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APU3039

SYNCHRONOUS PWM CONTROLLER WITH OVER CURRENT PROTECTION

Advanced Power Electronics Corp 

Advanced Power

Electronics Corp.

APU3039

IRMS = IOUT D(1-D)

---(9)

Where:

D is the Duty Cycle, D=VOUT/VIN.

IRMS is the RMS value of the input capacitor current.

IOUT is the output current for each channel.

For VIN=20V, IOUT=8A and D=0.165, the IRMS=3A

For higher efficiency, a low ESR capacitor is recom-

mended. Choose three Poscap from Sanyo 25TQC15M

(25V, 15µF, 90mΩ) with a maximum allowable ripple

current of 3A.

Inductor Selection

The inductor is selected based on operating frequency,

transient performance and allowable output voltage ripple.

Low inductor value results to faster response to step

load (high ∆i/∆t) and smaller size but will cause larger

output ripple due to increase of inductor ripple current.

As a rule of thumb, select an inductor that produces a

ripple current of 10-40% of full load DC.

For the buck converter, the inductor value for desired

operating ripple current can be determined using the fol-

lowing relation:

VIN

-

VOUT

=

Lx

∆i

∆t

;

∆t

=

D

x

1

fS

L

=

(VIN

-

VOUT)

x

VOUT

VINx∆ixfS

VOUT

; D = VIN

---(11)

Where:

VIN = Maximum Input Voltage

VOUT = Output Voltage

∆i = Inductor Ripple Current

fS = Switching Frequency

∆t = Turn On Time

D = Duty Cycle

If ∆i = 37%(IO), then the output inductor will be:

L = 4.65µH

The Coilcraft DO5022HC series provides a range of in-

ductors in different values, low profile suitable for large

currents, 4.7µH, 13A is a good choice for this applica-

tion. This will result to a ripple approximately 37% of

output current.

Output Capacitor Selection

The criteria to select the output capacitor is normally

based on the value of the Effective Series Resistance

(ESR). In general, the output capacitor must have low

enough ESR to meet output ripple and load transient

requirements, yet have high enough ESR to satisfy sta-

bility requirements. The ESR of the output capacitor is

calculated by the following relationship:

ESR [

∆VO

∆IO

---(10)

Where:

∆VO = Output Voltage Ripple

∆i = Inductor Ripple Current

∆VO = 100mV and ∆I ≅ 40% of 8A = 3.2A

This results to: ESR=31mΩ

The Sanyo TPC series, Poscap capacitor is a good choice.

The 6TPC330M, 330µF, 6.3V has an ESR 40mΩ. Se-

lecting two of these capacitors in parallel, results to an

ESR of ≅ 20mohm which achieves our low ESR goal.

The capacitor value must be high enough to absorb the

inductor's ripple current. The larger the value of capaci-

tor, the lower will be the output ripple voltage.

Power MOSFET Selection

The APU3039 uses two N-Channel MOSFETs. The se-

lections criteria to meet power transfer requirements is

based on maximum drain-source voltage (VDSS), gate-

source drive voltage (VGS), maximum output current, On-

resistance RDS(ON) and thermal management.

The MOSFET must have a maximum operating voltage

(VDSS) exceeding the maximum input voltage (VIN).

The gate drive requirement is almost the same for both

MOSFETs. Logic-level transistor can be used and cau-

tion should be taken with devices at very low VGS to pre-

vent undesired turn-on of the complementary MOSFET,

which results a shoot-through current.

The total power dissipation for MOSFETs includes con-

duction and switching losses. For the Buck converter,

the average inductor current is equal to the DC load cur-

rent. The conduction loss is defined as:

PCOND(Upper Switch) =2 ILOADxRDS(ON)xDxθ

PCOND(Lower Switch) =2 ILOADxRDS(ON)x(1 - D)xθ

θ = RDS(ON) Temperature Dependency

The RDS(ON) temperature dependency should be consid-

ered for the worst case operation. This is typically given

in the MOSFET data sheet. Ensure that the conduction

losses and switching losses do not exceed the package

ratings or violate the overall thermal budget.

9

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