Rainbow Electronics MAX15001 User Manual

Page 13

Choose a higher value for R1 than the one calculated in
the previous equation if a longer startup time can be
tolerated to minimize power loss on this resistor.

The above startup method is applicable to a circuit simi-
lar to the one shown in Figure 1. In this circuit, the ter-
tiary winding has the same phase as the output
windings. Thus, the voltage on the tertiary winding at any
given time is proportional to the output voltage and goes
through the same soft-start period as the output voltage.
The minimum discharge time of C1 from 21.6V to 9.74V
must be greater than the soft-start time of 5.6ms.

Another method for bootstrapping the power supply is
to use a bias winding that is in phase with the MOSFET
on-time (see Figure 5). In this case, the amount of
capacitance required at IN (C1) is much smaller.
However, the input voltage cannot have a range
greater than approximately 2:1 (primary winding volt-
age to bias winding voltage ratio).

For hiccup-mode fault protection, make the bias wind-
ing in phase with the output, then the power-supply hic-
cups and soft-starts under output short-circuit
conditions. The power supply does not hiccup if the
bias winding is in phase with the MOSFET on-time.

MAX15000/MAX15001

Current-Mode PWM Controllers with

Programmable Switching Frequency

______________________________________________________________________________________

COMP

GND

NDRV

UVLO/EN

OPTO

TRANS.

UFLG

R15

R12

MAX15000A

TL431

OPTO LED

OUT

R10

Figure 5. Secondary-Side Regulated, Isolated Power Supply