MC33363A
www.onsemi.com
10
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 300 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
primary side V
CC
voltage. When the 2.6 V threshold is
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 (UVLO) comparator has been
incorporated to guarantee that the integrated circuit has
sufficient voltage to be fully functional before the output
stage is enabled. The UVLO comparator monitors the V
CC
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
MC33363A. 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
internal bias as well as charge current to the V
CC
bypass
capacitor that connects from Pin 3 to ground. When V
CC
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.
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 MC33363A 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 8.0 W of continuous
output power at room temperature.
