MC33362

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10

The Power Switch is designed to directly drive the converter

transformer and is capable of switching a maximum of

500 V and 2.0 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 233 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 19. 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

mA, allowing it to be easily

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

applications that require secondary side voltage sensing,

Figure 22. In this application, the Voltage Feedback Input is

connected to the Regulator Output. This disables the Error

Amplifier by placing its output into the sink state, allowing

the optocoupler transistor to directly control the PWM

Comparator.

Overvoltage Protection

An Overvoltage Protection Comparator is included to

eliminate the possibility of runaway output voltage. This

condition can occur if the control loop feedback signal path

is broken due to an external component or connection

failure. The comparator is normally used to monitor the

exceeded, it will immediately turn off the Power Switch, and

protect the load from a severe overvoltage condition. This

input can also be driven from external circuitry to inhibit

converter operation.

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

MC33362. 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 14.5 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.

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

short−circuit protection. This output requires an external

bypass capacitor of at least 1.0

mF for stability.

Thermal Shutdown and Package

Internal thermal circuitry is provided to protect the Power

Switch in the event that the maximum junction temperature

is exceeded. When activated, typically at 155

°C, the Latch

is forced into a ‘reset’ state, disabling the Power Switch. The

Latch is allowed to ‘set’ when the Power Switch temperature

falls below 145

°C. This feature is provided to prevent

catastrophic failures from accidental device overheating. It

is not intended to be used as a substitute for proper

heatsinking.

The MC33362 is contained in a heatsinkable plastic

dual−in−line package in which the die is mounted on a special

heat tab copper alloy lead frame. This tab consists of the four

center ground pins that are specifically designed to improve

thermal conduction from the die to the circuit board. Figures

17 and 18 show a simple and effective method of utilizing the

printed circuit board medium as a heat dissipater by soldering

these pins to an adequate area of copper foil. This permits the

use of standard layout and mounting practices while having

the ability to halve the junction to air thermal resistance. The

examples are for a symmetrical layout on a single−sided

board with two ounce per square foot of copper. Figure 23

shows a practical example of a printed circuit board layout

that utilizes the copper foil as a heat dissipater. Note that a

jumper was added to the layout from Pins 8 to 10 in order to

enhance the copper area near the device for improved thermal

conductivity. The application circuit requires two ounce

copper foil in order to obtain 20 watts of continuous output

power at room temperature.