MP4026 – PRIMARY SIDE CONTROL, OFFLINE LED CONTROLLER WITH ACTIVE PFC

MP4026 Rev. 1.1

www.MonolithicPower.com

14

9/9/2015

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NON-ISOLATED APPLICATIONS

The isolated solution can prevent human body

from an electric shock by grid when touching

the load. But the power loss and the cost are

increased. Recur to safety enclosure frame of

the lamp, compared with isolated solution, the

non-isolated

solution

can

achieve

higher

efficiency and highly cost-effective.

Generally, the Flyback converter is common for

the offline isolated applications. As topology

transmutation, the non-isolated low-side Buck-

boost converter is also popular. Besides fitting

in isolated application, the MP4026 can also

operate in the offline non-isolated LED lighting

applications.

Figure 16 is a 30W low-side Buck-boost LED

driver with MP4026.

Operation of Low-side Buck-boost

The low-side Buck-boost can be treated as

Flyback converter with 1:1 turn ratio transformer.

So, the whole operation is absolutely same as

the description above. Different from isolated

solution, there are no separate primary- and

secondary-winding, so a smaller core size is

available for design. Without the impact of the

leakage

inductance,

the

snubber

is

unnecessary. All of these can save cost and

improve 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

CY1 in Figure 16 is added for RF EMI

improvement. The recommended value is from

10nF to 47nF with 630V rating.

Improvement of PFC & THD

The impact of non-turn-ratio is that the duty

cycle of the converter becomes smaller at same

spec. Based on MP4026 PFC principle, the PF

and THD of the converter drops compared with

isolated solution. So, generally, the non-isolated

solution is especially suitable for high output

voltage, since the higher output voltage can

extend the duty cycle to improve the PF and

THD and the efficiency can also be improved

meanwhile.

For the non-isolated solution with low output

voltage, the tapped-inductor can be applied to

improve the PF and THD.

Figure 11: Tapped-inductor for Low-side Buck-

boost Solution

Shown in Figure 11, 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 thru both of

the windings. But when the power FET is off,

just N1 conducts the current thru the rectifier

diode. The stored energy of N2 is released by

flux couple. So, the tapped-inductor features a

similar turn-ratio like the transformer in isolated

solution.

The nominal turn-ratio is

+

=

>

N1 N2

n1

N1

The duty cycle of the converter is obviously

extended by tapped-inductor, and then the

better PF and THD are available.

But, like transformer, the snubber is necessary

to clamp the voltage spike caused by leakage

inductance.