HV9805 datasheet(21/34 Pages) MICROCHIP

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 2015 Microchip Technology Inc.

DS20005374A-page 21

HV9805

The average value of a triangular-shaped current is by

nature half the peak amplitude. A low-pass filter and a

bridge rectifier shape the triangular boost inductor

current into a more or less ripple-free line current, with

an amplitude which is equal to half of the peak inductor

current.

Whereas the on-time period is constant, the off-time

period changes throughout the line cycle, with the

off-time becoming longer as the line voltage increases

toward the bus voltage. Accordingly, the switching

frequency varies over the line cycle, being lowest at

peak line voltage and highest near the zero crossing of

the line voltage.

4.11.1

VALLEY SWITCHING

The

driver

incorporates

valley

switching

(quasi-resonant switching), a technique for reducing

switching loss at the turn-on event of the boost FET.

Valley switching adds a third part to the basic boost

inductor waveform where the drain voltage is allowed

to swing down towards ground, as shown in Figure 4-3.

FIGURE 4-3:

Valley Switching Waveform Diagram.

Negative

Polarity Strobe

Valley Strobe

Boost

Inductor

Current

Drain

Voltage

1.25V

Current Sense

Amplifier Output

1.25V

RCSA

RLBS

RCSA

Current

Sense

Amplifier

Polarity

Detector

Valley

Strobe

Generator

IBST

VDRN

VREC

HV9805

~625 mV

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