1997 4-191
Application Hint 11 Micrel
4
V
REF
C3
3300pF
200V C7, C8, C9
3 x 1200µF
10V
R11
200Ω
V
OUT
+5V, 6A
D6
MBR1045
L1
10µH
C10
470µF
10V
C12
0.1µF
C6
1000pF
C11
1000pF
R15
1k
1/2 IC3
4N35
IC2
TL431A
R12
4.99k 1%
+5V
R13
4.99k 1%
R14
47k
R10
10Ω
1/2W
IC1
MIC38HC43
V
DD
V
REF
GND
COMP
V
OUT
R
T
/C
T
FB
I
S
2
1
4
3
8
5
7
6
R9
20k
2W
D2
MUR160
R1
330k
R2
330k
D3
MUR115
R8 33Ω
C2
22µF
25V
D4
16V
R6
22ΩD5
MBR030
C5
1000pF
R5
2.2k C4
0.1µF Q1
IRF830
CT1
R7
6.8Ω
R4
1.2k
R3
1.2k
D1
3N250
RT1 10Ω, 1A
C1
100µF
400V
EMI
FILTER
1/2 IC3
4N35
F1
85—264Vac
Line
Neutral
Return
T1
(LP = 40µH)
1
2
7,8
5,6
3
4
5
4
1
2
75% Efficiency at 115vac
Symbol Part Number
CT1 ETS92420
T1 ETS92423
L1 ETS92422
Note 1: Magnetic components are
available from Energy Transformation
Systems, Inc., tel: (415) 324-4949
Note 2: All resistors 1/4W unless
otherwise noted.
2A
(8 pin)
Application Hint 11
500kHz 30W Off-Line Switching Power Supply
by George Hall
Circuit Description
Line Input
Alternating line voltage is rectified by D1 and filtered by C1 to
provide a dc bus voltage for the main transformer T1 and
MIC38HC43 controller IC1.
Thermistor RT1 limits the in-rush current to C1, protecting D1,
and reducing the chance of an unacceptable momentary
voltage drop on the ac input line during turn-on.
PWM Operation
Resistors R1 and R2 charge C2 until its voltage exceeds the
UVLO (undervoltage lockout) of IC1 which causes output
drive to be applied to Q1. This lowers Q1’s drain voltage and
charges T1’s primary until the current sense voltage at pin 3
of IC1 exceeds 1V. IC1 then removes drive from Q1.
With Q1 off, T1 discharges into both the output (T1 pins 7,8
and 5,6) and tertiary (T1 pins 3 and 4) circuits and causes
Q1’s drain voltage to rise above C1’s voltage. IC1 voltage is
now supplied from the low impedance winding of T1 (pins 3
and 4).
The output voltage rises until IC2’s reference voltage reaches
approximately 2.5V where it begins drawing current through
the diode of optocoupler IC3. IC3’s detector transistor con-
ducts, raising the voltage on pin 2 of IC1. When the output
voltage equals 5V, pin 2 of IC1 will be 2.5V and current mode
PWM operation will regulate the output precisely over varying
line and load changes. R14 and C11 provide stability com-
pensation for IC2.
Q1 Protection
Components D2, C3, and R9 protect Q1 from avalanche
breakdown and possible destruction by clamping the leakage
inductance spike to a safe level. C6 and R10 suppress
parasitic oscillations from D6.
500kHz 30W Off-Line Switching Power Supply
Application Hint 11 Micrel
4-192 1997
EMI Filter
Electromagnetic interference feedback into the ac input line
from the operation of switched mode power supplies requires
EMI filtering to comply with national and international stan-
dards. Use these standards to determine the acceptable
levels of line conducted emissions for the specific application
and location.
EMI filtering may be simplified by procuring several packaged
EMI filters from a reputable source. Select the appropriate
filter by EMI measurement. Include the selected filter in the
final design or substitute the individual components (from the
filter’s parts list). Printed circuit board layout and component
placement will affect conducted emissions. If you are not
qualified in the this area consult an expert.
Circuit Layout
Care should be taken when designing high frequency con-
verters to avoid capacitive and inductive coupling of the
switching waveform into high impedance circuitry such as the
error amplifier, oscillator, and current sense amplifier. Avoid
long printed circuit traces and component lead lengths.
Locate oscillator and compensation circuitry near the IC. Use
high frequency decoupling capacitors on VREF and, if neces-
sary, on VDD. Return high di/dt currents directly to the source
and use large area ground planes where possible.
Safety
Always proceed with caution when working on off-line sup-
plies as lethal voltages are present. Never work on the supply
without someone nearby who is aware of the hazards and can
take steps to avoid serious injury to yourself in the event of an
accident.
5V REF UVLO
OSC
7(12)VDD
2.5V
2(3)
Feedback
/EA
1(1)
2R
Comp Analog
Ground
5(9) 3(5)
I Sense
S
RQ
R1V
35V
TQ
5(8)
Power
Ground
Output
6(10)
(Ckt) 7(11)
VD
(Output)
+
–
–
+
VREF
RT/CT
8(14)
4(7)
* MICx8C42, 43 / MICx8HC42, 43
†MICx8C44, 45 / MICx8HC44, 45
*
†
V
D
R /C
TT
R /C
TT
V
DD
V
REF
NC
V
REF
V
DD
FEEDBACK/EA
1
2
3
4
8
7
6
5
1
2
3
4
5
6
78
9
10
11
12
13
14
COMP
I
SENSE
OUTPUT
GND
NC
COMP
NC
NC
I
SENSE
OUTPUT
ANALOG GND
POWER GND
FEEDBACK/EA
Block Diagram Pin Configurations
8-pin DIP
14-pin DIP
(suffix -1, full featured parts)
Also available in 8-pin and 14-pin
SOIC packages.
