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VIPER25HD Datasheet(PDF) 20 Page - STMicroelectronics

Part No. VIPER25HD
Description  Off-line high voltage converters
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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VIPER25HD Datasheet(HTML) 20 Page - STMicroelectronics

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Operation description
VIPER25
20/40
Doc ID 15585 Rev 4
7.6
Quasi-resonant operation
The control core of the VIPER25 is a current-mode PWM controller with a the zero current
detection circuit designed for Quasi-Resonant (QR) operation, a technique that provides the
benefits of minimum turn-on losses, low EMI emission and safe behavior in case of short
circuit. At heavy load the converter operates in quasi-resonant mode: operation lies in
synchronizing MOSFET's turn-on to the transformer’s demagnetization by detecting the
resulting negative-going edge of the voltage across any winding of the transformer. The
system works close to the boundary between discontinuous (DCM) and continuous
conduction (CCM) of the transformer and the switching frequency will be different for
different line/load conditions. See the hyperbolic-like portion reported in Figure 27 on
page 21
.
At medium/ light load, depending also from the converter input voltage, the device enters in
Valley-skipping mode. The internal oscillator, synchronized to MOSFET’s turn-on, defines
the maximum operating frequency of the converter, FOSClim.
The VIPER25 is available as type ‘L’ or type ‘H’, depending from the value of FOSClim, see
Table 8 on page 8
. During the normal operation the converter works with a frequency below
FOSClim, so the ‘L’ type is suitable for application where the priority is on the EMI filter
minimization. The ‘H’ type is suitable when an extended QR operation range is a plus or the
priority is the transformer size reduction.
As the load is reduced, and the switching frequency tends to exceeds the limit FOSClim,
MOSFET’s turn-on will not any more occur on the first valley but on the second one, the third
one and so on, see Figure 29 on page 22. In this way a “frequency clamp” effect is achieved,
piecewise linear portion in Figure 27 on page 21.
When the load is extremely light or disconnected, the converter enters in burst mode
operation, see the relevant Section 7.14 on page 32. Decreasing the load will then result in
frequency reduction, which can go down even to few hundred hertz, thus minimizing all
frequency-related losses and making it easier to comply with energy saving regulations or
recommendations. Being the peak current low enough, no issue of audible noise.
The above mentioned way of operation is based on the ZCD pin. This pin is the input of the
integrated ZCD circuit which allows the power section turn-on at the end of the transformer
demagnetization. The input signal for the ZCD is obtained as a partition of the auxiliary
voltage used to supply the device, see Figure 28 on page 21.
When the integrated triggering circuit senses the negative going edge of the voltage VZCD,
going below the threshold VZCDTth, the power MOSFET is turned on with a delay that helps
to achieve the minimum drain-source voltage during the switch on. The mentioned triggering
circuit has to be previously armed by a positive going edge of the voltage VZCD, exceeding
the threshold VZCDAth. See the Table 8 on page 8.
After the MOSFET turn-off there is a typical noise generated by the transformer's leakage
inductance resonance ringing and coupled with the ZCD pin. The blanking time, TBLANK,
helps to filter this noise avoiding false triggers of the ZCD circuit.


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