MC33365

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9

The Power Switch is designed to directly drive the converter

transformer and is capable of switching a maximum of

700 V and 1.0 A. Proper device voltage snubbing and

heatsinking are required for reliable operation.

A Leading Edge Blanking circuit was placed in the current

sensing signal path. This circuit prevents a premature reset

of the PWM Latch. The premature reset is generated each

time the Power Switch is driven into conduction. It appears

as a narrow voltage spike across the current sense resistor,

and is due to the MOSFET gate to source capacitance,

transformer interwinding capacitance, and output rectifier

recovery time. The Leading Edge Blanking circuit has a

dynamic behavior in that it masks the current signal until the

Power Switch turn−on transition is completed. The current

limit propagation delay time is typically 262 ns. This time is

measured from when an overcurrent appears at the Power

Switch drain, to the beginning of turn−off.

Error Amplifier

An fully compensated Error Amplifier with access to the

inverting input and output is provided for primary side

voltage sensing, Figure 17. It features a typical dc voltage

gain of 82 dB, and a unity gain bandwidth of 1.0 MHz with

78 degrees of phase margin, Figure 6. The noninverting

input is internally biased at 2.6 V ±3.1% and is not pinned

out. The Error Amplifier output is pinned out for external

loop compensation and as a means for directly driving the

PWM Comparator. The output was designed with a limited

sink current capability of 270 μA, allowing it to be easily

overridden with a pull−up resistor. This is desirable in

applications that require secondary side voltage sensing.

Bulk Capacitor Voltage Comparator

In order to avoid output voltage bouncing during

electricity brownout condition, a Bulk Capacitor Voltage

Comparator with programmable hysteresis is included in

this device. The non−inverting input, pin 11, is connected to

shown in Figure 20 monitoring the bulk capacitor voltage

level. The inverting input is connected to a threshold voltage

of 1.25 V internally. As bulk capacitor voltage drops below

the pre−programmed level, (Pin 11 drops below 1.25 V), a

reset signal will be generated via internal protection logic to

the PWM Latch so turning off the Power Switch

immediately. An internal current source controlled by the

state of the comparator provides a means to program the

voltage hysteresis. The following equation shows the

network resistors.

RUpper + 20

[VBulk_H * VBulk_L ]

in K Ohm

RLower +

25

[VBulk_H * VBulk_L ]

VBulk_H * 1.25

in K Ohm

Figure 20. Bulk OK Functional Operation

BOK

11

50 mA

1.25 V

Protection Logic

Undervoltage Lockout

An Undervoltage Lockout comparator has been

incorporated to guarantee that the integrated circuit has

sufficient voltage to be fully functional before the output

voltage at Pin 3 and when it exceeds 14.5 V, the reset signal

is removed from the PWM Latch allowing operation of the

Power Switch. To prevent erratic switching as the threshold

is crossed, 5.0 V of hysteresis is provided.

Startup Control

An internal Startup Control circuit with a high voltage

enhancement mode MOSFET is included within the

MC33365. This circuitry allows for increased converter

efficiency by eliminating the external startup resistor, and its

associated power dissipation, commonly used in most

off−line converters that utilize a UC3842 type of controller.

Rectified ac line voltage is applied to the Startup Input,

Pin 1. This causes the MOSFET to enhance and supply

reaches the UVLO upper threshold of 15.2 V, the IC

commences operation and the startup MOSFET is turned

off. Operating bias is now derived from the auxiliary

transformer winding, and all of the device power is

efficiently converted down from the rectified ac line.

The startup MOSFET will provide a steady current of

The startup MOSFET is rated at a maximum of 400 V with

capacitor of 1000 μF or less.

Regulator

A low current 6.5 V regulated output is available for

biasing the Error Amplifier and any additional control

system circuitry. It is capable of up to 10 mA and has