LM5015

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SNVS538C – NOVEMBER 2007 – REVISED APRIL 2013

Power MOSFETs

The LM5015 switching regulator includes two N-Channel MOSFETs each with 450 m

Ω nominal on-resistance.

The drain of the high side MOSFET is the PVIN pin, and the source the HO pin. The drain of the low side

MOSFET is the LO pin, and the source is internally connected to the PGND pin via the 42 m

Ω internal current

sense resistor. The on-resistance of the LM5015 MOSFETs varies with temperature as shown in the Typical

Performance Characteristics graph. The typical gate charge for each MOSFETs is 4.5 nC which is supplied from

the VCC and BST pins, respectively, when the MOSFETs are turned on.

The maximum duty cycle of the power MOSFETs is limited less than 50%. This is achieved by an oscillator

divide-by-two circuit with an additional 50 ns of forced off-time introduced between the CLK and RS Flip-Flop.

Consequently, the maximum duty cycle is limited by the following equation:

DutyMax_Limit=(0.5 - 50 ns x FSW) x 100%

where

•

FSW is the switching frequency in Hertz (Hz)

(2)

The purpose of limiting the maximum duty cycle less than 50% is to ensure successful reset of the power

transformer in the Two-Switch Forward converter topology. See Application Information below for more detail.

Application Information

The following information is intended to provide guidelines for the power supply designer using the LM5015.

TWO-SWITCH FORWARD TOPOLOGY

Two-Switch Forward converter, like the conventional Single-Switch Forward converter, is derived from the Buck

converter topology. The main difference between a Forward converter and a Buck is that a power transformer is

introduced in the forward converter. The transformer realizes the input-output isolation, and the turns ratio

provides a means to optimize the duty cycle for the particular input and output voltage requirements of the

application.

The Two-Switch Forward converter employs two power MOSFET switches instead of the one switch of the

Single-Switch Forward converter. However the two-switch approach offers two major advantages over its single-

switch counterpart:

1. The voltage across the power MOSFET switches in a Two-Switch Forward converter is clamped to the input

voltage, allowing the input voltage range to approach the rating of the MOSFETs. Whereas, the maximum

operating voltage of a Single-Switch Forward converter is typically limited to half the MOSFET voltage rating.

2. The power transformer of a Two-Switch Forward converter is simpler, and hence costs less, than that of a

Single-Switch Forward converter, because the Two-Switch converter transformer eliminates the tertiary reset

winding that is normally required in the Single-Switch converter.

Figure 16 illustrates the Two-Switch Forward converter topology. The power circuit consists of an input capacitor

CIN, two MOSFET switches QH and QL, two clamp diodes DH and DL, a power transformer T1, two rectifier diodes

D1 and D2, an output inductor LO, and an output capacitor Co. Since the LM5015 integrates both QH and QL, a

low cost Two-Switch Forward converter can be realized without a need for discrete power MOSFETs. With a

slightly higher cost, the two rectifier diodes D1 and D2 on the secondary side of the power transformer can be

replaced with synchronous rectifier MOSFETs to improve efficiency in applications with relatively low output

voltage.

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