HR1000A – RESONANT HALF-BRIDGE CONTROLLER
To design the lossless current sensing network,
consider these two conditions:
1. R1 is smaller than several hundred ohms.
The sensing network acts as a capacitive
current divider. Use the equations below:
Cr
C1 ≤
100
0.8⋅ π
ICrpk
Cr
R2 =
⋅(1+
)
C1
2. R1 is ~10kΩ. The sensing network acts to
divide the ripple voltage on Cr. Design for
this condition as per the equations below:
Figure 10: Current Sensing with a Sense
Resistor
Cr
C1 ≤
There are two types of current sensing methods:
one uses a sense resistor in series with the low-
side MOSFET; the other uses a lossless
current-sensing network. The first method is
simple but causes some unnecessary power
consumption.
100
2
2
R1 + XC1
0.8⋅ π
ICrpk
R2 =
⋅
XCr
Calculating the reactance of C1 and Cr at the
frequency where the maximum peak resonant
current occurs. Empirically, the R2 and C2 time
constant is about 10/fmin.
Calculate the sense resistor using the following
equation:
4
RS =
Depending on the circuit, consider the
calculated value as a cut value that requires
adjustments based on experimental results to
meet the design target.
ICrpk
Where ICrpk is the desired peak current through
the primary MOSFET for the resonant capacitor
at low input voltage and full load.
The OCP can limit the energy transferred from
the primary to the secondary during over-load
or short-circuit period. However, excessive
power consumption due to high continuous
currents can damage the secondary-side
windings and the rectifiers. The HR1000A
provides additional protection to reduce the
average power consumption during OCP: When
OCP triggers, the converter enters a hiccup-like
protection mode that operates intermittently.
Since the circuit require an RC filter between
the sensing resistor and CS pin, select an RC
time constant at around 10/fmin.
Set the maximum over-load or short circuit
operating time (tOC) by selecting appropriate
CTimer and RTimer. During the first OCP level
when the CS voltage exceeds 0.8V, an internal
130µA current source turns on to charge CTimer
.
When the voltage on CTimer reaches 2V, the CSS
voltage drops below the OCP comparator
output. This forces the switching frequency to
equal fstart to minimize the transferred energy.
tOC is the time for the voltage on CTimer to rise
Figure 11: Current Sensing with Lossless
Network
HR1000A Rev. 1.01
8/30/2012
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17
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