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POWER PAGE PP43
Revised October 1996
TEL: 1-800-866-3590 FA X : 1 - 8 0 0 - 8 6 6 - 3 5 89
9340 OWENSMOUTH AVENUE C H AT S W O RTH C A L I F O R N I A 9 1 3 11
Theory of Operation
The PD Series modules are among the highest
power density modules in production today. Up
to 150 watts of output power is packed into a half
size industry standard footprint module only 0.4
inches high. The PD Series is based on
conventional, well understood technology.
Improvements in the magnetic element design,
and power and signal processing have been
applied to a forward converter topology.
The benefits of this approach are significant:
1.
Fixed frequency switching means that
the switching noise is predictable and
easily filterable.
2.
The power semi-conductors have l o w e r
v o l t a g e a n d c u r r e n t s t r e s s t h a n a r e
p r e s e n t u s i n g t h e r e s o n a n t or
quasi-resonant topologies. This means
that the power processing efficiency is
h i g h e r. I t a l s o m e a n s m o r e r e l i a b l e
operation.
3 .
F e w e r c o m p o n e n t s a r e r e q u i r e d .
Transformer reset, current limit, over-voltage
protection, bias supply, etc. are greatly
simplified, so fewer components are
needed. T h i s m e a n s t h a t t h e m o d u l e
s i z e i s reduced. It also means more
reliable operation.
4 .
I t s easier for Powercube to offer extra
f e a t u r e s l i k e s w i t c h i n g f r e q u e n c y
synchronization, module paralleling,
over-temperature protection and over-voltage
protection as standard features. T h e s e
features were normally available only in
full size modules. Also, the topology
u s e d a l l o w s f o r p a r a l l e l i n g o f u p t o
2 0 modules (for power boosting) without
t h e need for separate booster, driver
o r m a s t e r m o d u l e s . T h e s t a n d a r d
PD150 module is all that is required -
t h e r e b y eliminating the need for stocking
d i fferent types of modules.
Although most of the text in this
Power Page
TM
refers to PD150 converters, the discussion is
equally suitable for Model PD300, a 300 watt
DC-to-DC converter. Where applicable, the
differences are highlighted in the application.
1
POWER
PAGE
BASIC OPERATION AND FEATURES
PD SERIES
The operation of the PD Series DC-to-DC
converter is illustrated in the functional block
diagram of Figure 1. Input voltage is applied to
a low pass filter (formed by LIN, CIN) which
serves a dual purpose. First it filters source
transients and noise, and second, it reduces
(by 30dB) the amount of MOSFET switching
current which is reflected onto the input source.
The energy conversion process occurs when
MOSFET switch turn-on applies voltage from
the
input source to the
inductor L1.
S i m u l t a n e o u s l y, current increases linearly in
both the MOSFET and L1. MOSFET switch
turn-off causes current in L1 to decrease. An
output low pass filter (formed by L1, C1, L2, C2)
filters the output to provide a low ripple DC
output. Figure 2 shows the current and voltage
wave form in the MOSFET switch, the current in
the L1 inductor (output current) and SYNC
wave form (for Option P) and GA T E O U T
wave form (for Option V).
The conversion techniques used by the PD150
A NATEL COMPANY
POWERCUBE PP43
Revised October 1996
TEL: 1-800-866-3590 FAX: 1-800-866-3589
9340 OWENSMOUTH AVENUE C H AT S W O RTH C A L I F O R N I A 9 1 3 11
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2
design produces gently sloping current in L1, which
is more easily filtered, thereby producing lower
output ripple. The square wave switch current in the
MOSFET contains more energy, as compared to
peaked current of a half sinusoidal wave, thereby
making the conversion process more efficient.
Logic Control circuit provides a fixed switching
frequency of 375 KHz. The desired DC output
voltage is maintained by setting the ON- time of
the MOSFET. Logic Control receives output voltage
information from the Error Amplifier and load current
information from the Current Limit circuit in order to
maintain regulation of the DC output voltage.
Current Limiting: Current limit is accomplished by
using two independent circuits to protect the PD
Series from damage if the module is overloaded or
shorted. This series of DC/DC converters uses
current mode control and active trimming within the
current sense circuit in order to set a precision
current limit operating point. The output voltage
remains constant and within its specified limits up to
the current limit set point, which is 5 to 40% beyond
Figure 2.
Primary Current, Primary Voltage, Inductor Current,
SYNC and GATE OUT Wa v e f o r m s
POWER
PAGE
0
lL
lo
0
Vin
Vp
lp
Referenced to -Vin
Gate
Out
(V Option)
Sync
(P Option)
Output Current
(L1 Current)
Voltage
Current
MOSFET Switch
Isolation
MOSFET
Logic
Control
Thermal
Shutdown
Current
Limit
Opto -
Coupler
+ 2.5V
REF.
+ S
S
T
+ OUT
OUT
D2
C1
C2
L1
L2
Output Filter
Power Transformer
D1
Current
Sense
Transformer
Error
Amplifier
+ IN
IN
SD
SYNC
Input
Filter
OVP
1
4
3
2
5
9
7
6
8
C
IN
L
IN
Figure 1. Block Diagram PD150 Series TEL: 1-800-866-3590 FAX: 1-800-866-3589
9340 OWENSMOUTH AVENUE C H AT S W O RTH C A L I F O R N I A 9 1 3 11
Straight Line Current Limiting
Tbaseplate <80C
the maximum rated current. If the current at the
rated voltage is greater than the current limit set
point, the output voltage will decrease in order to
limit the output current. During an overload
condition, the Logic Control reduces the ON-time
of the MOSFET so that the output voltage is
reduced. If a short-circuit or very low impedance
condition, such as shorted capacitor, is present, the
ON-time of the MOSFET is reduced to an
extremely short time, or pulses may be skipped
e n t i r e l y, in order to limit the internal power
dissipation. When the temperature of the module is
greater than 80C, the narrow duty cycle
a n d
over-temperature protect circuits work
together to produce foldback current limiting, in
order to protect the internal components from
damage due to extreme heat. Figure 3 shows the
two modes of current limiting. Following removal
of the overload or short-circuit condition, the PD
Series will automatically resume normal operation.
When applied correctly, the Model PD150 is
virtually indestructible. If an abnormal and/or
Foldback Current Limiting
Tbaseplate >80
C
potentially destructive condition occurs, the
protection circuits Current Limit, Over-Voltage
Protection (OVP), and Thermal Shutdown cause the
Logic Control to turn-off the MOSFET, thus turning
off the DC-to-DC converter. The converter can also
be turned off externally by applying a Logic Low
signal to SD pin.
Sense Circuit: When the sense pins +S and S
are connected directly across the load, the
converter compensates for the voltage drop in the
load lines. This is accomplished by monitoring the
voltage across +S and S pins. This voltage is
then compared with an internal 2.5 Volt, temperature
stabilized band - gap reference (which represents
the ideal output) and amplified through the Error
A m p l i f i e r. Then through an opto-coupler (for isolation),
this signal is applied to Logic Control to adjust the
ON-time of the MOSFET, thereby maintaining the
output voltage. The DC output voltage of the Model
PD150 can be adjusted by using the trim pin T.
This is accomplished by simply adjusting the
reference voltage, which then changes the output
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3
POWER
PAGE
Vout
lout
Rated
Current
I short circuit
Current limit
trip point
Vout
lout
Rated
Current
I short
circuit
Current limit
trip point
1.5V
Figure 3. PD Series Current Limiting
PP43
Revised October 1996 PP43
Revised October 1996
TEL: 1-800-866-3590 FAX: 1-800-866-3589
9340 OWENSMOUTH AVENUE C H AT S W O RTH C A L I F O R N I A 9 1 3 11
A NATEL COMPANY
POWERCUBE
4
voltage proportionally. The output voltage can be
changed by any of several methods.
For more details ask for:
PP42, Output Voltage Trimming
For ease of application in existing sockets, the
PD150 sense and trim circuits are designed to be
compatible with the industry standard circuits, as
well as have same component values.
SYNC Pin (P Option): The SYNC pin, when
connected with other modules, causes all connected
Logic Control circuits to synchronize their switching
frequency to the module whose Logic Control has
the highest switching frequency. Similarly, if the
SYNC pin is connected to an external clock signal
whose frequency is higher than the connected
module, the Logic Control of that module will
synchronize with the external clock. The SYNC
function is available for P option only.
SD Pin (P