HF900 – 900V OFFLINE SWITCHING REGULATOR

HF900 Rev. 1.0

www.MonolithicPower.com

13

8/4/2015

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OPERATION

The HF900 integrates a 900V MOSFET for a

reliable switch-mode power supply solution. It has

burst-mode operation to minimize the standby

power consumption at light load. Protection

features such as auto-recovery for overload

protection (OLP), short-circuit protection (SCP),

over-voltage

protection

(OVP),

and

thermal

shutdown for over-temperature protection (OTP)

contribute to a safer converter design with minimal

external components.

PWM Operation

The HF900 employs peak-current-mode control.

On the secondary side, the output voltage is

divided by a voltage divider network. This voltage is

fed back to the primary side as voltage on the FB

using an optocoupler and a shunt regulator. The

which measures the MOSFET switching current.

The integrated MOSFET turns on at the beginning

of each clock cycle. The current in the transformer

magnetizing inductance increases until it reaches

the value set by the FB voltage, and then the

integrated MOSFET turns off.

Start-Up and VCC UVLO

Initially, the IC is driven by the internal current

source, which is drawn from the high-voltage

DRAIN. The IC starts switching, and the internal

high-voltage current source turns off as soon as

the supply of the IC is taken over by the auxiliary

winding of the transformer. When VCC falls

the internal high-voltage current source turns on

again (see Figure 2).

Driver

Switching Pluses

Auxiliary Winding Takes Charge

VCC

Figure 2: VCC Start-Up

The lower threshold of VCC UVLO decreases from

OLP, OVP, and OTP occur.

Soft Start

The HF900 implements an internal soft-start

circuit to reduce stress on the primary-side

MOSFET and the secondary diode and smoothly

establish the output voltage during start-up. The

internal soft-start circuit increases the primary

current

sense

threshold

gradually,

which

determines the MOSFET peak current during

start-up. The pulse width of the power switching

device is increased progressively to establish

correct operating conditions until the feedback

control loop takes charge (see Figure 3).

Figure 3: Soft Start

Switching Frequency

The switching frequency of the HF900 can be set

by FSET. The frequency can be set by a resistor

between

FSET

and

GND.

The

oscillator

frequency can be attained using Equation (1):

S

912

FSET

FST

1

f

R

200 10

112.5 10

V

−−

=

×+

×

×

Hz

(1)

Over-Voltage Protection (OVP)

Monitoring the VCC voltage via a 20µs time

constant filter allows the HF900 to enter OVP

during an over-voltage condition, typically when

operation once the fault disappears.