QPI-8LZ-01【PDF 8/17 页】IC HOT SWAP EMI FILTER 16LGA

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Sheet目录46

4页 5页 6页 7页 [8页]9页 10页 11页 12页

Picor Corporation ?picorpower.com

QPI-8

Rev 1.5, Page 8 of 17

QPI-8

QUIETPOWER

?/DIV>

Figure 15 - Simplified Hi-Voltage Boost Circuit for Charging Bulk Storage Capacitors.

The circuit in Figure 15 shows a simplified version of a

PWRGD enabled boost circuit. The bulk storage capacitance

(CBULK) is initially charged to BUS+, along with the CIN

capacitor, through the body diode of the PFET. Once these

capacitors are fully charged, PWRGD enables the boost

converter to slowly charge the bulk capacitance to the

desired hi-voltage. When a low QPI+ voltage is detected, the

BUS VOLTAGE MONITOR circuit enables the PFET to discharge

some of its energy into the load converters CIN capacitor and

to the load. When the QPI+ rail reaches a pre-determined

limit, it shuts off the PFET and the rail starts to drop again.

This circuit acts much like a ripple regulator and will maintain

the output load until either the BUS voltage is restored or the

energy of the bulk capacitor is depleted.

The QPI-8 has a current sensing circuit that shuts off the

internal pass FET if a high di/dt is sensed across its internal

inductor. To avoid triggering this function, the PFET should be

turned on in its linear region. Typically, a zener diode can be

used to regulate the V

G-S

of the PFET and the amount of

energy discharged from the bulk capacitance. The waveforms

in Figure 16 demonstrate a zero-volt bus transient and the

response of the hi-voltage boost circuit. The storage caps

provide energy to sustain the load, while the bus is at zero for

5ms. The input current (green, CH4) drops to zero while the

load current (violet, CH3) remains constant. The voltage

across the bulk storage caps (light blue, CH2) slowly decays

while input voltage to the converter (blue, CH1) is regulated.

It is critical to keep the load current on the converters input

capacitor to less than 25mA during the initial start-up. This

limit is set by the current limit level and the duty cycle of the

circuit breaker timer, which is about 99% off time. An

excessive load will discharge the bulk capacitors during the

off time to a level where the on time charging current will

never charge the capacitor to the full bus voltage. Only after

Powergood has been established should the load be enabled.

Figure 16 - 5ms, 0V transient with 200W Load. CH1 and CH2

are offset by 50V DC.

PI+PI-

PI+PI-BOOST

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