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G8MNY > TECH 19.12.05 19:43l 181 Lines 8812 Bytes #999 (0) @ WW
BID : 9304_GB7CIP
Read: GUEST DL1LCA OE7FMI
Subj: Switch Mode PS Principles
Path: DB0FHN<DB0FOR<DB0SIF<DB0CWS<DB0ZDF<DB0LJ<DB0RES<ON0BEL<GB7CIP
Sent: 051219/1213Z @:GB7CIP.#32.GBR.EU #:9304 [Caterham] $:9304_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Dec 05)
There are 2 types, they work slightly differently (when the secondary diode
conducts), but both use high frequency LF into a ferrite cored transformer.
With ferrite the eddy current losses are very small, & the BH curve magnetic
cycle area which determine the power loss per cycle is also much smaller than
for iron. (difficult to draw here).
__
IRON ³ __.--"/ A MODERN ³ ..-'
LAMINATE ³./' ./' FERRITE ³ //
./³ ./' ³ //
ÄÄÄÄ/ÄÄÅÄÄ/ÄÄÄ +H ÄÄÄÄÄÄ/Å/ÄÄÄÄÄ +H
./' ³/' // ³
./' _./'³ // ³
__.:..-" ³-B .-'' ³-B
The high frequency means smaller lighter transformers. For most ferrites the
power handling is proportional to the frequency. So a core running at 250KHz
can handle say 250W, but at 50KHz only 50W.
Transformer 8 ´ _ . -"""-._ eddy current &
Power 4 ´ _ . - ' `-. BH losses
2 ´ . - ' \ win out
1 ÁÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄ
5 10 20 40 80 160 320 640 KHz
The problems at these higher frequencies are the other circuit losses, not just
the increased eddy current & BH losses in the ferrite, but inter winding
capacitance, winding skin effect copper losses etc. External losses also
increase, such as transistor on & off times (FETs can be fast if driven well),
& diode rectifier on & off times & of course not to forget the increased
radiation losses from all the wiring as the frequency goes up.
MAINS CIRCUIT
Both types of SMPSU start with a MAINS FILTER (not always fitted despite QRM
laws!) the capacitors must be the "X" rated type for mains work, if earthed
there will also be 4n7 caps to earth. Sometimes a Voltage Dependent Resistor is
placed across the mains circuit in an attempt to protect the components from
line voltage surges.
The turn on current surge is limited by a resistor (say 5R 10W or a thermistor
& sometimes better put in the live feed) that feeds the bridge rectifier into
either 2x 200V electrolytics or 1x 400V one.
BRIDGE
SURGE ÚÄ´>ÃÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÂÄ +HT ÄÂÄ +HT
FUSE MAINS FILTER LIMIT ³ ³ ³ ³200v ³
LÄÄo-oÄÂÄÄÂÄÄÄ¿ ÚÄÄÄÂÄÄÄÂÄÄRÄÄÁÄ´<ÃÄ¿ ³ 100K === OR WITH ³
³ ³ (((() ³ ³ ³ ³ Link ³ ³ NO 110V ³ 300-380V
Anti ³ === ===== === VDR ÚÄÄÄÄÄÄÄ)Ä)ÄÄo oÄÄÅÄÄÄ´ OPTION === MAINS
shock R ³ (((() ³ ³ ³ ³ ³ 110V ³ ³200v WITH A ³ LIVE HT
NÄÄÄÄÄÄÁÄÄÁÄÄÄÙ ÀÄÄÄÁÄÄÄÁÄÄÄÁÄÂÄ´>ÃÄ)ÄÙ 100K === 400V CAP ³
BALANCED ³ ³ ³ ³ ³
CHOKE ÀÄ´<ÃÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄ -HT ÄÁÄ -HT
With the 2 caps a 110v mains option can be provided with just a wire link, that
uses only half the bridge as a voltage doubler. (OK if mains 60Hz & the 2 caps
are large value). All the smoothing caps primary & secondary must be low Z at
the pulse frequencies, so low equivalent series resistance ESR caps must be
used.
SINGLE ENDED
These use the back emf or "ringing up" of the stored energy in the ferrite
cored transformer. The transformer then does 3 jobs, provides isolation,
provides step down ratio, & provide choke input filter. As only DC pulses are
fed into the transformer only half the core BH curve energy can be utilised.
FAST Pi O/P Filter
+HT ÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄ¿ RECTIFIER ====
FERRITE )|ÚÄÄÄÄÄÄÄÄÄÄ)ÄÄÄ´>ÃÄÄÄÄÄÄÂÄ(((()ÄÂÄÄÄÂÄÄÄÄÄÄ +
TRANSFORMER )|( R === === ³ LOAD
³ ÀÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄ)ÄÄÄÄÂÄ -
___³ ÃÄÄÄÄ¿ ³ ³
³ \³ ÚÄÄÄÄÄÁÄ¿ ³+ ³ ³
R ÃÄÄÄÄ´Control³ === ³ ³
SNUBBER ³ e/³ ³ IC ³ ³ ³ ³
³ ÃÄÄÄÄÄÄ´ ³ ³ ________ set ³
=== ³ C ´ ÃÄÄ)ÄÄÄÄ´ opto ÃÄÄÄ>volts ³
³ R R ÀÄÄÄÂÄÄÄÙ ³ ³isolator³ pot ³
-HT ÄÄÄÄÁÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄ´________ÃÄÄÄÄÄÁÄÄÄÄÙ
OSC
The phase of the transformer is arranged so that when the transistor is turned
off the rectifier on the secondary conducts. The pulse width control IC is
powered from a HT dropper R, sometimes there is a separate mains PS or
secondary PS that takes over for this to give enough drive current.
The OSC CR sets the switching frequency, the IC also monitors the transistor
current with a small value emitter R. Voltage feedback from the output drives
an opto isolator to control the pulse width of the IC to alter the energy
stored in the transformer for the next pulse. Sometimes other windings can be
used but the regulation may be less accurate.
To stop the ringing up back emf from damaging the transistor a snubber CR is
used to terminate the +ve going HF pulse.
Due to the pulse current in the output is it usual to have a pi filter for the
DC output.
Rectifier
Secondary³ ringing³__ on ³
voltage ³ up ³ ³ load off ³
³ ³ ³º ringing ³
0v ´ÄÄ¿ pulse³ ³³ÀÄÄÄ¿ ³  Height due
³ ³ width³ º| ³ ³ ³ to mains V &
³ ³______³ ³______³ _³_turns ratio
³TRANSISTOR
³ ON OFF ON
In the single ended SMPSU the diode rectifier only conducts when the transistor
is turned off.
PUSH PULL
These are more complex & more powerful. The double action utilises the full
power handling of the transformer core, but that means the secondary must be
full wave & an additional HF pulse choke (about the same size of the
transformer) is now needed to convert the HT fixed height variable width
voltage pulse to the mean output load voltage.
+HT ÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ PULSE optional
ÚÄÄÄ)ÄÄÄÄ¿ diodes ³ CHOKE pi filter
³ ³ )|| ÚÄ´>ÃÄÂÄÄ)ÄÄÄ¿ ===== ====
³ ³_____)||( ³ R ((((()ÄÄÂÄ((((()ÂÄÄÄÂÄÄÄÄÄÄ +
³ )|| ÃÄÄÄÄÄ)ÄÄ)ÄÄ¿ ³ ³ ³ DC
³ )||( ³ ³ ³ === === ³ OUTPUT
³ ³ ÀÄ´>ÃÄÙ ³ ÀÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄ)ÄÄÄÄÂÄ -
³ ___³ ÃÄÄÄÄ¿ ³ ³
³ ³ \³ ÚÄÄÄÄÄÁÄ¿ ³+ ³ ³
__³ R T2ÃÄÄÄÄÄ´Control³ === ³ ³
³ \³ ³ e/³ ³ IC ³ ³ ³ ³
R T1ÃÄÄ)ÄÄÄ)ÄÄÄÄÄÄÄ´ ³ ³ ³ ³
³ e/³ ³ ³ ³ ³ ³ ³ ³
³ ÀÄÄÄÄ)ÄÄÄÅÄÄÄÄÄÄÄ´ ³ ³ ________ set ³
=== === ³ C ´ ÃÄÄ)ÄÄÄÄ´ opto ÃÄÄÄ>volts ³
³ ³ R R ÀÄÄÄÂÄÄÄÙ ³ ³isolator³ pot ³
-HT ÄÁÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄ´________ÃÄÄÄÄÄÁÄÄÄÄÙ
OSC
The control IC now has 2 drive outputs with not more than 45% on, as each power
switching transistor has to have some time to turn off before the other
antiphase one is turned on.
_____
³ ³ ³
Secondary ³ ³ ringing³T2 on³
voltage ³ º³ ³ ³³
0v ´¿pulseººÀÄÄÄÄÄÄÙ ººÚÄ
³³width³³ º³ringing
³³ ³both off ³
³³_____³
³ T1 on
Voltage ³ _____ _____ _
from ³³ ³ ³ ³ ³ height due
fast ³³_ _ _³ _ _ _ _³_ _ _³ mean ³ to mains V &
diodes ³³ ³ ³ ³ DC ³ turns ratio
ÁÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄ Á
on to off ratio depends
on load & supply voltage
The output diodes conduct when the transistors are on, pumping energy into the
pulse choke. As the pulse choke can be made big enough to still give current
between primary pulses (when there is no incoming source voltage so both diodes
may conduct) sometimes a pi DC output filter is not used.
SAFETY
1/ Working on them it is recommended you use a mains isolation transformer.
2/ Use a high voltage 1/10 or 1/100 scope probe, rated at 600V @ 100KHz not one
rated at 400v @ 100Hz!
3/ If in doubt if the HT is discharged when off, use a discharge R eg 10K 5W.
4/ Use the "one hand behind the back" safety precautions on HT circuits!
FURTHER READING
See my buls on "Stopping HF Tx/Rx SMPS QRM", "Reducing Electronic RF QRM" &
the crowbar part of "High AMP crowbar protected PSU".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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