SD4851 is designed for off-line SMPS, consisting of high voltage MOSFET, cable compensation and peak
current compensation without opto-coupler. SD4851 provides constant voltage/constant current (CV/CC) output
through primary current and auxiliary voltage. It adopts PFM technology and accurate constant voltage/constant
current (CV/CC) control loop for higher stability and efficiency.
A whole work period of SD4851 can be divided into two period sections:
When MOSFET is on, primary current is detected by sample resistor and voltage at pin FB is negative, load is
powered by output capacitor and output voltage VO decreases.
When primary current exceeds the limit, MOSFET is off and voltage at pin FB is detected. Output capacitor is
powered by secondary current and VO increases. MOSFET is on again after stop for TCV and hold for TCC.
And then, it comes to peak current detect again.
1. Under Voltage Lockout and Self-Start
At the beginning, the capacitor connected to pin VCC is charged via start resistor by high voltage DC bus and the
circuit start to work if voltage at Vcc is 16V. The output is shutdown if there is any protection during normal
operation and Vcc is decreased because of no powering of auxiliary winding. The whole control circuit is
shutdown if voltage at Vcc is 8V below to lower current dissipation and the capacitor is recharged for restarting.
2. Driver Circuit
The driver circuit is powered by VCC.
When VGATE＝1, the power MOSFET is on. When VGATE＝0, the power MOSFET is off. It is recommended to
set TLEB＝0.7μs to avoid error trigger caused by burr.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
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