Light Load Mode

In no load or light load condition, major power loss of total power consumption is from switching loss on the

MOSFET transistor switching loss, the core loss of the transformer loss and the loss on the external snubber

circuit loss become the majority in total power loss. The value of those power loss is proportional to switching

actions within a fixed period of time. So reducing number of switching actions leads to reduction of power loss.

SD4870 enters Light Load Mode in no load or light load condition. The gate drive output switches only when

output DC voltage drops below a present level and the switching frequency reduces. Otherwise the gate drive

remains at off state.

Oscillation Frequency Setting

The oscillation frequency is determined by resistor connected between RI and GND. The relationship between

the value of this resistor and frequency are shown below

(kHz)

R

6500

f

RI

Current Sense and LEB

At switching leading edge time, the current spike due to Snubber diode reverse recovery should be chopped off.

And this is available through internal LEB (Leading Edge Blanking) circuit. So that the external RC filter circuit on

SENSE input is no longer required.

During the blanking period, the PWM comparator and OC comparator are disabled and MOSFET transistor

keeps turn-on state if MOSFET turns on. The minimum on time of MOSFET is LEB time.

Gate Drive

GATE pin is connected to external MOSFET’s gate for switch control. Too weak the gate drive ability results in

more switch loss of MOSFET while too strong gate drive compromises the EMI performance.

A good tradeoff is achieved through the totem pole gate drive design with appropriate output ability and dead

time control.

Protection controls

SD4870 offers complete protection functions including cycle-by-cycle over current protection, over load protection,

Constant output power limit over universal input voltage range is achived with over current protection threshold

line voltage compensation to over current protection threshold.

startup sequence thereafter.

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

REV:1.0

2009.09.22

Http://www.silan.com.cn

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