HR1000A – RESONANT HALF-BRIDGE CONTROLLER
2. Under normal operation, the phototransistor
modulates the current flow through Rfmax to
modulate the frequency for output voltage
regulation. When the phototransistor is
saturated, the current flow through Rfmax is
at its maximum, setting the frequency at its
maximum
3. A series RC circuit connected between Fset
and GND determines the frequency shift at
start-up. (Please see the soft-start section
for details.)
Burst pin drops below 1.25V, HR1000A will shut
down the HG and LG gate drive outputs,
leaving only the 2V reference voltage on the
Fset pin and the SS pin to retain the previous
state and minimize HR1000A’s power
consumption. When the voltage on the Burst
pin exceeds 1.25V by 50mV, HR1000A
resumes normal operation.
Based on the Burst-mode operating principle,
the Burst pin voltage must connect to the
feedback loop. Figure 4 shows a typical circuit
connect the Burst pin to the feedback signal for
narrow-input–voltage range applications:
Based on the previous principles, the following
equations describe the minimum and maximum
frequency:
1
fmin
=
3⋅CT ⋅Rfmin
1
fmax
=
3⋅CT ⋅(Rfmin || Rfmax
)
Typically, the CT capacitance is between 0.1nF
and 1nF, so the values of Rfmin and Rfmax are:
1
Rfmin
=
3⋅CT ⋅ fmin
Figure 4: Bust-Mode Operation Set-Up
In addition to setting the oscillator, Rfmax also
determines the maximum switching frequency
that HR1000A operates in burst-mode. After
confirming fmax, calculate Rfmax as below:
Rfmin
Rfmax
=
fmax
−1
fmin
For the CT capacitance selection, here is a note
for low temperature and high switching
frequency application: when the temperature is
low, the source current capability of Fset pin
drops a little due to the property of internal
transistor circuit, which means there might be
not big enough current to charge/discharge the
large CT capacitor. So a small CT cap
(<=330pF) is recommended for such application.
Rfmin
3
8
Rfmax
=
⋅
fmax
−1
fmin
Here, fmax corresponds to a load point, PBurst
,
where the peak current flow through the
transformer is too low to cause audible noise.
The above introduction is based on a narrow
input voltage range. As a property of the
resonant circuit, input voltage also determines
the switching frequency. That means the PBurst
has a large variance over the wide input voltage
range. To stabilize PBurst over the input range,
use the circuit in Figure 5 to insert the input
voltage signal into feedback loop.
Burst-Mode Operation
Under light-load or in the absence of a load, the
maximum frequency limits the resonant half-
bridge switching frequency. To control the
output voltage and limit power consumption, the
HR1000A can enable compatible converters to
operate in burst-mode to sharply reduce the
average switching frequency, thus reducing the
average residual magnetizing current and the
associated losses.
Operating in burst-mode requires setting the
Burst pin on the HR1000A; if the voltage on the
HR1000A Rev. 1.01
8/30/2012
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14
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