MP020-5 – OFFLINE, PRIMARY-SIDE REGULATOR WITH CC/CV CONTROL AND A 700V FET

MP020-5 Rev. 1.05

www.MonolithicPower.com

14

1/23/2014

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*

+

VCC

CP

GND

MP020-5

Drain

FB

R

D

SN

+

-

Figure 11: RCD Snubber

Select

requirements and improve system operation.

The power dissipated in the snubber circuit is

approximately.

2

SN

SN

K

PK

S

SN

PS

O

V

1

PL I

f

2V

N

V

=⋅

⋅

⋅

×

−×

Where:

secondary side.

2

SN

SN

SN

V

R

P

=

The maximum ripple of the snubber capacitor

voltage is then:

SN

SN

SN

SN

S

V

V

CR

f

Δ=

⋅⋅

Generally, 15% ripple is reasonable, So the

less than 0.1ms for better CV sampling.

Therefore, adjust the resistor 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 200Ω to

500Ω to restrain the drain-voltage ringing.

Divided Resistor

For better application performance, the select

the resistor divider values from 10kΩ to 100kΩ

to limit noise from adjacent components on the

FB pin. If necessary, use a resistor between

1kΩ and 2kΩ connected between the FB pin

and resistor divider limit substrate-injection-

current effects, as shown in Figure 12.

Figure 12: Feedback Resistor Divider Circuit

For more accurate CV regulation, the accuracy

of these feedback resistors should be at least

1%.

Dummy Load

When system operates without any load and no

dummy load, the output voltage will rise above

normal operation because of the minimum

switching frequency limitation. Use a dummy

load for good load regulation. A large dummy

load will deteriorate efficiency and no-load

consumption, so the dummy load is tradeoff

between efficiency and load regulation. For

most applications, use a dummy load of around

10mW

as

it

also

satisfies

the

30mW

requirement.

Maximum Switching Frequency

Use a secondary-side diode conduction time

that exceeds 5.4µs, as per the following

equation.