C2472, C2473 and C2474
Datasheet
RDFC Controllers for Offline Applications
Product data
Page 5 of 18
DS-1423-0709C
© Cambridge Semiconductor Ltd 2007
26-Sep-2007
Start-up and Active State Power Conversion Modes
In the Start-up and Active states the C2472, C2473 and C2474 ICs have several modes for controlling power
conversion that are designed to achieve maximum efficiency and to limit power (current) across a wide range
of loads. Refer to Table 2 for a summary of each mode.
Mode
Typical Load
Range
Description
Standby
IOUT ≥ 0% to
~20% of rated
current
Standby mode reduces power consumption at low loads. It achieves this by
progressively reducing the on-time then by increasing the off-time as the load
decreases. As load increases, the converter duty is increased until the controller
returns to Normal mode. Typically, mains ripple causes change of operating mode
during each mains half-cycle, with the converter moving to lower-power modes
between peaks of the mains voltage.
Normal
IOUT > ~20% to
100% of rated
current
Normal mode is used for steady state power delivery. During Normal mode the power
device switches in a fully resonant minimum-voltage-switching waveform, with the off-
time determined by the transformer resonance (TRES) and the on-time being equal to
75% of the off time. A low level of primary switch current, sensed via the CS pin
voltage, causes the controller to change to Standby mode and a high level to Overload
mode.
Overload
IOUT >~100%
rated current
Overload mode is activated at high output loads. In this mode the on-time of the
primary switch is terminated early (before 75% of TRES) when the primary current
exceeds a preset maximum, thereby protecting the primary switch and limiting the
output current. This results in reduction of the output voltage. Heavy overload (sensed
by the on-period of the primary switch reducing below a preset time) causes Foldback
mode to be entered.
Foldback
VOUT < ~70%
rated output
Foldback mode is entered from the Overload mode. In this mode the controller reduces
the on/off duty cycle to protect the power supply and any connected load by both
shortening the on-period and increasing the off-period of the primary switch. Converter
cycles continue to maintain auxiliary power to the controller. The controller exits the
Foldback mode and enters the Power Burst mode after a fixed number of power
conversion cycles.
Power
Burst
VOUT < ~70%
rated output
Power Burst mode is entered periodically from Foldback mode in order to restart the
power supply output. In Power Burst mode, the controller operates at maximum
delivered power for a set number of power converter cycles. At the end of the burst, if
the load is not excessive, the converter goes to Normal mode; otherwise it reverts to
Foldback mode.
Table 2: Summary of Active Operating Modes
When the controller goes from Sleep to Start-up state, its power conversion mode is set to Standby. Typically
the converter output voltage is low at this time so the primary switch current is high during the first few
converter cycles. This causes the operating mode to change quickly to Normal or Overload mode.
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