MP4027 – PRIMARY-SIDE CONTROL, OFFLINE LED CONTROLLER WITH ACTIVE PFC

MP4027 Rev. 1.02

www.MonolithicPower.com

14

12/9/2014

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NON-ISOLATED APPLICATIONS

Although isolated solutions can prevent electric

shock from the grid when touching the load,

they cause power loss and increase costs.

Non-isolated solutions achieve higher efficiency

and are highly cost-effective.

Generally, the flyback converter is used for

offline, isolated applications. For the non-

isolated applications, a low-side buck-boost

topology is used. The MP4027 can operate in

both

offline isolated and non-isolated LED-

lighting applications (see Figure 19).

Operation of Low-Side Buck-Boost

The low-side buck-boost equates to a flyback

converter with a 1:1 turn ratio transformer. As

opposed to an isolated solution, there is not a

separate primary and secondary winding,

making a smaller core size. This saves cost and

improves the efficiency of the driver.

The Selection of FET & Rectifier Diode

Since it is just an inductor for non-isolated

solution, compared with isolated solution, at

same output voltage, the power FET can be

selected

with

lower

voltage

rating.

But,

oppositely, the voltage rating of rectifier diodes

for output and aux-winding must be increased.

Improvement of RF EMI

C12 in Figure 19 is added for RF EMI

improvement. The recommended value is from

10nF to 68nF with 630V rating.

Improvement of PFC & THD

The 1:1 ratio reduces the converter’s duty cycle

using the same specifications. Based on the

PFC principle in an isolated solution, the

converter’s PF and THD drops. A non-isolated

solution is suitable particularly for high-output

voltage since the higher output voltage can

extend the duty cycle to improve PF, THD and

efficiency.

For a non-isolated solution with low-output

voltage, the tapped inductor can be applied to

improve the PF and THD.

BUS

CS

GATE

N1

N2

Figure 13: Tapped-Inductor for Low-Side Buck-

Boost Solution

Shown in Figure 13, the tapped-inductor

includes two windings (N1 & N2) and a tap to

connect the rectifier diode. When the power

FET is turned on, the current goes through both

of the windings. When the power FET is off,

only N1 conducts the current through the

rectifier diode. The stored energy of N2 is

released by flux couple. The tapped inductor

features a turn ratio similar to the transformer in

an isolated solution.

The nominal turns ratio is







N1 N2

n1

N1

The duty cycle of the converter is extended by

the tapped inductor, which makes the improved

PF and THD available.

Like the transformer, the snubber is necessary

to clamp the voltage spike.

However, the non-dimmable solution usually

needs to cover the universal input range. The

input range is very wide, from 85VAC to

264VAC. MULT is used to detect the input-

voltage signal, but the resistor divider of MULT