MP024-10

– PRIMARY-SIDE CC/CV REGULATOR

MP024-10 Rev.1.0

www.MonolithicPower.com

20

4/17/2017

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RCD Snubber

The transfor

mer’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 (see Figure 13).

*

*

+

R

+

-

Figure 13: RCD Snubber

meet voltage spike 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

P

L I

f

2

V

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

C R f



(14)

Generally, a 15% ripple is reasonable.

using Equation (14).

The RCD resistor is a trade-off between 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.

Resistor Divider

For better application performance, select the

resistor divider

’s total value with in the range of

10kΩ to 100kΩ. Smaller resistors will draw

larger current from the auxiliary winding, which

increases the no-load consumption, and larger

resistors may pick up noise from adjacent

components.

If there is oscillation or noise disturbing the FB

sampling, an R-C filter can be inserted between

the resistor divider and FB to achieve a stable

substrate injection current effects (see Figure

14).

FB

Figure 14: 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 the 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 no-load consumption.

For most applications, a dummy load of several

mW is reasonable.