝑽
_𝑯𝑳𝑽𝑪𝑶
∶ 𝒇
_𝑯𝑳𝑽𝑪𝑶
(heavy load)
These two points define the mid-frequency range of VCO, which covers the normal operation ranges from
very light load to full load at nominal bus voltage.
To make the VCO slope lower and create better regulation performance, the covered voltage range should
be large. For example, for 0 to 2.4 V full range, the value could be set as,
𝑉
_𝐿𝐿𝑉𝐶𝑂
= 0.45 𝑉
𝑉
_𝐻𝐿𝑉𝐶𝑂
= 2.0 𝑉
It can be seen from the voltage gain curve in Figure 19 that, when the system is working around nominal bus
voltage (the resonant point), the frequency change is small while the load changes. Usually 15% deviation
from the nominal point is assumed, considering the tolerances of the circuit parameters that will affect the
voltage gain, i.e.
𝒇
_𝑳𝑳𝑽𝑪𝑶
= 1.1𝑓
𝑛𝑜𝑚
= 121 𝑘𝐻𝑧
𝒇
_𝑯𝑳𝑽𝑪𝑶
= 0.8𝑓
𝑛𝑜𝑚
= 88 𝑘𝐻𝑧
The above calculations are based on typical values of resonant components, a simplified transformer model
and LLC equivalent circuit. In reality, it is difficult to build an accurate system model upon which an accurate
gain/frequency curve and the consequent VCO frequency curve can be derived. Many factors come into play
including:
The tolerances of resonant tank component values will bring deviations of the switching frequency. For
example, there may be 7% tolerance for the resonant inductance
𝐿
𝑟
and 3.5% for the resonant
capacitance
𝐶
𝑟
.
Additional equivalent voltage drops caused by the transformer winding and PCB traces will affect the
actual voltage gain. Usually the transformer internal voltage drop is higher at heavy load than light load.
As such higher equivalent conversion gain is needed at heavy load.
The value of resonant factor
𝑚
has an effect on the shape of gain curves; larger values of m result in
flatter gain curves, which mean a wider operating frequency range.
Some of the factors above are difficult to quantify. So, the VCO curve can only be defined and optimized by a
combination of theoretical analysis and actual measurements.
4.3.2
Current sense and OCP
The LSCS pin of IDP230x controller senses the primary side current by detecting the voltage across the
sensing resistor in series with the low-side MOSFET. Typically, an RC low-pass filter is used to filter out the
switching noise in the sensing signal. The RC time constant should be around 1/10 of the switching period.
There could be several overcurrent protections (OCP) for different operating conditions, such as normal
operation, startup mode and burst mode. There are also OCPs with different threshold levels that trigger
different subsequent actions, such as increasing switching frequency and entering into an auto-restart
mode/latch mode.
The selection of current sense resistor is based upon the normal operation condition. An appropriate
sensing resistor value should:
Protect the LLC transformer from saturation and other components from over current stress.
Avoid mis-triggering in normal operation.
