CM6800 查看數據表（PDF） - Champion Microelectronic

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CM6800

LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO

Champion Microelectronic 

CM6800

LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO

where CSS is the required soft start capacitance, and the

tDEALY is the desired start-up delay.

It is important that the time constant of the PWM soft-start

allow the PFC time to generate sufficient output power for

the PWM section. The PWM start-up delay should be at

least 5ms.

Solving for the minimum value of CSS:

CSS = 5ms x 20μA = 80nF

1.25V

Caution should be exercised when using this minimum soft

start capacitance value because premature charging of the

SS capacitor and activation of the PWM section can result if

VFB is in the hysteresis band of the VIN OK comparator at

start-up. The magnitude of VFB at start-up is related both to

line voltage and nominal PFC output voltage. Typically, a

1.0 μ F soft start capacitor will allow time for VFB and PFC

out to reach their nominal values prior to activation of the

PWM section at line voltages between 90Vrms and

265Vrms.

Generating VCC

After turning on CM6800 at 13V, the operating voltage can

vary from 10V to 17.9V. The threshold voltage of VCC OVP

comparator is 17.9V. The hysteresis of VCC OVP is 1.5V.

When VCC see 17.9V, PFCOUT will be low, and PWM

section will not be disturbed. That’s the two ways to

generate VCC. One way is to use auxiliary power supply

around 15V, and the other way is to use bootstrap winding

to self-bias CM6800 system. The bootstrap winding can be

either taped from PFC boost choke or from the transformer

of the DC to DC stage.

The ratio of winding transformer for the bootstrap should be

set between 18V and 15V. A filter network is recommended

between VCC (pin 13) and bootstrap winding. The resistor of

the filter can be set as following.

RFILTER x IVCC ~ 2V, IVCC = IOP + (QPFCFET + QPWMFET ) x fsw

IOP = 3mA (typ.)

If anything goes wrong, and VCC goes beyond 17.9V, the

PFC gate (pin 12) drive goes low and the PWM gate drive

(pin 11) remains function. The resistor’s value must be

chosen to meet the operating current requirement of the

CM6800 itself (5mA, max.) plus the current required by the

two gate driver outputs.

EXAMPLE:

With a wanting voltage called, VBIAS ,of 18V, a VCC of 15V

and the CM6800 driving a total gate charge of 90nC at

100kHz (e.g. 1 IRF840 MOSFET and 2 IRF820 MOSFET),

the gate driver current required is:

IGATEDRIVE = 100kHz x 90nC = 9mA

RBIAS = VBIAS − VCC

ICC + IG

RBIAS = 18V − 15V

5mA + 9mA

Choose RBIAS = 214Ω

The CM6800 should be locally bypassed with a 1.0 μ F

ceramic capacitor. In most applications, an electrolytic

capacitor of between 47 μ F and 220 μ F is also required

across the part, both for filtering and as part of the start-up

bootstrap circuitry.

2008/10/23 Rev. 2.1

Champion Microelectronic Corporation

Page 13

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