MPQ18913

– 30V, 0.3A High Frequency Transformer Driver

PRELIMINARY SPECIFICATIONS SUBJECT TO CHANGE

MPQ18913 Rev. 0.8

MonolithicPower.com

12

12/8/2021

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

Preliminary Specifications Subject to Change

© 2021 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Selecting the Rectification Diodes

Schottky diodes are recommended to minimize

power losses and help achieve ZVS of MPQ18913,

because Schottky diodes feature low forward

voltage drop and fast recovery.

The current rating of the diodes can be calculated

with Equation (1):

Ipk≈π×Iload

(1)

Selecting the Output Capacitor

The output voltage ripple is determined by the

value of capacitance and the gate charge of power

device, e.g. SiC or IGBT. To maintain an

acceptable voltage ripple,

Vripple , the output

capacitance for the two output voltage rails can be

estimated with Equation (2):

Cout, positive rail=Cout, negative rail≥

Qg

Vripple

(2)

If the capacitance of each rail needs to be larger

than 22uF, MPQ18913A should be selected

instead of MPQ18913 due to its longer soft start

time.

Selecting the Switching Frequency

Higher switching frequency can reduce the size of

transformer, but it will lead to larger transformer

windings AC resistance, which limits the output

power. The recommended switching frequency is

as Equation (3):

Fsw= {

0.4MHz-0.75MHz, Pout≤6W

0.75MHz-2MHz, Pout≤3W

1.5MHz-5MHz, Pout≤2W

(3)

To setup the switching frequency, the FREQ

resistor can be calculated as

RFREQ=

100kΩ

Fsw(MHz)

(4)

Selecting the Magnetizing Inductance of the

Transformer

The design target of magnetizing inductance

based on ZVS achievement can be calculated

from equation (5).

Lm=

Dead Time

8×CSW×FSW

=

25ns

8×0.17nF×Fsw

(5)

If the magnetizing inductance is larger than the

target, it does not have enough magnetizing

current to achieve full ZVS. With the low input

voltage and small parasitic capacitance on the

switcher, partial ZVS will not raise concerns in

thermal or electrical stress. If the magnetizing

inductance is smaller than the design target, it will

lead to more magnetizing current than needed,

which results in extra conduction loss. Given the

small

Rds,on and a reasonable ACR of transformer,

the conduction loss will not raise thermal concerns

as well.

It

is

recommended

that

the

magnetizing

inductance be more than 10 times larger than

leakage inductance, in order to decrease the

output voltage’s sensitivity to the tolerance of

inductance and resonant capacitance.

Selecting the Resonant Capacitor

The value of resonant capacitor can be calculated

with equation (6):

Cr=

1

4×π2×Lr×Fsw

2

(6)

Where the

Lr is the leakage inductance of the

transformer, reflected to the primary side.