UM0920
Board details
Doc ID 17220 Rev 2
7/14
3.2
Description of main components
The complete converter application consists of an input section and the flyback converter
itself.
The input section contains:
–
Single diode rectifier (D3, D6)
The single diode rectifier is selected to allow simple connection of neutral to GND
and reduce components count.
–
Inrush current limiter (R1)
This component reduces the inrush current caused by connection of the SMPS to
mains (charging current of bulk capacitors) or by surge pulses.
–
EMI filter (L1, C4, C5)
This basic EMI filter using bulk capacitors and a simple inductor can be applied
due to the single rectifier.
–
Input bulk capacitors (C4, C5)
These capacitors store energy when the input AC voltage is low.
The flyback converter consists of:
–
VIPer16 (U1)
This device integrates a controller and high voltage power MOSFET in one
package. The controller works in current mode with a fixed frequency, in
discontinuous mode. Thanks to the built-in error amplifier, the VIPer16 can directly
sense the output voltage via a voltage divider. Mandatory components for proper
operation of the VIPer16 are V
DD
capacitor C10 and compensation network R4,
C8 and C9. Diode D1 is not mandatory but it connects the output voltage to V
CC
of
the Viper16, reducing standby consumption.
–
Peak clamp
(D5, R2, R7, C2)
This circuit absorbs energy from the voltage spike present after MOSFET turn-off.
This spike is generated by leakage inductance of the transformer.
–
Secondary side (D2, D4, C1, C7)
This consists of two rectifiers and capacitors for each output. The -5 V output is
used for feedback regulation. The main benefit of this solution is the connection of
the source of the VIPer16 to -5 V. This allows direct sensing of -5 V from the output
and simplifies the circuit. Another benefit of this method is the possibility to supply
the VIPer16 from 12 V secondary winding (7 V to -5 V). This configuration results
in a significant reduction in component count and, thanks to the internal error
amplifier, permits reaching a low tolerance of -5 V. The special feature of this
configuration is the fact that the current flowing from the input bulk capacitor
through the primary side of the transformer and power MOSFET is closed back to
the input capacitor via secondary side capacitor C7, and this capacitor is partly
discharged during the ON time. Due to this effect, the use of a low ESR capacitor
for C7 is highly recommended.
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