MP023 – PRIMARY-SIDE CC/CV CONTROLLER

MP023 Rev. 1.1

www.MonolithicPower.com

18

7/18/2016

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RCD Snubber

The transfomer’s leakage inductance causes

MOSFET drain voltage spikes and excessive

ringing on the drain voltage waveform, which

affects the output voltage sampling after the

primary MOSFET turns off.

The RCD snubber circuit limits the drain voltage

spike. Figure 14 shows the RCD snubber circuit.

Figure 14: RCD Snubber

requirements and improve system operation.

The power dissipated in the snubber circuit can

be approximated with Equation (12):

2

SN

SN

K PK

S

SN

PS

O

V

1

PL I

f

2V

N V





(12)

primary-to-secondary side.

2

SN

SN

SN

V

R

P



(13)

The maximum ripple of the snubber capacitor

voltage is then calculated with Equation (14):

SN

SN

SN

SN S

V

V

CR f



(14)

Generally, a 15% ripple is reasonable. The

previous equation can also be used to estimate

0.1ms for better CV sampling.

The RCD resistor is a trade-off based on the

power loss and the acceptable clamp voltage in

practical applications.

The damping resistor in series with the RCD

has a relatively large value to prevent any

excessive voltage ringing that can affect the CV

sampling and increase the output ripple. Use a

damping resistor value in the range of 100Ω to

500Ω to restrain the drain voltage ringing.

Divided Resistor

For better application performance, select the

resistor divider values from 10kΩ to 100kΩ to

limit noise from adjacent components on FB. An

RC filter can be inserted between the resistor

divider and FB to sense purified voltage. The

current effects (see Figure 15).

Figure 15: Feedback Resistor Divider Circuit

For accurate CV regulation, the accuracy of

these feedback resistors should be at least 1%.

Dummy Load

When the system operates without any load,

the output voltage rises above normal operation

because of the minimum switching frequency

limitation. Use a dummy load for good load

regulation. The dummy load is a trade-off

between efficiency and load regulation. For

example, a large dummy load can deteriorate

efficiency and the no-load consumption. For

most applications, several mWs for a dummy

load is reasonable.

Maximum Switching Frequency

The maximum switching frequency should be

limited by the sampling point. Figure 7 and EC

sampling point. The secondary on time must be