HR1000A – RESONANT HALF-BRIDGE CONTROLLER
Soft-Start Operation
Rfmin
Rss
=
fstart
For the resonant half-bridge converter, the
power delivered is inversely proportional to the
switching frequency. To achieve soft-start, start
the switching frequency with a high value until
the value is controlled by the closed loop.
−1
fmin
3⋅10-3
Rss
Css
=
The converter + HR1000A can easily achieve
soft-start using an external RC series circuit as
shown in Figure 8.
Select an initial frequency, fstart, at least 4×fmin.
Select CSS as a trade-off between the desired
soft-start operation and the OCP speed (see
the next section for details).
Current Sensing
Figure 9 shows the current sense block
diagram.
Figure 8: Soft-Start Block
When start-up begins, the SS voltage is 0V, so
the soft-start resistor (RSS) is in parallel to Rfmin:
Rfming and RSS determine the initial frequency as:
Figure 9: Current Sensing Block Diagram
There are two levels over-current protection.
1
fstart
=
The first level occurs when the CS voltage
exceeds 0.8V, the comparator triggers the
control logic to output a high-level control signal
that turns on the transistor connecting SS and
GND. Then the CSS voltage drop results a sharp
increase in oscillator frequency, and therefore
reduces the energy transferred to the output.
When the CS pin voltage drops back to
0.79V,the converter resumes normal operation
with the help of the 10mV hysteresis.
3⋅CT ⋅(Rfmin||Rss )
During start-up, the CSS charges until its voltage
reaches the reference–2V, and the current flow
through RSS drops to zero. This period takes
about 5×(RSS×CSS). During this period, the
switching frequency changes following an
exponential curve: the CSS charge initially
decays relatively quickly but the rate
progressively slows.
Generally, the CS pin voltage continues to rise
during a short circuit. The second level over-
current protection triggers when the CS pin
voltage rises to 1.5V. Then the IC is latched at
very low consumption (residual consumption in
EC table).
After this period ends, the output voltage is not
still close to the setting value, so the feedback
loop will take over start-up.
With soft-start, the input current increases
gradually until the output voltage reaches the
setting point with little overshoot.
For two-level protection, a very large CSS slows
the discharge to the point that the transformer
and the resonant inductor saturate and
damages the secondary diode.
Select the soft-start RC network as per the
equations below:
HR1000A Rev. 1.01
8/30/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
16
MPS [ MONOLITHIC POWER SYSTEMS ]
