Proper regulation is important in multiple output power supplies where the individual outputs must be
tightly controlled.
The continuing need for more compact and reliable switching power supplies has aroused renewed in-
terest in a well founded control technique: THE MAGNETIC AMPLIFIER.
THE MAGNETIC AMPLIFIER ( shortly written MAG -AMP ), means higher density, simpler control circuit
and higher running frequency in Switching Power Supply.
The MAG AMP is suitable for controlling individual outputs ranging from a few watts to over 100 watts in
power converters whose frequencies range from 20KHz to several hundred KHz.
Picture 1 shows the best application of the MAG AMP is in the Switched - Mode Power Supplies .
By using such a square - loop core to provide a controllable delay at the leading edge of the pulses at
the secondary of the transformer, one or more outputs can be independently and precisely regulated
without the losses inherent in linear regulators or the complexity of conventional switching regulators. In
cases where the load currents of the subordinate outputs are high ( more than 2 - 3 Amps ), the advan-
tages of the saturable regulators become more significant.
December 1997
AN904
APPLICATION NOTE
MAGNETIC AMPLIFIER WITH LPR30 CONTROLLER
by F. Lentini & T. Sampieri
V+
PWM
DRIVER
CR
MAIN Vout
-
+
FEEDBAK
CR
AUXILIAR 1
Vout
-
+
REGULATOR
CR
AUXILIAR 2
Vout
-
+
REGULATOR
Figure 1. Multiple-Output Switched-Mode Power Supplies.
1/3
LPR30 CONTROLLER
Using the LPR30 like controller (as shown in fig.2) , in the PWM (Auxiliary Outputs) the pulse width is
controlled by sensing the output voltage comparing it (pin 6) to the internal reference (pin 4) and using
the Error signal to adjust the pulse duration. Without any MAG AMP plus controller, in the Auxiliary cir-
cuits the outputs would be " semi - regulated ", since the primary control loop would provide line regula-
tion. But the other output would vary with load and temperature.
Besides, the LPR30 controller plus MAG AMP is particularly advantageous when independent current
limiting for the auxiliary outputs is desired, due to the fact the LPR30 is able to monitor even the current.
It is always active and together with MAG AMP provides an exceptionaly smooth transition as the output
loaded beyond the current limit and then returned to normal load condition.
To do this job, the LPR30 uses its second internal comparator ( CURRENT LIMIT AMPLIFIER , pin 2
and pin 3 see fig.3) that has an artificial offset of 50mV.
This means that when the voltage to the R
SENSE
is more than 50mV, the CURRENT LIMIT AMPLIFIER
recognises that the current is more than the nominal one generating then an error signal that is applied
to the MAG AMP RESET TRANSISTOR.
The increase in reset current decreases the pulses width at the output of the MAG AMP and thus op-
poses the increase in current which was sensed by R
SENSE
.
V+
PWM
DRIVER
VIN
5
LIM-
3
LIM+
2
ERR-
7
VREF
ERR+
8
1
6
4
LPR30
Rsense
SR
CR1
L4
CR2
AUXILIARY Vout
-
+
CR3
+6
+
CURR.
LIM .
GND
REF
AM P.
ERR.
CR2
MAIN Vout
-
+
FEEDBAK
Figure 2.
VIN
5
LIM-
3
LIM+
2
ERR-
7
VREF
ERR+
8
1
6
4
LPR30
Rsense
SR
CR1
L4
CR2
Vout
-
+
CR3
+6
+
V+
PWM
CONTROLLER
CURR.
LIM.
GND
REF
AMP.
ERR.
CR4
CURRENT
CONTROL
Figure 3.
AN904 APPLICATION NOTE
2/3
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the
consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No
license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specification men-
tioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied.
SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without ex-
press written approval of SGS-THOMSON Microelectronics.
1997 SGS-THOMSON Microelectronics Printed in Italy All Rights Reserved
SGS-THOMSON Microelectronics GROUP OF COMPANIES
Australia - Brazil - Canada - China - France - Germany - Italy - Japan - Korea - Malaysia - Malta - Morocco - The Netherlands -
Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A.
AN904 APPLICATION NOTE
3/3
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